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Departement Scheikunde
MICROSCOPIC ANALYSIS OF ROMAN VESSEL GLASS
Proefschrift voorgelegd tot het behalen van de graad van Doctor in de Wetenschappen aan de Universitaire Instelling Antwerpen te verdedigen door
Ann AERTS
Promotoren : Prof. Dr. F. Adams Prof. Dr. K. Janssens Antwerpen, 1998
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dankwoord
Zoals iedereen weet, is een doctoraat geen solowerk, daarom wil ik graag alle mensen bedanken die op hun eigen manier hebben bijgedragen tot het voltooien van dit werk.
Op de eerste plaats mijn beide promotoren Prof. Dr. F. Adams en Prof. Dr. K. Janssens omdat ze mij de mogelijkheid hebben gegeven mijn onderzoek uit te voeren in hun onderzoeksgroep, voor het aanbrengen van het onderwerp, de raad en de begeleiding.
Het Fonds voor Wetenschappelijk Onderzoek - Vlaanderen voor de financiele steun gedurende vier jaar.
Dr. M. Schreiner voor de hulp tijdens mijn bezoek aan de Technische Universitat Wien en Dr. D. Simons en Dr. D. Bright voor de goede samenwerking die ervoor gezorgd heeft dat mijn onderzoeksverblijf aan het National Institute of Standards and Technology, Maryland, USA zowel prettig als leerrijk was.
Dr. H. Wouters en C. Fontaine van het Koninklijk Instituut voor het Kunstpatrimonium te Brussel voor het bezorgen van de Qumran monsters en de hulp bij het interpreteren van de resultaten, de mensen van het Instituut voor het Archeologisch Patrimonium, Vlaanderen, W. Dijkman van Stadsontwikkeling en Grondzaken te Maastricht, Prof. R.C.A. Rottlander en Dr. B. Velde voor het beschikbaar stellen van uitgebreide reeksen Romeinse glasvoorwerpen en Dr. D. Fuchs en Dr. H. Rdmich van het Fraunhofer-lnstitut fur Silicatforschung voor het gebruik van de modelglaasjes en de hulp en raad tijdens de experimenten.
Verder ook nog C. Ferauge, W. Dorrine, K. De Cauwsemaecker en R. Saelens voor de hulp bij allerlei problemen met computers en/of toestellen, Laszlo voorzijn geduld en hulp bij alle mogelijke synchrotron problemen en Nathalie, Peggy en Fabiana voor de infrarood metingen.
Ine en Olivier voor de goede samenwerking en de interessante discussies, natuurlijk Monika, Rita, Begona, Hanna, Volodymyr, Liu en Marc die voor korte of lange tijd het bureau met mij gedeeld hebben en ervoor zorgden dat er altijd een prettige sfeer was en ook al diegenen die ervoor gezorgd hebben dat de middagpauzes altijd zeer gezellig waren.
Tenslotte wil ik al mijn vrienden en in het bijzonder mijn zus en ouders laten weten hoezeer ik het apprecieer dat zij mij altijd hebben gesteund, aangemoedigd en mijn geklaag hebben aanhoord als het al eens wat moeilijker ging. Bedankt, zonder jullie was het nooit gelukt!
Ann Aerts
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TABLE OF CONTENTS
Chapter 1 : Structure and Properties of Glass
1. The Glassy State ...... 1 2. The Structure of Silicate Glass ...... 2 3. Composition of Glass ...... 5 3.1. Network formers ...... 5 3.2. Network modifiers ...... 5 3.2.1. Network breakers ...... 5 3.2.2. Network stabilisers ...... 6 3.3. Intermediates ...... 6 3.4. Colouring oxides and decolourisers ...... 6
4. Reactions Between Glass Surfaces and Water ...... 7 4.1. Corrosion processes in aqueous solutions ...... 7 4.2. W eathering...... 9
5. Durability of G lass ...... 10 5.1. Composition of the bulk glass ...... 11 5.2. pH of the solution ...... 12 5.3. Composition of the solution ...... 14 5.4. Surface area of the glass, the ratio of the surface area of the glass to the volume of the solution and replenishment of the solution ...... 15 6. References 16
Chapter 2 : Instrumentation and Quantitation 1. Introduction...... 21
2. Scanning electron microscopy (SEM) ...... 22 2.1. Principle of scanning electron microscopy, interactions between electron beam and specimen ...... 22 2.1.1. Elastic scattering ...... 23 2.1.2. Inelastic scattering ...... 23 2.2. Instrumentation ...... 25 2.3. Homogeneity of the standard reference material ...... 28 2.3.1. NIST Standard Reference Materials ...... 28 2.3.2. Corning Museum of Glass standards ...... 30 2.4. Quantitative analysis by means of ZAF-method ...... 33 2.5. Accuracy and precision of major element analysis ...... 34
3. Micro synchrotron radiation induced X-ray fluorescence (p-SRXRF) ...... 39
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3.1. Principle of p-SRXRF ...... 39 3.2. Quantitative analysis by means of a Monte Carlo simulation model....42 3.3. SRXRF instruments at which experiments were performed ...... 42 3.3.1. NSLS beamline X26A ...... 43 3.3.2. Hasylab beamline L ...... 43 3.4. Accuracy and precision of trace element determinations ...... 44 4. Micro particle induced X-ray emission spectrometry (p-PIXE) ...... 47
5. Secondary ion mass spectrometry (SIMS) ...... 48 5.1. Principle of SIMS measurements ...... 48 5.2. The Cameca IMS 4F ...... 50 5.2.1. Primary ion beam system ...... •...... 50 5.2.2. Secondary ion beam system ...... 51 6. Fourier transform infrared spectrometry (FTIR) ...... 53 6.1. Principle of Fourier transform infrared spectrometry ...... 53 6.2. Instrumentation ...... 53
7. Summary ...... 55
References ...... 56
Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel 1. Introduction...... 59
2. The archaeological site and the glass collection ...... 60 2.1. The archaeological context of the site ...... 60 2.2. Description of the glass collection ...... 62 2.3. Aim of the study ...... 65
3. Experimental procedure ...... 66 3.1. Sample preparation ...... 66 3.2. Major element composition determination ...... 67 3.3. Trace element composition determination ...... 67 3.4. Multivariate data treatment ...... 67
4. Overall morphology of the samples cross-sections ...... 69 5. The bulk glass ...... 70 5.1. Major, minor and trace composition of the bulk glass ...... 70 5.2. Classification of data ...... 78
6. Prediction of durability ...... 82 6.1. Triangular diagram ...... 82 6.2. Thermodynamic approach ...... 84
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7. The leached layer ...... 86 7.1. The composition ...... 86 7.2. Layered morphology ...... 88 7.3. Intruded materials ...... 91
8. The surface ...... 94 8.1. Outlook of the surface ...... 94 8.2. The crust ...... 94 9. Conclusions...... 96
References ...... 97
Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 1. Introduction...... 101
2. Glass collections and sample material ...... 102 2.1. Origin of sample material ...... 102 2.2. Classification of the data ...... 104 2.3. The colour of glass ...... 104 3. Glasses from Cologne (Germany) ...... 105 3.1. Description of the glass collection and its location ...... 105 3.2. Classification of the data ...... 107 3.3. Decolourising agents in glass ...... 107 3.4. Correlations between major elements ...... 110 4. Glasses from Cologne (Germany), Trier (Germany) and Rouen (France)....110 4.1. Description of the glass samples ...... 110 4.2. Classification of the data ...... 111 4.3. Glass objects from Cologne ...... 113 4.3.1. Decolourising agents ...... 113 4.3.2. Correlations between major elements ...... 115 4.3.3. Correlations between trace elements : colouring agents 115 4.4. Glass objects from Rouen and Trier ...... 118
5. Glasses from Tongeren (Belgium) ...... 119 5.1. Description of the glass collection and its location ...... 119 5.2. Classification of the data ...... 121 5.3. Decolourising agents in the glass ...... 121 5.4. Correlations between major elements ...... 122 5.5. Correlations between trace elements : colouring agents ...... 122 6. Glasses from Oudenburg (Belgium) ...... 124
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6.1. Description of the glass collection and its location ...... 124 6.2. Classification of the data ...... 124 6.3. Decolourising agents in the glass ...... 126 6.4. Correlations between major elements ...... 127 6.5. Correlations between trace elements : colouring agents ...... 128
7. Glasses from Grobbendonk (Belgium) ...... 129 7.1. Description of the glasses and their location ...... 129 7.2. Classification of the data ...... 132 7.3. Correlations between major, minor and trace elements ...... 132 7.4. Opalising agents in glass ...... 132 8. Glasses from the southern part of Belgium ...... 133 8.1. Description of the glass objects and their location ...... 133 8.2. Classification of the data ...... 133 8.3. Correlations between major, minor and trace elements ...... 135 9. Glasses from Maastricht (The Netherlands) ...... 135 9.1. Description of the glass collection and its location ...... 135 9.2. Correlations between major elements ...... 137 9.3. Colouring agents in the glass ...... 139 9.4. Opacifying agents in the glass ...... 140
10. Glasses from Mancetter and Leicester (UK) ...... 141 11. General overview ...... 142
References ...... 146
Chapter 5 : Investigation of Some Historical Potash Based Glasses 1. Introduction...... 149
2. Description of the glasses ...... 149 2.1. Glass from Namen ...... 149 2.2. Glass from Fagnolle ...... 150
3. Quantitative analysis of the bulk glass ...... 150 4. Prediction of durability...... 153 4.1. Triangular diagram ...... 153 4.2. Thermodynamic approach ...... 154 5. Corrosion phenomena ...... 154 5.1. The leached layer ...... 155 5.2. The surface ...... 159
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6. Conclusions...... 160 References ...... 161
Chapter 6 : Burial Experiments with Model Glasses 1. Introduction...... 163 2. Production process and composition of model glasses ...... 166 2.1. Production process ...... 166 2.2. Composition of the model glasses ...... 167 3. Burial conditions ...... 167 3.1. Natural environments ...... 167 3.2. Controlled laboratory environments ...... 169 3.2.1. Humidity...... 169 3.2.2. pH...... 170 3.2.3. Temperature ...... 170 3.2.4. Exposure time ...... 170 3.2.5. Surface condition of the model glasses ...... 170 4. Analytical techniques used to study the corrosion of the model glasses 171
5. Results from IR and SEM measurements ...... 172 5.1. Infrared measurements ...... 172 5.2. Infrared results before burial ...... 175 5.3. Degree of corrosion : SEM versus IR measurements ...... 175 5.4. Results after exposure to soil in the laboratory ...... 177 5.5. Results after burial in test areas ...... 179 6. The effect of different parameters on the corrosionof the buried model glasses ...... 181 6.1. Humidity...... 181 6.2. p H ...... 181 6.3. Temperature ...... 182 6.4. Composition of the model glasses ...... 183 6.5. Surface condition of the model glass ...... 184 6.6. Composition of the soil ...... 185
7. Examination of cross-sections of the glasses ...... 186 7.1. Scanning electron microscopy ...... 186 7.2. Secondary ion mass spectrometry ...... 191 7.3. Micro synchrotron radiation induced X-ray fluorescence ...... 195 8. Model glass exposed to iron and copper solutions ...... 197 9. Crust formed on the model glasses ...... 199
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10. Conclusions...... 201 References ...... 203 Summary ...... 205 Samenvatting ...... 209 List of Publications ...... 215 Conference Contributions and Study Visits ...... 217
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Structure and Properties of Glass
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1. THE GLASSY STATE
In order to understand the processes of glass deterioration which may make restoration and conservation a necessity, a knowledge of the nature of glass, its chemical structure and physical properties, is essential.
Glass may be defined as a material which has been cooled to a solid condition while still retaining its liquid structure, without crystallisation taking place. In a liquid the atoms are joined to one another but not in a regular, symmetrical three-dimensional pattern; they form a random structure. When the liquid that is to become a glass is cooled down from very high temperatures to room temperature, no discontinuous changes take place. It simply becomes more viscous until it becomes a solid but keeps the internal structure of a liquid. Therefore glass can be described as a “supercooled” liquid in a metastable state which has been cooled far below the temperature at which freezing should have taken place. The very rapid increase in viscosity below the “melting” temperature explains why the glass can stay in this metastable state; the only way to pass into the stable crystalline state is by breaking the intermolecular bonds in the liquid and establishing new bonds to form a regular crystal lattice. However, not enough energy is available for these strong bonds to be broken and the glass-forming liquid fails to crystallise [1, 2, 3],
I •>
Temperature
Figure 1. Schematic representation of the changes in volume during cooling of a melt. Formation of a crystalline material: A-B-C and formation of a glass : A-E-F.
Figure 1 clearly shows the relationship between the liquid, solid and glassy states. At high temperature the system is in the liquid state (A). During cooling the volume decreases due to configurational shrinkage and according to the thermal coefficient of expansion of the liquid until point B, corresponding to temperature T L (the freezing temperature), is reached where freezing would be expected to occur. There will be
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a sudden decrease in volume (down to point C) due to crystallisation followed by further decrease (along line C-D) but now according to the thermal coefficient of expansion of the crystalline material. If the liquid is cooled very fast it can happen that the temperature TL is crossed without crystallisation taking place. The system stays in the liquid state and will continue to shrink due to configurational changes and according to the thermal coefficient of expansion of the “supercooled” liquid. The viscosity Increases in such a way that at the end configurational adjustments do not have time to occur. From point E on, which corresponds to temperature TG (the transformation temperature), the substance will only shrink by thermal contraction and is in the glassy state. The volume is larger than for the crystalline state, corresponding to a more open structure of the glass.
2. THE STRUCTURE OF SILICATE GLASS
The major constituent of glass is silicon dioxide (silica), S i02. In its crystalline form its basic structure is that of a tetrahedron, with four oxygen atoms surrounding a central silicon atom as represented in Figure 2 [1].
o 0
Figure 2. A S i0 4 tetrahedron, the basic structural unit of crystalline silica.
Each oxygen atom is shared between two silicon atoms. A three-dimensional network is built up by Si-O bonds at the comers of a tetrahedron. The four bonds from the silicon atom to the surrounding oxygens are strongly directional and always tend to keep the same relative orientations but the Si-O-Si angle between the two tetrahedra can vary and tetrahedra can rotate relative to one another [4]. A clear understanding of the structure of glass has only been achieved in the last 65 years. Zachariasen was the first to make a significant advance. In 1932 he proposed the random network in which the atoms are linked together by strong forces, essentially the same as those in crystals. In glass, however, the network is not periodical and symmetrical [5].
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From the two-dimensional representation in Figure 3 [5] it is clear that in both the crystal and the glass, corners are shared between oxygen triangles but in the glass the triangles are arranged irregularly and the structure is more open than in the crystalline form. In the case of silica glass the strong Si-O bonds form a tightly braced structure and this explains the high softening temperature (about 1700 C) necessary to disrupt the structural arrangements.
Figure 3. Structure of an imaginary two-dimensional oxide A20 3 in a crystalline form (a) and as a glass (b).
Addition of alkali metallic oxides, introduced as MzO (e.g. Na20 and K20), will bring, next to the positively charged metal ions, one extra oxygen ion with every two of them added. For each extra oxygen atom introduced, one of the Si-O-Si bonds in the network is broken and two non-bridging oxygens are produced which have a negative charge. As it is necessary to preserve the electrical neutrality within the structure, the positively charged metal ions occupy spaces in the network near to non-bridging oxygens. The continuity of the network is broken up and the new bonds are weaker and non-directional. The result is that the structure is less strong and the viscosity of the glass becomes much lower than that of pure silica [1, 6].
The structure of sodium-silica glass is represented in Figure 4 [7]. It is essentially the same as the one for potassium-silica glass except for the co-ordination number of the alkali ions which is five for sodium and ten for potassium. The difference is due to the size of the ions (the ionic radius of sodium : 0.98 A and of potassium : 1 .38 A). The distance between potassium and oxygen is therefore larger causing the potassium ions to be held more loosely in the network [8, 9].
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Figure 4. Two-dimensional representation of alkali-silica glass. Open circles represent oxygen atoms, filled circles silicon and shaded circles atoms of the singly charged metallic ion sodium or potassium.
Figure 5. Two dimensional representation of soda-lime-silica or potash-lime-silica glass. Open, filled and shaded circles as in Figure 4, cross-hatched circles represent calcium atoms.
Divalent alkaline earth ions M2+ introduce one extra oxygen when they are added as oxides (MO, e.g. CaO). They neutralise the negative charge on two non-bridging oxygen ions and thus tend to form a new link in the network which strengthens it again and by doing so they improve the chemical resistance of the glass. The result is shown in Figure 5 [10] which represents the basic structure of soda-lime-silica
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glass, the basic structure for Roman and modern glass in contrast to potassium- lime-silica glass, chiefly used in Europe during the Middle Ages [6].
3. COMPOSITION OF GLASS
The chemical composition of ancient glass objects is fairly complex but the components can be divided into four categories [1, 6, 11, 12].
3.1 Network formers
They form the most important part of the glass. Several inorganic oxides have the ability to form vitreous materials. The most common one in ancient glass is silica (Si02), other network formers are the oxides of boron (B20 3), phosphorus (P20 5) and titanium (TiOz). All of these can be found in combination with each other.
The main source of silica for ancient glass production was sand. As an alternative source, crushed flint or quartz pebbles were often preferred because the sand in certain areas contains considerable amounts of impurities.
3.2 Network modifiers
3.2.1 Network breakers The principal network breakers of which ancient glass was composed were the oxides of sodium (NaaO) and potassium (K20), but other components with general formula R20 (mainly alkali oxides) belong to this group. One disadvantage of the behaviour of alkali ions is that the glass is made less durable (even soluble in water) but the great advantage is that its viscosity and melting point are lowered because the network is made more open.
Alkalis for glassmaking have been obtained from many different sources in the past. Up until the medieval period both in Western Europe and the East the dominant alkali in ancient glass was sodium due to the use of soda (a sodium containing compound, usually Na2C 0 3). At some time prior to 1000 AD a great change came about in the type of glass produced in Western Europe north of the Alps : an almost complete change to the potash type glass, containing potassium as the major alkali, occurred.
Sources of sodium oxide available to the glassmakers included natural deposits resulting from naturally occurring evaporation and drying-up of land-locked seas and lakes and salts obtained by deliberate evaporation of sea or river water in pits or pans. The other major source of alkali was plant ash. This could produce a soda or potash type glass depending on the plant composition, coastal plants being relatively rich in soda and inland plants in potash.
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3.2.2 Network stabilisers Earth alkaline oxides, mostly CaO, are added to the batch to increase the durability of the glass. Besides CaO, other oxides belonging to the type RO, like MgO, BaO or CuO can be used.
Chalk, limestone or burnt shells have been used as convenient sources of lime (CaO). Its inclusion probably occurred as an impurity in the sand used in the eastern part of the Roman territory (sand from the Belus river which contains a considerable amount of CaO), since no reference is made to the deliberate use of lime to glasses produced there.
3.3 Intermediates
Oxides of type R20 3 such as Al20 3, Fe20 3 and Cr20 3 can either be incorporated in the network or can immobilise the alkali ions.
Aluminium oxide was an inevitable contaminant, being released by the crucibles’ walls themselves during glassmaking and the other raw materials used also yielded considerable alumina and varying amounts of iron oxide.
3.4 Colouring oxides and decolourisers
Colouring in ancient glass was produced in three ways. First by the presence of relatively small amounts of the oxides of transition metals such as cobalt (CoO), copper (CuO), iron (FeO), nickel (NiO), manganese (MnO), etc., which go into solution in the silicate network and form part of it in the same way as other multivalent cations do. Second, by the development of colloidal dispersions of insoluble particles such as those in silver stains or in copper and gold ruby glass. Third, opal and translucent effects were produced by the introduction of opalising agents (e.g. Ca2Sb2Or and Pb2Sb20 7) which gave rise to the formation of small crystallites in the glass that scattered the light rendering the glass opaque.
The production of colours in glass depends not only upon the inclusion of a specific metal oxide but also on the presence of other oxides in the batch and the temperature and state of oxidation or reduction in the kiln. Glass containing iron oxide for example could be decolourised by converting the blue colour of the reduced iron (Fe2+) to the yellow colour of the oxidised state (Fe3+) by altering the melting conditions or by adding oxidising agents such as the oxides of manganese or antimony to the glass batch.
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4. REACTIONS BETWEEN GLASS SURFACES AND WATER
4.1 Corrosion processes in aqueous solutions
Water is a primary agent of the environment which causes the deterioration of glass, its effect having been established by Lavoisier in 1770, but the earliest scientific examination of weathered glass was that by Brewster in 1863 who studied some iridescent glass vessels from Nineveh [6]. When a glass reacts with an aqueous solution, both chemical and structural changes occur within the glass surface. Reactions which corrode glass in water include ion-exchange processes, hydration, hydrolysis and condensation. In most cases, the rate at which water enters the glass structure controls the kinetics of the other glass water reactions, explaining the corrosion characteristics of different glass compositions [13].
Several early investigators [14, 15, 16] have already suggested that glass-water reactions should be considered in two stages :
Stage 1 : The initial or primary stage of attack is a process which involves ion exchange between alkali ions from the glass and hydrogen (or hydronium) ions from the solution, during which the remaining constituents of the glass are not altered [17, 18].
-S i-O -M (glass) + FI (solution) 'r_ ~Si-OH(g|assj + M (solution) (1)
Characteristics for the stage 1 process are : 1) The effective area of SiOz exposed to the corrosion solution is increased by the production of surface micropores resulting from alkali removal [17]. 2) The ratio of the alkali to silica in the extract is always higher than the same ratio in the bulk glass, thus showing that an alkali deficient layer (leached layer, gel layer) is formed on the surface of the glass. This layer forms a diffusion barrier through which further alkali ions must pass before they can be brought into solution [19, 20, 21]. 3) The pH of the corroding solution increases as a result of alkali ions replacing hydrogen ions in solution. 4) The rate and extent of ion-exchange depend on solution chemistry as well as on the glass structure [13] and the diffusion of HzO molecules is assumed to be the rate determining step [22, 23, 24],
Stage 2 : The second stage of attack is a process whereby breakdown of the silica structure occurs and total glass dissolution ensues.
~Si-0-Si~ + OH' ^ ~Si-OH + O-Si- (2)
~Si-0" + H20 ^ ~Si-OH + OH' (3)
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A hydroxyl ion in solution disrupts siloxane bonds in glass (reaction 2). The non bridging oxygen formed in this reaction interacts further with a molecule of water (reaction 3) producing a silanolgroup and a new hydroxyl ion which is free to repeat reaction 2 again, hence acting as a catalyst [19, 25]. These reactions break up the strong rigid glass network to a more weakly bound gel; however the gel still contains enough primary Si-O bonds to hold it together and delay dissolution.
In an infrared study on corroded thin films of glass Scholze et al. [26] have reported the existence of free w ater molecules inside the leached layer; however it is not clear from this study whether these have really diffused from the solution phase as H20 (hydration) or H30 ‘'’ species or formed in situ inside the leached layer due to an autocondensation reaction of the following type.
~Si-OH + HO-Si~ ^ ~Si-0-Si~ + H20 (4)
This water in silicate glass is mainly present as molecular HzO with an ionic co ordination similar to that in crystalline hydrates. The molecules may be present in two configurations, either as a complex with a strong hydrogen bonding or a complex with a weak hydrogen bonding [27].
Although no sharp demarcation exists between stage 1 and 2 processes, the latter effectively begin at pH > 9. This reaction involves complete breakdown of the silica network and eventually all species in the glass dissolve simultaneously.
A general equation for the corrosion of glass is given by Rana and Douglas [28] and Douglas and El-Shamy [29] involving both selective leaching and total dissolution mechanisms at constant temperature.
Q = at1'2 + bt (5) Q = total quantity o f alkali released by both selective leaching and total dissolution t = duration of the experiment a, b = empirically determined constants
The rate of silica extraction from the glass has also been found to obey a similar equation [30].
In the limit, as t approaches zero, the gradient approaches 1/2 and for t approaching infinity, the gradient approaches 1. Plots of log Q versus log t are close to linear over extended times with slopes varying between 1/2 and 1 with increasing time and temperature.
As the glass begins to leach, the first term of equation 5 dominates and the leaching behaviour is proportional to square-root time kinetics. This is expected to be most important for glass that has good chemical durability, low leaching temperatures and short leaching times and for glass being leached in acid or neutral solutions [18, 20,
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21, 31]. During this stage the rate of alkali extraction from the glass is therefore parabolic in character [28, 32, 33], Dependence upon t1/2 is indicative of a diffusion controlled process but it is not simply one of alkali ions, hydronium ions and water molecules diffusing through the bulk glass itself but, presumably, through the siliceous gel layer on the glass surface [19, 20].
As leaching continues, the second term in equation 5 dominates and linear time kinetics ensue. This is expected to be most important for glass of lower durability, higher temperatures and long leaching times and for glass leaching in alkaline solutions [18, 20, 21, 31]. Although this is usually supposed, leaching kinetics which are a linear function of time are not necessarily indicative of a uniform dissolution process. Instead, such kinetics can be indicative of rate controlling reactions at an interface. In that way, the deviation from t1/2 kinetics is caused by the formation of hydrated silicate phases which no longer serve as diffusion barriers [31].
A conclusion that can be drawn is that alkali leaching cannot be described as a simple ion-exchange process. Several reactions occur simultaneously during corrosion including ion-exchange, glass hydration and network hydrolysis. The extent and relative rates of these reactions control the mode of glass dissolution, the kinetics of alkali leaching (t1/2, t or intermediate) and the apparent nature of the species involved in the leaching process (alkali exchanging for H+, H 30 + or any other combination of H+ and H20 ) [13, 31].
Some investigators [34, 35] have extended this two stage process by adding more terms to equation 5. The processes taking place during this phase, called stage 3, are particularly important for corrosion of bioactive and nuclear waste glass. During intermediate and long times, surface layers can form during the corrosion process. These layers are produced mainly from precipitation of insoluble compounds formed near or within the leached glass surface. The layers can contain a variety of precipitated and/or absorbed species and can consist of highly crystalline phases as well as amorphous compounds. These layers and their associated precipitation kinetics can be very complex and they can act as protective barrier to subsequent glass dissolution of very effective or of little or no effectiveness, depending on the chemistry and properties of the layers and on the corrosion environment.
4.2 Weathering
The degradation of a glass surface due to interaction with the atmosphere is referred to as “weathering” [6, 17, 36, 37]. Weathering has been classified into two types according to the amount of water involved.
Type 1 is called condensation-runoff. Moisture collects on the glass surface until natural runoff occurs, carrying away the reaction products. This type of weathering is very similar to aqueous corrosion in which the solution is continuously replenished at a specified rate. During the time that water droplets remain on the surface,
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dealkalisation of the glass occurs with a simultaneous increase in the pH of the water. Rapid pH increases cause total breakdown of the glass in contact with the droplets. This results in localised deterioration and roughening of the surface.
In type 2, called condensation-evaporation, a thin layer of “fog” forms on the glass surface which evaporates before droplets form. This type of weathering, produced in cyclic temperature and/or cyclic humidity environments is a great source of concern to many glass manufacturers and is characterised by the presence of reaction products on the glass surface.
Newton [38, 39] has proposed a mechanism of damage based on the content of the most important ions in the glass, the leached layer and the surface crust. The mechanism is formulated as a series of reactions with several intermediary formed phases :
ion exchange —> hydroxides -> carbonates -» sulphates
Water, carbon dioxide and sulphur dioxide are the most important elements in the atmosphere responsible for the weathering of glass windows. Carbon dioxide does not attack glass directly but converts the hydroxides, produced by the interaction with water, into carbonates and is responsible for the calcite frequently found in the weathering crust [40, 41, 42].
2 Na (glass) + C O 2 +1/2 02 Na2C03 4- (6)
Sulphur dioxide is responsible for the formation of a crust mainly consisting of gypsum (C aS 04.2H20 ) and/or syngenite (K2S 0 4.C aS04.H20). Since these crystals absorb water from the air, they can attack the glass in the local areas where they have formed, leading to a pitted glass surface [43].
5. DURABILITY OF GLASS
Silicate glass is among the most chemically inert commercial materials; it reacts with almost no liquids or gases below 300 *C except water. Because of this chemical inertness, the chemical durability of glass is concerned almost entirely with its reactivity with water, aqueous solutions and water vapour.
The rate of attack on silicate glass by water depends on different factors which can be divided into three major categories each of them being further subdivided [17].
I. State of specimen bulk glass composition thermal history degree of annealing for stress and density phase separation (glass in glass)
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% crystallisation, if any prior corrosion exposure history surface features surface roughness surface composition homogeneity of the glass surface treatments II. Environmental factors temperature exposure time (continuous or cycled) relative humidity solution pH presence of inhibitors in the corrosion solution external stresses upon specimen radiation solution composition III. Physical factors weathering versus aqueous corrosion dynamic versus static corrosion exposed surface area-to-solution volume ratio (SA/V) corrosion behaviour of bulk glass versus powdered glass
The most important of these factors will now be discussed in detail.
5.1 Composition of the bulk glass
The chemical resistance of silicate glass shows a sudden drop as the silica content decreases below 66.67 mol%. This particular silica content corresponds to a point at which every silicon atom in the glass becomes associated with a basic ion as a second neighbour. Glass with more than 66.67 mol% S i0 2 has its -S i-O ' sites isolated by -S i-O -S i- groups which suppress the movement of ions involved in leaching. If the silica content is decreased, the glass shows an increased tendency to form a crusted layer [44].
For a given modifier, the reactivity of alkali silicate glass increases with the non bridging oxygen content. As the amount of alkali is increased, with the remaining constituents held in the same ratios, the rate of the reaction with water increases. However, two kinds of glass containing the same modifier content but different modifier cations (e.g., Na+ versus K+) do not react with water at the same rate. In general, the lower the charge-to-ionic radius ratio of the modifier cation, the more reactive the glass will be [13]. This means that binary potassium silicate glass has poorer durability than binary sodium silicate glass of the same molar alkali composition [45]. The most likely reason for this result is that the hydronium ions have a higher mobility in potassium than in sodium silicate glass because potassium and hydronium ions have about the same effective radius (1.3 A). K+ leaves behind
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larger voids in the network providing larger openings in which water can diffuse [36, 13]. If a second alkali oxide, such as potassium, is added to a sodium silicate glass, the durability of the glass is increased [45, 46]. This increase is a result of the “mixed-alkali effect", in which the mobility of an alkali ion is reduced when another alkali ion is added [40].
Addition of CaO to binary alkali silicate glass greatly enhances corrosion resistance. The diffusion coefficient of sodium ions in glass containing 5-10 % CaO is up to 50 times lower than in binary sodium silicate glass with the same soda concentration [40, 47]. This can be understood as resulting from a blocking of alkali ion motion by the doubly charged calcium ions that are bound more tightly in the silicate network. A number of other divalent metals such as magnesium, strontium, barium and cadmium lead to a similar enhancement of durability [20]. The result of an excess of alkali and a deficiency of lime in the composition leads to “sick glass” which is “weeping” (“sweating”) or has become crizzled. Weeping glass shows a slippery surface or droplets of moisture if it is exposed to humid conditions while the transparency of crizzled glass surfaces has diminished due to the formation of very fine surface crazing. They both suffer from the removal of alkali ions by the action of water vapour [48].
Addition of a few percent Al20 3 also increases durability because it immobilises an alkali ion and thus reduces ion exchange [49, 50]. The addition of other oxides such as ZnO, PbO and Zr20 was also studied but their influence depends strongly on the pH of the attacking solution [21, 48, 50].
5.2 pH of the solution
Solution pH is probably the single most important parameter affecting glass corrosion. When an alkali silicate glass is placed in pure water, the water instantaneously becomes a solution of alkali oxides and silica. The pH of this solution depends on the concentration as well as the relative ratio of alkali oxide to silica. Both the concentration and ratio change with time and so the pH of the attacking solution is also expected to change correspondingly [21].
In general, the rate of alkali extraction in a buffered system at a pH of less than 9 is more or less constant and independent of pH [13]. There are exceptions to this, for example in a soda-lime silica glass where the lime content is greater than 10 mol% there is a rapid increase in the rate of extraction when the pH drops below 3. The increased extraction is due to the increased solubility of lime, of which the extraction was otherwise negligible [48]. The rate of alkali extraction drops sharply between pH 9 and 11 and in highly basic solutions is almost as slow as network dissolution [13].
In contrast to alkali extraction, silica extraction is almost non-existent below pH 9, but increases noticeably above it. Above this pH, where silicate anions become
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stable and nucleophilic attack by hydroxyl ions is promoted, the dissolution rate of the silicate network increases for ail alkali silicates. The pH dependence for network dissolution is similar for that seen for silica [13, 48, 51].
The low solubility of silica in water is one of the main factors in the corrosion resistance of glass. When silica (quartz) is brought in contact with water at room temperature the value of the equilibrium solubility is very low (~ 6 ppm of quartz will pass into solution) and it is this extremely slow rate of hydration that is responsible for the glass having a high resistance to attacking water. The solubility of silica near the neutral point (pH = 7) is not greatly affected by changes in pH but it increases rapidly with alkalinity at pH > 9.
sio:
-4 HgSIQa
8 IO 12 14 pH
Figure 6. Stability diagram of fused silica in aqueous solutions in function of pH.
Figure 6 shows the equilibrium activity of different species of silica in aqueous solutions at 25 C at different pH values. This diagram can be divided in three distinctly different zones based on the predominance of each particular silica species. Below pH = 10 the minimum solubility is represented by the undissociated but soluble portion of H2S i0 3 which predominates in this range (independent of pH). In the second zone (pH 10 to 12) most of the silica which passes into the solution is due to the formation of H Si03' species and in the third zone (pH >12) S i0 32' predominates.
All silica based glass follows the same trend with respect to pH changes of the solution; the absolute magnitude on the Y-axis of Figure 6, however, varies with logaSi02, where a Sj02 denotes the activity of silica in the respective glass with fused silica as the standard state [21].
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Figure 7 shows the qualitative relationship between the corrosion rate and the pH for the broad family of silicate glasses. Experience indicates that a durable commercial glass survives water attack for a much longer time than one with a low S i02 content, such as, for example, medieval glass windows [52].
£ CO c o OC/3 ow low silica glasses o u> o f commercial glasses __
Figure 7. Relationship between pH and corrosion rate for a variety of glass sorts.
5.3 Composition of the solution
Ions in the solution have a strong influence on glass durability. The effects of solution ions on both the ion exchange and network dissolution have already been extensively reported by several investigators [53, 54, 55, 56]. The effect of the presence of such ions can be a complex function of the ion concentration in solution.
Many groundwaters are silica saturated from interactions with silica-rich rocks such as tuff, basalt, granite and/or silica-rich soils. Leaching experiments of silicate glass in such silica-rich groundwaters have shown that the dissolution of the silica glass matrix is minimised [57, 58].
In experiments on a soda-lime glass Ishikawa et al. [59] found that the corrosion rates at constant pH values depend on the type of cations in the corroding solution and that they decrease in the order Ba-Sr-NH4-Na-Li-Ca. The small corrosion rate with a Ca(OH)2 solution may be attributed to the low solubility of the calcium silicate produced. Addition of Al3+, Be2+, Zn2+ or Zr4+ to the solution leads to the formation of a protective layer on the glass surface [19, 55, 56, 60]. Wiegel [61] showed that 1 ppm of Cu in the solution can reduce the alkali release into hot water up to 40 % and also found that the alkali salts can reduce this release as well [62].
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An isotopic effect can be observed in acid solutions showing a smaller leaching rate in DCI than in HCI [19, 63],
In soils, particularly in peats and humic horizons, there are numerous chelating or complexing agents, for example, amines, citrates and acetates. Many of these have a detrimental effect on the durability of buried glass. If the complexes are water soluble then there will be an increase in the rate of glass corrosion [48, 64]. Next to these also various micro-organisms can cause deterioration. The pitting of the surface of medieval glass can provide a “foothold” for mosses etc. which will then continue to grow and trap water thus hastening the corrosion of the glass [42, 65, 66] or they may release organic acids which though “weak” themselves, can chelate ingredients from the glass [64].
5.4 Surface area of the glass, the ratio of the surface area of the glass to the volume of the solution and replenishment of the solution
The surface area is also an important factor, the amounts of the various constituents released by a glass under certain conditions are proportional to the surface area exposed.
The quantity of silica extracted per gram of glass powder after a given time increases as the ratio of the surface area (SA) of the glass to the volume (V) of the leaching solution increases. Dilmore [55] has shown that the concentration of Si4+ in solution at any given time is a linear function of SA/V. This can be attributed to the accompanying increase in the pH of the solution. Contrary to this it was found that the quantity of alkali removed in a given time did not vary appreciably in those experiments [21, 67]. The increase in the pH would be expected to suppress the exchange of alkali ions of the glass with protons from the solution. Increasing the pH, however, also favours the dissolution of silica and this has the opposite effect on the removal of alkali from the glass as it causes alkali to pass into solution through breakdown of the silica network. The apparent independence of alkali extraction on the ratio of the surface area of the glass to volume of the solution is probably due to these two factors counterbalancing one another.
There is also a marked increase in the extraction of silica as the number of replenishments of the solution is decreased which can again be contributed to the evident accompanying rise in the pH of this attacking solution. Alkali extraction again shows no definite trend [21, 67],
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REFERENCES
[1] Frank S., Glass and Archaeology, pp. 2-10, Academic Press, London (1982). [2] Grote Winkler Prins, 4, pp. 378-380, Elsevier, Amsterdam, 7th edition. [3] Holloway D.G., The Physical Properties of Glass, Chapter 1, Wykeham, London (1973). [4] Doremus R.H., Glass Science, pp. 23-31, John Wiley & Sons, London (1973). [5] Zachariasen W.H., The Structure of Glass, J. Am. Chem. Soc., 54, pp. 3841- 3846 (1932). [6] Newton R. and Davison S., Conservation of Glass, Chapter 1 and 3, Butterworths, London (1989). [7] Warren B.E. and Loring A.D., X-ray Diffraction Study of the Structure of Soda- silica Glass, J. Am. Ceram. Soc., 18, pp. 269-276 (1935). [8] Biscoe J., Druesne M.A.A. and Warren B.E., X-ray Study of Potash-silica Glass, J. Am. Ceram. Soc., 24 (3), pp. 100-102 (1941). [9] Autefage F., Etude de la Migration du Sodium et du Potassium dans les Mineraux et dans les Verres au Cours de I’Analyse a la Microsonde Electronique, J. Microsc. Spectrosc. Electron, 6, pp. 87-94 (1981). [10] Biscoe J., X-ray Study of Soda-lime-silica Glass, J. Am. Ceram. Soc., 24 (8), pp. 262-264 (1941). [11] Turner W.E.S., Studies in Ancient Glasses and Glassmaking Processes. Part III. The Chronology of the Glassmaking Constituents, J. Soc. Glass Tech., 192, pp. 39T-52T (1956). [12] Turner W.E.S., Studies in Ancient Glasses and Glassmaking Processes. Part V . Raw Materials and Melting Processes, J. Soc. Glass Techn., 194, pp. 277- 300 (1956). [13] Bunker B.C., Molecular Mechanisms for Corrosion of Silica and Silicate Glasses, J. Non-Cryst. Solids, 179, pp. 300-308 (1994). [14] Wang F.F. and F.V. Tooley, Detection of Reaction Products between Water and Soda-Lime-Silica Glass, J. Amer. Ceram. Soc., 41(11), pp. 467-469 (1958). [15] Tsuchihashi S. and Sekido E., On the Dissolution of Na20 -C a 0 -S i0 2 Glass in Acid and in Water, Bull. Chem. Soc. Japan, 32(8), pp. 868-872 (1959). [16] Bacon F.R. and Calcamuggio G.L., Effect of Heat Treatment in Moist and Dry Atmospheres on Chemical Durability of Soda-Lime Glass Bottles, Am. Ceram. Soc. Bull., 46(9), pp. 850-855 (1967). [17] Clark D.E., Pantano Jr. C.G. and Hench L.L, Corrosion of Glass, Chapter 1, Magazines for Industry, New York (1979). [18] Doremus R.H., Interdiffusion of Hydrogen and Alkali Ions in a Glass Surface, J. Non-Cryst. Solids, 19, pp. 137-144 (1975). [19] Scholze H., Chemical Durability of Glasses, J. Non-Cryst. Solids, 52, pp. 91- 103 (1982). [20] Das C.R. and Douglas R.W., The Reaction between Water and Glass, Part 3, Phys. Chem. Glasses, 8, pp. 178-184 (1967). [21] Paul A., Chemical Durability of Glasses; A Thermodynamic Approach, J. Mat. Science, 12, pp. 2246-2268 (1977).
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[22] Smets B.M.J. and Lommen T.P.A., The Role of Molecular Water in the Leaching of Glass, Phys. Chem. Glasses, 24, pp. 35-36 (1983). [23] Conradt R. and Scholze H., Glass Corrosion in Aqueous Media - A Still Unsolved Problem ?, Rivista della Staz. Sper. Vetro, 5, pp. 73-77 (1984). [24] Smets B.M.J., Tholen M.G.W. and Lommen T.P.A., The Effect of Divalent Cations on the Leaching Kinetics of Glass, J. Non-Cryst. Solids, 65, pp. 319- 332 (1984). [25] Charles R.J., J. Appl. Phys., 11, p. 1549 (1958). [26] Scholze H., Helmreich D. and Bakardjiev I., Glass Tech. Ber., 48, p. 237 (1975). [27] Wolters D.R. and Verweij H., The Incorporation of Water in Silicate Glasses, Phys. Chem Glasses, 22(3), pp. 55-61 (1981). [28] Rana M.A. and Douglas R.W., Reaction between Glass and Water, (I) Experimental Methods and Observations; (II) Discussion of the Results, Phys. Chem. Glasses, 2(6), pp. 179-195 (1961). [29] Douglas R.W. and El-Shamy T.M., Reactions of Glasses with Aqueous Solutions, J. Amer. Ceram. Soc., 50(1), pp. 1-8 (1967). [30] Ethridge E.C., Mechanisms and Kinetics of Binary Alkali Silicate Glass Corrosion, PhD Dissertation, Univ. Fla (1977). [31] Bunker B.C., Arnold G.W., Beauchamp E.K. and Day D.E., Mechanisms for Alkali Leaching in Mixed-Na-K Silicate Glasses, J. Non-Cryst. Solids, 58, pp. 295-322 (1983). [32] Lyle A.K., Theoretical Aspects of Chemical Attack of Glasses by Water, J. Amer. Ceram. Soc., 26(6), pp. 201-204 (1943). [33] Zagar L. and Schillmoller L., Gber die Physikalisch-chemischen VorgSnge bei der Wasserauslaugung von Glasoberflachen, Glastechn. Ber., 33(4), pp. 109- 116 (1960). [34] Hench L.L., Corrosion of Silicate Glasses : An Overview, Mat. Res. Soc. Symp. Proc., 125, pp. 189-200 (1988). [35] Clark D.E., Zoitos B.K. (eds.), Corrosion o f Glass, Ceramics and Ceramic Superconductors. Principles, Testing, Characterization and Applications, Noyes Publications, New Jersey (1991). [36] Bansal N.P. and Doremus R.H., Handbook of Glass Properties, Chapter 18, Academic Press, London (1986). [37] Baer N.S., Sabbiani C. and Sors A.I., Science, Technology and European Cultural Heritage, Butterworth, Oxford, pp. 16-18 (1991). [38] Newton R.G., Air-pollution, Sulphur Dioxide and Medieval Glass, CV-News Letter, 15, pp. 9-12 (1975). [39] Newton R.G., Sulphur Dioxide and Medieval Stained Glass, Sulphur Emiss. Environm. Int. Symp., pp. 311-313 (1979). [40] Doremus R.H., Glass Science, Chapter 9, Wiley, New York (1973). [41] Perez-Y-Jorba M., Tilloca G., Michels D. and Dallas J.P., Quelques Aspects du Phenomene de Corrosion des Vitraux Anciens des £glises Frangaises, Verres Refract., 29, pp. 53-63 (1975). [42] Perez-Y-Jorba M., Dallas J.P., Bauer C., Bahezre C. and Martin J.C., Deterioration of Stained Glass by atmospheric Corrosion and Micro-
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organisms, J. Mater. Sci., 15, pp. 1640-1647 (1980). [43] Schreiner M., Deterioration of Stained Medieval Glass by Atmospheric Attack, Glastechn. Ber., 7, pp. 197-204 (1988). [44] El-Shamy T.M., The Chemical Durability of K2O-CaO-Mg0-SiO2 Glasses, Phys. Chem. Glasses, 14, pp. 1-5 (1973). [45] Morey G.W., The Properties of Glass, Chapter IV, Reinhold, New York (1954). [46] Sen S. and Tooley T.V., J. Am. Ceram. Soc., 38, p. 175 (1955). [47] Terai R. and Hayami R., Ionic Diffusion in Glasses, J. Non-Cryst. Solids, 18, p. 218 (1975). [48] Newton R. and Davison S., Conservation of Glass, Chapter 4, Butterworths, London (1989). [49] Wassick T.A., Doremus R.H., Lanford W.A. and Burman C., Hydration of Soda-lime Silicate Glass : Effect of Alumina, J. Non-Cryst. Solids, 54, p. 139 (1983). [50] Newton R.G., The Durability of Glass - A Review, Glass Techn., 26, pp. 21-38 (1985). [51] El-Shamy T.M., Lewins J. and Douglas R.W., The Dependence of the pH of the Decomposition of Glasses by aqueous Solutions, Glass Technol., 13, pp. 81-87 (1972). [52] Adams P.B., Glass Corrosion. A Record of the Past ? A Predictor of the Future ?, J. Non-Cryst. Solids, 67, pp. 193-205 (1984). [53] Hudson G.A. and Bacon F.R., Inhibition of Alkaline Attack on Soda-Lime Glass, Bull. Amer. Ceram. Soc., 37(4), pp. 185-188 (1958). [54] lller R.K., Effects of Adsorbed Alumina on the Solubility of Amorphous Silica in Water, J. Colloid and Interface Science, 43(2), pp. 399-408 (1973). [55] Dilmore M.F., Chemical Durability of Multicomponent Silicate Glasses, PhD. Dissertation, Univ. Fla (1977). [56] Hench L.L. and Clark D.E., Physical Chemistry of Glass Surfaces, J. Non- Cryst. Solids, 28, pp. 83-105 (1978). [57] Wicks G.G., Robnett B.M. and Rankin W.D., Scientific Basis for Nuclear Waste Management V, Lutze W. (ed.), Elsevier Publ. Co., New York, p. 15 (1982). [58] Jantzen C.M., Prediction of Glass Durability as a Function of Environmental Conditions, Mat. Res. Soc. Symp. Proc., 125, pp. 143-159 (1988). [59] Ishikawa T., Kakagi T., Kawamoto Y and Tsuchihashi S., J. Ceram. Ass. Jap., 87, p.57 (1979). [60] Dilmore M.F., Clark D.E. and Hench L.L., Glass Durability in Aqueous Solutions of Aluminium, Amer. Ceram. Soc. Bull., 59(2), p. 410 (1979). [61] Wiegel E., Glastechn. Ber., 34, p. 259 (1961). [62] Wiegel E., Glastechn. Ber., 37, p. 141 (1964). [63] Pederson L.R., Comparison of Sodium Leaching Rates from a Na20.3Si02 Glass in H20 and D20, Phys. Chem. Glasses, 28, pp. 17-21 (1987). [64] Bacon F.R. and Raggon F.C., Promotion of Attack on Glass and Silica by Citrate and other Anions in Neutral Solutions, J. Amer. Ceram. Soc., 42(4), pp. 199-205 (1959). [65] Bettembourg J.M., Composition et Alteration de Verre des Vitraux Anciens,
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Verres Refract., 30, pp. 26-29 (1976). [66] Bettembourg J.M., Burck J.J., Orial G. and Perez-Y-Jorba M„ La Degradation des Vitraux de St. Remi de Reims (Marne), News Letter, 35/36, pp. 10-17 (1983). [67] El-Shamy T.M. and Douglas R.W., Kinetics of the Reaction of Water with Glass, Glass Techn., 13, pp. 77-80 (1972).
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Instrumentation and Quantitation
with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 2 : Instrumentation and Quantitation
1. INTRODUCTION
The scientific study of objects of art and archaeology often requires the use of analytical methods. This can be a complicated task since the materials are often strongly heterogeneous or show a wide range of compositional changes among apparently similar materials. In addition, corrosion phenomena resulting in diffusion of elements into or out of the objects could have occurred due to the conditions of burial or storage. The most important problem is probably the fact that these objects are often very valuable or rare so that sampling might be prohibited or restricted to microscopical fragments from less significant parts of the objects.
There is a tradition of elemental microanalysis in archaeometry, especially micro trace and surface analysis provides important information on the structure and origin of materials, the authenticity of archaeological and art objects, ageing and destruction or corrosion processes. The method of examination and analysis used must satisfy a series of conditions. First of all it should be non-destructive, which means that analysis can be carried out without altering the composition or appearance of objects and without the need to extract samples. If samples must be taken, they should be minimal in size. Furthermore, it has to be fully applicable to different materials and objects of any size and dimension (i.e., be universal) and provide both localised information from microscopic areas as well as average bulk information from heterogeneous materials (i.e., the method must be versatile). Next to these, the method used must be sensitive and multielemental. It must be fast to give a maximum of information for large collections of objects from museum collections or archaeological excavations.
Over the years a variety of analytical techniques has been used for the chemical analysis of ancient glass ranging from the use of conventional wet chemistry and X- ray diffraction in the early period up to the 1960s to more complicated and automated techniques including wavelength and energy dispersive X-ray fluorescence (XRF), electron-probe X-ray microanalysis (EPXMA) and scanning electron microscopy (SEM), neutron activation analysis (NAA), atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), secondary ion mass spectrometry (SIMS), particle induced X-ray emission (PIXE) and infrared spectroscopy (IR). Each technique has its intrinsic advantages and drawbacks and provides slightly different kinds of analytical information so that many analysts use a combination of more than one technique in order to cross-check and augment the validity of the results [1,2,3].
A selection of microanalytical techniques was made based on the availability of the instruments, the requirements mentioned above and the kind of information that could be obtained using these techniques. Suitable Standard Reference Materials were checked for homogeneity before a quantitation method was worked out to determine major, minor and trace composition of glasses using, as in this study,
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scanning electron microscopy and micro synchrotron radiation induced X-ray fluorescence.
2. SCANNING ELECTRON MICROSCOPY (SEM)
2.1 Principle of scanning electron microscopy, interactions between electron beam and specimen
When a focused electron beam strikes the surface of a specimen, the electrons penetrate the target. Along their trajectory, the electrons collide with the target atoms and as a consequence of these collisions, they are decelerated and scattered. Electron scattering (elastic or inelastic) is an interaction between the probe electron and the specimen atoms that results in a change in the direction of travel of the electron and/or its energy.
Different signals can be produced including backscattered electrons, secondary electrons, continuum and characteristic X-rays, Auger electrons, electromagnetic radiation of various energy outside the X-ray range and phonons [4-7]. These signals, which can be used to reveal the topography and local chemistry of the material investigated, are obtained from specific emission volumes within the sample. The depth of penetration of the electrons into the target is strongly dependent on the energy of the primary electron beam (E0) and on the average atomic number of the species (
10A-Auger electrons 50-500A 'Secondary electrons
Backscattered electrons
.Characteristic X-rays
E«EC
■E *0 Continuum X-rays
Secondary fluorescence by continuum and characteristic x-rays BSE spatial resolution
X-ray resolution
Figure 1. Pear-shaped interaction volume showing the range and spatial resolution for signals produced and used in a scanning electron microscope [4].
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2.1.1 Elastic scattering When a beam electron passes an atom at close distance, it can interact with the Coulomb field of this atom, formed by the positive field of the nucleus and the negative field of the bound electrons. As a result, the electron changes direction with hardly any loss of energy (< 1 eV, i.e., negligible compared to the incident energy). The elastic scattering angle can take any value in the range 0° - 180° so that the electron may hardly be affected at all or may travel back in the direction from which it came. Hence, in a single elastic scattering event or multiple scattering events, composed of many scattering interactions, a large change of direction of the impinging electron beam may be produced.
Beam electrons which have changed their direction in a series of elastic scattering events may travel back to the surface and escape. These electrons are called backscattered electrons; they provide an extremely useful signal for imaging in scanning electron microscopy since the probability for backscattering is directly related to
2.1.2 Inelastic scattering Inelastic interaction of the beam electrons with the atomic nuclei or the orbiting electrons from the target atoms both cause inelastic scattering. During these events energy is transferred to the atoms in the sample resulting in a decrease of the kinetic energy of the bombarding electron until it is captured by the solid, thus limiting the range of travel of the electron within the sample. Any amount of energy, ranging from a fraction of an eV to the entire energy carried by the incident electron, can be transferred during a single inelastic event, depending on the type of process. Contrary to elastic scattering, despite the loss of energy, the electron trajectory deviates only by a small angle. Inelastic interactions are coupled with the formation of a range of different signals which will now be discussed separately.
Interaction of the beam electrons with loosely bound conduction band electrons in a metal or the outer-shell electrons in semi-conductors or insulators, leads to the production of so-called secondary electrons. These electrons are produced along the entire path of the beam electrons and propagate through the specimen being themselves subject to inelastic scattering and energy loss. If the secondary electron retains enough energy to overcome the surface barrier energy when it reaches the surface, it will escape from the solid. The energy of the secondary electrons is typically less than 50 eV showing a maximum in the energy distribution curve at a few electron volts (2-5 eV). Due to this low energy, only secondary electrons generated close to the sample surface leave the specimen and can be detected.
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This low energy secondary electron signal is used in the scanning electron microscope to produce the secondary electron image and since the intensity of the signal also depends on the angle between the incident electron beam and specimen surface, it can be used to gain topographical information.
X-rays can be formed by two different processes during inelastic scattering of the impinging beam of electrons, namely bremsstrahlung or continuous X-rays and characteristic X-rays.
Passing through the Coulomb field of the target atoms, the beam electrons can undergo deceleration. The energy loss from the electron that takes place during such a deceleration event is emitted as a photon. This radiation is referred to as Bremsstrahlung or braking radiation. Most electrons give up their energy in a single step rather than in a series of unequal increments. Bremsstrahlung can therefore take on any value from zero up to the energy of the bombarding electron beam resulting in a continuous electromagnetic spectrum which forms the background in X-ray spectra produced in a scanning electron microscope. The intensity of this continuum is a function of both the atomic number of the sample and the beam electron energy.
Inelastic scattering can also result in ionisation processes caused by the interaction of the incident electrons with the inner-shell electrons of the atoms in the sample. One of the results of these processes is the production of characteristic X-rays. If the beam electron has enough energy, it may eject a K, L or M inner shell electron, leaving a vacancy in that shell. As a result, the atom is found in an ionised or excited state. The atom relaxes to its lowest energy or ground state through the transition of a higher-shell electron into the vacancy in the inner-shell. The energy difference of these transitions are sharply defined with values characteristic for the atomic species. The emission of characteristic X-rays appear in the X-ray spectrum as narrow peaks of high intensity superimposed on the continuous Bremsstrahlung spectrum. The intensity of these lines can be used to determine the composition of the sample.
The energy is not always released as X-ray photons during the relaxation process after the ionisation event. Sometimes the excess energy is transferred to another bound electron, which can then be emitted from the atom as an electron with specific kinetic energy. These are called Auger electrons. The Auger process is favoured for low atomic numbers. The Auger signal is only used in specialised analytical instruments to provide surface specific compositional information.
A significant fraction of the energy of the primary beam is transferred to the solid by small energy loss events that cause the excitation of lattice oscillations ( phonons). This process can produce considerable heating of the sample.
Next to all these processes, some other phenomena occur as a result of electron bombardment which are of minor importance in scanning electron microscopy such
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as cathodoluminescence in which the specimen emits long-wavelength photons in the visible or ultra violet part of the spectrum and electron energy loss when electrons pass through extremely thin samples without undergoing any interaction except inelastic scattering.
2.2 Instrumentation
Measurements were performed with a Jeol JSM 6300 scanning electron microscope equipped with a PGT (Princeton Gamma Tech) Si(Li) detector. The instrument can be subdivided into three major sections, the electron-optical column with the electron gun and the lens system, the vacuum system and the electronics and display system [4-7], The electron-optical column from the electron source to the sample chamber is represented in Figure 2.
The purpose of the electron gun is to provide a large stable current in a small electron beam. When a tungsten V-shaped filament is electrically heated to a temperature of about 2700 K, electrons become sufficiently energetic to escape from the material by a process called thermionic emission. The electrons are accelerated away from the filament (cathode) towards an anode plate by applying a strongly negative voltage. The filament is surrounded by a grid cap or Wehnelt cylinder with a circular aperture to shape the beam electrostatically to a diameter of 10 - 50 pm. Since electrons would be decelerated and scattered by gas molecules in air, a vacuum system is connected to the column.
Beyond the electron gun, the beam is focused and the diameter is further reduced by two electromagnetic condenser lenses. A third lens, the objective lens, ensures that the beam has a particular diameter when it impinges on the sample surface.
A double set of scanning coils is built into the column above the centre of the objective lens. The function of this deflection system is to displace the beam position and make a line scan or area scan over part of the sample.
The electron beam emerges from the final lens into the specimen chamber where many signals are generated due to interaction between the electrons and the sample (see previous section). The samples are mounted on a table which can be moved in X, Y and Z-direction (both manually and automatically), tilted and rotated.
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High tension cable Electron gun
Anode
Electron gun chamber
Alignment coils
Objective aperture selector Zoom condenser lens
Scan coils Column isolation valve
Specimen chamber
Specimen exchange chamber
Secondary electron detector ------
Specimen stage Backscattered electron detector Specimen holder o
Figure 2. Cross-section of the column of the Jeol JSM 6300 scanning electron microscope [8].
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The various signals which are generated are collected by different types of detectors. Secondary electrons are detected by an Everhart-Thornley detector which consists of a scintillator, a light pipe and a photomultiplier tube. When energetic electrons strike a scintillation material, photons (visible light) are produced. This light is transmitted through a light pipe by total internal reflection to the photomultiplier tube (PM). Here the light is converted into an amplified electrical signal when the photo-electrons are accelerated onto the successive electrodes of the photomultiplier, creating a cascade of electrons with a typical gain of 10s to 106. Backscattered electrons are sufficiently energetic to excite the scintillator directly but to make use of the secondary electrons a large positive potential (usually + 12 kV) is applied to the face of the scintillator. The scintillator is also surrounded by a wire cage which can be used in two ways. When kept at about + 250 V, it improves secondary electron collection by drawing these low-energy electrons from the specimen into the detector. When the cage is made slightly negative (- 50 V), it still collects (energetic) backscattered electrons which are travelling in the right direction while rejecting all (low-energetic) secondary electrons.
As the Everhart-Thornley detector is not very efficient for backscattered electron collection, this signal is detected by a solid state electron detector which operates on the principle of electron-hole pair production induced in a semi-conductor by energetic electrons which promote electrons from the filled valence band to the empty conduction band. When a potential is applied, the free electrons and the holes move in opposite directions resulting in a small current which can be used after being suitably amplified. This detector is insensitive to the low-energy secondary electrons. It has the form of a flat thin wafer and is placed at the bottom of the objective lens, allowing collection over a large conical angle.
The various X-rays which are generated by the sample are detected with a lithium- drifted silicon detector (Si(Li) detector). The active portion of this kind of detector consists of a reverse-bias p-i-n (p-type, intrinsic, n-type) silicon crystal. When an energetic photon is captured, a series of processes takes places finally resulting in the annihilation of the X-ray. The incident photon is first absorbed by a silicon atom and an inner-shell electron is ejected which, on its turn, will create electron-hole pairs while being scattered through the silicon. The excited silicon atom may release its excess energy either in the form of an Auger electron which can produce more electron-hole pairs or as a silicon X-ray which itself initiates the process again. The conduction electrons and holes which are formed in this way are swept apart by the applied voltage and collected on the electrodes to form a charge pulse which is then transformed into a voltage pulse by the preamplifier. For proper operation, both the detector and preamplifier must be cooled by liquid nitrogen. The signal is further amplified and shaped by a linear amplifier and is then passed to a computer where final processing of the data takes place. The detector is separated from the sample chamber by a thin beryllium window, protecting the cooled detector from contamination by impurities and blocking secondary and backscattered electrons which would cause noise in the detector.
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2.3 Homogeneity of the standard reference materials
2.3.1 NIST Standard Reference Materials
Five different Standard Reference Materials from the National Institute of Standards and Technology (NIST), namely Soda-Lime Flat Glass (SRM 620), Soda-Lime Float Glass (SRM 1830), Soft Borosilicate Glass (SRM 1411), Borosilicate Glass (SRM 93a) and Multicomponent Glass (SRM 1412) were tested for their homogeneity. The certificates indicate that these materials are tested for homogeneity using X-ray fluorescence spectrometry on different samples; for SRM 620, 1830, 1411 and 1412 no significant heterogeneity among the samples was found while in SRM 93a small longitudinal differences for S i02, Z r0 2 and B20 3 were observed. Samples were taken from the glass platelets, embedded in a methyl methacrylate resin, sanded with silicon carbide paper and polished down to 1 pm using diamond paste. 36 X - ray point spectra were collected (6x6 square) and quantitative analysis was performed. Table 1 shows the certified values with their uncertainty and the minimum and maximum values obtained for the different elements present in the Soda-Lime Flat Glass. The distribution of aluminium and silicon in the same Standard Reference Material is represented in Figure 3a and b respectively.
As can be seen in Table 1 and Figure 3, the results do not indicate large differences in composition between the various locations of the glass sample, especially not in the major components, although the variations are higher than the uncertainties in composition given in the certificates. The results for the other Standard Reference Materials were comparable except for the Multicomponent Glass which already showed to be heterogeneous on a backscattered electron image.
Table 1. Certified concentration and uncertainty and detected minimum and maximum values (w%) found for the elements of SRM 620 (Soda-Lime Flat Glass) after quantitative analysis of 36 spectra.
Elements Cert. ± uncertainty (w%) Det. Minimum (w%) Det. Maximum (w%) Na 10.65 ±0.04 10.1 10.6 Mg 2.22 ± 0.03 1.7 1.9 Al 0.95 ± 0.02 0.9 1.3 Si 33.68 ± 0.04 34.0 34.5 K 0.34 ± 0.03 0.3 0.4 Ca 5.08 ± 0.04 4.7 4.9 S 0.11 ±0.01 0.0 0.2
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Soda lime flat, Al
L 5 <
c o
c: 0 3 cO o O
b.
Soda lime flat. Si
34.6 34.5 Z 34.4 w 34.3 o c 34.2 o
ua> c 33.9 o o 33.8- 33.7
Figure 3. Distribution of aluminium (a) and silicon (b) in SRM 620.
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2.3.2 Corning Museum of Glass standards
At the Vlth International Congress on Glass, held in Washington D.C. in 1963, Turner outlined many problems in the precision of analyses of ancient glasses and suggested a co-operative venture between laboratories interested in such analyses [91-
In 1964, four experimental glasses, duplicating certain ancient compositions were produced by the Corning Museum of Glass, New York. These include two soda- lime-silica compositions (Glasses A and B), which serve well to duplicate typical compositions found among ancient Egyptian, Mesopotamian, Roman, Byzantine and Islamic glasses. A third glass (Glass C) is a high-lead and high-barium glass approximating some East Asiatic glass types. The fourth (Glass D) is a potash-lime- silica type with a composition resembling that of certain medieval glass artefacts and some glass from the 17th through the 19th centuries. Minor and trace elements were also introduced at levels which are comparable to those actually found in historic glasses. The compositions of these glasses are given in Table 3 e-h.
The glasses were prepared from carefully weighed batch ingredients of known purity. The batches were ball-milled to insure good mixing and to attain homogeneity in the glasses. They were then melted and held at 1450 °C for several hours while being stirred with a platinum stirrer. Afterwards they were drigaged by pouring them into deionised water, crushed, remelted and poured into thin sheets. These sheets were cut into small cubes.
Samples of these glasses were distributed to 29 laboratories all over the world. The analytical methods used varied greatly among the different laboratories and included among others atomic absorption, X-ray fluorescence, colorimetry, flame photometry and electrolysis. The results given in the tables as certified values are the averages of the results found by these different laboratories.
The advantage of using these standards in combination with NIST standard reference materials is the fact that these glasses contain the elements most often found in early glasses in the concentration ranges usually encountered when analysing historic specimens [10, 11].
The standards were tested for their homogeneity in the same way as the NIST Standard Reference Materials. The results for standard D are presented in Table 2 (certified value, range in certificate and minimum and maximum values of the elements detected) and in Figure 4 (distribution of sodium (a), calcium (b) and antimony (c)). Inspection of the results clearly shows that these standards are less homogeneous than the NIST SRM’s, especially for elements present in lower concentration (e.g. copper) or the elements which do not dissolve very well in the glass melt (e.g. antimony).
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Table 2. Certified concentration, range and detected minimum and maximum values (w%) for the elements of Corning Museum of Glass Standard D after quantitative analysis of 36 spectra.
Elements Cert. conc.(w%) Range in cert. (w%) Det. Min. (w%) Det. Max. (w%) Na 0.98 0.59 - 1.33 0.9 1.2 Mg 2.46 2 .3 4 -2 .7 3 2.0 2.2 Al 2.8 7 2.59 - 3.52 3.0 3.4 Si 25.81 25.17-27.47 25.5 26.3 P 1.75 1.59-2.17 1.9 2.1 K 9.55 9.08 - 10.19 9.3 9.8 Ca 10.76 9.30 - 10.96 9.8 10.1 Ti 0.24 0 .1 4 -0 .3 2 0.2 0.3 Mn 0.36 0.26 - 0.60 0.3 0.5 Fe 0.35 0.31 - 0.51 0.3 0.6 Cu 0.32 0.21 - 0.48 0.0 0.4 Ba 0.30 0.22 - 0.63 0.0 0.5 Sb 0.72 0.62 - 0.83 0.0 1.3 Pb 0.25 0.09 - 0.30 0.0 0.4
Standard D, Na
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b.
Standard D, Ca
10.05 5 (0 U 9.95
c o 9.85
c oa> 9.75 c o a 9.65
C.
Standard D, Sb
.a
c o c o o
Figure 4. Distribution of sodium (a), calcium (b) and antimony (c) in Corning Museum of Glass standard D.
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Although it was indicated on the certificates that no significant heterogeneities were present in the NIST standard reference materials, the variations in composition between different locations on the glass samples were found to be larger than the uncertainties given. The results of homogeneity tests further showed that the NIST Standard Reference Materials are more homogeneous than the glasses from the Corning Museum of Glass, especially when minor or trace elements are considered. In spite of the heterogeneity, these materials were used to check the quantitative method used in this work.
2.4 Quantitative analysis by means of ZAF-method
The net peak intensities of the characteristic X-ray signals, obtained by applying a deconvolution technique at the X-ray spectrum, can be converted into quantitative information by different methods. One of the most conventional ways to do this is to use the ZAF-correction procedure which is described in detail in many textbooks [4, 5, 7, 12].
First of all the ratio of the measured X-ray intensity lu of the elements in the unknown sample to the intensity of the same element in a standard ls is determined. This experimental ratio, called the k-value, is the basis for the quantitative analysis and is roughly equal to the ratio of the weight fractions of the emitting element if unknown and standard are very similar in composition. Both unknown specimen and standard must be measured under exactly the same experimental conditions.
Since unknown and standard are in general not equal in composition, the obtained X-ray intensity ratio must be corrected for a series of matrix or interelement effects. It is convenient to divide these into effects due to atomic number, Z„ X-ray absorption, A, and X-ray secondary fluorescence, F,. The atomic number effect arises from two phenomena, namely the electron backscattering R (representing the fraction of ionisation remaining in a target after the loss due to backscattering of the beam electrons) and the electron retardation or stopping power S (which is an approximation for the continuous energy loss due to inelastic scattering). X-rays produced in the sample have to pass through a certain amount of material before they reach the detector. Their intensity is reduced by the photoelectric absorption processes. Photoelectric absorption can lead to the third effect, secondary fluorescence. A characteristic X-ray can ionise another atom in the sample whose excitation energy is less than the energy of the original photon, resulting in the emission of a secondary characteristic X-ray.
Using these correction factors, the concentration of an element present in the unknown is calculated as follows :
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where Cu and Cs are the concentration of the element of interest in the unknown and the standard and Zh A-, and F, are the correction factors for the atomic number effect, the absorption effect and the fluorescence effect respectively. This equation must be applied separately for each element present in the sample.
Since the correction factors depend upon the true composition of the sample, which is unknown, the measured k-values are taken as a first estimate of composition. The obtained concentrations are normalised to 100 % and the results are used to calculate the initial ZAF-factors for each element. This leads to an iterative procedure which is performed until convergence of the results.
So far we have discussed the ZAF-procedure with the use of standards; it is also possible to carry out quantitative analysis without the need for standards. In the standardless procedure, first the intensity of X-ray lines generated in pure elements are calculated for a given set of experimental conditions. These data are corrected for absorption in the standard and modified further for the absolute collection efficiency of the detector to predict the X-ray intensity that would be obtained from this pure element standard. If no standards are used at all in the analysis, a normalisation must be carried out after every ZAF-iteration to bring the sum of the weight concentrations to 100 %.
2.5 Accuracy and Precision of Major Element Analysis
Two quantitative procedures, one using standards (ZAF AXIL) and one standardless (ZAF PGT) were employed and the results were compared. For the ZAF AXIL method the average X-ray intensity obtained for a set of standards was used to quantify another standard. Various NIST glass standards (SRM's 620, 1830, 1411 and 93a) and four Corning Museum of Glass Standards (A, B, C and D) in which N a+ is the major cation were used to validate the quantitative procedure. The ZAF AXIL procedure was carried out both at 15 and 20 kV operating voltage and the final measuring conditions were chosen to be an accelerating voltage of 15 kV and a current of 1 nA.
The reproducibility of the analysis was checked by collecting X-ray spectra from a homogeneous glass standard sample (NIST SRM’s) on 10 different locations; for elements yielding signals well above the detection limit (i.e. with a concentration larger or equal to 1 %), standard deviations equal to or below 1 % were obtained.
In Table 3, a to h, analysis results of the above-mentioned standards are compared to the certified composition.
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Table 3. Results of the quantitative analysis of different NIST and Corning Museum of Glass standards using a ZAF procedure with standards (ZAF AXIL) both at 15 and 20 kV and a standardless procedure (ZAF PGT) at 15 kV. The results are given in w%.
a. Soda-lime Flat Glass (NIST SRM 620)
Elements Cert. ZAF Rel. ZAF Rel. ZAF Rel. (w%) AXIL diff. AXIL diff. PGT diff. 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 10.65 ±0.04 8.44 21 8.73 18 10.59 1 Mg 2.22 ± 0.03 2.07 7 2.20 1 1.92 14 Al 0.95 ± 0.02 0.91 4 0.96 1 1.02 7 Si 33.68 ± 0.04 31.41 7 32.60 3 34.09 1 S 0.11 ± 0.01 0.10 9 0.08 27 0.11 0 K 0.34 ± 0.03 0.33 3 0.34 0 0.33 3 Ca 5.08 ± 0.04 5.05 1 5.16 2 5.00 2 Ti 0.01 ± 0.001 known concentration Fe 0.03 ± 0.003 known concentration
b. Soda-lime Float Glass (NIST SRM 1830)
Elements Cert. ZAF Rel. ZAF Rel. ZAF Rel. (w%) AXIL diff. AXIL diff. PGT diff. 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 10.20 ±0.07 9.19 10 9.01 12 10.59 4 Mg 2.35 ± 0.04 2.38 1 2.50 6 2.17 8 Al 0.06 ± 0.02 known concentration Si 34.16 ±0.04 33.90 1 34.85 2 34.72 2 S 0.10 ± 0.01 0.09 10 0.10 0 0.10 0 K 0.03 ± 0.02 known concentration Ca 6.12 ± 0.03 6.28 3 6.34 4 5.83 5 Ti 0.007 ± 0.001 known concentration Fe 0.11 ± 0.003 0.10 10 0.02 > 100 0.09 18
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c. Soft Borosilicate Glass (NIST SRM 1411)
Elements Cert. ZAF Rel. ZAF Rel. ZAF Rel. (w%) AXIL diff. AXIL diff. PGT diff. 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 7.52 ± 0.17 7.98 6 8.09 8 6.34 16 Mg 0.20 ± 0.02 0.22 10 0.23 15 0.05 75 Al 3.01 ± 0.06 3.03 1 2.85 5 3.09 3 Si 27.13 ±0.07 27.28 1 26.34 3 28.36 4 K 2.47 ± 0.08 2.48 1 2.62 6 2.62 6 Ca 1.56 ± 0 .0 4 1.57 1 1.55 1 1.56 0 Ti 0.01 ± 0.01 known concentration Fe 0.03 ± 0.01 known concentration Zn 3.09 ±0.15 3.22 4 2.49 19 3.84 24 Ba 4.48 ± 0 .1 3 5.54 24 6.03 35 3.45 23 Sr 0.08 ± 0.01 known concentration B 3.40 ± 0.07 known concentration
d. Borosilicate Glass (NIST SRM 93a)
Elements Cert. ZAF Rel. ZAF Rel. ZAF Ret. (w%) AXIL diff. AXIL diff. PGT diff. 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 2.95 ± 0.04 2.41 22 2.66 10 2.46 17 Al 1.21 ±0.03 1.15 5 1.29 7 1.32 9 Si 37.77 ± 0.05 35.47 6 38.42 2 38.08 1 Cl 0.06 ± 0.01 known concentration B 3.90 ± 0.02 known concentration
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e. Corning Museum of Glass Standard A
Elements Cert. Range in ZAF Rel. ZAF Rel. ZAF Rel. (w%) cert. AXIL diff. AXIL diff. PGT diff. (w%) 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 10.77 9 .9 8 - 11.11 9.21 15 9.13 15 10.27 5 Mg 1.69 1.42 - 1.96 1.60 5 1.54 9 1.39 18 Al 0.53 0.42 - 0.66 0.54 2 0.51 4 0.60 13 Si 31.12 30.71 - 32.32 30.73 1 30.60 2 32.44 4 P 0.06 0.03 - 0.09 known concentration S 0.06 0.04-0.10 known concentration Cl 0.10 0.10-0.17 0.13 30 0.10 0 0.07 30 K 2.43 2.11 - 2.87 2.47 2 2.41 5 2.44 1 Ca 3.79 2.86 - 3.92 3.71 2 3.62 5 3.86 2 Ti 0.48 0.28 - 0.48 0.40 17 0.41 15 0.37 23 Mn 0.75 0.69 - 0.86 0.73 3 0.68 9 0.81 8 Fe 0.76 0.63 - 0.84 0.75 1 0.75 1 0.71 7 Co 0.12 0.08 - 0.20 0.15 25 0.13 8 0.05 58 Cu 0.97 0.88- 1.19 0.89 8 0.89 8 1.02 5 Ba 0.48 0.31 - 0.65 0.45 6 0.09 > 100 0.44 8
f. Corning Museum of Glass Standard B
Elements Cert. Range in ZAF Rel. ZAF Rel. ZAF Rel. (w%) cert. AXIL diff. AXIL diff. PGT diff. (w%) 20 kV (%) 15 kV (%) 15kV (%) (w%) (w%) (w%) Na 12.81 11.28- 13.44 9.72 24 10.42 19 12.81 0 Mg 0.72 0.48 - 0.92 0.52 28 0.52 28 0.56 22 Al 2.23 1 .8 5 -2 .4 9 1.96 12 2.11 5 2.17 3 Si 28.77 27.71 - 29.89 26.80 7 27.74 4 29.56 3 P 0.37 0.25 - 0.42 0.19 49 0.17 54 0.27 27 S 0.22 0 .1 5 -0 .3 2 0.25 14 0.24 9 0.29 32 Cl 0.20 0.15-0.22 0.14 30 0.14 30 0.15 25 K 0.91 0.70 - 0.98 0.91 0 0.89 2 0.89 2 Ca 6.23 5.19-6.54 6.10 2 6.16 1 6.18 1 Mn 0.18 0 .1 2 -0 .2 9 0.16 11 0.17 18 0.09 50 Fe 0.24 0.21 -0.31 0.22 8 0.25 4 0.21 13 Cu 2.16 1.92-2.40 1.88 13 1.99 8 2.38 10 Zn 0.16 0.08 - 0.21 0.16 0 0.23 44 0.10 38 Ba 0.13 0.04-0.18 0.09 31 0.01 > 100 0.00 100 Pb 0.37 0.10-0.46 0.40 8 known conc. 0.24 35
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g. Corning Museum of Glass Standard C
Elements Cert. Range in ZAF Rel. ZAF Rel. ZAF Rel. (w%) cert. AXIL diff. AXIL diff. PGT diff. (w%) 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 0.89 0.60-1.01 1.06 19 1.06 19 0.61 31 Mg 1.71 1.46 - 1.90 1.95 14 1.76 3 1.53 11 Al 0.47 0.40 - 0.58 0.55 17 0.51 9 0.47 0 Si 16.30 15.75-17.76 17.94 10 16.99 4 17.86 10 P 0.04 0.03 - 0.08 known concentration S 0.06 0 .0 4 -0 .1 0 known concentration Cl 0.10 0.10 -0.10 known concentration K 2.27 1.93-2.58 2.18 4 2.35 4 2.49 10 Ca 3.62 3.29 - 3.89 3.54 2 3.63 1 3.88 7 Ti 0.49 0.42 - 0.93 0.62 27 0.66 35 0.90 84 Fe 0.23 0 .1 8 -0 .3 1 0.25 9 0.26 13 0.20 13 Co 0.13 0.09 - 0.20 0.10 23 0.12 8 known conc. Cu 0.93 0.88- 1.20 1.06 14 1.11 19 1.12 20 Ba 10.83 7.14-12.05 12.12 12 8.45 22 7.91 27 Pb 34.26 29.63 - 34.72 31.68 8 known concentration
h. Corning Museum of Glass Standard D
Elements Cert. Range in ZAF Rel. ZAF Rel. ZAF Rel. (w%) cert. AXIL diff. AXIL diff. PGT diff. (w%) 20 kV (%) 15 kV (%) 15 kV (%) (w%) (w%) (w%) Na 0.98 0 .5 9 -1 .3 3 1.21 24 1.10 12 0.60 39 Mg 2.45 2.34 - 2.73 2.64 7 2.44 1 2.20 11 Al 2.87 2.59 - 3.52 2.71 6 2.56 11 2.65 8 Si 25.81 2 5 .1 7 -2 7 .4 7 25.08 3 24.73 4 26.81 4 P 1.75 1.59-2.17 3.20 83 known conc. 1.72 2 S 0.12 0.08-0.15 0.06 50 0.04 67 0.13 8 Cl 0.40 0.19-0.40 0.09 78 0.12 70 0.14 65 K 9.55 9.08- 10.19 9.58 1 9.61 1 9.95 5 Ca 10.76 9.30 - 10.96 10.71 1 10.66 1 10.85 1 Ti 0.24 0 .1 4 -0 .3 2 0.19 21 0.21 13 0.18 25 Mn 0.36 0.26 - 0.60 0.40 10 0.41 14 0.44 22 Fe 0.35 0.31 -0.51 0.30 14 0.31 11 0.32 9 Cu 0.32 0.21 - 0.48 0.35 9 0.42 24 0.38 19 Ba 0.30 0.22 - 0.63 0.30 0 known conc. 0.22 27 Pb 0.25 0.09 - 0.30 0.30 20 known concentration
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For the concentration levels above 1 percent, in most cases deviations from the certified concentration of the order of a few percent are noted (indicated in the tables as “Rel. diff.”); larger errors were sometimes observed for Na and/or Mg. When Mg is present at low concentration (below 1 %), the PGT program overestimates the background, resulting in an underestimation of the concentration (see Table 3 c and f). For concentration levels close to the detection limit of the method (in the 0.1-1 % range) also large errors were found, that is why the concentrations of these elements were introduced in the quantitative PGT and AXIL programs; this is indicated in the tables as “known concentrations”. Also for heavier elements such as Zn and Ba a poorer accuracy is obtained. Similar trends can be observed in all the NIST standards. The Corning Museum of Glass Standards show worse results for some elements (e.g., Co, Cu and Pb), probably due to heterogeneity. The results obtained with the two methods (with and without standards) are comparable, except for magnesium when present in low concentrations. Therefore the standardless method was preferred because it is less time consuming. When using standards, it is necessary to remeasure them regularly to correct for changes in measuring conditions, especially the detector efficiency.
It can be concluded that the accuracy of the method is sufficient for an overall compositional analysis of the glass and for distinguishing glass materials with a manifestly different abundance of major elements such as Na+ and K+, but that the quality of the quantitative analysis results is too poor to detect small differences with sufficient confidence.
3. MICRO SYNCHROTRON RADIATION INDUCED X-RAY FLUORESCENCE (p-SRXRF)
Trace element determinations in silicate-type materials such as glass are usually performed by means of destructive techniques such as AAS (atomic absorption spectrometry) [13], ICP-MS (inductively coupled plasma-mass spectrometry) [14] and ICP-OES (inductively coupled plasma-optical emission spectrometry) [15]. Since these techniques require acid dissolution of the samples, the trace analysis of valuable samples by such technique involves the destruction of the glass samples. Therefore, preference was given to p-SRXRF which (a) is completely non destructive, (b) only requires minute samples and (c) can yield data on trace element concentrations with high quantitative reliability on both conducting and non conducting samples.
3.1 Principle of p-SRXRF
Synchrotron radiation induced X-ray fluorescence is a technique in which an intense (micro-)beam of polarised X-ray photons is employed as means of sample irradiation instead of photons generated by an ordinary X-ray tube.
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Synchrotron radiation is produced in synchrotrons or storage rings. This technique is based on the principle that electromagnetic radiation is emitted when electrons (or positrons) moving at relativistic speed are forced to change their direction. Practically, synchrotron radiation is produced when the electrons in the storage ring pass through a bending magnet, which is situated in between two straight sections, since this is the only place where they experience an acceleration. The radiation is emitted in a narrow cone tangentially to the circular particle path because the electrons are accelerated towards the centre of the ring; this facilitates the delivery of the radiation to a prefined sample area. It is used in experimental set-ups at the end of the beamlines.
One of the features that makes synchrotron radiation a useful source for XRF measurements is the fact that it is very intense, namely a factor 106-1012 more than that produced in conventional X-ray tubes. Bending magnets produce a polychromatic spectrum of which the energy range extends from the infrared into the hard X-rays (white beam). The characteristic energy of the radiation, defined as the median of its power spectrum, is given by 2.218 E3/R where E is the energy of the orbiting particles in GeV and R is the radius of the curvature of the beam orbit at the ending magnets in m. Quasi monochromatic X-ray microbeams can be generated from the white spectrum through the use of X-ray monochromators. Another advantage in comparison with the conventional X-ray sources is the extremely high brilliance achieved with synchrotron radiation sources, which is expressed as the number of photons emitted per unit source area over a unit angle of emission and per unit energy.
A particularly interesting property of synchrotron radiation is its high degree of linear polarisation. This means that the total emitted radiation can be considered to be a superposition of two independent components corresponding to photons whose electrical field vectors are situated perpendicular to (vertically polarised component) or parallel with (horizontally polarised component) the storage ring plane. Because of this polarisation, detection of scattering from a sample can be markedly reduced when the detector is positioned in the ring plane compared to when the detector is not placed in this plane. This results in spectra having a much lower scatter background and thus better sensitivity and detection limits (1-10 ppm). Careful alignment in height of the optical axis of the spectrometer to that of the storage ring is therefore very critical to obtain a high degree of linear polarisation.
Most of the presently operational synchrotron radiation sources (including the ones used in this work) belong to the so-called second generation facilities which were essentially designed to exploit the radiation produced from the bending magnets. This distinguishes them from the first generation ones in which the synchrotron radiation was produced as a parasitic phenomenon in high energy collision experiments with elemental particles. Of special interest for future activities are third generation storage rings which are specifically designed to obtain unprecedented intensity and brilliance. Significant is also the use of insertion devices that are placed in the straight sections of the storage ring (wigglers and undulators).
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Wigglers are magnetic structures that create multiple oscillations of the orbiting particles around the beam path and hence increase both the energy and intensity of the radiation. Undulators create smaller and more frequent deflections, giving rise to interference effects in the radiation produced, yielding coherent radiation concentrated around several specific energies (harmonics). In addition, X-ray optics of increased sophistication amplify considerably the flux and brilliance.
The fluorescent radiation is collimated by means of either a pinhole collimator or capillary optics so that only a microscopical volume of the sample material is irradiated. The lateral distribution of major, minor and trace elements can be measured by collecting spectra from different locations when the specimen is moved through the microbeam. The lateral resolution (10 pm in routine cases) is determined by two factors. On the one hand the beam size, which can only be reduced at the expense of a reduction of total beam flux (and thus of analytical sensitivity) and on the other hand the fact that the penetration and sampling depths of incoming and emerging X-rays are equal to or larger than the spot size. The combined use of third-generation synchrotron X-ray sources and more sophisticated X-ray optics such as conical and paraboloidal capillaries recently led to lateral resolutions of < 1 pm [16-21],
(a)
10' Ca Na a> c jcaC CJ 10* (A C3 oO
oc c
10'
10' 10 Energy. keV
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(b) 10 Ca Fe Mn 10'
Cu Zr+Sr 10: Zn Sb Sn Sb Ba
10‘ 20 25 30 Energy, keV
Figure 5. Typical (a) EPXMA and (b) white beam p-SRXRF (collected at the NSLS X26A station) spectra of the unaffected region of a glass sample from Qumran (#22).
From Figure 5, showing EPXMA and p-SRXRF spectra of the same glass sample (Qumran sample IRPA#22), it is evident that by using EPXMA only information on the major elements (in this case: Na, Al, Si, Cl, K, Ca, Mn, Fe) can be obtained whereas by means of p-SRXRF, additionally, minor and trace constituents such as Ti, Cu, Zn, Pb, Br, Rb, Sr, Mo, Zr, Sn, Sb and Ba are detected.
3.2 Quantitative analysis by means of a fundamental parameter method
The characteristic and scatter intensities in SRXRF spectra are influenced by many experimental parameters such as the excitation spectrum, its degree of polarisation, the excitation and detection geometry used during the experiments, as well as the composition and thickness of the samples. To obtain a better insight into the importance of these parameters and to determine the sensitivity and accuracy of the method, a detailed Monte Carlo simulation code was developed in our laboratory [22, 23] which is able to predict the complete spectral response of the spectrometer caused by interactions between a beam of polarised synchrotron radiation and a sample. The excitation conditions, the detector and the characteristics (composition, thickness and density) of the sample being measured, are introduced as input parameters and the model predicts the fluorescent line intensities.
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Taking advantage of the accuracy with which this can be done, the simulation code can be used in reverse to determine the composition of unknown samples from their experimental SRXRF spectra. In order to take into account the self-absorption characteristics of the sample correctly, knowledge of the matrix composition of the sample is essential; this is determined by previous measurements by a different analytical technique (SEM-EDX). The assumed composition of the unknown (provided as input for the simulation program) is changed during the iterative process until the simulated distribution matches the measured spectrum as closely as possible and the relative deviations have become smaller than a pre-set threshold value (e.g., 5 %). Next to this, the previously determined composition of Fe is used as an internal standard. Since the error on the Fe concentration, as determined by SEM-EDX, is about 10 %, this is also propagated into the uncertainties of the composition of the trace elements.
3.3 SRXRF instruments at which experiments were performed
p-SRXRF measurements were executed at the NSLS (National Synchrotron Light Source, Brookhaven National Laboratories, Upton, NY, USA) X26A beamline and at Hasylab (DESY, Hamburg, Germany) beamline L, DORIS III. They were both operated in the polychromatic radiation mode which has the advantage that (nearly) all elements are excited with comparable efficiency. Per sample, a spectrum collection time of 200-1000 s was used.
3.3.1 NSLS beamline X26A A 1.2 T bending magnet source produces the primary highly collimated photon beam which has a smooth continuous energy distribution. When the beam leaves the ultra-high vacuum in the storage ring, it is first collimated by four tantalum slits and then further defined by an 8x8 pm2 crossed slit system. The source to pinhole distance is 10 m. The sample, mounted on an XYZ stage, is positioned at 45° to the incoming beam and can be viewed by a horizontally mounted microscope which is equipped with a TV camera and which is used to bring the sample surface into the cross point of the primary beam and detector axes. A Si(Li) detector, situated at 4.5 cm from the examined material, detects the fluorescent and scattered radiation originating from the specimen. A Ag aperture (1 or 3 mm in diameter) is placed in between sample and detector in such a way that only the central part of the detector crystal is used. To decrease the intensity of low energy matrix lines, additional absorbers can be placed just behind the detector collimator.
3.3.2 Hasylab beamline L
The primary radiation, which also originates from a 1.2 T bending magnet, covers X- ray energies from 5-100 keV. The beam is first collimated by a crossed slit system consisting of four tantalum blades. Capillary optics are positioned immediately
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consisting of four tantalum blades. Capillary optics are positioned immediately behind this collimator in order to reduce the beam size to the 10 pm level. These glass capillaries focus and/or collimate X-ray beams by the repeated total reflection of X-ray photons on the inner walls of the tube. The sample is mounted on an XYZ stage and can be viewed by a long working distance microscope. The distance between bending magnet source and spectrometer is 18.5 m. The fluorescent and scattered radiation emitted by the sample, is again detected by a Si(Li) detector which, in this case, is positioned at 3 cm from the specimen surface. To reduce the scattering which originates mostly from the surrounding air, a Pb collimator is placed in front of the detector window. In Figure 6 a schematic drawing of this set-up is given.
Si(Li)-detector
Microscope Pb-shield
45 DORIS-III - Beam monitor | Capillary
SR Sample 45 holder Beam stop Cross Slits
Figure 6. Schematic drawing of the SRXRF set-up at beamline L in Hasylab.
3.4 Accuracy and Precision of Trace Element Determinations
In order to evaluate the accuracy with which the concentrations of minor and trace constituents can be determined, a series of geological glasses with known trace element composition was analysed. In Table 4, some of the obtained results are compared to the certified concentrations. The accuracy is most cases is of the order of the uncertainty caused by counting statistics. It can be concluded that next to having a much higher sensitivity for heavy trace elements (down to the 1 ppm level), p-SRXRF also permits the quantitative determination of trace elements with an accuracy of about 10 % [22].
The precision with which these analyses are done is similar to that found for the EPXMA measurements (around 10 % or less), except when the trace element concentrations are near the limit of detection of the method (0.1-10 ppm range).
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Table 4. Analysis results of three geological glasses obtained by (a-SRXRF (from [20]). The indicated 1s standard deviation refers to the variation in composition at different locations on the (homogeneous) sample and includes statistical variations.
ATHO KL2 STHS6/80
Literature Measured Literature Measured Literature Measured ppm (w%) ppm (w%) ppm (w%) ppm (w%) ppm (w%) ppm (w%) K (%) 2.34 1.64 ±0.17 0.37 0.53 ±0.16 1.11 1.0 ±0.1
Ca (%) 1.15 0.94 ± 0.09 7.8 7.1 ± 0.2 3.73 3.2 ± 0.1
Ti (%) 0.15 0.11 ±0.02 1.6 1.4 ± 0.1 0.43 0.38 ± 0.05
Cr 180 177 ±34 205 417 ±148 89 59 ±53
Mn (%) 0.08 0.10 ± 0.02 0.16 0.18 ± 0.03 0.07 0.07 ± 0.01
Fe (%) 2.76 reference 8.32 reference 3.4 reference
Ni - - 115 124 ±25 -- Zn 169 153 ± 10 120 123 ± 14 59 60 ± 8
Ga 25.6 28 ±4 20 23 ± 10 18 20 ± 9
Rb 59.4 65 ±4 9.4 10 ± 4 29.2 31 ± 4
Sr 92.3 99 ± 5 363 391 ± 33 480 472 ± 17
Y 103 105 ± 4 24.4 22 ± 6 - - Zr 567 557 ± 15 163 177 ±29 120 120 ± 6
Nb 65.5 52 ±3 17 13 ± 5 - 4 ± 2
Ba 557 592 ± 16 124 153 ±43 300 297 ±8
La 58.3 57 ± 3 13.7 12 ± 7 11 11 ±3
Ce 126 125 ±5 33.7 34 ± 13 23 23 ±3
Pr 14.9 13 ± 2 4.71 5 ± 4 --
Nd 58.9 72 ± 3 22.3 29 ± 11 14 14 ± 3
Sm 13 19 ± 3 7.8 13 ± 12 --
Eu 2.13 6 ± 4 - ---
Gd 19.2 16 ± 4 - -- -
Dy 15.4 13 ± 5 - -- -
Er 9.4 14 ± 5 - - - -
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Next to these geological glasses also Corning Museum of Glass standards A, B and D were analysed using the same method for quantification. The results (in ppm) are given in Table 5.
Table 5. Analysis results (in ppm) of the Corning Museum of Glass standards A, B and D obtained by p-SRXRF.
Oxide Standard A Standard B Standard D
Cert. Range in Meas. Cert. Range in Meas. Cert. Range in Meas. (ppm) cert, (ppm) (ppm) (ppm) cert, (ppm) (ppm) (ppm) cert, (ppm) (ppm) v2os 60 1 0 -3 8 0 4500 300 200 - 780 4600 150 100-470 4100
Cr20 3 10 1 0 -5 0 1700 50 3 0 -8 0 48 25 2 0 -5 0 41
CoO 1500 1 0 0 0 -2 5 0 0 2100 350 180 - 600 530 220 1 5 0 -3 0 0 260
NiO 300 200 - 400 380 900 700 - 1400 1200 600 480 - 740 580
CuO 12200 9900-13500 12000 27000 24000 - 30000 29000 4000 3400 - 4500 4100
ZnO 400 300-1000 420 2000 1000-2600 2200 1000 500-1500 1000
RbzO 100 1 0 0 -1 1 0 0 62 10 1 0 -2 3 4 50 3 0 -7 0 33
SrO 1000 800-1300 880 100 100-200 160 500 500 - 800 650
Zr02 50 1 0 -1 0 0 60 250 200 - 400 230 125 1 0 0 -1 3 0 150
Ag20 20 5 - 3 0 5 100 50 - 160 79 50 3 3 -7 0 43
Sn02 2800 2000 - 3300 2400 400 250 - 500 360 1300 1000-1600 1400
Sb2Os 17200 8300-19100 15000 4600 3200 - 6900 5300 9600 8400 - 12500 11000
BaO 5400 2900 - 6000 4000 1400 500 - 1800 960 3300 2500 - 4500 3900
PbO 800 500 - 900 660 4000 3600 - 5600 4900 2700 2000 - 3500 2700
Bi203 20 1 0 -3 0 5 50 3 3 -6 9 61 25 1 0 -3 0 48
The results obtained for the elements in these standards almost all fall in the concentration ranges which were found by the laboratories engaged in the comparison experiments. The fact that the concentration of vanadium is greatly overestimated can be explained by the fact that the background in that region of the spectrum was badly fitted, resulting in an overestimation of the signal of vanadium. This sometimes also affected the fitting of the chromium signal as can be seen for Standard A.
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4. MICRO PARTICLE-INDUCED X-RAY EMISSION SPECTROMETRY (M-PIXE)
Another technique for multielement analysis is particle-induced X-ray emission spectrometry (PIXE). A beam of charged particles, usually protons or, on occasion, heavier ions, accelerated to an energy of a few MeV, excites characteristic X-rays in the atoms of a specimen. The X-ray spectrum is recorded in energy-dispersive mode with the aid of a Si(Li) detector.
Microbeam PIXE, often called micro-PIXE, which is used in this work, is a variant of PIXE that has become important in recent years. It uses a highly focused proton beam leading to the options of both one and two-dimensional imaging of element distributions in a sample by scanning the beam across the surface of the specimen. The principle is analogous to that of the electron microprobe. MeV protons, however, are more difficult to focus than keV electrons. The beam from the accelerator is defined by the object aperture. The production of beam diameters less than 1 0 pm requires some form of focusing system in which the beam is further reduced in size by a system of demagnifying lenses, usually two, three or four quadrupole magnets.
A PIXE spectrum consists of the characteristic X-ray peaks from the elements in the sample and a continuous background underlying them. This background is the principal determinant of the limit of detection for any element and it arises from primary proton bremsstrahlung, from electron bremsstrahlung and from nuclear reaction gamma rays. The principal contributor is bremsstrahlung associated with the deceleration of electrons undergoing elastic collisions since the intensity of the primary, projectile bremsstrahlung due to protons is much less prominent. This bremsstrahlung background only predominates at the lower X-ray energies (< 7 keV).
PIXE has a truly multielemental capability, covering a large part of the periodic system with high sensitivity, although it depends upon the particular sample being analysed. The highest sensitivity is obtained at proton energies of 2-3 MeV and varies smoothly as a function of atomic number. The highest sensitivity is obtained for the atomic numbers 20
Other large advantages are the non-destructive nature of the technique and the rapid data accumulation; in routine work the measuring time is a few minutes. PIXE is also extremely versatile in terms of the type and size of samples that it can accommodate; different experimental arrangements may be used. It has matured into a fully quantitative technique for determining elemental concentrations down to part-per-million detection limits and the microbeam technique makes analysis possible with spatial resolution on the order of 1 pm [24,25].
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The ij-PIXE measurements were done at the Nuclear Microprobe facility of the University of Lund, Sweden [26], using a 5 pm proton beam of 2.55 MeV. During the image scans, a proton current of 330 pA was used.
5. SECONDARY ION MASS SPECTROMETRY (SIMS)
5.1 Principle of SIMS measurements
Secondary ion mass spectrometry (SIMS) is known as one of the most sensitive surface analytical techniques. The basic principle is schematically illustrated in Figure 7 [27]. The technique is essentially based on a combination of bombarding a solid sample surface with primary ions of keV energy leading to sputtering phenomena, the production of secondary ions characteristic for the sample and finally the separation and detection of ions by mass spectrometry.
is for mess M primary ions.... ip K\■ .... depth (zI
Mass Ion separation detection
sec. ions ...is sample Computer (target)
Figure 7. Schematic representation of the basic SIMS principle.
A beam of focused primary ions (Cs+, Ar+, O' or Oz+) with an energy in the keV range is used to bombard a solid target surface. These primary ions penetrate into the sample and set into motion a number of lattice atoms in the outer atomic layers of the sample. In turn, these recoil atoms will produce multiple collisions among the other atoms resulting in a collision cascade in the vicinity of the impact region. As a result of this energy transfer, the primary ions will come to rest at a certain depth, (the penetration depth) which is determined by the composition of the target material and the energy and mass of the primary ion, and they become implanted into the solid. Constituents from the sample which are displaced by these cascades, can be
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emitted when they are close enough to the surface and when they receive an impulse pointed away from the target with an energy which is higher than the surface binding energy. This process is called sputtering and is schematically represented in Figure 8 [27]. The particles emitted in this sputtering process consist of a variety of target species and are characteristic for the elemental composition of the bombarded specimen. These secondary particles can be positively or negatively charged or they can be neutrals, excited atoms or clusters of atoms. The depth where most particles are emitted from (the information or escape depth), is in the order of 0 . 6 nm and the number of secondary particles emitted per incident ion is defined as the sputter yield.
’a°,a+, a - primary ionI sputtered _ particle
a xb°,va xb;, a zb ~ Axl£,..., AxByIt sample surface emission zone
— intermixed region
Figure 8 . Schematic representation of the sputter process and the secondary emission in secondary ion mass spectrometry.
The secondary ionised fraction is extracted from the target region and accelerated into a mass analyser (magnetic sector, quadrupole or time-of-flight) where they are separated according to their mass to charge ratio (m/e). After this separation, the secondary ion current is measured by a suitable detector system (an electron multiplier or a Faraday cage).
Important advantages of secondary ion mass spectrometry are its broad elemental coverage (all elements from H to U are detectable) and the fact that also (low
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technique with detection limits typically in the (sub)ppm range, but with poor accuracy.
The lateral distribution of elements can be visualised because each ion originates from a point in the immediate vicinity of the bombarded region so that local analysis with a resolution of less than 1 pm becomes possible. The ion beam continuously sputters away target material resulting in the erosion of successive surface layers which means that depth analysis becomes possible. A depth resolution of a few nanometer can be achieved. By a combination of the two-dimensional localisation of the surface and the depth profiling, three-dimensional analysis of a sample can be carried out.
Important drawbacks of the technique are its destructive nature due to sputtering of the target material, the possible mass interferences and the fact that ion yields are matrix dependent making quantification of the data very difficult [28].
5.2 The Cameca IMS 4F
The secondary ion mass spectrometer used in this work is of the type Cameca IMS 4F. The instrument can be subdivided into two separate parts, namely the primary ion beam system with the ion source and the primary beam optics and the secondary ion beam system with the electrostatic and magnetic sector and the detection system. A schematic representation of the Cameca IMS 4F secondary ion mass spectrometer is shown in Figure 9 [29].
5.2.1 Primary ion beam system The instrument is equipped with two primary ion sources : a Cs microbeam source and a hollow cathode duoplasmatron source of which only the latter was used in this work. In the duoplasmatron, both positively (Ar+ and Oz+) and negatively (O') charged primary ions can be produced by a plasma which is formed in this source. Positive ions occupy the centre of the plasma while negative ions are mainly produced in the exterior parts of the plasma. The extraction position can be adjusted along the Z-axis. O' was chosen as primary beam since bombardment with negative ions charges insulating materials (such as glass) to a lesser extent.
The extracted primary ions are accelerated and first pass through a mass selector to obtain a clean beam, free from residual gas contamination and ionic contaminants of the ion source. Afterwards the beam is aligned and focused by a series of electrostatic lenses, deflectors and apertures which control the intensity, position and shape of the ion beam before it impinges on the sample surface under an angle of 30° relative to the normal of the sample. The primary beam can also be deflected towards a Faraday cage in order to measure the intensity of the primary current.
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Via a sample exchange room with an air lock system, samples with a diameter up to 25 mm and a maximum thickness of 13 mm can be placed in a special sample holder and then introduced into the instrument. A metallographic microscope is mounted onto the sample chamber to view the sample. Samples can be moved manually in X, Y an Z-direction and also automatically in X and Y-direction by computer controlled stepping motors.
The sample is held at a fixed sample voltage of +4.5 kV or -4.5 kV in positive secondary ion detection and negative mode respectively. The secondary ions emitted by the sample are accelerated towards the immersion lens system in a uniform electrostatic field and penetrate into the secondary column.
5.2.2 Secondary ion beam system The secondary ions are guided through the secondary ion transfer system, which consists of a series of electrostatic lenses, to the entrance slit of the mass spectrometer.
The instrument is equipped with a double-focusing mass spectrometer in which a spherical electrostatic analyser (ESA) is coupled to a magnetic prism via a spectrometer lens. Before entering the magnet, the secondary ions are energy- filtered by the electrostatic sector and an energy slit. Further they are separated in the magnetic sector according to their mass to charge ratio (m/e).
These mass-selected ions can be detected in different modes. After deflection the secondary ions can be detected by an electron multiplier (EM) for the detection of low ion currents (I < 10 6 c/s) or by a Faraday cup (FC) for high ion currents (I > 10 6 c/s). Both detection systems are used in the non-imaging mode. Ion microscope images can be visualised by focusing the virtual mass-filtered image onto a microchannel plate by the projector lens, where the secondary ion image is transferred into an electronic image which can be directly observed via a binocular after interaction with a fluorescent screen. A sensitive camera coupled to a TV monitor is normally used instead of visual observation. This is referred to as the imaging mode.
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© 15) J16
1. Duoplasmatron source 2. Cs source 3. Primary mass filter 4 . Immersion lens 5. Sample 6. Sample chamber 7. Transfer optics 8. Entrance slit 9. Electrostatic sector 10. Energy slit 11. Spectrometer lens 12. Mass spectrometer 13. Secondary mass filter 14. Exit slit 15. Projection lenses 16. Projection, display and detection unit 17. Deflector 18. MicroChannel plate 19. Fluorescent screen 20. Deflector 21. Faraday cup 22. Electron multiplier
Figure 9. Schematic representation of the Cameca IMS 4F secondary ion mass spectrometer.
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6. FOURIER TRANSFORM INFRARED SPECTROMETRY (FTIR)
6.1 Principle of Fourier transform infrared spectrometry
The infrared region is split up into three parts : the near-IR (v = 12820 - 4000 cm'1), the mid-IR (v = 4000 - 400 cm'1) and the far-IR (v = 400 - 40 cm"1) of which only the mid-IR is used in this work. When a beam of infrared light hits a sample, energy of certain selective wavelengths can be absorbed depending on the molecules present in the material. The absorption bands of IR-spectra appear at well-defined wavelengths and the spectra are a reflection of the characteristic molecular rotations and vibrations containing both compositional and structural information of the materials studied.
A way to examine a beam of radiation spectroscopically is by way of interference techniques. The IR-radiation emitted by the source is converted into an interference pattern by Fourier transformation before hitting the sample. After interaction with this material, a new interferogram is created which can be transformed into a spectrum via calculation [30-32].
6.2 Instrumentation
Infrared transmission spectra were recorded on a Nicolet 20DXB FTIR spectrometer and infrared reflection spectra on a FTIR Microscope (Spectra tech) coupled to a Nicolet 5DXB spectrometer. 200 scans were collected in the mid-IR range (400 - 4000 cm'1) with a resolution of 4 cm'1. Figure 10 shows a schematic view of a Nicolet 20DXB FTIR spectrometer.
The heart of the FTIR spectrometer is the interferometer part, which is of the Michelson type in the instrument described here. Infrared light is emitted by a Globar source, consisting of a rod of synthetic SiC, raised to a temperature of 1500 K by means of electric current and is directed to a beam splitter which is mounted in the interferometer at an angle of 45° to the incident beam. The beam splitter is built in such a way that it allows half of the light to pass through while it reflects the other half. The latter part is reflected by a fixed mirror which is placed at a distance L so that the beam hits the beam splitter again after a pathlength of 2L. The transmitted part, however, is falling on a reflecting mirror which is not fixed but can be moved around L by a distance x resulting in a total pathlength of 2(L+x) for this beam when it again reaches the beam splitter.
After reflection, the two beams are recombined on the beam splitter where interference occurs as a result of the optical pathlength difference ( 2 x) between the fixed mirror and the movable mirror. This interference can be constructive (when the pathlength difference is a multiple of the wavelength X) yielding a maximum detector
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signal or destructive (when the pathlength difference is an odd multiple of half of the wavelength 7J2) resulting in minimum detector signal. In this way the interferogram is produced which is a measure for the radiation intensity as a function of the mirror displacement (or the optical pathlength difference).
A Beamsplitter Moving Mirror Input r M ir r o r /V ------
Electronics Laser Beamsplitter S o u rc e
Sample M lrro^ S a m p le Mirror
Figure 10. Schematic representation of a Nicolet 20DXB FTIR spectrometer.
The interferometer measures all frequencies of the incident radiation simultaneously and converts them into an interferogram which is thus a quenched and modulated signal. This modulated signal is passed through the sample compartment where the sample selectively absorbs certain frequencies resulting in a new interferogram which is finally focused on the detector. This interferogram is subjected to a Fourier transform process which describes it by a series of cosine and sine functions to yield the final spectrum of the sample.
The signal is also accompanied by a He/Ne laser signal with exactly known frequency to control the change in optical path difference. Since the spectrum produced by an FTIR instrument is a single beam spectrum, a background spectrum must be scanned and stored separately before the sample spectrum is recorded. These two spectra are ratioed afterwards to produce the sample spectrum free from the profile of the source and atmospheric absorptions.
A Deuterated Triglycine Sulphate (DTGS) and a Mercury Cadmium Telluride (MCT) detector, both pyro-electric detectors, were used for the normal FTIR and the microscope respectively. The detector produces an electrical signal, proportional to the incident radiation intensity, over the spectral range of the spectrometer. The DTGS detector operates at room temperature while the MCT detector is working at
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very low temperatures requiring cooling by liquid nitrogen, resulting in higher cost but also a faster response time.
Infrared analysis of small samples had posed considerable difficulty but a solution was found in coupling an infrared microscope with an FTIR spectrometer allowing microscopic analysis of sample sizes down to the diffraction limit. The sample is mounted on a sample holder and placed on the stage of the microscope. It is brought into focus and the image can be studied using either transmitted or reflected visible light. A knife-blade aperture system is used to isolate the area of interest and this part of the sample is analysed in-situ in the FTIR microscope in transmission or reflection mode [30-32].
7. SUM M A RY
In this chapter the different microanalytical techniques used in this work are discussed. All the techniques have their own advantages and drawbacks but they all supply partial information about the objects under investigation on extremely small sampling material.
Optimal measuring conditions were stipulated and quantitative procedures for the determination of major, minor and trace elements from scanning electron microscopy and micro synchrotron radiation induced X-ray fluorescence measurements of glasses were examined using a series of NIST and Corning Museum of Glass standards which were first investigated on homogeneity. Major (and some minor) elements could be determined with an accuracy in the order of a few percent using a standardless ZAF program; larger errors were observed for heavier elements such as Zn or Ba and for concentrations close to the detection limit of the method (between 0.1 and 1 %). p-SRXRF permitted quantitative determination of the trace elements with an accuracy of about 1 0 % by means of a fundamental parameter method.
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REFERENCES
[1] Brill R.H., The Scientific Investigation of Ancient Glasses, Proceedings of the Vlllth International Congress of Glass, London, pp. 47-48, Society of Glass Technology, Sheffield (1967). [2] Henderson J., The Scientific Analysis of Ancient Glass and Its Archaeological Interpretation, Henderson J. (ed.), Scientific Analysis in Archaeology, and Its Interpretation, pp. 245-254, Oxbow Books, Oxford (1989). [3] Henderson J., The Analysis of Ancient Glasses Part I : Materials, Properties, and Early European Glass, JOM : The Journal o f the Minerals, Metals and Materials Society, 47(11), pp. 62-64 (1995). [4] Goldstein J.l. and Yakowitz H., Practical Scanning Electron Microscopy. Electron and Ion Microprobe Analysis, Ch, Plenum Press, New York (1977). [5] Heinrich K.F.J., Electron Beam X-Ray Microanalysis, Van Nostrand Reinhold Company, New York (1981). [6 ] Lawes G. and James A.M., Scanning Electron Microscopy and X-Ray Microanalysis. Analytical Chemistry by Open Learning, John Wiley & Sons, Chichester (1987). [7] Goldstein J.I., Newbury D.E., Echlin P., Joy D.C., Romig A.D. Jr., Lyman C.E., Fiori C. and Lifshin E., Scanning Electron Microscopy and X-Ray Microanalysis. A Text for Biologists, Materials Scientists, and Geologists. Second Edition, Plenum Press, New York (1992). [8 ] Jeol JSM 6300 Scanning Electron Microscope, Mechanical Parts List. [9] Turner W.E.S., The Precision Attainable in the Chemical Analysis of Ancient Glass, Advances in Glass Technology, Part 2, pp. 384-387, Plenum Press, New York (1963). [10] Brill R.H., Interlaboratory Comparison Experiments on the Analysis of Ancient Glass, Proceedings of the Vllth International Congress on Glass, Paper No. 226, Brussels (1965). [11] Brill R.H., A Chemical-Analytical Round-robin on Four Synthetic Ancient Glasses, Proceedings of the IXth International Congress on Glass, Section Bl, pp. 93-110, Versailles (1971). [12] Scott V.D., Love G. and Reed S.J.B., Quantitative Eiectron-Probe Microanalysis [Second Edition], Ellis Horwood, New York (1995). [13] Brill H., Excavations at Jalame, Site of a Glass Factory in Late Roman Palestine, University of Missouri Press, Chapter 8 (1988). [14] Calvi M.C., Tornati M. and Scandellari M.L., I Vetri Romani del Museo de Aquileia, (1968). [15] Jackson C.M., Hunter J.R., Warren S.E. and Cool H.E.M., The Analysis of Blue-Green Glass and Glassy Waste from two Romano-British Glass Working Sites, Archaeometry’90, Birkhauser Verlag, Basel, pp. 295-304 (1991). [16] Koch E.E. (ed.), Handbook on Synchrotron Radiation, North-Holland Publishing Company, Amsterdam (1983). [17] Blewett J.P., Synchrotron Radiation - 1873 to 1947, Green M.A., Scott J.P. and Woodruff P.R. (eds.), Synchrotron Radiation Instrumentation, North- Holland, Amsterdam (1988).
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[18] Kunz C. (ed.), Topics in Current Physics. Synchrotron Radiation. Techniques and Applications, Springer-Verlag, Berlin (1979). [19] Winnick H. (ed.), Synchrotron Radiation Sources. A Primer, Series on Synchrotron Radiation Techniques and Applications, vol. 1, World Scientific, Singapore (1995). [20] Janssens K., Vincze L., Vekemans B., Aerts A., Adams F., Jones K.W. and Knochel A., Synchrotron Radiation Induced X-Ray Microfluorescence Analysis, Mikrochim. Acta [Suppl.J, 13, pp. 78-115 (1996). [21] Adams F., Janssens K. and Snigirev A., Microscopical X-ray Fluorescence Analysis and Related Methods with Laboratory and Synchrotron Radiation Sources, JAAS, in press. [22] Vincze L., Janssens K., Adams F. and Jones K.W., A General Monte Carlo Simulation of ED-XRF Spectrometers : III Polarized Polychromatic Excitation, Homogeneous Samples, Spectrochim. Acta, 50, pp. 1481-1500(1995). [23] Vincze L., Monte Carlo Simulation of Conventional and Synchrotron X-ray Fluorescence Spectrometers, PhD dissertation, University of Antwerp (1995). [24] Johansson S.A.E. and Campbell J.L., PIXE, A Novel Technique for Elemental Analysis, John Wiley & Sons Ltd., New York (1988). [25] Johansson S.A.E., Campbell J.L. and Malmqvist K.G., Particle-Induced X-Ray Emission Spectrometry (PIXE), John Wiley & Sons, Inc., New York (1995). [26] Tapper U.A.S., Hellborg R., Hult M.B., Larsson N.P.-O., Lovestam N.E.G., Malmqvist K.G., Pallon J. and Themmer K., Nucl. Instr. and Meth. B, 49, p. 425 (1990). [27] Werner H., Introduction to Secondary Ion Mass Spectrometry (SIMS), Fiermans L., Vennik J. and Dekeyser W. (eds.), Electron and Ion Spectroscopy o f Solids, Plenum Press, New York (1977). [28] Benninghoven A., Rudenauer F. and Werner H. (eds.), Secondary Ion Mass Spectrometry : Basic Concepts, Instrumental Aspects, Applications and Trends, Chemical Analysis Series, vol. 8 6 , John Wiley & Sons, New York (1987). [29] Cameca IMS 4F, User’s Guide. [30] Griffiths P.R. and de Haseth J.A., Fourier Transform Infrared Spectrometry, Chemical Analysis, vol. 83, John Wiley & Sons, New York (1986). [31] George W.O., McIntyre P.S. and Howthorpe D.J. (eds.), Infrared Spectroscopy, Chichester (1987). [32] Vansant E.F., Gillis-D’Hamers I, Molinard A. and Vanhoof C., Textbook FTIR, IREC, UIA, Wilrijk.
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Investigation of a Roman Glass Collection from Qumran, Israel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel
1. INTRODUCTION
Glass objects which have been buried for extended periods of time show corrosion phenomena on their surface through the contact with groundwater. This causes the original clear glass to become covered with an opaque and flaky coating. Fragments of this top layer can be easily removed. In some cases the corrosion layer can become so extensive that the mechanical strength of the glass is severely affected. As a result, many ancient glass artefacts have become very fragile.
Brewster was the first to describe in detail the outlook of naturally decomposed glass; his work dates back from 1863 [1]. All speculative suggestions about iridescence in glasses were put to rest by his physical explanation of the cause of the observed colours. Brewster made a detailed examination of a variety of specimens of iridescent glass found in Nineveh and Rome. His observations led him to conclude that the decomposition of ancient glass usually begins at a large number of isolated centres on or below the surface and that the process of decomposition then proceeds in all directions, though probably with greater velocity down into the glass. The colours in the altered glass could be satisfactorily explained as being the result of interference phenomena exhibited by extremely thin plates of air trapped between transparent or translucent films of colourless altered glass. Brewster also observed the presence of a variety of crystals on decomposed glass; he speculated about the composition of these crystals but made no attempt to discuss the chemical composition of decomposed ancient glass.
The first quantitative analysis of decomposed ancient glass was made by Geuther, the results of which were published by Hausmann [2]. Geuther analyzed the outer layers of more or less decomposed glass on a specimen from excavations in Rome as well as the underlying unaltered glass. He found that alkalis were absent from the alteration products and that these contained 19.3 % water as contrasted to the unaltered glass which contained no water at all.
Fowler [3] arrived at a classification of the modes of decay of glass on the basis of his inspection of a large number of objects. He made a distinction between filmy, blistering or iridescent decay on the one hand and granular decay on the other hand, the latter being further subdivided into superficial creeping decay, deep creeping decay, spotty or pitting decay and splitting, cracking or crumbling decay.
The investigation made by Geilmann into the alteration or decomposition of glass long buried in the soil is a very detailed and thorough account of the complex physical and chemical changes that occur [4]. By means of many analyses he showed conclusively that the alkalis, the alkaline earths and a few other components are more or less completely removed during decomposition of glass in the soil, whereas aluminium, iron, titanium and some other elements that form insoluble decomposition products, remain behind to a variable extent together with the silica. The leached-out parts of the samples are hydrated to an extent of about 20 %. This
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water content poses severe conservation problems because the crust may become very friable when the water dries out and any cleaning of the surface must be done before it is fully dry.
Other early discussions of excavated glass are given by Neumann [5], Laubengayer [6 ], Varshney [7], Turner [ 8 ] and Caley [9].
The nature of the crust formed on glass samples during burial differs from the one formed on glass panes in the atmosphere. In the soil normally a lamellar dark coloured structure develops in the leached layer. These layers have attracted much attention over the years but they are not yet fully understood. They were first described by Brewster [10] and then by Fowler [3], Geilmann [4, 11] and Raw [12]. These thin layers are typically of the order of 5-20 pm for potassium based glasses and less than 1 pm for sodium glasses [4, 11, 13]. Brill and Hood suggested that these layers could be compared to annual tree rings, one deposited per year but starting from the surface on [14], but this was later refuted by Newton and Malow [15, 16].
Next to glass corrosion studies performed by archaeologists, also in the nuclear safety and waste disposal sector, the interaction of soil/water and glass has been studied extensively. The study of the long-term durability of nuclear waste glasses, particularly in relation to the environmental disposal conditions, is of great importance in storing nuclear material buried in geological sites. Since the 1970s many studies have been conducted on the behaviour of natural and synthetic glasses and the French R7T7 glass [17-22]. The use of natural analogues to model waste glass durability was proposed by Ewing [23].
In this chapter a Roman glass collection is studied which was excavated at the archaeological site of Khirbet Qumran in Israel. Because this rather large group of glasses is well-dated (1st century AD) and belonged to one particular community, it presented an opportunity to find out which information about the glass objects themselves and/or the site they were found in can be gained by investigating in detail the major, minor as well as trace composition of the different objects. Moreover, it was expected that a study of the multilayered corroded parts of the material could lead to a better understanding of the chemical and physical processes taking place during prolonged burial.
2. THE ARCHAEOLOGICAL SITE AND THE GLASS COLLECTION
2.1 The archaeological context of the site
The archaeological site of Khirbet Qumran (“Khirbeh” means “ruin”) consists of a collection of ruins of various buildings situated on the north-western shore of the Dead Sea in Jordan (until 1967, now under Israeli occupation), on a terrace of marl
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at the eastern edge of the Judean desert (See Figure 1). The site is periodically flooded by the neighbouring Wadi Qumran. The place was excavated for the first time in 1951 by a small team led by Father R. de Vaux; five other archaeological campaigns followed regularly in the period 1953-1958.
MONTNEBO ▲
h y r c a n ia ▲ » QUMRAN ------A IN FESHKHA
• /E iN EL-GHUWEJR
CAUJRHOt ••f/KHIRBET MAZIN
DEAD SEA
Figure. 1. Location of Qumran at the north-western shore of the Dead Sea.
Few archaeological sites around the Mediterranean have stirred up so many questions as Khirbet Qumran. A lot of controversies exist about the role of this site in early history. The ruins were excavated in the context of and as an extension of the excavations of the nearby caves where numerous manuscripts, known as the famous “Dead Sea Scrolls”, were discovered. This discovery was unique since the passages of the Old Testament on the scrolls were in a well-known stylistic form but they were written on parchment which was centuries older than all comparable manuscripts known until then. Based on the interpretation of other ancient texts and on partly ambiguous archaeological observations, the leader of the archaeological
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team performing the excavations, R. de Vaux, came to the conclusion that the human group that occupied the old fortress until the arrival of the Roman troops was a community of a dissident Jewish religious order, named the Essenes who considered the arid environment an appropriate location to lead a monastic and ascetic life. That is why the site was interpreted as the monastic residence of these people. It would have been this religious community that, faced with the imminent danger of the Roman troops between 4 BC and 6 8 AD, had hidden its 'library' in the nearby caves. This hypothesis was avidly advocated by R. de Vaux until his death in 1971 [24-28],
Contrary to established ideas about the site, the first results obtained by a team from the Catholic University of Louvain (UCL), commissioned to publish the results of the Khirbet Qumran excavations, tend to suggest that instead of a religious centre, the site was characterised by industrial and commercial activities in the field of the perfume industry which were centered around a well-appointed villa. The presence of a large collection of coins and luxury goods at the site serves to strengthen this counter-hypothesis [29].
2.2 Description of the glass collection
Next to an extensive series of terracotta oil lamps and a collection of stone objects, also a group of 90 (fragments of) glass objects, such as bottles, pearls, goblets and cups was recovered during the excavations. The objects are assumed to date back from the period 4 BC - 6 8 AD and therefore remained buried for about 1900 years.
Through contact with water in the soil the objects show extensive surface corrosion phenomena. With the exception of a few, all the glasses are in a state of advanced decomposition with, as a consequence, an extreme fragility and a partial or total loss of transparency. The range of manifestations of the alteration is very broad : iridescence, pitting, opacification, thick crust, leaching and flaking away of surface layers. The material has lost its mechanical strength, some of the glasses are even decomposed to the point of acquiring an earthenware outlook or stacks of flakes.
The reasons for this unfavourable state of conservation are difficult to determine. R. de Vaux has pointed out that during the periods when the site was deserted (from 73 until 132 AD and from 135 AD on since all other more recent trace of occupation is accidental and occasional), the soil probably was flooded periodically due to overflows of the aqueduct, the sewers and the nearby river Wadi Qumran. One also needs to take into account the fact that (as a result of natural phenomena such as the earthquake of 31 BC or due to the invasions of Roman troops) massive amounts of soil were moved in and out of the site in between the different periods of occupation. These filling in and clearing of the site had become apparent in the study of the lithic objects and earthenware lamps found at the site. It can be assumed that in particular the glass objects had suffered from this repeated movement of soil and rubble inside the site.
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The excavators have distinguished between a number of loci, each designated by a number which is usually associated with an architectural entity within the site [30], Most of the discoveries of glass artefacts were done in the excavation campaign of 1953; only six glass objects are reported to have been found in 1954. An explanation presents itself when the topographical distribution of artefacts throughout the site is considered. Almost all glass fragments are found in the central quadrangle of the site; only six of the fragments were found outside this perimeter. Within the perimeter of the central quadrangle, the largest number of discoveries were concentrated in the eastern half of the tower (locus 1 0 ) and in small rooms to the south of the open area (locus 37) which occupied the central part of the quadrangle. The purpose of these rooms, in which small basins for the distribution of water were found, is uncertain (Figure 2).
" M 1 — — US
1 3 2
1129'
12 0 W/ I24< 27 26 £46
4 0 41
27 24 JO 23 33 144
•0
77 to
90
78 9 4 90
Figure 2. Map of the Qumran site showing locus 10, the place where most of the glasses were found.
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Prof. R. Donceel (UCL) confided the glass fragments from the excavations of Qumran to the Royal Institute for Cultural Heritage (IRPA-KIK) in 1988 where examination and restoration was carried out by C. Fontaine-Hodiamont immediately afterwards. First of all, the fragments were sorted, the still adhering soil was removed and the glass pieces were cleaned. The second intervention was consolidation and fixation of the alteration products. Different solutions based on the acrylic resin Paraloid B72 were used. Some fragments could be regrouped and glued together [31]. In Figure 3, a photograph of some of the objects is shown after restoration.
Figure. 3. A few Roman glass objects found at Qumran, Israel (after restoration).
Parallel to the restoration, a descriptive card-index was worked out in which each specimen was described individually by eighteen properties covering the form, the dimensions, the fabrication technique, the condition of conservation and the restoration treatment [32]. It is important to remark that in most cases the determination of the form proposed in the card-index follows an extrapolation beginning from a single preserved fragment (a base, a fragment of the body, a neck, a handle or a lip). Of the whole of eighty nine identified objects, only four recipients presented a complete profile after restoration. Table 1 shows the classification and numbers of the fragments which were analysed.
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Table 1 Typological classification of the glasses presented in the card-index.
Type Subtype Numbers Ointment vessels 4, 9, 11, 13, 15, 16, 26, 28, 31, 37, 36, 38, 41, 42, 45, 47, 51,56 Goblets with a flat base 6 , 20, 29, 40 ribbed 3, 49 with a widened lip 10, 21b, 23b, 44, 54 moulded 12, 60, 65, 6 6 , 73, 74 with fluted body 14 decorated with glass strings 39 incised 43, 6 8 with a vertical flattening 75 with a band at the top 2 1 Cups ribbed with flat base 2, 18, 30, 49b, 55 moulded ribbed 35 with decoration (?) 8 , 48, 63, 76 ribbed with rounded base 7 (?) 17, 53, 69 Flasks 27, 32, 50, 62 Biconical recipients 5, 23, 34 Bottles 19, 22, 25, 27b, 33, 59, 61 Chalice on foot 24 Indeterminable shapes 52, 70, 71, 72, 77
One must point out that dating of the glass objects of Qumran on the basis of their typology and manufacturing technique is fairly difficult; the same is true for glass objects in general. Also, no clear and precise information on the occupation periods of the site is available at present. At best, one can attempt to verify whether there are no manifest contradictions between the probable manufacturing dates of the most characteristic (and best conserved) glass objects and the 'official' chronology of Qumran.
2.3 Aim of the study
This series of corroded glass objects is of interest from two points of view. On the one hand for material scientists and glass restoration specialists, these samples can be used to obtain a better understanding of the complex chemical and physical processes that give rise to the multilayered corrosion crusts, information which eventually can contribute to the refinement of currently employed restoration and conservation procedures for archaeological glass.
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On the other hand, the composition of the original glass itself, its variability within the extensive series of glass objects found at Qumran and its possible correlation with age, provenance and object type may reveal interesting information about two aspects : one is the history of the site itself and the other is the evolution of glass making techniques. The fact that the glass artefacts originated from Syria (in the broad sense of the word) and from the period just before and after the birth of Christ justified an in-depth study of its material nature. It is clear that at the borders of the Dead Sea, one is fairly close to the areas of glass fabrication of which the production found its way throughout the Roman empire, whereas the time-period they were attributed to is one which is characterised by the introduction of innovating techniques, which with high probability were developed in the north of Egypt or in the coastal region of Syria and Phoenicia [33]. This study may also be relevant in the framework of finding answers to important questions about the economical history of the region and more specifically about the manufacturing and export activities of the coastal workshops.
Whatever the typology and age of the glass objects may finally appear to be, it seems clear that the occurrence of this glass collection is hard to reconcile with hypothesis of the presence at the Qumran site of a community seeking poverty and detachment from worldly affairs. In this respect, obviously, the question about the cost and rarity of the decorated glass objects at that period is a fundamental one.
In the specialised literature, there are a number of reports on the chemical analysis of Glass from the Roman period, excavated at various locations in Europe [34-40]. In most cases, however, only a few objects were found or analysed per site. In the present case, where an extensive series of objects is available, a unique opportunity presented itself to evaluate to what extent detailed investigations on the composition of the various glass objects can reveal information on the provenance and history of the objects themselves and/or of the site they were found in. Therefore, next to the use of the conventional method of electron probe X-ray microanalysis (EPXMA), which is suitable for determining the major composition of the glass, microscopic synchrotron radiation induced X-ray fluorescence analysis (p-SRXRF) was employed to determine, in a completely non-destructive manner, the trace composition of a selected set of the Qumran glass samples.
3. EXPERIMENTAL PROCEDURE
3.1 Sample preparation
Samples were taken from each of the glass objects. Most glass samples were 1-10 mm2 in size and were embedded into resin blocks which were subsequently ground and polished down to 1 pm grain size using diamond paste to expose a cross- section perpendicular to the corroded glass surface. Most measurements were done using samples prepared in this way; for the EPXMA measurements, the resin
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blocks were carbon coated; no such additional preparation was necessary for the p- SRXRF analyses.
3.2 Major element composition determination
EPXMA measurements were performed with a Jeol JSM 6300 Scanning Electron Microscope, equipped with an energy-dispersive Si(Li) X-ray detector. For the measurements, an electron beam current of 1 nA and an acceleration voltage of 15 kV were employed; a low magnification setting of the instrument (300x) and a limited analysis time ( 2 0 0 s) were used to insure that no significant diffusion of sodium occurred during the irradiation. The obtained X-ray intensities were quantified by means of a standardless ZAF procedure (see Chapter 2). Various glass standards of known composition were used to evaluate the accuracy of the analysis (see Chapter 2).
3.3 Trace element composition determination
p-SRXRF measurements were executed at the NSLS (National Synchrotron Light Source, Brookhaven National Laboratories, Upton, NY, USA) X26A beamline using a 8 x 8 pm white synchrotron X-ray beam and at the DORIS III (Hasylab, Hamburg, Germany) beamline L-station. Per sample, a spectrum collection time of 200-1000 s was used. Quantification of the obtained SRXRF spectra was done using a Monte Carlo simulation model. Similar to the EPXMA measurements, the accuracy of the method was checked by means of a series of glass standards with certified trace element composition (see Chapter 2).
3.4 Multivariate data treatment
In order to elucidate the major variations in the set of compositional data obtained using both above-described methods from the series of glass samples, hierarchical clustering of the data matrix was performed by means of the IDAS software package developed in our laboratory [41].
Cluster analysis is a multivariate statistical data analysis technique which is used extensively for the interpretation of analytical data. The main purpose of cluster analysis is to find groups of similar objects in a data set containing the properties (variables) measured on a large number of samples. The groups are called clusters [42],
The results of a quantitative experiment are commonly represented as an M x N data matrix X where xik is the value of the k-th variable for the i-th object.
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"*1 1 x 12 ... * 1N "xr x 21 x 22 X2N x 2 x = (1 ) X i
i XM2 XMN_ X M_
The first step in carrying out a cluster analysis is to measure the similarity or dissimilarity of every pair of individuals. There are several ways of doing this but Euclidean distance, which is used here, is one of the most common measures of dissimilarity. The distance dy between two data points in an N-dimensional space is represented a s :
(2) k=1
Of the variety of clustering techniques, hierarchical cluster analysis is still the most widely used due to the relative computational simplicity and the ability to visually represent the results in the form of a dendrogram. The agglomerative hierarchical methods start with as many clusters as objects. At each step the two most similar objects (or clusters) are merged into a single cluster. After m-1 such steps all objects belong to one large cluster.
There are several clustering strategies differing in the criteria used to decide which individual elements or clusters should be merged together. The IDAS-program uses Ward’s method which is considered to be one of the best agglomerative hierarchical clustering methods [43]. At each step of the analysis the two groups which give the smallest possible increase in the total within-group sum of squares are combined. Lance and Williams [44] showed that at any step of the clustering the distances between a newly formed cluster r and any other cluster or data point k can be computed according to the formula :
d * = a idik + a jdjk +pdij + y|dik -dj, (3)
The coefficients are different for the different clustering algorithms. In Ward’s method they are calculated as follows :
n; +n„ nr + n k _ n, + n k nr + n k (4) c I II P nr + n k y = 0
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4. OVERALL MORPHOLOGY OF THE SAMPLE CROSS-SECTIONS
Figure 4. Secondary electron image of cross section of Object Qumran #22.
Figure 5. X-ray Intensity line profiles corresponding to Figure 4.
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Figure 4 shows a secondary electron image of a cross section of a fragment from one of the Qumran Objects. As in all glass samples, inspection of this polished cross-section using the secondary and back-scattered electron images formed in the scanning electron microscope reveal three distinct regions : a central layer of original, unaffected glass, a leached layer (or gel layer) approximately 1 0 0 to 150 pm thick which has formed on the inside of the original glass panes and a precipitation crust about 10 pm in thickness on top of the original glass surface. In some cases (Figure 4), the leached layer has evolved quasi-parallel to the original surface, whereas in others, much more complex morphologies are encountered (Figure 10b). X-ray intensity line profiles taken across the cross-section (Figure 5) show marked differences between the composition of crust, corrosion layer and bulk glass. This is further discussed in sections 7.1 and 8 .
5. THE BULK GLASS
5.1 Major, minor and trace composition of the bulk glass
In Table 2a-h, the major element composition (as obtained by EPXMA from 75 samples) and the minor/trace composition (as obtained by p-SRXRF from 64 samples) are listed. The data pertaining to each sample are organised according to the different typological categories (See section 2.2) the corresponding glass objects fall in : (a) ointment vessels (18 objects), (b) different types of goblets (23 objects), (c) different types of cups (14 objects), (d) flasks (4 objects), (e) biconical recipients (3 objects), (f) bottles (7 objects), (g) a chalice (1 object) and (h) indeterminable shapes (5 objects). The average composition per category is also indicated, although some outliers (numbers 31, 32 and 33) are left out from the calculation of the average concentrations for the trace composition. Taking the accuracy of the EPXMA method (see Chapter 2, Section 2.5) and p-SRXRF method (see Chapter 2, Section 3.4) into account, on first sight, these averages all appear to be (nearly) identical and only convey the information that all glass is of the typical soda-lime- silica type, as is the case for most glass from ancient times. During this period a stable glass was made from calcareous sand and natron.
Table 2. (See next 6 pages) Composition of different types of objects in the Qumran series (in w% in top part, in ppm in bottom part of each table).
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a. Ointment vessels (18 objects). 0 0 9 ZYZ
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r> n n in o ■g- to 71 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel b. Goblets (23 objects). o O CD o 3 CO f«. 0) CO co § 95 632 309 0.94 0.76 7.34 0.68 235 0.16 2.51 0.05 0.14 o 16.70 o Mean 70.32 I CD CM 0> O co o> 73 46 o cm 20 193 0.17 0.08 0.18 0.82 j 7.79 0.03 0.42 j 226 726 0.84 2.60 CD O 1860 16.34 70.25 ! | 1050 | 406 o CM f~. ID CO ro CO CO CO R 2 26 76 496 0.77 0.03 291 0.23 2.54 o 0.13 0.92 CD 0.43 O - = 15.76 70.12 I ■ ■ 183 | 105 CM 00 9 90 0 /0 6 CM CM o LT 68 0.02 O 2.46 o 7.91 0.49 d 16.45 70.28 90*0 CO 69 CO (O co CO *tr 43 24 33 26 CM - in 52 125 1.07 557 0.62 6.18 0.02 0.67 0.03 2.18 0.14 o* 17.21 71.52 10 9 S90 CO o> s o CM o co CO CO CO CM 85 co CO cn 22 s CD - 1.17 747 179 1.99 0.00 6.41 d 16.97 d o d 71.90 180 co m rr e«. CD m - CO r*. CO CM 29 fs- 143 548 257 0.07 0.43 0.12 2.57 0.05 0.19 0.85 7.77 0.51 16.50 70.07 I I I 120 901 ■O’ r - ® CD in © 53 R 23 CD 33 197 493 0.84 0.09 0.91 0.02 3.10 0.05 0.12 4.93 0.33 - 16.48 72.13 0 0 0 9S0 0 6 6 100 S o> © C0 ID - CD R eo 68 70 8 in 166 1.24 562 2.52 0.12 4.99 0.03 - 17.07 d 72.11 o o 000 090 S00 o CD 66 1.03 0.05 5.03 0.67 d CM 0.37 16.91 72.35 o o z 65 1.24 0.53 0.03 2.46 0.02 0.14 4.97 0.88 0.27 17.18 72.23 0 0 £ 09 CD O o CD O’ o CD 40 - 8 - 133 321 421 0.03 2.48 0.04 0.26 0.63 7.47 0.01 0.32 17.07 70.57 00 0 E00 CM ** © o CO CM CO CO CO CD 39 42 CM 122 503 1.23 0.04 2.50 0.22 0.56 4.94 0.87 0.29 17.33 71.84 = CM vft CM CO © CD o o CM in o CM 3 22 8 £2 159 662 673 351 0.03 CO 0.26 2.43 0.10 o 0.81 8.41 0.39 0.52 16.30 o 69.63 60*0 00 6 CO CM CD 3 r^ ID CO © «D o CD 44 26 3 CM 146 8 479 0.82 0.85 0.15 2.53 7.79 o 0.41 0.49 16.50 70.24 ID S0’0 S O ID r— g o CO CO 69 s 152 121 118 281 499 0.10 0.86 0.84 0.37 CM 0.22 2.41 d 0.52 - 16.23 CD 69.83 oo e o o z 60 0 o O o o - m o r*. 23 40 CO a> 8 21b 345 754 208 1.19 2.64 0.92 8.03 3.89 0.22 d 17.58 66.54 ee*o 10 0 © o CD CD ««• 26 33 83 8 120 170 138 = 599 233 2.54 0.16 0.71 0.93 8.87 0.04 0.48 0.57 16.52 68.59 E90 CD 66 ov M- o 26 25 127 178 108 149 - 414 0.13 2.52 0.02 0.07 0.83 8.50 0.05 0.42 - - § 16.41 O 69.73 0 9 6 000 co 0) CO g cn 0> m 95 R 137 190 131 364 = 564 0.64 2.53 0.14 0.68 0.07 0.48 9.45 0.63 16.47 67.86 80 9 o CM o £S0 o CM O’ o o CO CM o 36 - 8 139 132 619 376 243 566 2.56 0.10 0.85 7.99 0.07 16.52 o o o 69.80 CM o r - CO in 29 o © 85 35 28 *- 896 228 647 0.27 0.12 437 2.41 0.22 0.83 0.85 7.88 0.05 0.44 0.54 17.03 69.34 00 9 00 9 o> CO CD CO CM c*. 20 r** r—• CO 28 |M 152 418 226 896 0.16 2.45 0.85 0.79 0.20 7.94 0.37 0.45 - 16.64 69.92 CD CO 00 s CD - ot © CM 26 89 CO 164 294 0.34 2.55 0.75 0.86 0.15 8.75 0.04 0.57 16.67 o o 68.53 I I 553 I I 156 | 9 , O o 9 o o (0 <5 o O 3 $ m £ e 9 NiO © a> SfO BaO PbO MgO £ ZnO C/3 CaO MnO CuO i Z 1 co F u. 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel c. Cups (14 objects). o> CO LO N* CM 0 5 CO O CO O pf\ fn T f /v-i CM r - Is- 'R CM m- to Or-COCOOO^lO CO o > CM ^ CO ^ 03 CO (NJ r - o cvi c o O O O O Is-* O O O CO 0 5 i n CO CO 0 5 O o CO 0 c o T— CD Tf 05 n N N V— M- O CM 0 0 r - o M- CO ID 0 5 0 0 CO g L O O C 5 co’ 0 5 CM CM CO O o o d LO ^ CO CM O CM CO O o ’ O o o CO CO CM CO o 05 CO CM CO h - t o M 1 CO t o ' . Is- CO LO o t o c d 0 5 d CM CO d c > o d 0 0 d d d 0 5 0 5 CO Is- . i . LO 0 5 CO CO CO 0 5 LO 00 CO CO 00 CM CO . OO CO CO CO o CO lO £ LO CO CM CO* 0 5 = ? ^ - ; § 2 s N i n CM CM d CM CD d d o d c o d d d CO r - . CO CO CM ^CDOCMNM-CDO) M* CM t- CO 05 05 CO O LO CO Is-* 0 5 O CM CO OOOOCOOOO 00 r^. T“ T_ Is- t o LO CM M* CO CO 0 5 Is- o CO n 0 5 ‘ T— LO o 00 r * - T— M- r - c o c o - 2 co m co' 0 5 d CM CO d d o d c d d d d CO Is- 0 5 CO CO 0 5 CO LO o CO N CO CO o T— CO h - CO o M* c d d d CM Is - d d o d d d d co CO CO CO T— LO o T— N- CO o c o c o c o CD Is- t o § CO § v - o V CO o o o CO LO 00 CO CO 0 5 CM LO CO co 0 5 CM CO LO o o ° i n ° d cm’ CO d d d d c d d d d CM CO CO CM CO CO o 0 0 0 0 CM CO r— oo 0 5 Is- O 05 CO CM r - t o t — r - 0 5 CD o M" M- CO CO M* CM T f CO CM 0 5 CO M" 05 O O O CM c d CM c d CO d CO d o d d o d d d d CO O M- N N n CM CM 3 CO © CM ^ o cm d) co cn o t- t - CO Jg CO 0 CM N - CO OJ CO O C \i g O O O CD h~' O O CM ^ CO O LO 0 5 M* CO o 0 5 CO o _ _ 0 0 q >J CM CM CD CM CM x— CO GO 0 5 o r r LO c o O 05 O T - c d 0 5 O ° g « » 8 g c o r N ^ CM d CM CO d o d d 1 ^ d d d LO LO CO CO c o CO Is - CO 05 ' LO OO 00 (O CO M* LO . o o o o c o Is- o M; LO r * r- 0 5 0 5 O O ( O r N N N CO o CO CO t - CM d c m 1 ^ d o d d d d O OO CD CM CO 0 0 CD CO LT5 Is- 0 5 Is- 0 5 _ Is - c q o CO r - O O CO r- M* CO 0 5 CO co co o CM 0 5 CO CM T- d CO d o d d 0 0 d d d r- CO CO CO M* 0 5 CO oo CO iq CO CO o cm o in CM LO o I s- o o O LO OO Is - CO CO o 0 5 co O O) co 0 5 CM co 0 5 O) ■ CO CM 0 5 CD CM CD CO M* CO CM CM in o 0 5 OO O o M; LO o d c s i Is- d d d d Is- ’ d d d CO <*5 Tj- in co CM o o O o O O O O - 2 0 0- 9,0 0 0 0 CM o CM o o o eg CO a> - EQ -Q CaO MgO CO MnO < 0 . i - LL. (j ^ O N ££ 7 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel d. Biconical recipients (3 objects). 5 23 34 Mean NazO 16.94 16.06 16.31 16.44 MgO 0.19 0.26 0.12 0.19 a i2o 3 2.45 2.61 2.52 2.53 S i0 4 69.38 69.78 69.80 69.65 P2Os 0.08 0.11 0.10 0.10 so3 0.17 0.16 0.12 0.15 Cl 0.83 0.80 0.84 0.82 k2o 0.82 0.86 0.87 0.85 CaO 8.02 8.49 8.36 8.29 TiOz 0.06 0.07 0.05 0.06 MnO 0.37 0.36 0.47 0.40 F e ,0 , 0.45 0.53 0.48 0.49 Cr20 3 20 0 0 7 NiO 7 11 0 6 CuO 160 270 768 399 ZnO 24 41 69 45 Br 6 7 30 14 Rb20 10 15 71 32 SrO 493 611 3090 1400 v 2o 3 8 9 44 20 Z r0 2 73 65 420 186 Mo20 3 4 2 14 7 S n 02 56 77 1380 504 Sb20 5 241 318 905 488 BaO 132 181 946 420 PbO 91 230 607 309 e. Bottles (7 objects). 19 25 27b 33 61 22 59 Mean Na20 16.40 16.16 16.54 16.42 16.22 16.24 16.33 16.32 MgO 0.14 0.44 0.12 0.27 0.15 0.07 0.11 0.19 Al20 3 2.50 2.49 2.52 2.48 2.41 2.46 2.50 2.49 SiO* 69.15 69.21 70.36 70.08 69.88 70.02 69.93 69.80 P2Os 0.14 0.06 0.02 0.11 0.05 0.04 0.11 0.07 so3 0.20 0.16 0.07 0.15 0.11 0.19 0.14 0.15 Cl 0.80 0.78 0.83 0.90 0.83 0.85 0.87 0.85 k 2o 0.96 0.81 0.80 0.80 0.84 0.81 0.88 0.84 CaO 8.70 8.86 7.69 7.62 8.34 8.25 8.29 8.25 T i0 2 0.03 0.03 0.04 0.05 0.08 0.06 0.06 0.05 MnO 0.46 0.47 0.47 0.56 0.38 0.43 0.39 0.46 F e ,0 , 0.45 0.50 0.51 0.54 0.58 0.49 0.46 0.51 Cr20 3 26 0 0 0 0 6 NiO 10 0 0 5 18 8 CuO 83 302 879 244 237 217 ZnO 29 41 98 33 46 37 Br 3 1 1 2 3 2 RbzO 16 16 52 20 15 17 SrO 658 571 2381 996 632 714 y 2o 3 10 8 13 16 8 11 Z r0 2 76 57 326 94 50 69 Mo20 3 1 2 10 2 2 2 S n 02 95 221 835 220 106 161 Sb20 5 97 441 1481 359 136 258 BaO 97 250 1797 451 155 238 PbO 106 186 371 341 235 217 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel f. Flasks (4 objects). 27 32 so 62 Mean NazO 16.64 16.35 16.30 15.91 16.30 MgO 0.28 0.28 0.06 0.16 0.20 a i2o 3 2.61 2.54 2.48 2.47 2.53 S i0 4 69.21 68.61 69.37 70.12 69.33 P2° s 0.12 0.15 0.06 0.02 0.09 so3 0.17 0.04 0.07 0.05 0.08 Cl 0.79 0.61 0.79 0.76 0.74 k2o 0.88 1.13 0.84 0.84 0.92 CaO 8.25 9.25 8.97 8.50 8.74 T i0 2 0.06 0.06 0.05 0.04 0.05 MnO 0.42 0.44 0.47 0.46 0.45 F e ,0 , 0.54 0.55 0.54 0.58 0.55 0r2O3 0 0 29 15 NiO 0 0 10 5 CuO 291 2380 94 191 ZnO 39 10800 31 35 Br 2 0 3 3 RbzO 25 0 13 19 SrO 948 2474 700 824 v 2o 3 15 156 9 12 Z r0 2 102 19600 88 95 Mo20 3 1 105 3 3 S n02 129 739 189 159 Sb2Os 518 49 161 339 BaO 279 377 192 235 PbO 328 267 136 232 g. Chalice (1 object). 24 NazO 16.24 MgO 0.26 a i2o 3 2.58 S i0 4 70.01 P2Os 0.09 so3 0.13 Cl 0.76 k2o 0.82 CaO 8.05 TiOz 0.06 MnO 0.49 Fs?04 0.54 Cr20 3 0 NiO 5 CuO 222 ZnO 42 Br 0 RbzO 16 SrO 626 y2o 3 9 Z r0 2 69 Mo20 3 2 S n02 169 Sb20 5 447 BaO 333 PbO 136 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel h. Indeterminable shapes (5 objects). 52 70 71 72 77 Mean NazO 17.09 16.54 15.64 18.20 15.89 16.67 MgO 0 . 1 2 0 . 1 0 0.43 0.08 0.04 0.15 AI2 O3 2.57 2.55 2.52 2.54 2.55 2.55 S i0 4 69.38 69.80 6 8 . 1 0 68.38 69.35 69.01 P2 O5 0 . 0 0 0 .0 1 0 . 1 2 0 . 0 0 0 .1 1 0.05 S 0 3 0.14 0 .0 1 0.08 0.30 0 .1 1 0.13 Cl 0.90 0.78 0.65 0 . 8 6 0.77 0.79 K20 0.72 0 . 8 8 1 .0 2 0.82 0 . 8 8 0 . 8 6 CaO 8 . 2 0 8.24 10.27 7.84 9.41 8.79 T i0 2 0.04 0.04 0.05 0.03 0.07 0.05 MnO 0.31 0.46 0.50 0 . 6 6 0.32 0.45 Fe 2 0 3 0.48 0.54 0.67 0.33 0.51 0.51 Cr2 0 3 0 16 0 13 7 NiO 1 0 1 0 2 2 8 1 2 CuO 410 144 139 1 0 1 198 ZnO 42 30 25 24 30 Br 5 3 4 18 7 Rb20 14 1 2 17 1 2 14 SrO 676 582 735 548 635 y 2 o 3 1 1 9 9 8 9 Z r0 2 65 89 55 76 71 Mo2 0 3 2 1 3 4 3 S n 0 2 79 1 0 0 48 91 80 Sb2 0 5 334 265 203 2 0 206 BaO 304 159 313 6 6 2 1 1 PbO 282 163 103 71 155 The two types of sand suitable for glass-making which are specifically mentioned by location in classical writings are those at the mouth of the river Belus on the Syrian coast and the seashore deposit [Pliny (AD 23-79)] near the mouth of the river Volturnus, north-west of Naples and the ancient harbour of Pozzuoli. Turner found that it contained about 9 % of lime [45] and other analyses have given similar lime contents [46, 47]. Next to this also 3.6-5.3 % of alumina and about 1.5 % of magnesium carbonate is present so, when mixed with alkali, it will have been possible to make a quite durable glass. Undoubtedly, as alkali, natron, a natural sodium sesquicarbonate (Na 2CO 3.NaHCO 3.2 H2O) from the Wadi Natrun, north-west from Cairo, would have been used in the glass-making activities in those surroundings. The composition of natron is complex and variable : the sodium carbonate content varies from 22.4 to 75.0 per cent, sodium bicarbonate from 5.0 to 32.4 per cent, sodium chloride (which explains the presence of Cl in the glass) from 2.2 to 26.8 per cent, sodium sulphate 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation o f a Roman Glass Collection from Qumran, Israel from 2.3 to 29.9 per cent plus water and insoluble material. It had already been employed from very early times as detergent, in medicine and for embalming [48, 49]. The colour of the glass produced would not have been up to the standards of modern colourless glass because, although the iron oxide content was low in comparison with other sands that Turner examined, it was high in comparison with that of present day glass [45]. The concentration of iron and manganese in the glass is approximately the same. Iron in the Fe(ll) state results in a blue-green colour in glass which can be partially decolourised by adding manganese due to the following equilibrium reaction. Fe2+ + Mn3+ ^ Fe3+ + Mn2" When conditions during melting are such that the purple from the manganese just compensates for the colour of the iron, a colourless glass will result [49]. Object # 2 1 b on the other hand shows the typical purple colour due to a high manganese concentration (3.9 % MnO). Table 3. Composition of Roman glass found at different locations throughout the Roman Empire (w%). Location Wroxeter Mancetter Jalame Kdln C. West. Aosta Aquileia Country Britain Britain Palestine Germany France Italy Italy Reference [34] [35] [36] [37] [38] [39] [40] NazO 17.4 17.5± 1.1 15.8 16.55 16.59 16.72 17.56 MgO 0.6 0.54 ± 0.05 0.7 0.59 0.09 0.385 0.69 AI2O3 1.9 2.44 ±0.13 2.7 2.22 2.29 2.33 2.72 Si02 70.8 70.4 ± 0.7 - 71.17 70.02 70.09 67.74 p 2o 5 0.1 0.14 ±0.02 0.12 - --- S03 0.3 - 0.1 - -- 0.19 Cl 0.8 - 0.6 - 1.15 -- k 2o 0.9 0.70 ±0.14 0.7 0.54 0.55 0.86 0.83 CaO 6.3 7.09 ± 0.79 9.0 7.76 8.86 7.79 8.11 Ti02 - 0.08 ± 0.02 0.1 0.09 - 0.056 - MnO 0.5 0.41 ± 0.21 0.12 0.26 1.01 0.015 0.54 Fe 20 3 0.5 0.48 ± 0.09 0.39 0.91 0.07 0.219 0.783 CuO - - - -- < 0.005 0.079 BaO - - 0.1 -- 0.043 - Sb20 5 - 0.19 ±0.20 ---- 0.081 SrO - - - -- 0.056 - PbO - 0.03 ± 0.01 0.004 ---- 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel For comparison, in Table 3, the major element composition of Roman glass excavated in Britain [34, 35], Israel/Palestine [36], Germany [37], France [38] and Italy [39, 40] is listed. In the Qumran samples, very similar compositions are found. It is a pity that for most of the glass materials listed in Table 3, only the major element composition is known, since, as explained below, knowledge of the trace element fingerprint of the materials might have permitted to see more subtle differences and/or similarities among them and the Qumran glass. 5.2 Classification of data Instead of distributing the samples a priori into typological categories, we have found it more useful to keep all data together and to employ the multivariate statistical technique of Hierarchical Cluster Analysis (HCA) to emphasize the structure in the data. As mentioned before (Section 3.4) the result of HCA is a dendrogram, showing in a graphic form the similarity of the composition of the various Qumran samples. In Figure 6 , the dendrogram obtained on the basis of the major element composition alone (75 samples) is shown. A clear distinction between a large group comprised of 64 objects and a small group of 11 samples can be observed. The difference between the two groups becomes more clear when within each group, the average composition is calculated. As already mentioned earlier, the distinction in major element composition is almost not significant: only the CaO abundance features a significant difference between the large group (8.4 ± 0.5%) and the small group (5.9 ± 0.8%) at the 1s (standard deviation) level. This could indicate that within each of the two groups of objects which were identified, all objects of that group originate from the same batch of bulk glass using slightly different raw materials. In the histogram of Figure 6 , also the typological category corresponding to each of the samples is indicated. It can be observed that the small group of objects is exclusively composed of the types “O” and “G”, respectively ointment vessels and goblets. In the last category, especially the 'moulded goblet' type is well represented. Although in the dendrogram, the large group displays a definite structure and is further divided into sub-groups, no straightforward correlation between this substructure and the object typology can be made. 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel H oo Q 0> £ j O) O q ^ 2 O bO g O qzJJD -O O • • • ♦ •• •* ♦ * O O ^ cj fepq pi ffi Q\ < S I CSJ c v l oo 'Ooo 'OM OO O s «o VO 0 G GOOOGGGGGI 0 GGIFOO G C OFGBO CB OC C GGFR0GB G BH0 CG GGCBR BCGB 0FC I GO GR GOOOOCGGC CC I IC CO Figure 6 . HCA dendrogram obtained using major elements composition of the samples. 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation o f a Roman Glass Collection from Qumran, Israel Table 4. Average concentration and standard deviation for oxide concentration of major, minor and trace elements in the glass fragments belonging to the three groups of Qumran objects. Concentrations above the horizontal line are given in w %, below it in ppm. Oxides Group 1 (n=45) Group II (n=9) Group III (n=5) Na20 16.48 ± 0.40 17.20 ±0.35 16.28 ± 0.60 MgO 0.23+0.13 0 .0 1 ± 0 .0 1 0.07 ± 0.13 AI2O3 2.51 ±0.07 2.35 ± 0.34 2.42 ± 0.10 S i0 2 69.46 ± 0.62 71.69 ±0.42 70.92 ± 1.62 P2O5 0.08 ± 0.04 0.02 ± 0.04 0 . 0 0 ± 0 . 0 0 S 0 3 0.16 ± 0 .1 1 0.17 ±0.07 0.20 ± 0.07 Cl 0.82 ± 0.07 1.16 ± 0 .0 5 1.06 ± 0 .0 5 k 2o 0.84 ± 0.06 0.58 ± 0 .1 2 0.61 ± 0 .1 1 CaO 8.41 ±0.55 5.52 ± 0.61 7.54 ± 0.42 T i0 2 0.05 ± 0.04 0.04 ± 0.04 0 . 0 2 ± 0 . 0 2 MnO 0.43 ± 0.06 0.84 ± 0 .1 3 0.09 ± 0.09 Fe 20 3 0.52 ± 0.06 0.33 ± 0.04 0.39 ±0 .13 Cr20 3 1 2 ± 1 2 23 ± 2 7 9 ± 8 NiO 7 ± 5 15 ± 7 8 ± 8 CuO 209 ± 95 50 ± 4 3 83 ± 137 ZnO 35 ± 19 26 ±7 18 ± 13 Br 7 ± 11 8 ± 7 7 ± 4 Rb20 14 ± 4 12 + 3 13 ± 3 SrO 637 ±145 5 3 4 ± 130 5 7 0 ± 137 y 2o 3 9 ± 2 6 ± 1 8 ± 3 Z r0 2 79 ± 17 63 ± 16 59 ± 15 Mo20 3 3 ± 2 3 ± 1 0 ± 0 SnOz 113 ± 4 9 56 ± 34 42 ±21 Sb20 5 354 ± 190 29 ± 31 1 ± 1 BaO 234 ± 127 151 ± 5 9 151 ± 92 PbO 156 ± 6 4 17 ± 14 13 ± 9 From EPXMA-spectra, only information on the concentration of major elements could be obtained. In order to gain a better insight into the origin of the various glass samples, p-SRXRF was used to obtain a trace-element 'fingerprint' of each glass sample. A close inspection of the major together with the trace element composition reveals that the situation is in fact more complicated than just the two groups mentioned before. Within the large group of objects (group I), another small subset (group III) can be distinguished, containing 5 objects. As can be seen from Table 4, where the average major, minor and trace composition within these three 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel groups is listed, the difference between the latter 5 objects and group I is due to the copper, tin, antimony and lead concentrations which are all lower in group III. Within the series of 45 objects that constitute the large group, in all cases some antimony is found to be present while in the objects of group III no antimony is detected. Next to the previously mentioned elements, also the manganese concentration in the objects of group III is significantly lower than in group I and II. In group II the lower CaO concentration is also reflected in the lower copper, tin, antimony, barium and lead concentration than in group I. These values, however, are similar for groups II and III. Because the trace fingerprints of the three groups differ only significantly in a relatively limited number of elements, one can still reasonably assume that all the objects were made from batches of bulk glass which were closely related or they may have been manufactured using two different procedures, each introducing slightly different concentrations of some trace elements into the glass objects being made. When correlations between elements were considered, copper was found to be positively correlated with both zinc (see Figure 7) and lead, meaning that copper was probably added to the glass melt as a Cu-based alloy which was softened by lead. Antimony also showed a positive correlation with lead and manganese and iron were positively correlated up to a manganese concentration of 0 . 6 %, suggesting that above this concentration manganese was added intentionally as a decolouriser (or colouring agent in very high concentration). 140 120 100 £ Q. n. 80 O c M 60 ♦ 40 . SJS* 20 | ♦ ♦ 0 100 200 300 0 500 600 700 80040 900 1000 CuO (ppm) Figure 7 Positive correlation between the concentration of CuO and ZnO in the glass objects from Qumran. Regarding the question of the provenance of the objects, the above-discussed composition information suggests that almost all objects found at Qumran were 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel made from the same batch of glass. Either they were made locally in a glass workshop (e.g., by remelting and working of a large stock of bulk glass) or they were imported ready-made in large quantity from elsewhere. In either case, at Qumran, there seems to have been a relatively large demand for glass vessels of various types such as goblets and ointment pots. This is unusual since glass objects in ancient times were hard to make and thus fairly rare. This tentative conclusion appears to be consistent with the hypothesis that at the Qumran site some industrial activity in the field of ointment/perfume manufacture was located during ancient times, the glass vials and bottles being used as receptacles for perfume, ointment, etc., as proposed in [29]. 6. PREDICTION OF DURABILITY 6.1 Triangular diagram Much insight can be gained into the relative durability and weathering (or corrosion) behaviour of different glass types through the use of a triangular diagram representation [50]. The compositions of the glasses (usually expressed in weight percent) are first converted to molecular percentages of the constituent oxides, these are then combined into three categories, the network formers, the alkaline earth network modifiers and the “effective” alkali content (see Chapter 1) and the results are represented in a triangular diagram, the three groups of constituents being the sides of the triangle. Table 5. Results of the calculations to convert weight percentages to molar percentages for the oxides building up the glass in the three groups of Qumran. Oxide Molec. W % Molar Mol % W% Molar Mol % W% Molar Mol % weight equiv. equiv. equiv. Group I Group II Group III Na20 62.0 16.48 0.266 16.39 17.20 0.277 17.0 16.28 0.263 16.31 M g O 40.3 0.23 0.006 0.40 0 .0 1 0 .0 0 0 0 .1 0.07 0 . 0 0 2 0 .1 2 AI2 O 3 1 0 2 .0 2.51 0.025 1.54 2.35 0.023 1.5 2.42 0.024 1.49 Si02 60.1 69.46 1.156 71.23 71.69 1.193 73.7 70.92 0.180 73.16 P 2 O 5 141.9 0.08 0 .0 0 1 0.06 0 .0 2 0 .0 0 0 0.00 0.000 0.00 k2o 94.2 0.84 0.009 0.55 0.58 0.006 0.4 0.61 0.007 0.43 CaO 56.1 8.41 0.150 9.24 5.52 0.098 6.5 7.54 0.134 8.31 Ti0 2 79.9 0.05 0 .0 0 1 0.06 0.04 0 .0 0 1 0 .0 2 0 . 0 0 0 0 .0 0 MnO 70.9 0.43 0.006 0.40 0.84 0 .0 1 2 0.7 0.09 0 .0 0 1 0.06 Fe 20 3 159.7 0.52 0.003 0.19 0.33 0 .0 0 2 0 .2 0.39 0 . 0 0 2 0 .1 2 Total 1.623 1.612 1.613 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel The constituents of the glass exercise their effect on the weathering of that glass by virtue of the numbers of their molecules in association with each other and not by the weight of those molecules. The results of the calculations are given in Table 5. Columns 3, 6 and 9 represent the weight percentages of the oxides which make up the glass for group I, II and III respectively. Molar percentages are calculated by first dividing the weight percentage of the oxide by its molecular weight (column 2 ) and the results obtained (molar equivalent) are given in column 4, 7 and 10. Finally the molar percentages are obtained by dividing each molar equivalent by the total (1.623, 1.612 and 1.613 for group I, II and III respectively) and multiplying by 100. These molar proportions of oxides are then combined according to the following rules : The alkaline earth component, RO, is obtained by adding together all the oxides with this formula, For the Qumran glasses this is : Group I : RO = MgO + CaO + MnO = 0.40 + 9.24 + 0.40 = 10.04 Group II : RO = MgO + CaO + MnO = 0.00 + 6.08 + 0.74 = 6.82 Group III : RO = MgO + CaO + MnO = 0.12 + 8.31 + 0.06 = 8.49 The network former is almost entirely silica (Si02) but some glasses contain P 20 5 as well as T i0 2, Z r0 2, S n 0 2> etc. and this must be added to the S i0 2. In addition, it was shown in Chapter 1, section 3.3, that Al 20 3 occupies a special place, each molecule being able to immobilise an alkali ion and it can also be incorporated into the network. Thus the total of the network oxides has to be increased by twice the Al 20 3 (and similar oxides such as Fe 20 3, Cr20 3, etc.). For the Qumran glass the total network forming oxides (called S i0 2) is given by : Group I : Si0 2 = S i0 2 + P 2Os + T i0 2 + 2(AI2 0 3) + 2(Fe 20 3) = 71.23 + 0.06 + 0.06 + 2(1.54) + 2(0.19) = 74.81 Group II : Si0 2 = S i0 2 + P2O5 + T i0 2 + 2(AI20 3) + 2(Fe 20 3) = 74.00 + 0.00 + 0.06 + 2(1.43)+ 2(0.12) = 77.16 Group III : S i0 2 = S i0 2 + P2O5 + T i0 2 + 2(AI20 3) + 2(Fe 20 3) = 73.16 + 0.00 + 0.00 + 2(1.49) + 2(0.12)= 76.38 The remaining component in the triangular diagram is the alkali oxide, called R 20 , which represents only the available alkali and not the total alkali, some of the alkali being immobilised by the alumina. The result for the Qumran glasses is : Group I : R20 = Na20 + K20 - Al 20 3 - Fe 20 3 = 16.39 + 0.55 - 1.54 - 0.19 = 15.21 Group II : RzO = NazO + K20 - Al 20 3 - Fe 20 3 = 17.18 + 0.37 - 1.43 -0.12 = 16.00 Group III : RzO = NazO + KzO - Al 20 3 - Fe 20 3 = 16.31 + 0.43 - 1.49 - 0.12 = 15.13 These values are plotted in a triangle of which the sides are co-ordinates for the three different categories of components. In the triplot in Figure 8 the highly durable glasses are placed near the centre. Modern float glass is represented by A, M represents the remarkably durable Saxon window glass and R some Roman window glass. Some examples of less durable 83 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C hapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel window glass are situated more to the right bottom comer of the triangle (B, C, D, E, P, Q, H and J). The composition of the “weeping” glasses falls at point W, whereas those which “crizzle” are at Z. It can be seen from this representation that both of them contain too little lime for good stability. Figure 8 shows that the average durability of the original Qumran glass is comparable to that of durable Saxon, float and Roman window glass as was expected [50]. A : modem float glass M : Saxon window glass R : Roman window glass W : weeping glass Z : crizzled glass B, C, D, E and P : less durable window glass H, J and Q : least durable window glass G I, G II and G III: Qumran glass Figure 8. Comparison of average durability of Qumran glass with other glass types. The triangular diagram is of great advantage in helping to display the effects of differences in composition but it has its limitations. It does not discriminate, for example, between soda and potash, yet the soda glasses have about twice the durability of the potash glasses neither does it discriminate between the effects of lime and magnesia, nor does it incalculates mixed alkali effect. 6.2 Thermodynamic approach Another approach of predicting durability was that of Newton and Paul [51] which is based on thermodynamic grounds. They related the loss in thickness of the glass in water (in millimetres per century) to the free energy of hydration as calculated from the composition. The principle has been developed further by Plodinec and Jantzen [52, 53], 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel In 1977 Paul [54] predicted the durability of glass on the basis of its thermodynamic stability assuming that glass-water reactions may be described in terms of the summation of the free energies of hydration (AG° values) of individual oxide or silicate groups, weighted in proportion to the mole fraction present. The most positive free energies represent the more durable species. It can be seen from tables with free energies of hydration and/or ionisation [51, 53, 54] that the value for K2S i0 3 and Na 2S i0 3 are, respectively, -173.9 and -120.0 kJ/mole, so that potash glasses will be represented as being appreciably less durable than the equivalent soda glasses. The calculation for the three groups in the Qumran glasses is given in Table 6 . The glass is considered, for the purpose of the calculation, as being a physical mixture of the orthosilicates and the uncombined S i0 2. The free energies of hydration, AG°, in kJ/mole are given in the second column and AG° per mole of glass is the product of the quantities given in the third, fourth and fifth column, for group I, II and III respectively. Table 6 . Calculation of average free energy of hydration AG° of the three groups in the Qumran glass. Comp. AG* AG* per mole glass AG* per mole glass AG* per mole glass kJ/mole Group 1 Group II Group III Na2O.Si02 -120.0 0.164 x (-120.0) =-19.67 0.172 x (-120.0) = -20.62 0.163 x (120.0) = -19.57 MgO.Si02 -57.9 0.004 x (-57.9) = -0.23 0.00 x (-57.9) = -0.00 0.001 x (-57.9) = -0.07 K20.S i0 2 -173.9 0.006 x (-173.9) =-0.96 0.004 x (-173.9) = -0.64 0.004 x (-173.9) = -0.75 CaO.Si02 -67.1 0.092 x (-67.1) =-6.20 0.061 x (-67.1) = -4.08 0.083 x (-67.1) = -5.58 unc. SiOz 23.3 0.446 x (23.3) = 10.40 0.504 x (23.3) = 11.74 0.480 x (23.3) = 11.18 AG tot -16.66 kJ/mole -13.60 kJ/mole -14.79 kJ/mole Newton and Paul [51] subsequently showed that AG° calculated in this manner correlated well with the amount of alkali leached from various glasses, the latter quantity being used as a measure of durability. Short term laboratory tests on a variety of glasses, including ancient man-made and natural specimens, have demonstrated a linear relation between AG* and the release of structural silicon into the leaching solution [54]. If archaeological glasses are investigated, aqueous attack of the glasses has already occurred so a measure of their relative durabilities can only be determined in terms of the mean rate of corrosion of the glasses, as revealed by the thickness of the corrosion crusts which have formed during a known period of exposure to groundwater. The average corrosion layer thickness of 50-150 pm corresponding to a contact time of approx. 1900 year yields a linear rate of corrosion in the range 25- 80 nm/year. In Figure 9, the relation between free energy of hydration AG* and the 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel corrosion rate for the two groups in the Qumran series is compared to other Na- based Roman glasses (WRX) and to K-based medieval window glasses (EP, RA, SC, SAA) found in Britain. The average AG° and corrosion rate found for the Qumran glass agree with the approximately linear relation found by Cox & Ford [34], 10 R A 6 S C 6 + E P 4 EP1 k. 4>ra -S C 1 1 RA4 >» SAA1 ZLE c o ’55 o o o Increasing durability Group LGro“P j' *5 \ Group II & \W R X 1 (Qc V^LX6 K 0.01 WRX9~*VWRX2 W R X 1 0 0.001 -45 -40 -35 -30 -25 -20 -15 -10 ■5 0 Free energy of hydration (kj/mol) Figure. 9. Approximately linear relation between free energy of hydration and corrosion rate of Roman glass from Qumran and glass from several sites in Britain. 7. THE LEACHED LAYER 7.1 The com position In Table 7, the composition of the bulk glass of Qumran Object #22 is compared to that of the corrosion layer and that of the crust. These results are typical for most corroded fragments which were investigated. As already shown in Figure 5, the Si concentration in bulk glass and leached layer is almost the same; Na and Ca are severely depleted in the corrosion layer while Fe is more concentrated in this part and Mg gradually rises towards the surface. Whereas usually a depletion of K is observed both in ancient and medieval glasses, in the Qumran samples K is enriched in the corroded layers; as observed in all samples using different analytical techniques (EPXMA, p-PIXE, p-SRXRF and SIMS). Other investigators have already published bulk analyses and elemental profiles of glasses whose leaching 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel behaviour closely resembles the one reported here, with the exception of the K enrichment [4, 9, 34, 55], Clarck and Zoitos discovered the presence of a K-peak in the interaction zones of nuclear waste glasses which had been buried for two years [56]. The fact that the K+ abundance is increased in the leached layer may be the result of a high K concentration in the soil the glass was buried in due to the vicinity of the burial site to the Dead Sea. Table 7. Composition (% w) of the bulk glass, the corrosion layer and the crust of Qumran Object #22. Bulk Leached layer Crust Na20 16.24 0.43 < 0 . 1 0 MgO 0.07 7.04 7.93 a i 2o 3 2.46 4.19 4.51 S i0 2 70.02 81.47 43.17 p 2o 5 0.04 - 0.53 so3 0.19 0.81 0.52 Cl 0.85 1.23 0.50 k 2o 0.81 2.80 2.14 CaO 8.25 0 . 8 8 37.39 T i0 2 0.06 0.14 0.45 MnO 0.43 0.26 0.32 Fe 20 3 0.49 0.92 3.05 To study the behaviour of minor and trace elements next to major elements, p- SRXRF line scans were taken along polished cross-sections of the samples. Three groups of elements showing similar behaviour can be discerned in Figure 10 : K, Ti, Cr, Fe, Zn and Br being enriched in the leached layer, while Ca, Mn, Sr, Pb and Mo are clearly depleted in concentration; the elements Zr, Sn and Sb appear not to be affected by the corrosion process. The reason for the existence of these different groups of elements may be connected with the size of the ions and/or the way these interact with the silica network in the glass. 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel 10' 10' ,:C r :C a Mn (x 3.0) Fe : S n (x 10) M o (x 2.0) 10- Br : S b (x 0.1)' ,:Sr (x 2.0) ,:PtJ E 10‘ 10‘ Q. Q. C o S.c io: 10' <1) eo Oo 10‘ 10! 10 10( 10' 0 50 100 150 200 250 300 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Vertical distance, pm Vertical distance, pm Vertical distance, pm Figure. 10. Major and trace element line-scans obtained by p-SRXRF, collected along a line perpendicular to the glass surface, through a parallel section of the corrosion layer. 0-60 pm : original glass; 60-240 pm : leached layer; 240-300 pm : crust/embedding resin. 7.2 Layered morphology When alteration has taken place on an archaeological glass object during burial in damp soil or immersion in rivers or the sea, over a period of some hundreds of years, examination in cross-section under the microscope shows that the crust shows a laminar structure, the fine sublayers running parallel with the outer edge of the glass. The individual layers of these structures range in thickness from about 0.5 to 30 pm depending on the composition of the bulk glass. As was mentioned before, Brewster was the first to describe this phenomenon [1] and Fowler [3] called it filmy, blistering or iridescent decay. Brewster found out that the more or less continuous films of altered glass on the surface could be detached by means of a sharp knife and that such films were commonly compound since they usually could be split into a number of much thinner films. Though these sublayers adhered to each other strongly, he did not believe that they were close enough together to be in optical contact since water and other liquids were admitted freely between the films. He also observed that the separated layers exhibited a variety of colours by transmitted light as well as reflected light. Admission of liquids, which could be done by simple immersion, resulted in complete extinction of the transmitted light, which, however, returned as soon as the films dried out. That is why he explained the colourful iridescence so familiar for archaeological specimens 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel of glass as being the result of interference phenomena due to air being trapped in the small spaces between the sublayers in the altered glass. Brill and Hood [57] suggested that the appearance of these minute individual layers in crusts on archaeological glass objects which have been formed over extended periods of time might be a reflection of some periodic or cyclical change taking place during the long time exposure. According to them, for example, seasonal variations in temperature or alternating periods of rainy and dry seasons might be responsible for the laminated effects. Since such phenomena occur in yearly cycles, the possibility arose of dating archaeological specimens of glass with extreme accuracy simply be counting the number of the individual layers comprising the alteration crust. In this way, the layers could constitute a historic record of time of burial like tree rings. The hypothesis was tested by counting the number of layers in glasses that had been buried or submerged for known periods of time and it was found to be successful for a few glass objects of the late Roman and Islamic periods and for several pieces of more recent times. The authors state that it is of course important that the crust is intact on the fragments to date and, unfortunately, this is not often the case because the layers are so thin and fragile that they usually flake away in the soil long before they are excavated. In spite of the positive results this view has been met with scepticism by glass chemists. One reason for doubt is the complexity of hydrolytic attack on glass. These layers can also be observed when the object has been submerged in the sea where annual temperature changes, if they occur, are very small and water is always present. Hampton was able to produce ten layers in a sample of glass by autoclaving it for four hours at 144 C [12]. Newton showed that there can also be very much variation in the number of layers from one part of the specimen to another, hence, that little reliance can be placed on a single count [15, 58]. Shaw [13] showed that the layers were not amorphous and thus complicated chemical changes had occurred during their formation. Douglas [59] has suggested that the layers might be produced by the alternation of two chemical reactions, producing (a) an ion exchanged layer which then (b) becomes porous enough to permit water molecules to penetrate to the unaltered glass, starting process (a) again. The completion of this cycle might require about a year at ordinary temperatures. In a number of Qumran samples, the corrosion layer is not continuous but shows this typical layered morphology (see Figure 11a and b). In some cases, when the corrosion front has progressed in a regular, undisturbed manner into the body of the specimen, stacks of thin parallel sublayers have developed parallel to each other and to the surface of the original glass (Figure 11a). The thickness of each individual layer is in the range 0.5-1 pm, which is typical for soda glasses in contrast to potash glasses where the thickness is much larger (from 2 to 30 pm). The process of attack by moisture is influenced by irregularities in the glass (e.g., cracks, inclusions of air or trapped dirt particles). Inclusions, such as the remains of a gas 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel bubble, can disturb the contours of the corrosion layers which, in many cases, results in the hemi-spherical morphology shown in Figure 11b. (a) Figure 11a and b. Layered corrosion morphology in some corrosion layers. 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel 7.3 Intruded materials The leached layer of buried glass is frequently dark brown or black in colour, with the result that the glass is either covered with dark spots (pits) or completely opaque. In cross-section, a brown-black dendritic invasion of the interior of the glass may also be observed, apparently following the lines of cracks into the glass. One of these regions is represented in Figure 12. The dark colour is usually the result of precipitation in the crust of compounds of iron and manganese. These appear to enter with the attacking solution through the cracks and channels of the corrosion crust [49]. Manganese and iron in the Mn(ll) and Fe(ll) states, respectively, are soluble in water but are thermodynamically unstable. They are readily oxidised to a higher, more stable state to give insoluble compounds such as hydroxides, oxyhydroxides and/or oxides. Both elements are common constituents of soils as a result of the breakdown of silicate minerals. In natural waters, manganese concentrations as high as 1 0 ppm have been recorded [60]. Thus, under anaerobic conditions, as in waterlogged soils, Mn(ll) and Fe(ll) ions are highly mobile, facilitating their migration in decomposing glass. However, should the oxygen content of the environment increase, due, for example, to the respiration of plants or biological activity, both elements may be precipitated as insoluble oxides [34, 61, 62]. Other investigators, though, state that the iron and manganese originate from the glass itself and do not diffuse into the glass from the soil since the iron and manganese oxides in the soil are only soluble at very low pH. The pH prevailing adjacent to the glass surface is 8 or 9 as a consequence of the leaching of the cations and at that pH value those compounds are completely insoluble [63]. In a few cases the blackening of buried glass has been found to be caused by black lead sulphide [63]. In order to obtain a better insight in the migration of various elements between the glass, the corrosion layer and the surrounding soil, also trace element distributions of such an area were recorded. Since, in view of the brittle character of the material, it was hard to obtain sections thinner than 100 pm, p-PIXE as preferred over p- SRXRF for obtaining these maps. The results are shown in Figure 13, these data show a fairly complex behaviour of the various trace elements. Next to a high Mn content, the precipitates are also enriched in elements such as Cr, Fe, Ni, Br and Mo but depleted in K and Ti, while alternate sublayers show higher concentrations in Ti, Fe, Cu and Zn. Also, broad alternate bands rich in respectively K and Ca can be observed, the latter also showing a higher Sr content. Although these different layers reappear in most samples of similar morphology, the order and relative thickness of the different sublayers vary considerably among the different samples. 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran , Israel Figure 12. X-ray maps of major and some minor elements showing the distribution in the bulk glass, corrosion layer, manganese deposition and the crust. 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel Figure 13. p-PIXE maps from a 1200 x 1200 pm region of the corrosion layer on one of the samples; the analysed area is also visible in Figure 12. 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel 8. THE SURFACE 8.1 Outlook of the surface To get a detailed outlook of the surface of the Qumran glasses, light microscopic images were taken with a magnification of 100. An example is shown in Figure 14. On the surface a colourful crust with shades of green and brown has formed. The multilayer structure underneath gives the objects the iridescent outlook. Multiple cracks are visible and on the right hand bottom corner a part of the upper layer has flaked off resulting in the exposure of the underlying gel layer. Figure 14. Light microscope photograph of the surface of Qumr§n object #22. 8.2 The crust In most samples, a calcium-rich crust (see Table 7) has formed on top of the corrosion layer. Whereas the corrosion layer is situated inside the original glass pane, the crust clearly was formed by precipitation of various Ca (and other cation) salts which have probably been leached out of the glass. It is not clear however which crystalline materials have actually precipitated. ED X-ray spectra collected from samples having a thick enough crust (>10 pm) feature on some locations Ca-K 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel and S-K peaks of equal intensity, indicating the presence of C aS 0 4.2H20 (gypsum, see Figure 15a) at other places, however, only a Ca-K line is visible (Figure 15b), which points to the presence of other Ca salts like C a C 0 3 or C a(H C 03)2. It is known that calcite occurs on porous materials which have been in contact with groundwater for a long time. According to changes in soil pH, water content and biological activity, calcite can be reprecipitated from solutions which are saturated in calcium bicarbonate [64]. Unfortunately microscopic FTIR (Fourier transform Infrared) measurements did not allow us to confirm this assumption, probably because there was not enough crystalline material present. (a) Si 0.0 2.0 4 . 0 6.0 8.0 keV (b) Si Mg Cl 2.0 4 .0 6.0 8.0 keV Figure. 15. ED X-ray spectra from crust of Qumran Object #22. 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran , Israel 9. CONCLUSIONS By using two non-destructive and complementary X-ray emission techniques (EPXMA and p-SRXRF) respectively, the major and minor/trace composition of a series of about 90 Roman glass objects of different types excavated from the ruins of Qumran, Jordan was determined. Bulk analysis of the unaffected regions of the glass showed that the glass has similar properties (overall composition, durability, rate of corrosion) to that of Roman glass objects found at other sites in Europe and indicated that at Qumran large quantities of glass objects were being used, possibly in the perfume industry based there. These glasses were also studied with respect to the corrosion phenomena that have occurred as a result of contact with groundwater during a period of ca. 1900 years. Cross-sectional profiles of the glass fragments showed that the thickness of the corrosion layer may vary between a few pm to 0.5 mm. In the leached layers, elements such as Na and Ca were depleted relative to the original bulk glass; on the other hand, the concentration of Mg, Al and K was found to be higher than in the original glass. In a number of cases, a layered morphology was observed in the affected regions of the glass. In some cases, salt precipitates (sulphates, carbonates) of the elements that were leached out of the glass were found on the glass surface, giving the glass surface its typical corroded outlook. Since the composition of these glass vessels is very similar to other Roman glass, it is relevant to analyse more Roman glasses with a different age and from different locations throughout the Roman Empire to get to a better insight into the variations and/or similarities in composition, to obtain an idea about raw materials used and to find possible explanations for the three groups which are present in the series of Qumran objects. In Chapter 4 the major, minor and trace composition of various Roman glass collections with different age and provenance is studied and comparisons are made. Since not enough is known about the burial conditions (e.g., humidity, temperature, differences in composition of the soil, depth and orientation of burial of the glass fragments), it is rather difficult to make conclusions about alteration processes based on the observations of corrosion phenomena on the Qumran glasses. That is why simulation experiments in well controlled environments should be carried out which could lead to a better understanding of the different processes taking place and the importance of various parameters on the corrosion of glass during burial. Results of such experiments are given in Chapter 6 . 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel REFERENCES [1] Brewster D. Sir, On the Structure and Optical Phenomena of Ancient Decomposed Glass, Roy. Soc. Edinburgh, 27, pp. 193-204 (1863). [2] Hausmann J.F.L., Bemerkungen Qber die Umanderungen des Glases, nebst den Resultaten der von dem Herrn Doctor Geuther im Hiesigen Akademischen Laboratorium in Beziehung darauf Ausgefuhrten Chemischen Analysen, Nachrichten van der Georg-August-Universitat und der Koniglichen Gesellschaft der Wissenschaften, pp. 114-120 (1856). [3] Fowler J., On the Process of Decay in Glass, and Incidentally , on the Composition and Texture of Glass at Different Periods, and the History of its Manufacture, Archaeologia, 46, pp. 65-162 (1880). [4] Geilmann W., Beitrage zur Kenntnis Alter Glaser IV. Die Zersetzung der Glaser im Boden, Glastech. Ber., 29, pp. 145-168 (1956). [5] Neumann B., Der Babylonisch-Assyrische Kunstliche Lasurstein, Chem. Zeit, 51, pp. 1013-1015 (1927). [6 ] Laubengayer A.W., The Weathering and Iridescence of Some Ancient Roman Glass Found in Cyprus, J. Amer. Ceram. Soc., 14, pp. 833-836 (1931). [7] Varshney Y.P., Glass in Ancient India, Glass Ind., 31, pp. 632-634 (1950). [8 ] Turner W.E.S., Studies of Ancient Glasses and Glass-Making Processes. Part II. The Composition, Weathering Characteristics and Historical Significance of Some Assyrian Glasses from the Eight to Sixth Centuries B.C. from Nimrud, J. Soc. Glass Techn., 38, pp. 225-456T (1954). [9] Caley E.R., Analyses of Ancient Glasses 1790-1957. A Comprehensive and Critical Survey, The Corning Museum of Glass, Corning Glass Center, New York, Chapter 6 (1962). [10] Brewster D. Sir, Discoveries in the Ruins of Nineveh and Babylon, Layard A.H. (ed.), pp. 674-676 (1853). [11] Geilmann W., Beitrage zur Kenntnis Alter Glaser VI. Eine Eigenartige Verwitterungserscheinung auf Romischen Glasscherben, Glastechn. Ber., 33, pp. 291-296(1960). [12] Raw F., J. Soc. Glass Techn., 39, pp. 128-133T (1955). [13] Shaw G., Weathered Crusts on Ancient Glass, New Scient., 29, pp. 290-291 (1965). [14] Brill R. H. and Hood H.P., A New Method for Dating Ancient Glass, Nature, 189, pp. 12-14 (1961). [15] Newton R.G., Some Problems in the Dating of Ancient Glass by Counting the Layers in the Weathering Crust, Glass Techn., 7, pp. 22-25 (1966). [16] Malow G., Mater. Res. Soc. Symp. Proc., 11, p. 25 (1982). [17] Thomassin J.H., Nogues J.L. and Touray J.C., Iztude de la Couche Developpee a la Surface d’un Verre Nucleaire lors de I’Alteration en Milieu Aqueux : Comparaison des Donnees de I’Analyse des Solutions et de la Microscopie £lectronique, C. R. Acad. Sci. Paris, 297(H), pp. 857-862 (1983). [18] Nogues J.L., Les Mecanismes de Corrosion des Verres de Confinement des Produits de Fission, PhD Dissertation, Univ. Montpellier (1984). [19] Godon N., Thomassin J.H., Touray J.C. and Vernaz E., Experimental 97 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel Alteration of R7T7 Nuclear Model Glass in Solutions with Different Salinities (90°C, 1 bar) : Implications for the Selection of Geological Repositories, J. Mater. Sc/., 23, pp. 126-134 (1988) [20] Petit J.C., Della Mea G., Dran J.C., Magonthier M.C., Mando P.A. and Paccagnella A., Hydrated-layer Formation during Dissolution of Complex Silicate Glasses and Minerals, Geochim. Cosmochim. Acta, 54, pp. 1941- 1955 (1990). [22] Goldschmidt F., Corrosion Aqueuse des Verres Silicates. Analogie entre les Verres Volcaniques et fe Verre de Reference Frangais R7T7, PhD Dissertation, Univ. Paris Xl-Orsay (1991). [23] Ewing R.C., Natural Glasses : Analogues for Radioactive Waste Forms, Mat. Res. Soc. Symp. Proc., I, pp. 57-68 (1978). [24] Baillet M., Milik J.T. and de Vaux R., Discoveries in the Judean Desert, volume 3, Clarendon, Oxford (1962). [25] Fritsch C.T., The Qumran Community : Its History and Scrolls, MacMillan, New York (1956). [26] Laperrousaz E.M., Les Manuscrits de la Mer Morte, PUF, Paris (1961). [27] Milik J.T., Qumran, Encyclopaedia Universalis, 13, pp. 896-898 (1972). [28] de Vaux R., Archaeology and the Dead Sea Scrolls, The Schweich Lectures of the British Academy, XLI b, London (1973). [29] Donceel-Voute P., Les Ruines de Qumran Reinterpretes, Archeologia, 298, pp. 24-35(1994). [30] Donceel R. and Donceel-Voute P., Methods of Investigation of the Dead Sea Scrolls and the Khirbet Qumran Site. Present Realities and Future Prospects. The Archaeology of Khirbet Qumran, Annals of the New York Academy of Sciences, 722, pp. 1-38 (1994). [31] Fontaine-Hodiamont, Quatre-ving-neuf Verres Tres Fragmentaires Provenant du Site de Khirbet Qumran (Cisjordanie, ler s. ap. J. -C.), Bulletin de I'lRPA/ van het KIK (Selectie/Selection), XXV, pp. 277-280 (1993). [32] File IRPA2L/123 [33] Forbes R.J., Studies in Ancient Technology, second edition, pp. 145-155, Leyde (1966). [34] Cox G.A. and Ford B.A., The Long-term Corrosion of Glass by Ground-water, J. Mat. Sci., 28, pp. 5637-5647 (1993). [35] Jackson C.M., Hunter J.R., Warren S.E. and Cool H.E.M., The Analysis of Blue-green Glass and Glassy Waste from Two Romano-British Working Sites, Archaeometry’90, pp. 295-304, Birkhauser Verlag, Basel (1991). [36] Brill R.H., Excavations at Jalame, Site of a Glass Factory in Late Roman Palestine, Chapter 8 , University of Missouri Press (1988). [37] Van Rolf C. and Rottlander A., Naturwissenschaftliche Untersuchungen zum Romischen Glas in Koln, Kolner Jahrbuch fur Vor- und Fruhgeschichte, 23, pp. 563-582 (1990). [38] Velde B. and Cedron C., Chemical Composition of Some Gallo-Roman Glass Fragments from Central Western France, Archaeometry, 22, pp. 183-197 (1980). [39] Mirti P., Casoli A. and Appolonia L., Scientific Analysis of Roman Glass from 98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel Augusta Praetoria, Archaeometry, 25, pp. 225-240 (1993). [40] Calvi M.C., Tornati M. and Scandellari M.L., Ricerche Technologiche, I Vetri Romani del Museo de Aquileia (1968). [41] Bondarenko I., Treiger B., Vand Grieken R. and Van Espen P., IDAS, A New Windows Based Software Package for Cluster Analysis, Spectr. Acta Elec., 46B, p. E00 (1995). [42] Massart D. and Kaufman L., The Interpretation of Analytical Data by the Use of Cluster Analysis, Wiley, New York (1983). [43] Ward J.H., Hierarchical Grouping to Optimize an Objective Function, J. Am. Statist. Ass., 58, pp. 236-244 (1963). [44] Lance G.N. and Williams W.T., A General Theory of Classificatory Sorting Strategies : 1. Hierarchical Systems, Comp. J., 9, pp. 373-380 (1967). [45] Turner W.E.S., Studies in Ancient Glasses and Glass-making Processes. Part V. Raw Materials and Melting Processes, J. Soc. Glass Techn., 194, pp. 277-300(1956). [46] Engle A., Readings in Glass History, No. 1, Phoenix Publications, Jerusalem, pp. 1-26(1973). [47] Engle A., Readings in Glass History, No. 10, Phoenix Publications, Jerusalem, p. 10(1978). [48] Frank S., Glass and Archaeology, Academic Press, London, pp. Chapter 4 (1982). [49] Newton R. and Davison S., Conservation of Glass, Butterworths, London, Chapter 3 (1991). [50] lliffe C.J. and Newton R.G., Using Triangular Diagrams to Understand the Behaviour of Medieval Glasses, Verres Refract., 30(1), pp. 30-33 (1976). [51] Newton R.G. and Paul A., A New Approach to Predicting Durability of Glasses from their Chemical Compositions, Glass Techn., 21(6), pp. 307-309 (1980). [52] Plodinec M.J., Jantzen C.M. and Wicks G.G., Proc. 2nd Int. Symp. Ceramics in Nuclear Waste Management, Chicago, April (1983). [53] Jantzen C.M. and Plodinec M.J., Thermodynamic Model of Natural, Medieval and Nuclear Waste Glass Durability, J. Non-Cryst. Solids, 67, pp. 207-223 (1984). [54] Paul A., Chemical Durability of Glasses; a Thermodynamic Approach, J. Mat. Science, 12, pp. 2246-2268 (1977). [55] Macquet C. and Thomassin J.H., Archaeological Glasses as Modelling of the Behaviour of Buried Nuclear Waste Glass, J. Appl. Clay Sci., 7, pp. 17-31 (1992). [56] Clarck D.E. and Zoitos B.K., Corrosion of Glass, Ceramics and Ceramic Conductors, pp. 222-268, Noyes Publications, New Jersey, USA (1991). [57] Brill R.H. and Hood H.P., A New Method for Dating Ancient Glass, Nature, 189, pp. 12-14 (1961). [58] Newton R.G., Some Further Observations on the Weathering Crusts of Ancient Glass, Glass Technol., 10, pp. 40-42 (1969). [59] Newton R.G., Archaeometry, 13, pp. 1-9 (1971). [60] Morgan J.J. and Stumm W., J. Amer. Water Works Assoc., 57, p.107 (1965). [61] Marshall K.C., Biochemical Cycling of Mineral-Forming Elements, Trudinger 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3 : Investigation of a Roman Glass Collection from Qumran, Israel P.A. and Swaine D.J (eds.), Elsevier Scientific Publishing, Amsterdam, p. 261 (1979). [62] Collins J.F. and Buol S.W., Effects of Fluctuations in the Eh-ph Environment on Iron and/or Manganese Equilibria, SoilSci., 110(2), pp. 111-118 (1970). [63] Knight B. Archaeological Conservation and its Consequences, Roy A. and Smith P. (eds.), The International Institute for Conservation of Historic and Artistic Works , London, pp. 99-104 (1996). [64] Courty M.A., Goldberg P. and Macphail R., Soils and Micromorphology in Archaeology, Cambridge University Press, Cambridge (1989). 100 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 Provenance Analysis of Roman Glass from the 1st-6th Century AD Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 1. INTRODUCTION In general, glass produced in early times and up to the Middle Ages was obtained by melting together a siliceous constituent (e.g., quartz sand), a fluxing agent (e.g., natural soda or sodium-rich plant ashes) and one or more colourant, decolourant and/or opacifying substance(s). It is logical to assume that the use and type of these constituents varied, depending on the manufacturing centres and the historical period [1]. Roman vessel glass from the period 1st-6th century AD, however, shows the remarkable property that its major composition is very similar, containing approximately 66-72 % Si02, 16-18 % Na20 and 7-8 % CaO, irrespective of the geographical location and time period the material originates from; even post-Roman glass (9th century) maintains this composition for the most part. This was already shown in Table 3 in Chapter 3 where the major (and minor) composition of some Roman glasses excavated at different places throughout the Roman Empire is given. The reasons for this high degree of compositional similarity are unclear and are at present subject to speculation in the literature. The consistency in composition suggests a certain discipline in the manufacturing technique drawing on a constant supply of raw materials of high purity for sodium, silica and aluminium which were used in quite constant proportions for a long period of time over a large geographical area [ 1 , 2 ]. Either all the bulk glass (in the form of ingots) was manufactured in one or a few locations in the Middle East or all the glass was made with raw materials originating from the same source. A Middle-Eastern source seems to be the most logical explanation of the pure soda component [3]. It was assumed that Roman galleys loaded the natron in Alexandria and brought it to the head of the Adriatic from where it was further transported up the Rhine. Since natron-type glass continued to be made long after the breakdown of the Roman imperial power in the west, it seems that traders and merchants continued to import natron into the north-west of Europe using these or other trade routes (via the Black Sea, up the Danube, the Vistula and via the Baltic sea), even after the fall of the Roman Empire [4]. The hypothesis that this material was already fused into glass near the site of extraction and that this raw glass was then transported in the form of frit or cullet to the site where production of objects took place, is also very plausible since it eliminates the necessity of finding the pure sources of CaO, Al 20 3 and S i0 2 at various places, situated in various climates throughout Europe. Furthermore, an insoluble, highly stable material (glass ingots) is much easier to handle during transportation than to treat natron which is corrosive when wet. A third hypothesis is that for the manufacture of glass during the later period of the Roman Empire, glass from earlier periods was extensively recycled [1, 5]. The analysis of glass can potentially provide information about the raw materials used, the time at which the objects were made and the technology of ancient glass production [6 ]. The basic technology used can be established by the determination of the major components such as silica, sodium or potassium oxide and calcium 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD oxide. With these major components, minor constituents are introduced into the glass batch. Magnesia, phosphorus pentoxide and chlorine help to identify the use of a particular alkali while titanium and alumina provide insight into the nature of the silica source. Colourants, decolourants and opacifiers are often deliberately added bringing other trace elements with them which can help to identify the kind of material used or sometimes even their sources [7]. Analytical groupings in a series of data are often not performed since analysts have not determined the presence of enough oxides, such as antimony and tin together, on a routine basis. As mentioned before (Chapter 3, Section 5.2) three groups were found in the Qumran data set. The composition of the glasses was compared to the results found for a series of Roman glasses from Jalame which is situated in the neighbourhood of Qumran (Palestine). 40 samples from glasses, representative of glass made at Jalame were analysed and it turned out that all vessels had approximately the same composition : no clear subclassifications could be distinguished when coloured glasses or glasses with deliberate addition of manganese were excluded. The glass has the typical soda-lime-silica composition, more specifically they belong to the low K 20-low MgO type [ 8 ] and the results are very comparable to the ones found for the Qumran glasses except that no Sb 2 0 5 was found in any of these glasses [9]. In order to obtain a more detailed picture of the differences and similarities of Roman glass from different periods and geographical locations, an extensive set of Roman glass samples (about 350 samples, including the Qumran series) was analysed for its major, minor and trace composition. For these analyses, a combination of scanning electron microscopy and micro synchrotron radiation induced X-ray fluorescence was employed. The aim was to deduce whether the groups which were found in the Qumran data set, could also be found in glass collections originating from other locations and/or other time periods in the Roman Empire or whether they are typical for glasses from the Middle-East. The concentrations found for the different elements in the various series were compared in order to obtain more information about raw materials used which could lead to a better interpretation of the subdivisions in the set of objects. 2. GLASS COLLECTIONS AND SAMPLE MATERIAL 2.1 Origin of sample material Based on information found in the literature [5, 10, 11, 12] and personal communications with Dr. R. Brill (Corning Museum of Glass), a number of analysts who had investigated Roman glass and determined its composition (mainly major elements) were selected and contacted to ask if they could send us some sample material. Prof. R. Rottlander from the University of Tubingen, Germany and Dr. B. Velde from the Laboratoire de Geologie de l’£cole Normale Superieure in Paris, 1 0 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD Figure 1. Part of Europe showing the locations where the glass collections originate from. 103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD France were both willing to provide sample material from Cologne (Germany), Trier (Germany) and Rouen (France) and Dr. C.M. Jackson from the University of Sheffield, Great-Britain sent us her results from trace element analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES) from samples found in Mancetter and Leicester (Great-Britain). Together with the previously studied Qumran glasses, this already provided us with a geographical range covering a wide part of the Roman Empire. Later on these samples were complemented with glasses from various locations in Belgium (Oudenburg, Tongeren, Grobbendonk and some locations in the south of Belgium) and the Netherlands (Maastricht) in order to cover also a wide time range including the period in which the Qumran glasses were made. These samples were obtained from The Royal Institute for Cultural Heritage (IRPA), The Institute for the Archaeological Patrimonium, Flanders (IAP) and the Department of Town Development and Ground Matters in Maastricht. Figure 1 shows the locations in Europe where the glass collections originate from. 2.2 Classification of the data All these samples were analysed for major, minor and trace element composition and the results were compared. A wide range in geographical locations, time as well as colour of the glasses was present. The results of the different geographical locations will now be discussed separately. For each site a t least one table was made showing the average composition of the different subgroups in the series of samples together with their standard deviation and the minimum and maximum value for each element in each group. Just as in the Qumran data set, this classification in subgroups was not done using hierarchical cluster analysis or principal component analysis since these methods did not give results that were easily interpretable. The reason is probably the fact that the values for all elements are used together (giving the same weight to every element in some cases) leading to subgroups which have no straightforward connection with raw materials used. That is why preference was given to perform the classification manually by visual inspection of the data and mainly concentrating on elements which are directly connected with additives used in the glass batch such as colourants, decolourants or opacifiers. In that way subgroups are created which can explain the use of different raw materials, sources and/or additives and which mainly lead to subgroups containing glasses with (nearly) the same colour. Groups with comparable elemental concentrations can be found in all the glass collections investigated. 2.3 The colour of glass At first only “naturally” coloured and decoloured glasses were selected and studied, later on intentionally coloured samples were added. The “natural” aqua colour, which visually is a pale greenish blue, is typical for Roman glass. A concentration of 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD iron oxide of about 0.3 to 0.6 w%, known to be the level of iron impurities in the raw materials used for glass production, explains this colour. The iron is present in glass as a mixture of two ionic species, the chemically reduced ferrous ion (Fe2+), giving a bluish colour to the glass and the oxidised ferric ion (Fe3+), resulting in a much weaker yellowish colour; by convention, the Fe content is always reported as Fe20 3 however. The effect of the ferric ion can usually be neglected since the ferrous ion is a much stronger colourant. The colour of any particular piece of glass depends on its chemical composition and on the oxidising or reducing conditions during melting. In glass melts, redox reactions occur in which ions can be converted from one to the other. In neutral or reducing conditions, iron is predominantly present in the Fe2+ form which results in the commonly observed aqua colour. The origin of the colouration due to the presence of ferrous iron is a strong absorption peak in the infrared region which tails out into the red region of the visible spectrum when enough ferrous iron is present. The absorption of the red light results in the greenish blue aqua colour in the glasses. The more ferrous iron present, the bluer the glass appears. The result is that the colour can shift, creating a range of greenish and bluish variants of aqua when the melting conditions become slightly more oxidising or reducing [9]. 3. GLASSES FROM COLOGNE (GERMANY) 3.1 Description of the glass collection and its location The Romisch-Germanisches Museum in Cologne has an extensive collection of Roman glass objects. Cologne, the capital of the province Germania Secunda, was an important centre from the 2nd century onward until it was captured by the Franks in 457 [13]. It is thought that in ancient times high quality glasses were produced in large amounts in Cologne itself during four centuries or longer because of the rich and luxury glass found in Roman graves and supposed to have been made locally [14] and the discovery of remains of several glass-furnaces together with frits and bluish-green glass [15, 16, 17]. This of course does not mean that all glass found in Cologne was produced locally since a rich city, as it was, could have easily imported glass objects from elsewhere and other workshops may have co-existed in the Rhineland. The samples we have measured were taken from bottles and ribbowls but in most of the cases the form of the original objects was undetermined as the samples were often quite small pieces. The glasses were all naturally coloured blue-green (aqua) or decolourised glasses and they cover a range from the 1st to the 4th century AD The glasses were pulverised to powders. 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD o CD © CM CM CM © in CO O CO fx- CO 32 3 46 42 to © 182 198 113 142 231 r — Max 0.74 0.61 0.81 8.74 2.36 CT O 2350 19.75 TO 75.03 X M- 03 © Y— CO © O CO - © © to M* to O © CM 20 43 162 Min 271 0.07 0.12 0.47 0.46 © 5.24 15.05 tt 69.94 3 CO CM to O CO © CO CO M* h-. © CO CO CO 29 33 55 CO 23 45 49 LO CO o - 494 1.32 0.14 CM 0.14 0.09 0.14 o* O X StDev -H +1 +1 +1 +i •H -H -H +1 -H -H -H -H -H •H -H -H -H •H -H ■H -H -H -H CM CO CO CO CO o CO CO CM Ti" 03 CO 98 30 48 20 39 48 CO to CO CO 925 521 0.52 0.77 O 2.18 © O 17.51 n=15 Mean 72.01 o T}- © 99 CM to © CO T“* © to O 74 32 20 87 CO CO 192 3 174 164 323 1.20 CO 0.77 Max 8.20 1180 1020 1180 2.66 © © 5980 73.10 Cf 20.48 (0 CO zoo 990 CO © © © © © Y“ o © 52 83 54 33 26 257 365 Min lO 1.25 0.17 3.56 0.21 0.46 ~Ci 69.12 © CM o CO 03 © © CM CO in f^. N. 29 82 55 28 29 CO u 195 217 258 0.14 0.14 0.11 0.93 1370 1.16 0.13 © © © X StDev Y— -H ■H -H -H -H +1 -H •H +1 -H -H +1 *H -H •H -H -H •H -H -H -H -H -H -H -H ■o* 600 © - © N- 56 57 23 Tj- CM OO CO o> 108 327 625 451 0.37 0.54 6.26 0.62 2.23 CM n=23 18.09 Mean | | 0.85 © o CM CO 99 © © CO o © r— •O' in to to to 72 23 26 24 to 324 576 CM 0.19 6.52 Max 0.37 © © © 2513 73.81 20.03 CM CO eoo CO © © © CO CO o y “* CO © o 00 to CO CO o CO CO © T" © 48 - © 159 214 Min CO 5.45 0.25 © © o ’ OO 3 18.14 72.14 .Q O > 06 © O © 03 in T“ to in hx- CM CO o CO h*- O O © © o CO h- CO to CO CM to CM in OO © 39 X Q CM 87 CM CM O © © © o © © © © in © lo to -H -H -H -H +1 -H ■H -H -H -H -H -H -H -H -H +i -H -H *H -H •H -H -H o r z 0 Z 6 © CM CO N © © co to o o © © CM CO © to CO 1— 22 v— CO - 54 CM CM M- CO 215 03 1150 0.16 0.03 n=5 to © Mean I 98'OZ 960 CO CO © h*. 68 CO Tj- o O 03 to © CO 59 52 29 CM CM CO 124 CO CO 432 0.23 7920 18.2 0.59 0.61 2.04 CO © aoua CO K. CO o 66 to o o CO CO © O © © CO 74 65 46 CO 833 506 o 253 CM 0.16 0.03 0.46 6.52 2550 C3 >1 © 5120 CM 1872 CM 73.25 31900 colourl. to CO Y— to © CO o in © CO CO CO 56 57 58 43 CO CM 111 169 921 754 CO 1150 0.52 0.94 Max 8.64 4140 0.91 2.48 © © 71.58 20.39 090 CO CO © © © © CO CM CO © 56 37 30 CO 33 109 531 201 209 Min CO 0.72 1.95 0.52 0.22 2 4.82 © o 16.54 70.17 CO X > r-»- O 03 CO CO CO © o CO CO y — in to o> CM * CM © o © 03 CO © CO CO CM CO © © © 47 o © 37 CM CM CM to © © © © © © o ■*“ -H -H +1 -H -H +t -H -H -H ■H -H +1 -H +1 -H -H -H -H -H -H -H -H -H 03 890 00 CO © to Oi m CO CO CO © to n to © CO 03 in © in CO h- CO 79 Y“* 46 © 47 CM to 393 CO © 435 to 0.37 c 0.69 6.96 2.25 © © Mean r*. m n eg in cn CM in CuO 03 CoO CO CaO MnO MgO M Z < to Q. 1- Li. >■ O < C£ 2 CO CO Table 1. Average concentration, standard deviation and minimum and maximum concentration for the subgroups in the samples from Cologne. Concentrations are given in w% (above horizontal line) and ppm (below horizontal line). 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 3.2 Classification of the data The average concentration and standard deviation for every element together with the minimum and maximum values in the subgroups, originating from the differences in composition between the samples of this data set, is shown in Table 1. The concentrations above the horizontal line (Na20 to Fe20 3) are given in weight percentages (w%); these are the results obtained by Dr. Rottlander. The ones below the line, in parts per million (ppm), are the results from our synchrotron XRF measurements. The first group consists of 9 naturally coloured glasses, the second and third contain only 1 fragment, colourless and naturally coloured respectively; in the fourth subdivision 5 colourless glasses are found, 1 colourless and 22 naturally coloured glasses make up class five and finally the sixth one is formed by 1 colourless and 14 naturally coloured glasses. The elemental concentrations which are highlighted will be discussed further. Unfortunately no information about the age of the separate objects was given so this is not mentioned in the Table. It should be noted that the classification of the objects into the 6 classes of Table 1 was only performed on the basis of their composition and not on the basis of their colour. 3.3 Decolourising agents in glass The samples can be subdivided into naturally coloured and colourless glasses. According to Plinius [18] colourless glass was highly valued by Romans; in Cologne it can already be found from the first half of the first century. Normally, vessels from colourless glass were more expensive than those made from naturally coloured glass. In the literature [3, 5, 8, 19-23] manganese and antimony are mentioned as decolourising agents in ancient glasses; both are used to oxidise Fe(ll) and eliminate the colour introduced by iron impurities. It is not obvious to decide whether these elements were introduced as impurities or intentionally added but the fact that they are often present simultaneously or individually in concentrations which are too high to result solely from accidental inclusions suggests an intentional addition. Rottlander [10] gives an explanation for the decolourisation of the glasses. He compares the composition of the colourless glasses to that of the naturally coloured glasses and concludes that the colourless glasses contain more S i0 2 and therefore less MgO, Al20 3, CaO and Fe20 3. The abundance of Na20 is comparable. This can be clearly seen in the second and fourth group of Table 1 which both consist of colourless glass objects and therefore show significantly different concentrations for the above mentioned elements compared to the groups with the naturally coloured ones. The colourless glasses contain on average half of the amount of MnO of the naturally coloured ones, which confirms the hypothesis of Sellner [24] that not the manganese content but the furnace atmosphere, i.e., its reducing power, is the determining factor for the colour. He suggested that the addition of manganese does not need to be high, when glass is produced using sand with a low iron content, if melting takes places under strongly reducing conditions. Mirti, however, who found a MnO concentration of less than 0.03 % in some colourless Roman 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD glasses from Augusta Praetoria, Italy [21], excluded that manganese was used as decolouriser and concluded that oxidation of iron must have been achieved either through a strict control of the furnace atmosphere or through the addition of decolourants which serve the same purpose as manganese. Antimony is known to be such an element. Inspection of Table 1 clearly shows that one of the most striking differences between the various subgroups is the concentration of Sb20 5. Therefore, antimony was probably used as decolourising element and the hypothesis of the furnace atmosphere being decisive for decolourisation must be treated with caution. Antimony was not only used as a decolourant or clarifying agent in antiquity but also as opacifying agent (for opaque white glasses) and as an additive to remove air- bubbles from the glass melt [3, 21, 22, 25]. X • • • o j © column 1 j © ! ocolumn 2 I X X © X A column 3 | | x column 4 | * * X ym x © x I x column 5 i • m X © © j • column 6 , x * J> • * • £ • • X x S 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 Fe20 3 (w % ) Figure 2. Binary plot showing the slightly positive trend between the MnO and Fe20 3 concentration in the glasses from Cologne. “Column" in the legend refers to Table 1. Manganese can be introduced into the glass with the sand or the flux but sometimes it was added to the melt intentionally. Concentrations of MnO (especially above 0.6 %) which are independent of Fe20 3 can most certainly be attributed to intentional addition. Addition of manganese is able to introduce some Fe, Cu, V, Ni and Ba and sometimes also Mg and Al which are present as impurities in the original manganese-containing ingredient [9], In the data set no clear positive correlation could be found between manganese and any of these above mentioned elements except for a slightly positive trend with iron (Figure 2, the subgroups from Table 1 are represented with different symbols) but this can be expected since they can be both introduced as impurities in the sand. Together with the relatively low MnO 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman G/ass from the 1st-6th Century AD concentration, this suggests that manganese was not an intentional additive. Nevertheless, the Mn/Fe ratio in most of the glass is much higher than in natural sources [26] which means that manganese could be added systematically to glass batches in such a way that the positive trend between the Fe20 3 and the MnO content is antropological and not chemical : when more iron was present in the raw materials, a larger amount of manganese was needed to decolourise the glass melt [20], 35000 30000 R = 0.95 25000 ! © column 1 ! □ column 2 ; E 20000 an . A column 3 ou> X column 4 ■o 15000 jx column 5 , 10000 5000 0 500 1000 1500 2000 2500 3000 PbO (ppm ) Figure 3. Binary plot showing the positive correlation between Sb20 5 and PbO in the groups of glasses from Cologne (columns in Table 1) and the possible use of two raw materials sources for antimony (represented by the line and the circles). Turner states that the co-presence of antimony and sulphur points to the fact that the antimony was added in the form of the mineral stibina which was used in the Orient and in the Roman Empire as an eye-cosmetic. Mines of this mineral are found in Syria and Mesopotamia [24]. Hahn [27] reports the analysis of glass samples from Pompeii, Vindonissa and Redania, Italy and found a constant ratio between Pb and Sb. She supposes that antimony was introduced into the glass via a Sb-Pb mineral, as a sulphide of Sb or as native Pb-Sb alloy. Another possibility is the evaporation product of Sb-oxide or of hydrated Pb-antimonate. These yellowish minerals can be found in Algeria or northern Italy [28]. Since no values for S 0 3 were given, the correlation between this element and antimony could not be proven but for a large part of the glasses there is an obvious positive correlation between lead and antimony (shown by the line in Figure 3) suggesting that these two elements were introduced into the glass together. Considering a correlation coefficient of 0.95, we can assert that there is a true positive correlation between these elements at a 99.9 % level of significance (and even higher). The other group, showing no 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis o f Roman Glass from the 1st-6th Century AD correlation between the antimony and lead content (encircled in Figure 3), points to the use of another raw material which might be the previously mentioned stibina. 3.4 Correlations between major elements Correlations between major elements were investigated by Rottlander [10] leading to some conclusions about raw materials used. The silica ingredient used in Cologne for glass production was most probably the white sand from Frechen. It was preferred to Rhine sand since it has a lower iron content (0.02 and 4.5 % respectively). Although Plinius [29] has never mentioned the addition of chalk to the glass melt which contains sand and soda, contrary to the sand near the River Belus, the one from Frechen does not contain enough calcium to result in a durable glass. Negative correlation between S i02 and CaO points to the addition of chalk or lime as a separate component and this was confirmed by the discovery of non-local chalk- lumps from bends of the Rhine or the Eifel mountains near the glass-furnace in Hambacher Forst where sand was used from the same sources as in Cologne. Next to this, positive correlation of MgO and CaO also makes suspect rock origin of calcium. The fourth most important component of the glass is Al20 3, which is negatively correlated with Si02, meaning that it did not enter the glass through the sand. The consistent abundance of this component is striking; it indicates that its presence was quite premeditated, no impurities would be so consistent in quantity. It is known [30, 31] that the addition of alumina increases the resistance of glass to aqueous attack and generally also the mechanical strength of the material. The very constant concentration of Al20 3 in Roman glasses suggests that the Imperial Romans were aware of the use of this compound as a stabiliser in chemical and mechanical properties. The summary of analyses given by Sayre and Smith [8] indicates that the value of Al20 3 in glass was not widely known before or after this period. The precise source of Al20 3 is difficult to determine since it is rare to find alumina as a pure oxide in temperate latitudes. A possibility would be a feldspar, either sodic or calcic, since such minerals melt relatively easily and have been used by ceramicists for several centuries in Europe [5]. The use of a calcic feldspar could also explain the positive correlation between CaO and Al20 3 (although the proportions are not right for a feldspar with molecular formula CaAI2Si20 8). Correlations between trace elements will be discussed in the next section where more results from glasses from Cologne are given. 4. GLASSES FROM COLOGNE (GERMANY), TRIER (GERMANY) AND ROUEN (FRANCE) 4.1 Description of the glass samples The first series of glass samples supplied by Velde consisted of 55 small glass chips taken from bottles, beakers, dishes or plates and drinking glasses as well as smaller 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD flasks and containers for more precious products. The objects also originate from Cologne and like the previously described ones form part of the collection of the Romisch-Germanisches Museum in Cologne. They cover a time range from the 1st to 6th century AD. The glasses were all clear, naturally coloured or decolourised [1], Next to these, some samples from so-called frontinian bottles were present. Such bottles or jugs have the form of a barrel with an open top and a funnel mouth on which one or two handles are attached. They are mould blown with an inscription (a glass-makers’ stamp or trade-mark) found imprinted in the bottom which often includes the letters FRO in some form. The form is typical of tomb finds in the Rouen area in Normandy (France) and zones to the east (Trier, Germany). Our samples were taken from objects which form part of the collections of the Musee des Antiquites, Rouen (11 samples of which 5 frontinian bottles, 1 other bottle, 1 urn, 1 box and 3 unidentified forms) dated to the 3rd to 4th century and of the Landesmuseum, Trier (5 samples from frontinian bottles) originating from the 1st to 2nd century [2]. In the north of France later glass is predominant, although early finds are also known. It is generally supposed that factories were present in Normandy and Poitou [30] and that the north of France was one of the chief centres of glass-making from the 2nd century onward [33]. Trier may have been another glass centre in western Germany besides Cologne. Although only bead-manufacture is proved thus far [34], there may have existed other manufacture such as the typical frontinian jug which seems to belong to this district. 4.2 Classification of the data After analysing all the glass samples, their compositions were compared and divided into subgroups. Table 2 again shows the average concentration and standard deviation for every element together with the minimum and maximum values in the different subgroups found. Again the classification was done on the basis of the composition only. The first group consists of 4 glasses, 3 from Cologne (1 from the 1st and 2 from the 2nd century) and 1 from Rouen, the second has 12 samples including the 5 frontinian bottles from Trier, 1 unknown form from Rouen and 6 from Cologne (1 from the 1st, 2 from the 2nd and 3 from the 3rd century), 3 glasses from Cologne (1 from the 1st century and 2 from the 2nd century) make up class three, in the fourth subdivision next to 18 glasses from Cologne (4 from the 1st, 6 from the 2nd, 6 from the 3rd, 1 from the 4th and 1 from an unknown century) the 9 remaining objects from Rouen can be found, the 23 glasses in group five all originate from Cologne (4 from the 2nd, 5 from the 3rd, 9 from the 4th, 1 from the 5th, 2 from the 6th and 2 from unknown century) and finally the sixth one is formed by 2 4th century glasses from Cologne. The elemental concentrations of special interest are again highlighted. 111 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD W o 03 o CO o r- in co CO 160 100 - 140 95 385 373 60 24 CM o' d CD o p in 63 CM CO in CO in CM O o o CO lO o CM co O CO o f- CO r- CM co CO M1 CO TT in CM TT m 138 - © o co o - 371 i- in in CO CM CO CM CO CM o 73 o> 30 69 30 35 - - 174 o o o d d o' o' a s -H ■H -H -H ft +1 +1 -H +1 -H ■4H ■H ft ft ft +i ft -H ft ■H •ft -H ft ft in co in CM CO 601 is co r*- o CO r*- O’ CO m CO CM 03 CO 45 261 372 s 5- O eg r* d o cd o »• 3 in o in co © CO Si tT CM 2 d - CM CO o o o> f S s? ? 8 o o CM O O ID CM CO CO CM «(— CM CM lO CM 5-5 © o o CO o o o CSI CO CO s-a m in o co ^ CM 0 3 CO 5 4 2 3 2 3 8 - o d o d o’ O 2 4 3 ■H ■H *H f t ■H ■H -H •ft -ft ft ■ft M ft +t +t ft ■ft + t CO CM CO CM c o eo co CO 5? 7 CO co 1 2 3 7 CM 3 5 7 9 » - CM 5*2 o cm - d d o O 0 6 0 o 6 9 CO r - CO co 8 9 £ 4 2 6 3 co 2 9 6 3 3 9 8 1 1 7 1 8 8 3 5 4 9 1 4 7 0 4 0 2 . 0 5 8 .4 1 0 . 3 3 3 . 3 8 3 . 7 5 3 . 1 0 0 .7 1 1 8 .4 4 7 4 . 6 8 03 o o O in i-* i n CO o o T~ co o O) h - N 5 CM co m - CO o o o • c CM o CM CO o 03 o Q o o CM CM § 2 9 eo i n o o CM d co o o d o r - CM 9 9 CO 0 3 CM o o r r CM f - a CM CM •M- co. 03 m CO r - CM co CO 0 3 h - CO CM t n eo 3 CM CM 8 CO 8 0 03 d o *- o o d d d o d ■ft •ft -ft •ft •ft ■H -H -ft f t ■H ■H +t •ft + t •H ■ft f t -H -H -H •ft -ft f t •ft f t -ft •ft •ft 0 9 6 N. 03 0 0 6 CM O CM O co Tj- 0O r - o m 9 5 5 9 3 2 2 4 in CM CO 1 1 4 9 1 2 2 CO 4 8 6 1 . 0 4 0 . 4 4 0 . 7 3 6 . 9 0 c o 0 . 2 9 2 . 5 9 7 0 . 2 6 660 CM 000 r- CO in o 5 eo CO CD © 78 47 46 CO Si 159 268 871 s 1.70 8750 0.37 3.15 2.00 6.95 Max 0.33 o 17.38 74.56 000 000 000 OS'O o O’ o o in co Tf CO 03 CO CO r- o CM in CM CM 5 73 R CM 652 Min 8210 0.46 0.44 6.45 2.26 14.16 70.67 r~ r-» h- 03 O co 03 tn o 03 03 CM CO 5 CO CO CM CM co CM CM o o o o CO 87 ■o S o © CO co e n Si 43 172 CO d o CM o o o o o o o •ft •ft f t •ft ■H ■ft -ft -tt ■h •h •ft -H ■h •ft ft •H •ft •ft ■H •ft •ft f t ft ft f t f t ft ft 8S0 990 co , 00 0 o eo CO V co co in CO co co 03 CO f 90 35 CO II O CM 28 67 CO r“ 775 1.40 0.20 1.08 c in 0.14 2.61 6.71 CD 72.17 Mean 680 X CO O co in co CO 73 28 85 65 40 236 130 204 590 1010 1.12 1.16 2960 1.50 0.96 7.59 2.61 0.33 1450 0.83 5 18.49 73.40 080 000 000 CM 03 tt- |S - o © CM 74 CD co 48 30 27 in 144 227 180 368 Min 1.70 0.13 0.38 *5 0.25 o 5.88 15.48 68.85 > CO CO 03 t»- r- CM CO h - CO % C3 CM CO f - co co CM co £ co in 46 CO SB 5 in 45 co 03 CO 577 T3 32 eo 260 o d o o o d o o o d CO K‘ CO £ •ft -H f t f t -H f t + t +t f t + t -ft ■H •ft f t -ft -ft -w •H +t + t ■ft f t f t ■H f t f t f t f t 090 CM in 880 o CO CM o Si CM o CO s 25 45 158 M* CD - s 111 is.' S 457 1.05 - 03 0.51 0.64 6.53 0.14 2.15 0.47 C Mean co 1475 X 680 m O CD CO f»- o 62 24 276 317 179 631 297 3090 ? 4040 1.75 1470 2.60 0.75 7.67 0.33 0.83 2.37 5 0.71 17.75 69.34 000 o CO o 03 m CD CO o 03 48 03 CM 178 405 568 - s 2200 Min 3040 1.89 1.00 0.49 0.17 0.57 0.75 6.17 0.19 15.41 68.40 co CO CO co co CO M 1 f - CM 09 CM 03 CM CD CM co O 03 CO CO co o co £ -o o CM Tj- r - o 03 CD 03 1 55 o o o d d d o o o o CO f t ■ft f t •ft +t ■ft •ft ■ft f t f t ■tt •ft ■H •ft •ft •ft •ft -H •H f t ■ft + t f t f t f t f t f t f t 6689 CO £90 O CO CO CO s eo is. CO CO o 1 o 182 35 in 5 133 503 3 CM cd 1.48 n -4 1.13 0.49 0.71 2.08 6.76 0.42 o 5 Mean O O O O o’ o O O O o o O o o CO O) o’ O* CO* d jS1 c j PbO SrO S* o NiO tS* ZnO CO CoO CuO z S 5* c7) CO CaO P MnO ll? £ U 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD When this table is compared with the previous one it turns out that similar groups (with comparable concentrations for the different elements present) are found in these set of glasses. The division into subgroups is again mainly based on elements such as antimony, manganese and also copper and lead. A vague correlation between compositional group and age can be seen, the first three groups only contain glasses from the 1st until 3rd century while all the objects from later centuries fall in one of the three last categories. Glass from the 5th and 6th century tends to have a higher concentration in manganese and iron as well as some trace elements but this will be further discussed in section 9 (Glass objects from Maastricht). 4.3 Glass objects from Cologne 4.3.1 Decolourising agents As mentioned in the previous section, both manganese and antimony were used as decolourant in Roman glasses. Although it is stated [22] that both elements could be used together, an unclear negative relation was found between the two elements in these glasses from Cologne (Figure 4) indicating that preference was given to the use of only one decolourising component at a time. A closer inspection of the results demonstrates that apparently a substitution from the use of antimony to manganese took place during the Roman period, probably around the end of the 3rd century. In Figure 5 a few results for each century (except for the 5th century because only one glass is present) are clearly showing this evolution. Velde [22] mentions that it is generally accepted that the use of antimony was dominant in the Hellenistic and early Roman period while a greater predominance of manganese is found in later glass. It is relevant to investigate whether the first decolourant, manganese, was deliberately added or whether is was introduced as an impurity in raw materials used. There is again a tendency towards positive correlation between manganese and iron. Addition of manganese could introduce elements such as magnesium, aluminium, vanadium, barium, nickel and copper present as impurities in the manganese-containing ingredient [9]. A tendency towards a positive correlation was found for the first 4 elements, especially in the glasses from later centuries, again an indication that in the later period manganese was indeed deliberately added as a decolourising element. In these glasses, the other decolourant, antimony, was probably introduced by a mixture of raw materials. On the one hand some glasses show a positive correlation between antimony and sulphur suggesting the use of antimony sulphide (stibina) but on the other hand also a tendency towards positive correlation between antimony and lead was found for some other glasses which points to the addition of an antimony-lead mineral or alloy as source for antimony. The glasses containing the highest concentration of antimony (group 3) have a lower magnesium and manganese content similar to the previously described glasses but 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD their concentration of Al20 3, CaO and Fe20 3 is comparable to that in the other groups of this collection. 9000 8000 □ A 7000 O unknown 6000 p 1st century 4 2nd century | 5000 X 3rd century X 4th century ~ 4000 ' * X X □ O 5th century + 6th century * A 3000 X x X 2000 ' xc 1000 O XX -e-x— Xc?St x A 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 M nO (w%) Figure 4. Binary plot showing the possible negative relation between MnO and Sb205 in the second series of glasses from Cologne. Glasses from different centuries are indicated with a different symbol. 90 0 0 1.40 8 0 0 0 1.20 _ 7 0 0 0 ■ £ Q. <> 1.00 S? a 6ooo in oN « 5 0 0 0 0.80 = |S b 2 Q 5 I o c 0 4 0 0 0 - - 0.60 gs 1c o0) 3 0 0 0 - c 0.40 o oo 2000 - - - 0.20 1000 - 4 0.00 0 1 2 3 4 5 6 C entury Figure 5. Plot showing the substitution in time from Sb20 5 to MnO as decolourant in Roman glasses from Cologne over a time period between the 1st to 6th century AD. 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 4.3.2 Correlations between major elements No elements (Al, Ca, Fe, Ti, Cr, Zr, Y and Ba) were found that are positively correlated with S i0 2; this implies that a very pure and clean raw material was used, probably the Frechen sand to which raw materials for calcium and alumina were added. Contrary to the previous samples, no positive correlation was found between MgO and CaO, indicating that dolomite (C aM g(C03)2) was not used as source for calcium. CaO is clearly positively correlated with Al20 3 which might again point to the addition of a calcic feldspar to introduce alumina to the glass melt. 4.3.3 Correlations between trace elements : colouring agents One of the transition metals which is widely known to produce colour in glass is copper [3, 19, 21, 23, 35, 36]. It either goes into solution into the silicate network and forms part of it in the way other multivalent cations do or it does not dissolve but it is dispersed in the glass as minute particles (i.e., as a colloid). Copper oxide produces green or blue tints under oxidising conditions depending on the structural environment (i.e., the glass modifiers which are present) of the ion [35, 36]. With lead oxide, the copper oxide will colour with variations of green; with sodium or potassium oxide the colour will rather become turquoise blue. Using copper in a reducing furnace atmosphere results in a brilliantly opaque red glass. Dispersed copper colours are known as copper ruby glasses, e.g., haematinum (or haematinon) which is an opaque blood-red glass in which under magnification cuprous oxide crystals become visible against a colourless background [23], Since no opaque glasses are present in this series of objects, copper is only present in its dissolved form in the network and not as colloid. On the one hand, copper oxides may be introduced to the glass melt as any of the common copper ores, although the oxides and carbonates are the most usual. On the other hand the simultaneous occurrence of copper and tin may suggest that slag from bronze founding (i.e., an oxidation product of bronze) is added to the glass. When instead of tin zinc is present together with copper, probably brass instead of bronze was used to produce the green-bluish colour. In order to investigate this, the correlation between the copper oxide content and these compounds was checked and the results are given in Figures 6, 7 and 8. Glasses with an exceptionally high amount of copper or tin are left out in order to better show the correlations. For some glasses a positive correlation was found between CuO and ZnO and for others between CuO and S n 0 2, suggesting that probably both, brass and bronze respectively, were used as source for copper. 115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis o f Roman Glass from the 1st-6th Century AD 100 90 uncorrelated 80 O unknown a 1st century R = 0.72 A 2nd century 50 X 3rd century O X4th century 40 - O 5th century + 6th century °2<. uncorrelated 20 10 o 50100 150 200 250 300 350 400 450 500 CuO (ppm) Figure 6. Binary plot showing the positive correlation between CuO and ZnO for some glasses in the second series from Cologne. 200 • 180 - R = 0.73 160 140 O unknown a 1st century 120 E A 2nd century a . a . 100 R = 0.91 X 3rd century oc x4th century CO 80 O 5th century + 6th century 60 ,.+ X 40 20 to 0 50 100 150 200 250 300 350 450400 500 CuO (ppm) Figure 7. Binary plot showing the positive correlation between CuO and S n 0 2 for some glasses in the second series from Cologne. 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD Figure 6 demonstrates that the glass objects can be divided into two groups, the ones that show a positive correlation between the CuO and ZnO content (represented by the solid line) and the ones that do not show this correlation (encircled). Based on the correlation coefficient of 0.72, the theory of positive correlation can still be accepted at a 99.9 % significance level. The Cu/Zn ratio of about 3/1 which is present in this first group, strongly points to the use of brass since this is a frequently occurring ratio for these two elements in this material [37, 38]. The second group can be found in Figure 7 showing a positive correlation between the CuO and SnOa content (represented by the dotted line with a correlation coefficient of 0.91) with a Cu/Sn ratio of about 4/1 which is also normal for bronze [37, 38]. The other group is represented by the solid line with a correlation coefficient of 0.73 in Figure 7. This tendency is mainly followed by the glass objects from the 2nd century, except for one which can be considered to be an outlier. The line shows a Cu/Sn ratio which is much too low for being caused by bronze, meaning that tin must have entered the glass in another way. The two positive correlations can be accepted at a 99.9 % level of significance. Next to these, there are also some glasses with a relatively high concentration of tin but almost no copper at all. 1000 R = 0.87 800 • O unknown ] □ 1st century ; A 2nd century I 600 a x3rd century o X4th century ja a. 05th century 400 + 6th century 1 CP 200 A A. 0 50 100 150200 2 5 0 300 350 4 0 0 450 CuO (ppm) Figure 8. Binary plot showing the positive correlation between CuO and PbO for the second series of glasses from Cologne. Figure 8 shows that there is also a clearly positive correlation between CuO and PbO (with a correlation coefficient of 0.87, meaning that it can again be accepted at a 99.9 % level of significance) which can be explained by the fact that lead is used as a softener in brass and bronze, although the Cu/Pb ratio is too low for most of the 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD glasses. Arsenic, another element that can be present in these alloys, especially in the early bronzes, also shows a slightly positive correlation with CuO [37, 38]. Positive correlations between other trace elements are mostly due to the natural presence of these elements in some raw materials used. 4.4 Glass objects from Rouen and Trier The glasses from Rouen and Trier were considered separately from the ones from Cologne. The clear negative correlation between Na20 and CaO, which is also present in the other glasses was already mentioned by Velde [2]. He states that, since these are the only fusible elements present with a high concentration, there was probably a replacement of one by the other to fuse the material into a liquid. Besides this negative correlation with a coefficient of - 0.75, Figure 9 shows that, considering these components, the frontinian bottles from Trier (left upper corner in Figure 9) can almost be separated from the glasses from Rouen except for three objects but these also include two Frontinian bottles next to one box. 7.50 R = - 0.75 7.00 6.50 i O On 6.00 5.50 5.00 15.00 15.50 16.0016.50 17.00 17.50 18.00 18.50 19.00 Na20 (w%) Figure 9. Binary plot showing the negative correlation between N a 20 and CaO in the glasses from Rouen and Trier. The correlations between other elements such as the negative correlation between Si and Ca, Si and Mg and Si and Al and the partly positive correlations between Cu and Zn and between Cu and Sn, were comparable with the ones found for the glasses from Cologne and will therefore not be discussed further. 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 5. GLASSES FROM TONGEREN (BELGIUM) 5.1 Description of the glass collection and its location Tongeren was the only Roman city within what is now Belgium. During the entire Roman period this settlement was given different names such as Atuatuca, Atuatuca Tungrorum, Civitas Tungrorum, Tungri Oppidum or simply Tungri, pointing to the various people that lived there and who were probably all descendants of the Teutons [39]. The history of the Roman city knew three important phases which all have their own characteristic features : (1) the development starting from the origin around 10 BC until the fire due to the revolt of the Batavians in 69-70 AD, (2) the evolution from the last quarter of the 1st century until the second half of the 3rd century and (3) the situation during the late-Roman period until the beginning of the 5th century when the population strongly diminishes and the Roman troops pull out [40]. Tongeren must have been an important centre in Gaul, it was densely populated and wealthy until the end of the 4th century and it was one of the main road-junctions of roads coming from e.g. Bavai, Tienen, Nijmegen and Cologne [41, 42]. Romans buried their dead outside the city-walls, along the access roads. This was also the case in Tongeren; the cemeteries were located on the north-eastern and south-western side of the city in the neighbourhood of the axial high road connecting Bavai and Cologne [43]. The graves generally contain urns with burnt ashes, some earthen bowls, dishes, jars and also coins. In richer graves glasses, bronze objects and even jewellery were found [41]. The graves were used during the entire Roman period so the glass finds cover a range from the beginning of the Roman rule in these regions until the 5th century. It is unknown how much of the fine glass found in rich Belgian graves was made locally or might have been imported from e.g. the Rhineland. The fact that so much glass was found in Tongeren as well as the role that this city played as both economic and cultural centre in North-Gaul, implies that the existence of glass-furnaces in the capital of the civitas must not be rejected a priori. Sand to produce glass could be found in the Kempen and the city was well situated for export. Nevertheless, Tongeren seems to have mainly been a residential city since few traces of crafts have been found [44]. The Institute for the Archaeological Patrimonium in Flanders, Belgium (IAP) provided glasses from Tongeren that were all found in graves outside the city. Unfortunately no detailed information about age or stylistic characteristics was available but the objects probably cover the 1st to 3rd century. Next to naturally coloured and colourless glasses, also some fragments with bright colours (purple, blue and amber) were available. 119 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 0 9 9 'O E O t n o o z o o m c n o © o o c n 1 0 3 9 3 8 a 2 2 - 3 a 1 4 5 8 0 5 r*-' o ' 5 6 2 1 . 3 5 0 . 1 3 6 . 3 3 M a x 0 . 0 7 0 . 1 7 i 4 . 7 4 tx . 0 0 9 t n 0 0 0 c o CO c o o O t n CD - o o CM Tj- 7 9 2 4 2 2 - - 3 5 0 t o 0 5 5 1 0 M in 1 . 1 8 1 . 0 3 0 . 1 2 0 . 3 7 0 . 1 0 4 . 3 1 0 . 1 0 3 . 8 6 c o 0 0 9 o o z o s o o o t - o c o © o c n t o o o CM t n 3 7 4 7 2 6 - CM c n 1 . 4 3 0 . 1 3 0 . 2 0 0 . 1 3 0 . 0 2 0 . 1 2 0 . 4 5 0 . 6 3 S t d e v 44 44 44 44 -H -H 44 -H 44 44 44 -H 44 44 -H + i 41 41 41 41 41 41 41 41 41 41 44 44 C\| O CM II CN © CO c n o «n o CO 05 c o i n o o © CM c S - 3 9 2 0 1 1 2 5 7 8 r-.' 5 3 6 1 . 1 7 1 . 2 7 0 . 4 6 U5* 0 . 1 3 0 . 2 1 0 . 1 1 0 . 0 1 o ' 6 9 . 6 8 ^ e a n 0 0 9 c o 0 0 £ p - O X p x (0 c p CO t n c n to oo t n CM CM c n 1 3 2 8 7 0 2 8 8 5 5 5 5 2 1 r x 9 6 4 1 . 2 9 8 . 9 3 0 . 5 3 0 . 7 7 0 . 1 5 0 . 2 0 2 2 . 5 5 o 0 9 6 o CO 0 0 0 o 0 0 0 o QJ CO o © 05 o c o o © o o O CM eo o 1 2 C 6 4 7 4 9 - M in t n 1 . 0 2 0 . 3 6 6 . 5 0 0 . 0 7 1 . 8 7 0 . 1 3 o • 2 s ' 0 0 9 0 0 9 0 0 £ o o z 0 0 6 s 0 8 0 .to cn i n t n c n CM CM to t o 3 3 2 0 - 8 2 -- - 0 . 7 8 0 . 2 4 0 . 0 3 0 . 5 4 0 . 0 2 5 0 . 2 1 S t d e v 44 44 44 44 ■H 44 + i -H 44 -H 44 +1 44 44 44 44 41 41 41 41 41 41 41 41 41 41 44 41 CO 0 1 9 0 9 9 N. 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CO n - o o - 2 9 2 4 - £ © 7 3 4 M in Tf" 1 . 4 3 8 . 7 9 1 1 1 0 0 . 6 7 2 . 2 7 CO 0 . 0 3 1 o o z 0 0 8 0 0 9 0 0 0 o CO 0 0 9 0 0 9 o t n © o o e g c n CM c o e g t n c n © CM - © 4 0 - u 3 4 5 0 . 3 4 0 . 0 5 0 . 1 1 0 . 0 2 0 . 1 7 o S t d e v 44 +1 +1 44 41 44 +1 44 44 44 41 44 41 44 44 41 41 41 41 41 41 41 41 41 41 41 44 44 0 0 9 e g 0 0 9 CM h - r - 0 0 r j- o c n 0 5 to to O 7 5 CM - It 3 6 83 9 7 8 1.67 1 . 0 3 0 . 5 4 1 1 8 0 0 . 7 3 8 . 8 3 0 . 0 1 d 2 . 3 9 c 1 5 .0 1 6 9 . 7 1 M e a n 0 9 6 O O S 'O E O Q> o r o o ’tf © © o CM © t n © to CO CM 7 0 8 5 6 4 4 6 3 3 9 CD 1 . 0 4 2.31 & 0 . 7 6 8 . 6 0 0 . 1 3 1 4 8 0 2 . 2 3 Q. 1 7 . 8 9 ; 6; 5 . 9 5 0 6 8 N. 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CM fx . a h - o CO CM e g CM 9 3 5 7 7 7 4 9 140 ! 9 1 7 1 5 2 5 5 o * 5 8 3 1 0 3 0 1 . 2 6 0 . 6 7 8 . 2 3 0 . 2 3 M a x 0 . 1 3 2 . 3 8 0 . 3 8 o o 1 5 3 0 0 35 1 8 . 7 5 e - .O) 0 9 0 0 0 8 0 0 0 0 0 0 c o CO CM t n to % CO c o O O i n c n fx . h - O o 3 5 s 3 6 7 o> 1 7 . 0 1 c o ! > O o CD o i n 05 CO c n t n 05 CO CD CM «*■ CM a o o e g c o o o O h - O CM t n 0 5 CM h - g - CD 1 9 3 ■ o 3 0 a - s 1 0 0 o 1 6 6 3 3 1 o to o © d o ' d o o d o o o o 41 41 41 41 41 44 44 44 c 44 41 -H 44 41 44 41 44 41 44 44 44 44 44 +1 41 41 41 41 41 i n CO O) © 05 t n Cg c n c o e g O) i n 05 CO c n it 0 ) Q T v © N. fxl o e g c n CO c Z 0 5 ? TT © e g CO o ' o W o n : d d d t n i n © 5 ? c n CO £ CM CO © TT to o & O O O d o d O O d O <5* O O CO <0* 0 5 o U in* B r O © j£* CD* N i O S- S r O P b O Z n O C u O C o O m Z 2 . < CO CL CO C a O }— M n O li. < cc £ Z CO to m Table 3. Average concentration, standard deviation and minimum and maximum concentration for the subdivisions in the glass objects from Tongeren. Data above the horizontal line are given in w% and below it in ppm. 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 5.2 Classification of the data Again a division into different subgroups was performed manually and the results for the seven classes are given in Table 3. In the first group, 5 glasses, all light blue or colourless, are found; the second consists of 3 blue glasses, including a bright blue one; only one object, of purple colour, forms the third class; 3 colourless glasses make up class four; in the fifth subgroup 5 glasses which are all light blue can be found; the 7 samples in group six show different bluish and greenish variations of the natural aqua colour and finally the last class is formed by 2 amber (brown) coloured glasses. The elemental concentrations which are characteristic for each group are highlighted. The concentration of magnesium turns out to be very low in all glasses but, as mentioned in Chapter 2, Section 2.5, this is most probably underestimated and can be considered to be below 0.5 w%. The differences between the different subgroups are again mainly related to decolouring or colouring agents such as Mn, Sb, Fe, Cu and Co and will be discussed in the following sections. 5.3 Decolourising agents in the glass 14000 12000 R = 0.97 10000 O column 1 □ column 2 E 8000 A column 3 Q. a. X column 4 X column 5 ■Q 6000 tn O column 6 + column 7 4000 2000 0 200 400 600 800 1000 PbO (ppm) Figure 10. Binary plot showing the positive correlation between Sb20 5 and PbO suggesting the use of a Sb-Pb mineral or alloy as source for antimony. “Column” in the legend refers to the columns in Table 2 containing the different subgroups. 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD In this set of glasses both antimony and manganese seem to have been used as decolouriser. The colourless glasses in the first group in Table 3 all have a high Sb20 5 content (and a low concentration of Al20 3, CaO and MnO) which strongly suggests that this component is deliberately added to the glass melt while the colourless glasses in the fourth group almost do not contain any Sb20 5 but clearly show a higher amount of MnO. Antimony was probably added as a Sb-Pb mineral or alloy since a strong positive correlation (R = 0.97) was found between these two elements (Figure 10). To find out whether manganese was deliberately added to the glass melt, again the correlation with elements that can be introduced together with the manganese were checked and this time a positive correlation was found with cobalt, nickel, copper, barium and magnesium, especially for the objects containing a rather high amount of manganese. This implies that manganese did not just enter the glasses as an impurity. The only object belonging to the third group in Table 3 has a purple colour due to the high manganese concentration. Manganese was not only used as decolouriser in the Roman period but also as a colouring agent. Indeed, manganese oxide, introduced to the glass batch as either the dioxide or carbonate, will produce purple-brown or shades of violet depending upon which other modifiers are present [23]. 5.4 Correlations between major elements As in the previously described glasses a very clean and pure or purified sand must have been used since no positive correlation could be found between the S i0 2 content in the glasses and elements such as Zr, Cu, Ni and Fe which are naturally present as traces in sand. The results of the correlation diagrams between the major elements were comparable to the ones described earlier, the most important being : a negative correlation between Si and Mg, Si and Al, Si and Al and a positive correlation between Al and Ca, Na and Cl and Fe and Mn. 5.5 Correlations between trace elements : colouring agents Copper was most probably used as a colourant in these glasses since many objects show variations of a blue colour, especially the ones in the second group of Table 3, of which the average concentration of copper is the highest. It is not very clear which material was used as the source for copper since the correlation with neither tin nor zinc is completely conclusive although there is a tendency towards positive correlation between CuO and ZnO suggesting that brass could have been added to the glass melt. Next to copper and iron, another element was used to produce a blue colour in glasses during the Roman period, namely cobalt. Tetra-co-ordinated Co2+ (i.e., as a network former) produces a strong blue absorption even in trace amounts (above 20 122 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD ppm) [35, 36, 45). Cobalt was used to make blue or violet glass but also mixtures of cobalt, copper and manganese were commonly used [23]. Using cobalt as a colouring agent will introduce other elements into the glass batch since cobalt minerals are often associated with minerals and compounds that contain copper iron, nickel, arsenic bismuth, aluminium, zinc, lead and manganese. As a result, some of these elements will find their way into the final glass melt [3, 6]. Cobalt occurs in few places in the world so it must have been traded in some form over long distances. Cobalt could be added to the glass melt either as a Co-mineral or as a highly concentrated glass cullet. It is believed that Persia was the source of most cobalt used in ancient times [9, 46]. A search for correlations between cobalt and all of the above mentioned elements was carried out. With some elements (As, Ni, Pb, Cu and Bi) a clear tendency towards positive correlation was found and with others (Mn, Fe and Zn) a very clear positive correlation was present of which one example is shown in Figure 11. The positive correlation between the CoO and ZnO concentration can be accepted at a 99.9 % significance level, based on a correlation coefficient of 0.86. The bright blue glass present in this series has a high content of cobalt, copper and manganese indicating that probably a mixture of these elements was used to produce the blue colour. R = 0.86 O column 1 □ column 2 A column 3 ;X column 4 IX column 5 I •c o lu m n 6 j + column 7 0 20 40 60 80 100 120 C o O (ppm ) Figure 11. Binary plot showing the positive correlation between CoO and ZnO indicating that zinc was present as a trace constituent in the cobalt containing ingredient. “Column" in the legend refers to the columns in Table 2. The last group in Table 3 consisting of only 2 glasses, shows yet another colour, namely brown (amber). What is remarkable in the composition is the exceptionally 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD high amount of Al20 3 compared to the other glasses. It is known that alumina has the tendency to enhance the colouring of glass by Fe-sesquioxides [47]; it was also used for this purpose to produce dark coloured wine bottles in the 19th century [48]. 6. GLASS FROM OUDENBURG (BELGIUM) 6.1 Description of the glass collection and its location In Oudenburg remains were found belonging to a quite large settlement, probably founded in the second half of the 1st century, which flourished during the 2nd century and was abandoned by its inhabitants in the middle of the 3rd century due to floods and regular invasions by Franks and Saxons. After this period, the new inhabitation showed a completely different and purely military character. At the highest point of Oudenburg, which was situated at the coast at that time, a Roman fortress or castellum was constructed at the end of the 3rd century. It is presumed to have been an important part of the defence system of the coast installed by the Romans at the north-western edge of the Empire. The fortress was occupied until the beginning of the 5th century. The sudden appearance and interruption of the occupation points to the arrival and departure of a body of soldiers [49-52]. Two large graveyards were discovered in Oudenburg, dated to the end of the 3rd, beginning of the 4th century. Here an important number of glass objects were found, the forms correspond to the ones which are usually found in late-Roman graves. Small hemispherical bowls (Isings type 96) and bulbous flasks with a long neck and rounded rim (Isings type 101) are the most common objects, next to these beakers on conical, pushed-in foots (Isings 109b); some frontinian bottles and different kinds of plates were also found. Most of the objects are greenish in colour, have a rather thin side and are very often defaced by air-bubbles [50]. 14 objects, both aqua coloured and colourless, were selected, sampled and prepared for quantitative analysis. 6.2 Classification of the data The composition of this series of glasses was determined and again, based upon differences in their composition, manually subdivided in different classes resulting in Table 4. The first, third and fifth group are all formed by just one object which shows an unusual concentration for at least one element; the second group consists of 6 decoloured glasses and in the fourth one 5 glasses with variations of bluish and greenish aqua colour are found. The remarkable differences in composition, mainly between naturally coloured and colourless glasses, in this rather small collection of objects will now be further discussed. 124 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD S 0 0 0 5 o r - to -rr CO CD 04 N O co © co co CO o T- © cn CO O CO tO CN CO o IO CO N • t - ^ r CO 0 5 0 0 t - lO © co o co ^2 co 2 “ o i §- y CO 2 CO S 0 5 O O T- 0 5 “ O' S ▼“ S2 oo CN © cvi CO O O T~ o ^ S! Is- ^ ® o 00 0 5 CN o CO CD 05 ID CO CO CD co o 0 5 I D ID © o CO N. N; N* 0 5 co co O' CN CO ^5 LO 05 N OO 0 5 0 5 © co' CO r- to CN ^ CD r*- tj- 05 CN I D o ’ CN CD o o © O 1^ o o O co CD CO o CO CO CO CO Is- O CO nj- ID p N- o o O CN o o o OXO^M-NCNO^CNS^ggS O o o o o o o o O o o ' © +1 +1 +! +1 +1 +1 +1 +1 +1 +i +1 +1 +1 -H +1 +1 -H *H +1 +1 +1 +1 -H -H -H o T— t n o o o CO CO o h - ID ID CO o CN p c o CD p 00 W CO w CM r- g s § 9 05 o 5 0 g CO CD o CN CD © o o o ’r_ o o o © CO CN CO O Is— CN CN CD . o cq © CO CN Is- CO CO N - CD CN CO CO Is- N» o LO CO 05 co © CN CD o o o o © O o r - CN CO r - CN 05 CO CN CD O to CN o> p © ID CO r^. CO ID ID S N in O M- o CO N1 CO S r* O f CN h-t- -O'-t- CNh- o T- © O T_ o cd o o ' cd o CO © o ID o CO CO o s N; © CN O oo © © CO Is- cd T“ CD © T~ CD © ©' o ID © o* © N CO o CO CO - Is- CO CN o> CN 05 CN ID t o ( n t - C D O ) o p © © © CO CO 00 CN ® N 1 < D O ) o o ’r~ © O o o © © o © CO +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 -H -H +1 +1 «H -H -H +1 o © CN CO © CO Is- CN CO CN to ID O CN O o CO o CO CN ID r - N* CO ID N ’ CO CM ID 00 IDm S 5> 05 CO 05 CN CN N - LO t - h- 00 ^N CN CN t— © CO o © T-‘ O CD © O © © ^ C\l © £ O CO O) T- O) CN to CO CN O LO ^ O ^ N CO ^ T— O CO CN Is- 0 5 * “ CO CN ^ c o o n O O CN CN •N* CN 0 5 N! O *r-' g o o O Is- O O O CO ^ o 0 ood.Ooondoooo Q . Q 0" O#— o' (M <15 O Z O N | m S W>NN wgCQQ. Table 4. Average concentration, standard deviation and minimum and maximum concentration for the different subdivisions in the series of glasses from Oudenburg. Data above the horizontal line are given in w% and below it in ppm. 125 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 6.3 Decolourising agents in the glass Both decolourising elements, manganese and antimony, are present in all the glasses from Oudenburg, antimony at rather high concentration levels. Because no conclusive positive correlations were found between MnO and any of the elements which are present in the raw materials used and which were mentioned before, it is not completely clear whether manganese entered the glass as an impurity or whether is was deliberately added, except for the glass in the last group of Table 4 (MnO : 1.32 w%) in which it was definitely used as a decolouriser. Contrary to most of the previous glasses, the source for antimony was not a Sb-Pb mineral or alloy but more likely stibina, the antimony sulphide Sb2S3 since positive correlation with a correlation coefficient of 0.80 (which means that it is accepted at a 99.9 % level of significance) between Sb20 5 and S 0 3 exists as is shown in Figure 12. 0.60 0.50 R = 0 .8 0 0.40 « blue-green i •S-« 0.30 o □colourless co 0.20 ■ 0.10 0.00 0 2000 40 00 60 0 0 10000 12000 1 4 000 16000 180008000 Sb20 5 (ppm) Figure 12. Binary plot showing the positive correlation between Sb20 5 and S 0 3 indicating the use of stibina as source for antimony. Although the glass in the third group of Table 4 contains a high amount of antimony, it is not completely colourless but still shows a blue-green glimmer just as the glass in the last group which is also not completely decolourised by manganese. This proves that both elements only have an effect on iron (by oxidising it to the ferric state and neutralising the yellow colour) and not on the other colouring oxides such as copper [23], which is present in these glasses with a significantly higher concentration than in the completely colourless glasses of the second group. 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 6.4 Correlations between major elements These glasses, like all the others considered so far, have a composition that obviously belongs to the low magnesium/low potassium soda-lime-silica type. This means that the concentration of major elements is rather comparable for all glasses and does not provide any surprises, except for the object of the first class in Table 4. This particular glass has a significantly higher content of silica together with a lower content of soda while none of the other elements shows an unexpected concentration. The reason for this is not clear and no glasses with comparable compositions were found in the other collections. 1.40 □' 1.30 R = 0 .8 7 1.20 • | « blue-green { S t I □ colourless ; ° 1.00 - 0.90 0.80 0.70 16.00 16.50 17.00 17.50 18.00 18.50 19.00 19.50 20.00 20.50 21.00 Na2Q (w%) Figure 13. Binary plot showing the clear positive correlation between Na20 and Cl indicating the use of natron as source for soda. Correlations between major and minor elements were as in the previously described glasses. Again a very clean or purified sand must have been used since no positive correlation was found with any of the trace elements which are present in this material and can be simultaneously introduced into the glass. Calcium does not seem to have entered the glass as an impurity of the sand but was probably added separately because no positive correlation exists between CaO and S i0 2. A tendency towards positive correlation between CaO and MgO was found, pointing to a rock material, but the magnesium content is very low and hence the results are not thrustworthy. The source for Al20 3, negatively correlated with S i0 2 but positively with CaO, once more appears to be a calcic feldspar. In this series of glasses the positive correlation between NazO and Cl, which was also found in the previous glasses, is very clear (Figure 13), showing a correlation coefficient of 0.87. This means that, taking also the low potassium and magnesium concentration into 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD account, there can be no doubt about the fact that natron was used as source for soda since this salt deposit contains NaCI [23], 6.5 Correlations between trace elements : colouring agents This set of glasses could already be visually split up into two groups, the coloured (to various degrees) and the completely colourless glasses. This was not reflected in the major composition and it did not even show up when decolourants were considered but it became visible when plots were made for some trace elements. To find out which copper-containing ingredient was added to the glass melt, correlations were checked and they lead to two conclusions. First of all, in all plots (CuO against S n 0 2, ZnO, PbO and As20 3), the data points split up into two groups, belonging to the coloured glasses on the one side and the colourless on the other side. Secondly, the plots show a different behaviour for these two groups. Considering Figure 14, it becomes clear that bronze was added to the coloured glasses since a positive correlation was found between the copper and tin content and the SnOz/CuO ratio of about 2/5 in these glasses is still reasonable for bronze. Only five glasses were included in the calculation of the correlation coefficient, the others were considered to be outliers. In the colourless ones, although a positive correlation with a coefficient of 0.88 was found between CuO and ZnO (Figure 15), the addition of brass to the glass melt does not seem very likely since a ZnO/CuO ratio of 7/5 is much too high for this material. For both groups a positive correlation was found with PbO and As20 3. 100 90 = 0 .9 6 80 70 _ 60 ♦ ♦ £ CL * blue-green s 50 O □ colourless " 40 30 20 CP 10 0 20 40 60 80100 120 140 160 180 200 CuO (ppm) Figure 14. Binary plot showing positive correlation between CuO and Sn02 for the coloured glasses and no correlation for the colourless glasses. 128 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 7 0 R = 0.88 6 0 5 0 E 4 0 a . • blue-green I □ colourless i 3 0 • 20 10 0 20 40 60 80 100 120 140 160 180 200 CuO (ppm) Figure 15. Binary plot showing positive correlation between CuO and ZnO for the colourless glasses and no correlation for the coloured glasses. 7. GLASSES FROM GROBBENDONK (BELGIUM) 7.1 Description of the glasses and their location Grobbendonk, founded in the 1st century, was situated at the territory of the Civitas Tungrorum, in the neighbourhood of the high road which connected Tongeren with Dordrecht (The Netherlands). Vici or settlements often came into existence at halting-places along the high roads at a distance of 45 km from each other and they supplied passing merchants and their animals with shelter and food. In Grobbendonk, traces of the 3rd century are rather limited, the youngest remains point to the year 250 as the probable end of the Roman vicus [53, 54]. In several graves at Grobbendonk which are dated in the 2nd century, next to South- Gallic terra-sigilata earthenware, fragments of glass objects were found such as ribbowls (Isings type 3, 1st century [55]) which show the typical blue-green colours and also brown and dark blue millefiori. The quite rich finds which are rather unexpected for the north of Belgium suggest a certain wealth in the settlement [56]. A series of colourless, naturally coloured and brightly coloured (blue, green, brown and black) glasses, which were found in Grobbendonk, were provided by the Institute for the Archaeological Patrimonium in Flanders, Belgium (IAP). 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD o 05 05 CO CM o CO CO o CO o O O LO CM 05 p c co n o 5 ^ | 5 ^ “ | ° § § | S 5 d cvi r - d ■*“ n ! d o **“ o h- r - LO o r - CM CO CO O S ^ co o 00 p p 05 CO CM CO 05 oo P r- S N CM 05 CM ^ m CD d CM* CO d d cd d d d « ° fe o LO LO 05 M- r - CO o CM o O o o o co p p CO 0 5 LO o in co eg o r*- CO 77 CO CM 718 T— CtJ co d d d d d d d d d d +i +i +i +i +i +i + i +i *H +i + i + 1 +1 +1 +1 -H +i -H -H -H 00 o GO 05 CO CO CO © ift co o p o p O) CM N. 05 CM t o o CO CO CM LO - 175 628 284 05 752 d CM CO d cd d d d *- 00« § 2 9 7 0 CO o 05 o CO CM o CO co o CO o p p LO CM CO oo o ' CO cd 2 o> lO CO d cm’ h- d T— •*“ r-* d d d ° oo o o co CO CM CO CO CO CO r*- CO o LO o O 00 00 T“ CO co ^ 5 CD LO d £2 LO CM d CM Is- d T- d cd d d d LO tj - 05 CO o r - co h- CO CO 0 0 CM o o CM o o o CO o •*“ o CO LO to CM CO CM - 50 CO 5 0 197 s 907 co d d d d d d d d d d d 210 +1 *H +1 +1 -H +i +i +i +i +i +i +1 +1 +1 +1 -H -H •H +1 +1 +1 +1 -H +1 -H +1 CO CO o LO CD CO LO LO *r~ r - CO CO 0 5 p o o p 05 CM r - p-. 00 CO 05 OO CM r - 27 CO 45 139 626 468 cd d 446 2 3 2 0 d CM Is- d d d d d 1150 oScoioinoiococo LO T- ° r-^CMfOCOCOfOO)r- coo}Pa5cocOTt*§co*,coto a> W - O ...... MN^M-COT-W COCMgl^10 CM CM r- © *«- o r*- o o CO o co o oo cm o CO LO 05 CO o o p o co r - t- © r - o 05 Tf r - (DS g § cd LO CO CO o d cm* co d d CD O O O ® g 8 ? r - CM 00 05 CM ***■ 00 CO P— CO ^ -K CO CO CO o 5 o O O CM o CO o r - CO *<■ LO o T - S T - X t 0 CO Js» T— °° 00 CO T- CO d d 'r - d d o o d d d +i •H +i +1 +i +i +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 •H +1 +1 +1 +1 «H +1 +1 +1 +1 CO 0 9 Z CO T— CO oo LO * - p o CM CO r — CO CO o o ^ 20 5 42 05 175 CO 00 1.22 7.09 0.87 1 2 1 0 d 2.08 CO 0.18 0.83 d o CO CO o M* CM CO CM m M O o cq CO CO LO CM 00 ^-oSNM-CDoSt- s 2 CO o CM 0CO°^T-r-CMCDCM CM CO d CM r - d 05* d 0 5 o O o CO s o 00 CO CO CM o p o 0 5 CO o o O 0 0 ■M- CO O CM 0 0 CO T“ 79 410 CO 902 d -c - CO d o d r-’ d d o CM Tfr CO CO CO 05 CM h - o o o o LO o CM 0 5 0 5 CO CO CM CM 66 26 103 T“ 169 d d V d d d d d d +i +i +1 +i +i -H +i +i -H +1 +1 +1 + 1 -H -H -H + 1 -H +1 -H -H +1 +1 -H o 05 o CM CO CO CM CM O O T» o T“ CM Is- T~- LO CO 0 5 *r~ CM *3* CM 23 35 30 CM CO o 103 595 d CM h- d d co d d 1130 O o O o o o o o o o o := 3=i c co ° 4'.« 2 9< 2 fS* C rS* « -O O 2 o N 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD <0 CO o SOCOSWOCNJCVJMO^ O O CN LO 3 ^O^.CMCNNIOCNCOCD LO CN ^ CM LO O CN O CO IT) .O jSd^jSd^dcdddd N* LO CO LO t— CN ^ N* oO ^i2iocNOincNCoco O^CNOCOO^CNCOr- g^S^-OCOM-SS SE (O CNcn _r; ns r-Q co 03 se(n 3 r T- ^ ^ to CO Q. CNi§C O Is- O CN O c o O 05 CO to CO CN to O 00 o CO o o 05 O LO - ■«-; O N- O O) N- O CO N- 05 CO CN 5* t - t - M K CO CO cn ■*“ s r 1 cvi CO o ^ N N* CN r o O e o TTlOCOO^ON-LOCO I ^ o o . O CO CN 0> CN CN tO h- .s^ O Q to 05 CN to o o> lO o> J: o ^ CO O o CO X— t o CO o o CO ^ T— Tfi CO O CN 05 c o c o *^r N o 0 CN •2 ■ g 0 o ) ^ i n ; r o 0 5 o fe § %! ™ -O d *d d d CO f— 05 .2 •Q o S O t- fe 00 r- CO CO J- S CO § T T o t/5 9 O S S 05 . O CN . ^ CO 05 N* N ^ CO O S g 05 N. § CN 2 to K Nf to CN <0o 5 o cvi J o' o o o d 5 CN C5 «5 CN § CO CN O to 00 OO CN CN hs r> ' . o CNtn o CN CO m CN o> N § o S n s N w § co JG c n n - ^ ? gj M h- d -O o CN Is- o *- o Is- O d 0 5 CN £! o co 0 5 i n c o n - i o o tn co o © o ^ o r^- O O U O 0 5 CN N to ^ 00 >■ CO N* S 10 I! oo oo CO CO CO 05 21 05 r- c £ O CO o o i n d d ^ CN 00 ^ N* in CN CO r* o i O LO CO CN co CO O O o T- O CN CO CN CN CO N* t o CO in o n 0 5 p lJ CN ^ O j w » o N- o T_ Is- O O o 05 CN o «*> t n CO Is- 0 5 CO 05 CO oo o II o t o P o CO Is— t n O O CO CO t o Tj- CO o o LO o o Is- O CN LO c o CN N. d o N" d o d LO O O « o 0^*0 O5O o O o o o o o {g1 0 5 ^ Q O Q N CO Q C OJ O 3 C esi m JQ -i2 Table 6. Average concentration, standard deviation and minimum and maximum concentation for the colourless and intentionally coloured glasses from Grobbendonk. Data above the horizontal line are given in w% and below it in ppm. 131 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 7.2 Classification of the data The composition of the glass samples was determined and the results of the classification are given in Tables 5 and 6 containing the naturally coloured and intentionally coloured ones respectively. The subdivision into four groups in Table 5 is again mainly based on the Sb20 5 content leading to classes with comparable compositions to the ones found for the previous glasses. The yellow glasses which are present in the first group next to the blue-green ones, cannot be separated from them on the basis of the composition which means that they probably show this colour due to the interaction of iron and manganese in a strictly controlled oxidising atmosphere in the glass furnace. The only difference between the third and fourth group (both have a moderate Sb20 5 content) is the concentration of CuO which is significantly higher in the fourth one, although this is not reflected in the colour of the glasses. 7.3 Correlations between major, minor and trace elements The naturally coloured glass samples from Grobbendonk show the normal composition; the usual correlations between major, minor and trace elements are observed such as negative correlation between Si and Na, Ca, Al, K and some trace elements present as impurities in sand, positive correlation between Mn and Fe, Na and Cl and Sb and Pb (already described in detail in the previous sections). More attention will be paid to the intentionally coloured glasses in Table 6. Some of the colours were already present in the other glass series such as brown (first group in Table 6) which is again due to alumina (2.53 and 4.12 w%) enhancing the effect of the iron. A combination of cobalt (1840 and 2360 ppm), copper (0.23 and 3.11 %), iron (1.56 and 1.73 w%) and manganese (0.75 and 0.97 w%) results in the bright blue colour of the glasses in the second group while the high manganese content (2.68 w%) is responsible for the purple colour of the glass in the last column but one. The bright green glass has a somewhat higher manganese (1.03 w%) and iron (2.12 w%) content but copper (4.21 %) was obviously used as a colouring agent added to the glass melt as bronze since the amount of tin (0.37 %) is significantly increased compared to the other glasses and the ratio of the two elements is possible for this material. An excess of iron oxide, that is, 10 per cent or more of the glass batch, produces a black glass as can be seen in the fifth group of Table 6 (Fe20 3 : 11.33 w%). 7.4 Opalising agents in glass So far, only clear glasses were considered. Next to these, also opal, opalescent or opaque glasses exist. They owe their characteristics to the presence of gas bubbles or, in most cases, crystalline particles in the glassy matrix, which disperse the light. The density of the opaqueness is influenced by the size, number and regularity of distribution of the bubbles or crystalline particles. When a suspension of solid or 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis o f Roman Glass from the 1st-6th Century AD crystalline substances in the glass melt is used to produce the opacification of glasses, the result depends on the solubility of the opalising agent, the concentration used and on the temperature and time o f heating involved. One opaque light blue glass is present in this series. The composition of this glass is comparable to that of the other blue glasses except for the calcium and antimony content. This points to the use of calcium pyroantimonate (Ca 2Sb 20 7) as an opalising agent in ancient blue glasses as proposed in [22]. The black and brown glass (columns 6 and 7) also show a rather high content of antimony suggesting that some opalising agent might be present here as well. Different opalising agents are known to have been used in ancient glasses; they will be further discussed in section 9. 8. GLASSES FROM THE SOUTHERN PART OF BELGIUM 8.1 Description of the glass objects and their location Samples from glass vessels belonging to glass collections in two museums in Belgium, (Musees Royaux d’Art et d’Histoire, Brussels and the Musee Curtius, Liege) were provided to us by H. Wouters and C. Fontaine from the Royal Institute for Cultural Heritage (IRPA), Brussels. All the glasses (except for one, coming from Neerhaeren which is located in the northern part of Belgium) originate from Roman Tumuli (burial mounds) located in the southern part of Belgium (Wallonia), namely Warnant-Dreye, Celles, Omal, Blehen and Avennes. The glasses from Warnant- Dreye, Celles and Omal are dated to the end of the third, beginning of the fourth century and the others to the end of the first, beginning of the second century AD. All the glasses are colourless or natural blue-green, samples were taken from different objects such as bottles, cups and jugs. 8.2 Classification of the data The major composition of the glass samples coming from the museum in Liege ( 8 samples) was determined in the Royal Institute for Cultural Heritage. These results, together with the ones obtained by us constitute the data of Table 7. From the Table it becomes immediately clear that every subdivision is formed by glasses coming from various places and with different ages. The first group is formed by 1 glass from Avennes, the second consists of 3 objects from Celles and Omal. In the third class again only 1 glass is found; this is the only one coming from Warnant-Dreye. The fourth subdivision has seven glasses from Celles and Omal, the fifth one 4 glasses from Neerhaeren, Blehen and Avennes and 3 objects from Celles and Avennes are placed in the last one. 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 0 9 0 CD CO CO h - T~ CO CM CM © o CM 3 3 3 0 5 0 CO 4 7 Is- h - - 8 2 5 7 .8 5 0 .1 7 0 .7 7 0.41 0 .4 4 M ax 2 .6 5 © © 1 9 .5 3 7 1 .2 8 CO GO o CM LO CO o to h- CM © CO s C h - LO CO h - o CO CO o> CO N- © to O LO CO © CM © © CO O CM o © CM S LO d csi O) O © d LO d d d Is- CO <5 CO OS CD to CM CO 00 oo O © CO © Co 5 o © O Tf r - M* CM 0) 3 g 03 p CO p o © o CM CM © © © © tn C O © © © © © d c: CD co d d 2 -H -H •H -H +1 +1 ■H -H •H +i -H -H +1 +1 +1 -H ■H -H +1 -H -H *fl -H +1 ■H -H CO o CM © CM CO © © CM f*— O © - CO o 03 © © M* CM CO © CM CO 7 .1 4 0 .5 8 0 .3 5 0 .6 7 n = 3 2 .3 8 0 .1 5 © d 1 7 .4 4 M ea n h - I I 0 .5 4 co CO 0 S 6 © CO © Is- © © Q) Ip o © CO © © © © M* CO r— - r - CO CO m C 6 7 2 CO t o 7.31 0.51 0 .7 8 2 5 4 0 M ax 0 .5 5 2 .4 9 © d d 1 7 .3 7 c 7 0 .6 2 5? 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Average conc., standard deviation and min. and max. concentration for the 1 PbO glasses from the Roman tumuli in Neerhaeren and the southern part of Belgium. Data above the horizontal line are in w% and below it in ppm. 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 8.3 Correlation between major, minor and trace elements The compositions of these glasses is comparable to those discussed before and they do not show any unexpected correlations between major, minor or trace elements so they will not be discussed in detail. 9. GLASSES FROM MAASTRICHT (THE NETHERLANDS) 9.1 Description of the glass collection and its location The name, Maastricht, is derived from the Latin “Mosae Trajectum" which means crossing of the Maas, after the wooden bridge built by the Romans at this location. Soon after the construction, a settlement was created in the first century which was a junction of high roads. The main road came from Gaul, passing Tongeren and going to Maastricht. At the other bank of the river the road was split into four with the most important one going to Cologne. The city of Maastricht originates from a settlement of Germanic merchants and workmen. After the revolt of the Batavians in 69-70 AD followed a long period of relative rest during which the city flourished. During this period trade became especially important until finally the Romans had to abandon the field for the Franks in the 5th century [57]. The Department of Town Development and Ground Matters in Maastricht owns a large collection of (fragments of) glass objects and they provided us with material from colourless, naturally and intentionally coloured clear glass as well as opaque glass samples. The objects cover almost the entire Roman period (from the first until the fifth century AD) but most of them are late Roman (fourth and fifth century). Except for 3 glasses in Section 4 (1 from the fifth and 2 from the 6 th century), only glass samples from the earlier centuries of the Roman Empire were discussed so far in this work. Towards the end of the third century in the West, the decline in glass-making had set in causing ordinary household glasswares as well as the majority of tablewares to be badly made during the fourth century. The glass from this period usually has a greenish or even brownish colour and contained bubbles and streaks. At the end of the fourth century the Western glass even starts showing signs of a new tradition in which the taste of the Frankish tribes predominated, the number of forms drastically diminished to tableware only [55]. 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD CV ,X CO 0 0 0 o CM CO © o CO CO 5 25 67 47 53 262 265 436 490 O i in 1.13 1.99 7.83 2.72 Max 0.86 0.57 1160 in 0.61 2.51 1 2 0 1 3550 CO 8040 CM CO S90 M- 0 0 0 in o o r** 03 co r j - CM o 03 28 35 43 - CO c o 107 162 191 c d 961 812 1.15 1.70 Min c d - 0.43 0.97 0.32 1030 2 . 1 2 1140 X 0 . 1 2 CO CD' CO 0 0 0 § co CO o o o CM o CO CO 03 CO CM CD 15 94 CO r j- 8 20 160 co 0.37 0.31 0.40 1030 0.47 0.04 0.15 5220 0 . 2 0 0.19 S, d d o CM 6840 o> Stdev ■s -H 44 44 -H +4 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 • 5 690 r - . 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T f co CO CO 83 62 2 0 44 27 199 574 1.38 1.33 0.14 0.47 0.34 1.06 0.92 2400 2450 0.11 o o o " 5 Stdev u 44 +4 44 44 44 44 41 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 S CM t n 0E0 660 O o > <35 3 co CO O 94 30 o 24 CO i CM 8 170 454 837 r*«-' CO* 923 1.12 0.84 0.25 1150 2.24 0.42 3530 0.13 co* CM c Mean CO 001 890 X O 09 CD q CM co O 0 3 CO 2 0 97 47 3 N 128 in 919 239 2.45 X 7.71 4.42 3.67 0.54 0.72 1650 1450 5 2.63 o 19.29 CO t o 69 et3 m X 5 c o r* - o 24 60 48 42 5 s 538 764 Min 1.66 2.39 0.18 0.53 6.69 0.70 4.39 1040 0.91 2.57 0.38 18.05 CD SOO SOO 900 o o t CM T f i O o c o o M- m CO CM CO c o 0 5 1 2 p 8 69 CM 269 431 489 1.42 0.72 0 . 0 2 0.04 0.88 cd o o -Q Stdev 44 +4 -H 44 +4 +4 44 44 44 44 44 41 44 44 44 44 44 44 44 44 41 44 44 44 44 41 +4 44 h - c o 960 O CM CO c q r**. CO CO II § CM 0 3 r«- CO 47 83 88 S S CO co 190 8 728 2.42 4.40 1110 2.67 0.19 0.53 0.71 1340 c 0.53 CM Mean CO 696 CM 09 c o r» O 23 132 10? 8 687 307 3 681 993 1.62 0.97 0.28 0.97 Max 2.32 0.44 0.55 0.97 1340 7080 0 . 1 2 4920 17.401 c o 408Q0 6S0 000 SOO CO CD 66 o r** CO tn CO in CO 14 o 52 53 CM O 125 1 0 0 313 650 664 Min in 1.17 0 . 2 2 2.23 0 . 2 0 0.84 8.90 1280 16.81 c o o 600 SOO SOO 200 800 O o O § CO o CM co co CM h - co f - CD X» i n 23 0 3 40 185 a 449 232 207 co 1.32 0.26 0.55 0.31 blue 0.31 0.25 C/3 0.42 CM CM 44 44 44 44 +1 44 44 44 41 44 44 41 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 © 05 CM CO CO CM § 03 II O O) £ in I 16 i 58 26 116 103 161 370 665 co' 828 1.39 0.25 © c o 2.28 0 . 2 2 0.38 o 0.91 9.30 d 1210 p ) Mean CD CM j j 680 LLZ 890 CO 03 r- 03 co 0 3 CM co CM o> O in 28 23 37 t T 153 in CO 1.05 4.69 1460 1200 7740 0.76 2450 I 2450 Max 2.51 5.02 0.48 3600 2 0 1 0 19.83 CO LZ'Z 000 oeo co 880 C CO lO oo r**. CM o c o c o t n CO o 14 13 CO 10 to CD 550 d 910 CM 1.84 1.47 0.40 is 0.20 0.55 6.89 > ■ 18.43 CD 600 5 800 SOO O CM m CO Q 03 32 63 33 164 in 155 423 924 741 929 1.15 1.97 1.11 0.08 0.15 0.32 0.25 0.51 0.13 CM cQj Stdev s> +i 44 44 ■H 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 03 990 660 0 CO 66 O i TT <*» 0 3 03 15 24 23 67 3 284 750 4 5 0 § II 01 649 1.89 2.57 1390 2.35 7.35 0.58 CM 0.28 © 1180 c Mean 63.41 i co 990 N. 0 3 c o O CO 0 3 -M- CM CO 78 43 59 23 - 32 CM 3 600 415 1.33 2.08 3.78 0.76 0.25 0.77 0.45 8.23 Max © 1231 19800 I 19.75 s 010 99'L 000 StO 000 CM X CO ID t J- c o O CD o CM 03 27 23 27 40 3 0 3 5 387 cd h -' 887 Min 1.14 0.61 6.86 0.18 CO 0.49 8980 600 CO 600 900 a> CM c o co r*. CM CO CM o co CM r-. 14 in s 2 107 147 1.33 1.37 1.25 0.30 0.57 0.03 3860 0.17 0.15 CO 0.03 o o 44 44 44 44 +4 44 44 44 44 41 44 44 44 44 44 44 44 44 44 44 44 44 44 44 41 44 44 44 CM CM CO 99 8 10 O CO CO in r - 03 O o> tn 37 35 27 49 163 111 11 cn 470 cd 1.24 2.16 0.36 - 1047 0.68 7.36 0.21 o 0.29 18.83 c Mean CD ! ! 0.58 i «/> o o O 0 o cT O O to* SrO ZnO CoO CuO CaO MnO imj CO MgO 05 z <* CO CL p £ < QC 2 01 CO ffi Table 8. Average concentration, standard deviation and minimum and maximum concentration for the clear glasses from Maastricht. Data above the horizontal line are given in w% and below it in ppm. 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD 9.2 Correlations between major elements Samples from these later glass objects offer the possibility to find out whether, next to these stylistic variations and changes in colours, also changes in composition took place during these periods. The results of the quantitative analysis of the clear glasses is given in Table 8 , subdivisions were made on the basis of the colour of the glasses, namely, colourless, green, bright blue, brown, naturally coloured and dark green. 3.00 2.50 R = 0.85 2.00 © unknown □ 2nd century A 3rd century o 1.50 0>CM X4th century + 5th century 1.00 0.50 0.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 MnO (w%) Figure 16. Binary plot showing positive correlation between MnO and Fe 2 0 3 for all the glasses from Maastricht demonstrating the change in composition during the fourth century AD. From the chemical composition of the glasses and some correlation diagrams it immediately becomes clear that indeed a major change has occurred somewhere during the second half of the fourth century. The glasses from the fifth century show a markedly higher concentration of manganese, iron and titanium compared to glasses from the first to the third century which probably explains their variation in green and brown colours. Figure 16 shows the positive correlation between the MnO and Fe 20 3 content in the various glasses from Maastricht. Based on a correlation coefficient of 0.85, the positive correlation can again be accepted on a 99.9 % level of significance. The composition of some glasses from the fourth century overlaps with the older ones while others have a composition which is comparable to the later ones (higher concentration of both manganese and iron) suggesting that major changes in glass-making must have occurred during this century in which invasions of Roman settlements by Saxon and Frankish tribes were common in the surroundings of Gaul. The five glass objects with unknown age, 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD probably date back from either the second half of the fourth or the fifth century AD since they show the typical late Roman composition. © unknown □ 2nd century A 3rd century I ! x4th century j ; + 5th century ' 60.00 62.00 64.00 66.00 68.00 70.00 72.00 S i0 2 (w % ) Figure 17. Binary plot showing negative correlation between S i0 2 and MnO for all the glasses from Maastricht. 9.5 0 R = 0.90 9 .0 0 8.50 : © unknown 8.00 jO 2nd century j 7.50 A 3rd century j x4th century 7.00 + 5th century R = - 0.86 6 .5 0 6.00 5 .5 0 1.50 1.70 1.90 2.10 2.30 2.50 2.70 2.90 A l20 3 (w %) Figure 18. Binary plot showing positive correlation between Al 20 3 and CaO for the glasses from Maastricht dated to the second, third and some of the fourth century and negative correlation for the late Roman ones. 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C hapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD Most of the late Roman glasses also show a higher content of trace elements such as zinc, copper and zirconium which might point to the use of non-purified or impure raw materials introducing these trace elements into the glass batch. Unfortunately no positive correlations could be found between these elements and any of the major compounds such as S i0 2. Figure 17 shows the negative correlation (correlation coefficient of - 0.92) between Si0 2 and MnO. This could be explained by the fact that during later periods, people started recycling older glass fragments by re-melting and re-working them, introducing impurities originating from the crucibles and tools used during this process and not from the raw materials used. On the other hand, if we look at the plot of Al 20 3 against CaO (Figure 18), it seems that, during the fourth century, there might have been a change in raw materials as well. A positive correlation with a correlation coefficient of 0.90 exists between these two compounds for the older glasses (until the fourth century) suggesting that they had entered the glass melt together e.g., as a feldspar, but it changes into a negative correlation with a correlation coefficient of - 0 . 8 6 for the later glasses which suggests that probably next to recycling of older glass also new glass production took place using different supplies of raw materials. Since Al 20 3 can also be introduced in the glass from the crucible walls, the higher Al 20 3 content might also indicate remelting of the 4-5th century glass samples. Both correlations can be accepted at a 99.9 % significance level. 9.3 Colouring agents in the glass 250 R = 0.94 200 R = 0.89 O unknown 150 £ □ 2nd century Q. Q. X A 3rd century oc x4th century N 100 X 5th century 0 1000 2000 3000 50004000 CuO (ppm) Figure 19. Binary plot showing positive correlation between CuO and ZnO for the glasses from Maastricht. Different slopes suggest the use of different brass sources. 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD The green colour of the late Roman glasses is probably not only caused by the iron present but also by the copper content. Although some of the glasses have a considerable amount of Sb 20 5 (up to 2 w%), they still show a green colour, caused by copper, which again proves that antimony only acts as a decolouriser for iron and not for other colouring agents present in the glass. The copper seems to be added as brass since a positive correlation between CuO and ZnO was found (Figure 19). Apparently more than one source of brass was used since different slopes are present in the p lo t: one with a ZnO/CuO ratio of 1/3, represented by the solid line and one with a ratio of 1/15, represented by the dotted line, which are both possible for brass. Next to this correlation also a positive correlation was found between copper on the one hand and arsenic and lead on the other hand. For some glasses from the fifth century a positive correlation was found between CuO and S n 0 2 suggesting that copper was also added to the glass batch as bronze. Based on the correlation coefficients of 0.89 and 0.94, the correlation can be accepted at a 99.9 % significance level. 9.4 Opacifying agents in the glass As mentioned before, next to the clear glasses also various opaque glasses and opaque decorations on glasses were present in this collection. Since the opacity is caused by the presence of crystalline particles in the matrix, it means that the material is no longer homogeneous, so it is rather hard to obtain reliable quantitative results from these glasses. The heterogeneity of the material is visible on a backscattered electron image (Figure 20). Figure 20. Backscattered electron image of one of the opaque white lines in a blue glass from Maastricht, clearly showing the crystalline particles in the glass matrix. 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD Over the years many different opacifying agents have been used by glass-makers and they have been extensively studied by Turner and Rooksby [25]. The authors claim that during the pre-Roman and Roman period the only opalising agents used were reduced copper compounds for red or red-brown glasses, compounds of antimony for the white, blue and greens and lead pyroantimonate for the yellow opaque ones. They did not find any evidence for the use of tin as an opacifying agent until well on in the Christian era, their oldest specimen to contain it being an eleventh century yellow mosaic glass from Novgorod. In later years, occasionally calcium fluophosphate was used to obtain white opaque glasses, e.g. by Kunckel [25]. Lead oxyarsenate was introduced in the eighteenth century and fluorides came into use in the nineteenth century and have been extensively used during the twentieth century. In modern glass-making, various other elements such as titanium, selenium, nickel and chromium have been added to the melts to produce a wide range of colours in opaque glasses. Scanning electron microscopy measurements were performed on the different opaque glasses but because of the heterogeneity of the material, the results could only be used in a qualitative way. As was expected, the red glass contained a high concentration of copper; addition of a copper compound leads to a red or red-brown glass under reducing conditions which means that the copper is present as red cuprous oxide, metallic copper or a mixture of both of them. Most of the whites show a high concentration of calcium and antimony meaning that Ca 2 Sb20 7, which is the oldest known glass opacifier, or CaSb 20 6 (less frequent), was used; this compound is also present in opaque green and blue glasses together with the colouring agents, copper and/or iron and copper and/or cobalt respectively. Lead pyroantimonate (Pb 2C a 20 7) was definitely used in all the yellow opaque glasses since they all have a high concentration of lead and antimony. Contrary to what was stated by Turner and Rooksby [25], tin oxide was found in many of these opaque glasses even up to a few percent in weight percentage. Since tin was also found by Henderson [58] in glass beads from the Merovingian time (6-7th century AD) and Rehren [59] even mentioned the presence of this element in a Egyptian red opaque glass (14-12th century BC), the start of the use of this material should probably be regarded to be much before what was supposed by Turner and Rooksby. Next to all the opacifying agents, most of these glasses also show a higher content of at least one of the following elements : chromium, arsenic, cadmium and bismuth. These elements have probably entered the glass batch as impurities present in the source materials used for the opacifiers. 10. GLASSES FROM MANCETTER AND LEICESTER (UK) Vessel glass and glass-blowing waste from two Romano-British settlements (Mancetter and Leicester) were investigated by Jackson et al. from the University of Bradford, UK. [12]. Based on the remains found there, being cullet, melted glass and glass waste associated with a glass furnace (in Mancetter), both these places were considered to be glass-working sites. The glass from Mancetter (M) was dated 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD to the mid second century and that from Leicester (L) was primarily third century in date. Almost all the glass on both sites was blue-green in colour with also a small fraction of colourless, yellow-green and yellow-brown glass present. In the literature only information about major and minor elements was given but C. Jackson provided us her results of the trace element analysis performed by inductively coupled plasma atomic emission spectroscopy. This glassy material shows the typical soda-lime-silicate composition which was also found for all the previously described glasses. The chemical composition of the glasses from the two sites is very similar; it only shows some slight differences in the levels of iron (M : around 0.5 w% and L : around 0.8 w%), manganese (M : around 0.5 w% and L : around 0.3 w%) and phosphorus (M : around 0.16 w% and L : around 0.11 w%). In the colourless glass, which was used for higher quality tablewares and which was especially present in Leicester, the element used as decolouriser was clearly antimony. This glass contains significantly less aluminium, calcium, phosphorus and iron compared to the blue-green glass from the same site. On first sight nothing unexpected was found in the composition or correlation diagrams of these glasses. Again a pure or purified sand appears to have been used (no positive correlation with certain trace elements) to which a fluxing agent (soda), lime and some colourant or decolourant were added. The concentration levels of the trace elements are comparable to the ones found for the glasses investigated by us, except for zirconium which is present in a significantly higher amount in these glasses (around 280 ppm). One would think that it had entered the glass batch as an impurity of the sand (meaning that another sand was used as raw material in Great-Britain) but no positive correlation was found between S i0 2 and Zr02. So, it seems that it must have been present in another source material. Besides this, all the glasses, even the ones which are not colourless, have a relatively high concentration of Sb 2Os (almost all of them contain more than 1000 ppm Sb20 5); the same fact was noticed in the glasses coming from Oudenburg, which is the site situated most western in Belgium, closest to the UK. This might mean that more antimony was used in the more western part of the Roman territory, even in the third and fourth century, compared to the east since Brill did not find any antimony in the glasses from Jalame [9] and the level of this element in the Qumran glasses is very low as well. A tendency towards positive correlation was found between Sb 20 5 and PbO so probably a Sb-Pb alloy or mineral was added to the glass melt. The use of stibina cannot be ruled out; unfortunately the correlation between antimony and sulphur could not be checked since the concentration of S 0 3 was not available. 11. GENERAL OVERVIEW Major, minor and trace composition of 1st-4th century Roman glass objects If we compare the chemical compositions obtained for all the Roman glasses described in this work, dated to the fourth century or before, it becomes clear that, 142 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD they are indeed very similar, especially in major elements, as was mentioned in the literature; sometimes only slight differences can be seen which are not sufficient to differentiate between various locations or age. All the glasses show the low magnesium soda-lime-silica composition which is typical for the Roman period. Although it was assumed that these glasses were produced by merely adding together a siliceous compound (sand), a fluxing agent (soda) and some colourant or decolourant, it seems that lime and alumina were also added separately since no positive correlation was found between the concentration of CaO and Al 20 3 and the silica content. Next to this, the sand which was probably used in the Western part of the Roman Empire does not contain enough calcium to produce a durable glass and some lumps of chalk were found near a glass furnace in Germany. The sand appears to have been very pure (or even purified) since no positive correlations were found with trace elements such as zirconium or titanium which can be introduced into the glass batch as impurities of this raw material. When minor and/or trace elements are considered, differences, which are mainly related to the colour of the glass, can be seen between the various objects. Since comparable concentrations of trace elements (at these different levels) can be found in all the groups of glass objects, it is not completely clear whether they can be used to classify glasses by age or geographical location. Yet subtle differences are present such as the higher concentration of zirconium in the glasses from the UK. Decolourising agents in the glass One of the elements covering the largest range of concentration levels is definitely antimony, which in ancient times was not only used as decolouriser but also as opacifying agent and as means to remove air-bubbles from the glass melt. Its use as decolourant seems to have been largely taken over by manganese during later centuries except for the most western part of the Roman territory (Mancetter and Leicester in Great-Britain and Oudenburg in Belgium) where it is present in relatively high concentrations in almost all glasses, even the ones dated to the third century. The raw materials used for antimony were on the one hand stibina, a Sb-S mineral, and on the other hand a Sb-Pb mineral or alloy, of which the latter seems to have been used more often (evidence for the use of stibina was only found in some of the glasses from Maastricht and in the ones from Trier, Rouen and Oudenburg). Colouring elements in the glass The most common colouring elements are iron, manganese, copper and cobalt. Iron is present in all the glasses and has probably entered the glass batch as an impurity present in the raw materials used. In its reduced state it is responsible for the blue- green aqua colour which is very typical for Roman glass vessels. The colouring effect of this element can be diminished or even ruled out by adding a decolourant, namely antimony or manganese. The resulting colour depends both on the conditions in the glass furnace (oxidising or reducing) during glass production as well as on the ratio Mn/Fe in the glass, an excess of manganese results in a purple colour. The use of copper can lead to a red, blue or green glass, depending on the atmosphere in the glass furnace. Copper can be added to the glass melt as either 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD bronze or brass and it seems that both were used during the different centuries and all across the Roman Empire since positive correlations were found between CuO and ZnO for some glasses and between CuO and Sn0 2 for others. Colouring with cobalt (even from 50 ppm on) results in a brightly blue glass; this element can be added to the glass batch as either a cobalt ore or a blue glass cullet rich in cobalt. Composition of late Roman glass Somewhere during the second half of the fourth century a drastic change in glass production appears to have occurred; this is reflected in stylistic characterisations, colour as well as composition of the glass vessels,. The number of forms is strongly reduced to only common tableware, most of the glass shows a dark green or even brown colour and the concentration of iron, manganese, titanium and some trace elements such as copper, zinc and zirconium is significantly higher. The reason for this change in composition could be explained by recycling of older glass introducing impurities to the glass during this process and/or the use of less pure or unpurified raw materials. Correlations between major, minor and trace elements in the glass The most important correlations between major, minor and trace elements in the different glass collections are summarised in Table 9. Table 9. Overview of the most important correlations (positive, negative or no correlation) between major, minor and trace elements in the glass objects from Cologne (C), Rouen and Trier (R & T), Tongeren (T), Oudenburg (O), Grobbendonk (G), Wallonia (W), Maastricht 1st to 4th century (M 1-4), Maastricht 4th and 5th century (M 4-5) and Mancetter and Leicester (M & L). c (1) c (2) R & T W M (1-4) M (4-5) M & L Si-AI neg. neg. neg. neg. neg. - neg. neg. neg. - Si-Ca neg. - neg. - neg. neg. neg. neg. - - Si-trace neg. neg. neg. neg./- neg./- neg. neg./- neg. neg./- neg./- Na-CI ? pos. pos. pos. pos. pos. pos. pos. pos. ? Na-Ca neg. neg. neg. neg. neg. neg. - neg. neg. neg. Al-Ca pos. pos. pos. pos. pos. pos. pos. pos. neg. pos. Mn-Fe pos. pos. pos. pos. pos. pos. - pos. pos. pos. Sb-S ? pos. pos. - pos. -- pos. pos. ? Sb-Pb pos. pos. - pos. - pos. pos. pos. pos. pos. Cu-Zn pos. pos. pos. pos. pos. pos. pos. pos. pos. pos. Cu-Sn pos. pos. pos. - pos. pos. pos. - pos. ? Cu-Pb pos. pos. pos. pos. pos. pos. pos. pos. pos. pos. Conclusions Since the major composition of all these glass objects is so similar, it is likely that probably only a few workshops produced glass from raw materials, which was then transported as ingots throughout the Roman Empire. At different sites these were remelted, colourants, decolourants and/or opacifiers were added, introducing 144 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD different amounts of the various trace elements, and objects were formed. During later centuries older glass seems to have been recycled. Probably further information can be gained if more glass samples are measured, compared and especially correlations between trace elements are studied in detail providing indications about raw materials used, which might lead to a better understanding of trade routes and/or techniques in glass production during the Roman period. According to our results, what can also be stated is that the glass-makers during that time appear to have had a quite good understanding about how to produce a durable glass and which elements or compounds to use as colourants, decolourants and/or opacifiers. 145 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD REFERENCES [1] Velde B., Alumina and Calcium Oxide Content of Glass Found in Western and Northern Europe, Oxford J. Archaeology, 9, pp. 105-117 (1990). [2] Velde B. and Sennequier G., Observations on the Chemical Compositions of Several Types of Gallo-Roman and Frankish Glass Production, Ann. 9th Cong. Assoc. Int. Hist. Verre, pp. 127-147 (1985). [3] Henderson J., The Raw Materials of Early Glass Production, Oxford J. Archaeology, 4, pp. 267-291 (1985). [4] Newton R.G., Recent Views on Ancient Glasses, Glass Techn., 21(4), pp. 173-183 (1980). [5] Velde B. and Gedron C., Chemical Composition of Some Gallo-Roman Glass Fragments from Western-France, Archaeometry, 22, pp. 183-187 (1980). [6 ] Henderson J., The Analysis of Ancient Glasses Part I : Materials, Properties, and Early European Glass, JOM, pp. 62-64 (1995). [7] Henderson J. and Holland I., The Glass from Borg, an Early Medieval Chieftan’s Farm in Northern Norway, Medieval Archaeology, 26, pp. 29-58 (1992). [8 ] Sayre E.V. and Smith R.W., Compositional Categories of Ancient Glass, Science, 133, pp. 1824-1826 (1961). [9] Brill R.H., Scientific Investigations of the Jalame Glass and Related Finds, Excavations at Jalame, Site of a Glass Factory in Late Roman Palestine, Weinberg G.D. (ed.), Chapter 9, pp. 257-294, University of Missouri Press (1988). [10] Von Rottlander R.C.A., Naturwissenschaftliche Untersuchungen zum Romischen Glas in Koln, Kolner Jahrbuch fur Vor- und Fruhgeschichte, 23, pp. 563-582 (1990). [11] Cox G.A. and Ford B.A., The Long-term Corrosion of Glass by Ground-water, J. Mat. Sci., 28, pp. 5637-5647 (1993). [12] Jackson C.M., Hunter J.M., Warren S.E. and Cool H.E.M., The Analysis of Blue-green Glass and Glassy waste from Two Romano-British Glass Working Sites, Archaeometry ‘90, pp. 295-304, Birkhauser Verlag, Basel (1990). [13] Fremersdorf F., Neue Beitrage zur Topographie des Romischen Koln, Romische-Germanische Forschungen, 18, p. 6 8 (1950). [14] Harden D.B., Snake-thread Glasses found in the East, J.R.S., p. 50 (1934). [15] Fremersdorf F., Die Anfange der Romischen Glashutten Kolns, Kolner Jahrb. Vor- und Fruhgesch., 8 , p. 28 (1965-1966). [16] Doppelfeld O., Romisches und Frankisches Glas in Koln, pp. 12-16 (1966). [17] Neu S., Romische Glaswerkstatt Helenenstrafte, Koln II. Fruhgeschichtlichen Denkmalern, 38, pp. 179-183 (1980). [18] Plinius, Naturalis Historia, 36, pp. 65-70. [19] Sayre E.V., The Intentional Use of Antimony and Manganese in Ancient Glass, Advances in Glass Technology, Matson F.R. and Rindone G.E. (eds.), 2, pp. 263-282, Plenum Press, New York (1963). [20] Newton R.G., Colouring Agents Used by Medieval Glassmakers, Glass Techn., 19, pp. 59-60 (1978). 146 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD [21] Mirti P., Casoli A and Appolonia L., Scientific Analysis of Roman Glass from Augusta Praetoria, Archaeometry, 35(2), pp. 225-240 (1993). [22] Velde B. and Hochuli A., Correlations Between Antimony, Manganese and Iron Content in Gallo-Roman Glass, personal communication (1996). [23] Newton R. and Davison S., Conservation of Glass, Chapter 3, Butterworths, London (1989). [24] Sellner C., Ol H. and Camara B, Untersuchung Alter Glaser (Waldglas) auf Zusammenhang von Zusammensetzung, Farbe und Schmelzatmosphare, Glastechn. Ber., 52, pp. 255-264 (1979). [25] Turner W.E.S. and Rooksby H.P., A Study of Opalising Agents in Ancient Opal Glasses Throughout 3400 Years, Glastechn. Ber., 32K, pp. 17-28 (1959). [26] Collins J.F. and Buol S.W., Effects of fluctuations in the Eh-pH environment on iron and/or manganese equilibria, Soil Sci., 110(2), pp. 111-118 (1970). [27] Hahn P., Qber Spektrochemischen Untersuchungen an Romischen Fensterglasern, Glastechn. Ber., XXVII, p. 459 (1954). [28] Forbes R.J., Studies in Ancient Technology, V, p. 174, Leiden (1957). [29] Knoll H., Glasherstellung bei Plinius dem Alteren, Glastechn. Ber., 52, pp. 265-270 (1979). [30] Morey G.W., The Properties of Glass, sec. edition, American Chemical Society Monograph Series, Reinhold Publishing Co., New York (1954). [31] Damour E., Cours de Verrerie, Ch. Berange, Paris (1929). [32] Gerspach, TArt de la Verrerie, p. 18, Paris (1885). [33] Morin-Jean, La Verrerie en Gaule sous TEmpire Romain, p. 252, Henri Laurens, Paris (1913). [34] Loschke S., Fruhchristliche Werkstatte fur lasschmuck in Trier, Trierer Heimatbuch, p. 337,. Trier (1925). [35] Sanderson D.C.W. and Hutchings J.R., The Origins and Measurements of Colour in Archaeological Glasses, Glass Techn., 28, pp. 99-105 (1987). [36] Orlando A., Olmi F.f Vaggelli G and Bacci M., Mediaeval Stained Glasses of Pisa Cathedral (Italy) : Their Composition and Alteration Products, Analyst, 121, pp. 553-558 (1996). [37] Riederer J., Archaologie und Chemie - Einblicke in die Vergangenheit, pp. 99- 129, Rathgen-Forschungslabor, Berlin (1987). [38] Tylecote R.F., A History of Metallurgy, sec. edition, pp. 69-71, The Institute of Materials, London (1992). [39] Vanvinckenroye W., Tongeren Romeinse Stad, Provinciaal Gallo-Romeins Museum, Tongeren (1975). [40] Vanderhoeven A. and Vynckier G., De Archeologie van de Stad Tongeren, Archeologie in Vlaanderen, 3, pp. 17-20 (1996). [41] Baillien H. and Ulrix F., Tongeren, Belgie’s Oudste Stad, Prisma (1948). [42] Helsen J., Moermans W. and Severijns P., 222 Jaar Tongeren, 15 vddr Chr. tot 1985, Concentra, Hasselt (1988). [43] Vanvinckenroye W., Gallo-Romeinse Grafvondsten uit Tongeren, Provinciaal Gallo-Romeins Museum, Tongeren (1963). [44] Vanderhoeven A., De Romeinse Glasverzameling in het Provinciaal Gallo- 147 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 : Provenance Analysis of Roman Glass from the 1st-6th Century AD Romeins Museum, Provinciaal Gallo-Romeins Museum, Tongeren (1962). [45] Weyl W.A., Coloured Glasses, Dawson’s of Pall Mall, London (1959). [46] Brill R.H. and Barnes I.L., The Flight into Egypt, from the Infancy of the Christ Window (?) : Some Chemical Notes, The Royal Abbey of St.-Denis in the Time of Abbot Suger (1122-1151), McK. Crosby S (ed.), p. 81, New York (1981) [47] Bailly M., Le Verre, La Conservation en Archeologie, Berducou M.CI. (ed.), Chapter IV, Masson, Paris (1990). [48] Poire P., A Travers I’lndustrie, Hachette, Paris (1897). [49] Mertens J., Laat-Romeins Graf te Oudenburg, Arch. Belgica, 80 (1964). [50] Mertens and Van Impe L., Laat-Romeins Gratveld van Oudenburg, Arch. Belgica, 135 (1971). [51] Creau I., De Gallo-Romeinse Nederzetting onder het Laat-Romeins Grafveld van Oudenburg, Arch. Belgica, 179 (1975). [52] Mertens J. and Crabbe R., Oudenburg : Romeinse Legerbasis aan de Noordzeekust, Arch. Bel. Spec., 4(1987). [53] Van Dyck G., Grobbendonk : Het Historisch Verhaal van een Kleine Gemeenschap, De Roerdomp, Brecht (1982). [54] Goetschalckx P.J., Geschiedenis van Grobbendonk, Genootschap voor Geschiedenis en Volkskunde, Antwerpen (1994). [55] Isings C., Roman Glass from Dated Finds, J.B. Wolters, Groningen (1957). [56] Janssens P., Het Gallo-Romeins Grafveldje van Grobbendonk, Arch. Belgica, 93(1966). [57] Borromeus B., Geschiedenis van Maastricht. De Oudste Stad van Nederland, J. Schenk, Maastricht. [58] Sablerolles Y., Henderson J. and Dijkman W., Early medieval glass bead making in Maastricht (Jodenstraat 30), The Netherlands. An archaeological and scientific investigation, Perlen. Kolloquien zur Vor- und Fruhgeschichte, Band 1, pp. 293-313, Dr. Rudolf Habelt GmbH, Bonn (1997). [59] Rehren Th., Ramesside Glass-colouring Crucibles, Archaeometry, 39(2), pp. 355-368 (1997) 148 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 Investigation of Some Historical Potash Based Glasses Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses 1. IN TR O D U C TIO N As mentioned in Chapter 1, during the Middle Ages the production of glasses with potash instead of sodium as the major alkali was introduced in Europe. Newton [1] states that a possible explanation for this rather sudden change is the fact that the marine plants from coastal sites of which the ashes were used as alkali source for glass production, could no longer meet the demand for window glass for churches and cathedrals after 1000 AD. The glassmakers moved to the extensive beechwood areas in continental Europe where they used the wood as fuel for their furnaces and the ashes as the source of alkali for glass production as was already described by Theophilus [2]. Next to the archaeological glass samples described in previous chapters, which all date back from the Roman era, some medieval glasses were also analysed. The composition of the model glasses which were used in the burial experiments described in the next chapter is closer to that of medieval glass (potash glass). Therefore it was found relevant to examine whether or not the corrosion of this type of glass, which was mostly used during the Middle Ages, is comparable to that of soda-lime-silica glass, which is the typical composition for Roman glass. 2. DESCRIPTION OF THE GLASSES Two series of medieval glass objects, which had been buried for extended periods of time, were analysed. The first one, originating from the ruins of a church in Namen, Belgium, belong to the 14th-15th century AD. The other series of glasses was recently excavated from the ruins of a castle in Fagnolle, Belgium. After a fire in 1555 the objects were covered by rubble and now all show a black corrosion crust. The description of the glasses is given in Tables 1 and 2. 2.1 Glass from Namen Table 1. Description of the glass samples originating from Namen, Belgium. Sample Colour Surface state 40 light green brown layered corrosion morphology 43 very light green brown-black layered iridescent corrosion morphology 45 very light green light brown layered crust and some iridescence 46 yellow-brown coloured layered crust with pitting and iridescence 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses 2.2 Glass from Fagnolle Table 2. Description of the glass samples originating from Fagnolle, Belgium. Sample Colour Origin Surface state 1 / 2 very light green - black layered crust, easily removable 1/3 white opaque - white, black and brown layered crust 2 / 2 yellow drinking glass heavily corroded, black layered crust 3/3 colourless - black-green crust 2/4 colourless drinking glass black crust and iridescence 2/5 colourless drinking glass brown-black layered crust 2/7 colourless - black crust 3/8 blue - brown-black layered crust 3/9 deep-blue - brown-black layered crust 2 / 1 0 colourless - no visible corrosion 2/13 colourless bottle black crust 3/15 colourless drinking-glass brown-black layered crust 3. QUANTITATIVE ANALYSIS OF THE BULK GLASS Quantitative analysis of medieval glass samples can reveal information about the raw materials used. A difference is made between the use of wood ash and fern ash as raw material to produce glasses called forest-glass and fern-glass respectively. It does not seem to be useful nor correct to call these glasses potash glasses since potash is only a part of the raw material used for their production. Based on their major/minor composition and their K 20/CaO, CaO/MgO and MgO/MnO ratios, Wedepohl [3] divided medieval glasses into six subtypes : soda- lime, soda-ash, wood-ash, wood ash-lime, wood ash-lead and lead glasses. Further subdivisions in the wood ash glass types were made by other investigators : in beechwood ash and ash from other trees, oaks for example, the ratio KaO/CaO is 0.5 to 0.7, whereas a K 20/CaO ratio larger than 1 is found in fern ash and ash from grasses [4-6]. It is assumed that no lime was added to the melt during the production of simple forest-type glasses, hence the magnesium and phosphorus content (two elements present in the ashes) of the glasses can be unambiguously correlated with the calcium content. These type of glasses contains on average 0.7 % FeO, giving them their typical greenish-yellowish colour. During the late Middle Ages a modified wood ash based recipe appeared [1]. The S i0 2 content of the glasses which are produced on the basis of this recipe, remained more or less the same but the K 20/C aO ratio was clearly reduced; these glasses are called forest-lime glasses. An increase in the CaO/MgO ratio probably points to an addition of lime to the glass melt. 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses The quantitative results for major, minor and trace components in the Namen and Fagnolle glasses are shown in Table 3 and 4 respectively. Except for one sample, F2/10, these two series of glasses obviously belong to the forest-lime glasses. They all show the typical major composition and a K 20/C aO ratio below 0.8. Also, the trace composition is different from the one found in Roman glasses. On the one hand a overall low Sb 2 0 5 content is found which may indicate that this material was no longer used as a decolouriser during the Middle Ages. On the other hand a higher concentration was found for barium and lead in most of the samples. The deep-blue colour in the Fagnolle glasses 3/8 and 3/9 was brought about by the addition of cobalt which probably also introduced some other trace elements such as copper, nickel and lead. Table 3. Composition of the Namen glasses, %w as determined by EPXMA above the horizontal line and ppm as determined by p-SRXRF below it. N40 N43 N45 N46 NazO 0.71 0 .8 6 0.77 1.41 MgO 2.05 2.30 2.47 3.02 ai 2 o 3 2.75 3.52 3.79 2.16 Si04 58.33 51.84 52.34 52.72 p2 o 5 3.42 4.00 3.84 4.15 s o 3 0.97 0.43 0.51 0.87 Cl 0.51 0.59 0.58 0.78 k2o 6.29 6.99 8 .0 1 11.30 CaO 2 2 .1 0 25.68 24.19 20.38 Ti0 2 0.56 0.67 0.64 0.65 MnO 0.93 2.42 2.14 1 .6 8 Fe 20 3 1.39 0.71 0.72 0.87 CoO 166 96 87 NiO 67 76 6 6 CuO 190 166 157 ZnO 490 298 279 As20 3 2 0 1 2 7 Br 2 3 3 RbzO 1 1 1 151 170 SrO 965 2185 1684 y2o 3 30 15 15 Zr02 280 175 172 Mo20 3 2 3 1 Sn02 246 38 18 Sb20 5 4 0 0 BaO 2179 5115 3899 PbO 2540 62 37 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation o f Some Historical Potash Based Glasses Fagnolle sample 2/10 has a different composition which is more comparable to the Roman glasses discussed in the previous chapters. The ashes from marine plants at the seashore, such as Salicornia Herbacea, have a composition suitable to be used as alkali source for glass production. Analysis shows that their major component is sodium carbonate, which will eventually produce a soda-glass. These plants contain as minor constituents calcium, magnesium and also potassium. Their magnesium and potassium content are higher than in terrestrial salt deposits such as natron which were mostly used during the Roman period. This distinction makes it possible to decide on the raw materials used after chemical analysis of ancient glass samples. Table 4. Composition of the Fagnolle glasses, %w as determined by EPXMA above the horizontal line and ppm as determined by p-SRXRF below it. F1/2 F1/3 F2J2 F3/3 F2J4 F2J5 F2/7 F3/8 F3/9 F2/10 F2/13 F3/15 Na20 1.39 0.92 0.56 0 .0 1 0 .0 1 0.77 0.14 1 .1 1 1.59 11.87 0.69 1.50 MgO 4.05 2.79 4.24 1.59 1 .6 8 4.25 3.24 4.45 5.21 0.40 2.95 4.10 ai 2o 3 3.48 1.08 2.18 2.15 1.27 1.47 2.48 2.46 2.36 0.79 3.27 1.57 Si04 57.68 63.09 57.11 63.56 64.46 57.78 62.67 59.47 56.37 71.63 60.05 57.25 p2o 5 3.48 1.79 3.21 1.40 0.80 3.28 1.75 2.74 3.11 0.32 2.42 2.90 so3 0.56 0.05 0 .0 0 0.23 0.23 0 .0 1 0.27 0.08 0.03 0 .0 0 0 . 1 0 0.13 Cl 0.31 0.36 0.24 0 .0 0 0 .0 0 0.38 0 .0 1 0.23 0.28 0.89 0.32 0.39 k2o 7.91 10.54 1 0 .1 0 5.83 12.51 10.55 7.40 9.17 9.74 4.62 7.46 8.92 CaO 19.39 18.87 19.89 23.46 16.48 19.53 18.96 18.16 18.70 7.67 20.79 2 0 .2 2 Ti02 0.19 0 .0 0 0.31 0.55 0.36 0 .2 0 0.41 0.23 0.13 0 .0 2 0.38 0.42 MnO 0.90 0.31 1.27 1.15 1.73 1.14 2 .0 2 0.84 1 .1 1 1.35 0.92 1 .0 1 Fe 20 3 0.84 0.54 0.90 0.27 0.47 0.64 0.77 0 .8 6 1.26 0.43 0.79 0.61 CoO 51 51 45 162 15 123 259 3686 1538 15 165 89 NiO 23 17 0 50 33 74 135 1311 786 0 78 62 CuO 128 58 50 96 137 289 140 489 1061 71 383 226 ZnO 268 258 80 209 307 413 277 368 348 1 0 0 354 388 As20 3 23 7 14 108 1 0 217 3437 11 0 62 0 Br 3 13 4 0 1 4 0 2 4 39 4 5 Rb20 5 93 7 71 340 245 187 207 169 42 140 186 SrO 57 807 71 954 1401 1079 1158 871 1 1 0 2 354 887 987 y2o 3 6 7 9 11 16 2 2 15 15 2 2 7 28 16 ZrOz 89 45 116 228 292 217 216 172 148 153 2 1 2 186 Mo20 3 0 5 0 0 0 3 0 69 51 1 2 1 SnOz 47 3 8 8 1 59 161 160 551 174 317 315 188 Sb20 5 0 1 2 0 0 0 0 2 1 0 0 1 0 BaO 116 347 119 4682 3902 2407 2550 2806 3658 647 3572 3592 PbO 543 138 785 12 51 1707 254 2 2 0 1 1 1 0 2 306 2127 1088 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses 4. PREDICTION OF DURABILITY 4.1 Triangular diagram The durability of the Namen and Fagnolle glasses was tested using the triangular diagram. The background and the calculations were explained in Chapter 3, Section 6.1. The results are shown in Figure 1. Since these medieval glasses contain potassium as their major alkali it was expected that they would be situated near the right hand bottom corner of the triangle near other less durable glasses which show pitting and crusting corrosion phenomena (B, D, E, F and G). This is true for all the glasses except sample Fagnolle 2/10, the soda-glass, which is situated in the middle together with the durable Saxon and Roman window glass and the modem float glass. This sample is the only one of the series which does not show any visible corrosion whereas the surface of the others is pitted, iridescent or has a brown-black crust. A : modern float glass M : Saxon window glass R : Roman window glass W : weeping glass Z : crizzled glass B, C, D, E and P : less durable window glass H, J and Q : least durable window glass * : Namen glass # : Fagnolle glass R 2O F2/10 RO Figure 1. Comparison of the durability of the Namen and Fagnolle glasses with other glass types. 153 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses 4.2 Thermodynamic approach The free energy of hydration and the rate of corrosion of the Namen and Fagnolle glasses were calculated as was explained in Section 6.2 of Chapter 3 and the values were compared to the ones found for medieval and Roman glasses excavated in Great Britain. The results for the glasses from Namen are represented in Figure 2 by “+” and the ones from Fagnolle by “x”. Except for sample 2/10 from Fagnolle, which falls right in between the Roman glasses WRX, all the glasses are situated around the medieval glasses EP1, EP4 and SAA1 as could be expected from their major composition. RA6 S C 6 - E P 4 EP1 w S c T l RA4 + a>re sa AI *5* XC E XX S. c o in o fc 0.1 durabilityIncreasing o o F2/10 ■S \W RX1 X 2 c \ “ WRX6 WRX 10 0.001 -45 -40 -35 -30 -25 -20 -1 5 -10 ■5 0 Free energy of hydration (kJ/mol) Figure 2. Comparison of free energy of hydration and corrosion rate between glasses from Great-Britain (Na-based Roman glasses (WRX) and K-based medieval window glasses (EP, RA, SC, SAA)) and glass from Namen (+) and Fagnolle (x). 5. Corrosion phenomena Inspection of cross-sections of the medieval glasses reveals that also in these samples the three important areas, namely the bulk glass, the leached layer and the crust have formed. The bulk glass is dealt with in section 3, the two other regions will be discussed in detail in what follows. 5.1 The leached layer X-ray maps were collected from of the embedded glass Fagnolle 2/5 and the results are shown in Figure 3. Nothing unexpected occurred during the time of burial; alkali 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses such as Na and K as well as the alkaline earths Mg and Ca are clearly depleted in the leached layer, on the contrary, Al is slightly enriched in this area. Figure 3. X-ray maps of the cross-section of glass Fagnolle 2/5 showing the distribution of elements across the bulk glass, the leached layer and the crust. 155 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses Layered corrosion morphology, common in glasses which have been buried for a long period of time, is also encountered both in the Namen and the Fagnolle glasses. In contrast to the Qumran and other Roman glasses whose individual layers are less than 1 pm in thickness, the thickness of the sublayers in these medieval glasses is between 4 and 8 pm. In most cases the layers have evolved parallel to the glass surface and to each other. Figure 4a and b represent a secondary electron and a backscattered electron image respectively showing in detail this structure in sample Fagnolle 2/2. a) X 1 0 0 1 8mm Figure 4. Secondary electron (a) and backscattered electron image (b) of the layered corrosion morphology in the leached layer in Fagnolle glass 2/2. 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses Figure 4b also demonstrates that high Z elements have intruded from the surrounding soil into the glass since the outer area became lighter in colour. This phenomenon is even more striking in the Namen glass 46. Figure 5a shows a backscattered electron image from a part of the cross-section of this sample in which a dendritic deposition of a higher Z element, which has probably entered the glass from the environment, along a crack in the glass can be observed. In Figure 5b, showing one of these structures in detail, it appears that the layered morphology is not disturbed but followed by these precipitations. a) b) Figure 5. Backscattered electron image of Namen glass 46 showing intruding materials along a crack (a) and the deposition of these elements in detail (b). 157 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Gfasses V' {y J ' \ ■,}[ J' <■ ■ - s Figure 6 . X-ray elemental maps of the same area of glass Namen 46 as represented in Figure 5a. 158 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses To have an idea about which elements are entering the glass, elemental X-ray maps were collected from the area represented in Figure 5a and the results are shown in Figure 6 . Manganese is the major constituent which supports the view that these intrusions might consist of manganese oxide. Next to manganese, lead, probably in the form of lead sulphide, is also present, especially along cracks. Other maps show that also elements such as iron, phosphorus and chlorine are enriched in these regions, which may indicate that a mixture of different salts has precipitated into the gel layers. Unfortunately, the more resistant glass, Fagnolle 2/10, did not show any leached layer so no comparison could be made between this glass and the others. 5.2 The surface From the X-ray maps of Figure 3 it becomes clear that a calcium-rich crust has formed; other elements such as iron, manganese, phosphorus and lead are present in this outer layer as well, which probably explains its black colour. Unfortunately no information about the corrosion products formed could be obtained using X-ray diffraction nor by means of infrared measurements. 1 0 fr-1 m X950 Figure 7. Secondary electron image of a pit on the surface of Namen glass 46. Next to the formation of a crust, a type of alteration typical for medieval glasses called pitting corrosion, is visible on these glasses. This type of corrosion is generally observed For glasses containing between 57 and 63 mol% S i0 2 (see Tables 3 and 4) [7, 8 ]. The mechanism of the formation of these structures is still not well understood. Figure 7 shows a secondary electron image of a pit found on the surface of sample Namen 46. Although the surface around the pit appears 159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses not well understood. Figure 7 shows a secondary electron image of a pit found on the surface of sample Namen 46. Although the surface around the pit appears intact, a circular area of ca. 50 pm diameter shows material deterioration. Cracks have formed in this area and corrosion products are accumulated in the cavity. 6. CONCLUSIONS Two series of potash based medieval glasses from Namen (14th-15th century AD) and Fagnolle (15th-16th century AD), Belgium were studied to compare their composition and corrosion behaviour with the Roman glasses described in previous chapters. The quantitative results for major and minor components clearly show that the glass objects (except for Fagnolle sample 2/10) belong to the potassium-rich forest-lime glass type, which means that, compared to the Roman soda-lime-silica glasses, they have a much higher concentration of K20 since potassium was used as the major alkali instead of sodium. Moreover, they also contain more magnesium, phosphorus and calcium. When trace elements are considered, an overall low Sb 20 5 content and in most objects a higher concentration for barium and lead is found. Fagnolle sample 2 / 1 0 , which is a soda glass, shows a composition which is comparable to the Roman glasses except for the higher magnesium and potassium concentration, pointing to the use of ash from marine plants (rich in sodium) as alkali source. Cross-sections of the glass samples demonstrate that, just as in the Roman glasses, three regions, bulk glass, leached layer and crust, can be distinguished. Alkali (Na and K) as well as alkaline earths (Mg and Ca) are depleted in the leached layer while Al is slightly enriched in this region. Layered corrosion morphology is also encountered in these glasses but the individual layers are between 4 and 8 pm thick in contrast to the Roman glasses where they are only 1 pm or less in thickness. Higher Z elements such as manganese, lead and iron have intruded into the glass from the environment, leading to dendritic depositions, especially along cracks. Next to the formation of a crust, rich in calcium, iron, manganese, phosphorus and lead, also pitting corrosion was present on the surface of these glass objects. This is a type of alteration which is typical for medieval glasses containing between 57 and 63 mol% of S i0 2. 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5 : Investigation of Some Historical Potash Based Glasses REFERENCES [1] Newton R.G., Recent Views on Ancient Glasses, Glass Techn., 21(4), pp. 173-183. [2] Hawthorn H.J. and Smith C.S., On Divers Arts : The Treatise of Theophilus, 2nd Ed., University of Chicago, pp. 55-56 (1976). [3] Wedepohl K.H., Krueger I. and Hartmann G., Medieval Lead Glass from North-western Europe, J. of Glass Studies, 37, pp. 65-82 (1995). [4] Geilmann W., Beitrage zur Kenntnis Alter Glaser. III. Die Chemische Zusammensetzung Einiger Alter Glaser, Insbesondere Deutscher Glaser des 10. bis 18. Jahrhunderts, Glastechn. Ber., 28, pp. 146-156 (1955). [5] Meiwes K.J. and Beese F., Ergebnisse der Untersuchung des Stoffhaushaltes eines Buchenwaldokosystems auf Kalkstein, Ber. Forschungszentrum Waidokosysteme Reihe B, 9, pp. 1-142 (1988). [6 ] Wedepohl K.H., Die Herstellung Mittelaterlicher und Antiker Glaser, Akademie der Wissenschaften und der Literatur, Mainz, Franz Steiner Verlag, Stuttgart, Germany, pp. 1-38 (1993). [7] Schreiner M., Deterioration of Stained Medieval Glass by Atmospheric Attack. Part 1. Scanning Electron Microscopic Investigations of the Weathering Phenomena, Glastech. Ber., 61(7), pp. 197-204 (1988). [8 ] Bettembourg J.M., Conservation Problems of Early Stained Glass Windows, Proceedings of the Stained Glass Symposium, Conservation Paper 6 , Crafts Advisory Committee, London (1977). 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 Burial Experiments with Model Glasses Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses 1. INTRODUCTION The glass fragments which have been analysed and described in the previous chapters, have all been buried for a considerable amount of time (1900 years for the Roman glasses from Qumran, between 1900 and 1500 years for the other Roman glasses and 500-600 years for the medieval glasses). Due to the contact with groundwater the glass was affected and corrosion and leaching processes took place. Although extensive research is done on test materials in weathering chambers to study the importance of parameters in the atmosphere, little is known about the factors which determine the alteration of glass in damp soil. An attempt was made in 1963, when a British Association burial experiment was started at Wareham in Dorsetshire, United Kingdom in order to provide archaeologists and technologists with information about the rates of deterioration of materials. Various types of material were buried and samples of nine types of glasses were included. It was intended that samples should be excavated for examination of the deterioration after certain periods of time [ 1 ]. The types of glasses which were included in this experiment were as follows : 1 : Newly made glass of typically Roman composition. 2 : Newly made glass of typical Medieval composition. 3 : Newly made glass of composition similar to that of the linen-smoother found at Hangleton [2]. 4 : Modern plate glass with two surfaces ground and polished. 5 : As No. 4 but with the surfaces left in the “as produced” condition. 6 : Glass spheres, about 2 cm diameter, made of “E” glass, a low-alkali glass used for making electrical insulation glass fibre. 7 : Heat-resisting borosilicate glass used for making glass cooking ware. 8 : High-quality soda-lime silica optical glass in the form o f approximately 2.5 cm cubes with all surfaces ground flat. 9 : Lead optical glass also in cube form and with ground surfaces. The chemical composition of the glasses (in w%) is given in Table 1. Fletcher realised that the acid soil at Wareham (pH 4.5 to 5.0) was unlikely to produce much effect on even the least durable of the glasses (which was confirmed later on) and therefore took the initiative in establishing another burial experiment in carboniferous limestone (pH 9.55 to 9.75) at Ballidon, Derbyshire, United Kingdom in 1970, including the same nine glass types [3]. The aim of the investigation was to excavate and examine the Wareham and Ballidon samples at various times over a number of years. Comparison among them and with the unburied original samples made it quite evident that even after several years only the most sensitive of the glass samples and only the ones buried at Ballidon shows any noticeable deterioration. In all the other cases the effects of burial were insignificant [3-8]. The glass pieces of type 3 have shown markedly 163 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses different behaviour depending on the soil. Those buried in acid soil showed only very slight iridescence after nine years of exposure whereas the samples buried in limestone showed clear crazing of the surface after only one year. Some parts of the surface showed flaking and pitting after two years in this environment and after eight years most of the surface was completely flaking away. The simulated medieval glass and Roman glass, the modern plate glass and high lead optical glass showed varying amounts of iridescence and the soda-lime-silica optical glass, the borosilicate ovenware and the E glass marble (used for making glass fibres) were mainly unaffected even after nine years in both environments. Table 1. Composition of the glasses (%w) used in the burial experiments in Wareham and Ballidon. Oxide 1 2 3 4 5 6 7 8 9 Roman Medieval Medieval plate glass E-glass borosil. soda-lime lead gl. polished rough B 2 O 3 - ---- 8 . 0 1 4 .3 0 . 6 - F - --- - 0 .5 -- - N a zO 18.2 2 . 6 0 .7 13.3 13.3 0.25 4.2 1 0 . 6 8 . 0 M g O 0 .5 4 .7 5 .4 3.1 3.1 4 .4 - 1 . 0 - AI 2 O 3 3 .6 4 .3 3 .5 1 . 1 1 . 1 1 4 .4 1 . 8 0 . 8 - S i0 2 6 8 .5 50.1 46.5 72.7 72.7 54.3 79.1 70.1 54.2 - --- P 2 O s - 4 .6 -- - k 2o 1.3 1 6 .7 1 6 .4 0 .5 0 .5 0 .3 5 0 .3 5.1 4.1 C a O 7 .3 1 8 .6 2 1 . 6 9 .2 9 .2 1 7 .4 0 . 2 9 .5 - M n O 0 .3 4 0 .6 3 ------ F e 2 0 3 0 .4 9 0 .9 5 1 . 2 0.09 0.09 0.42 0.03 - - C o O - 1 .0 3 ------ C u O - 0.08 ------ AS 2 O 3 ------0 .0 6 0 . 1 Sb20 5 ------0 .3 0 .0 5 B a O ------2 . 0 - PbO - 0 .1 ------3 3 .6 It can be concluded that most of the samples are far too durable to provide much information of interest to archaeologists during short term exposure experiments. Accordingly, in the experiments discussed below, much more sensitive, i.e., rapidly corroding glass types were used which allow to observe changes in composition after exposure times of 3-6 months. Other burial experiments were carried out by the Study Centre for Nuclear Energy (SCK), Mol, Belgium. In the Belgian Programme on “Management of Vitrified High- level Waste” the intermediate storage of this vitrified nuclear waste is considered before final geological disposal. A clay formation, the Boom clay, is being evaluated as a possible host formation for eventual disposing. Studies were started to investigate the interaction between high-level waste glass and the clay disposal 164 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses medium. Both the dissolution of the inactive glass matrix constituents and the leaching behaviour of the radionuclides of the elements of interest (Pu, Am, Np, Tc, Cs and Sr) were studied. Pamela and R7T7 Cogema type high-level waste glasses, which are all high in boron and lithium, were used in the experiments. Next to experiments in the laboratory at 90 and 170 °C, in-situ tests in the clay were carried out at 16 °C to verify the glass corrosion in a realistic environment. The glass samples were studied with different microanalytical techniques after 2, 3.5 and 7.5 years. The main observation at clay temperature (16 °C) was the quasi-absence of corrosion for most of the relevant waste glasses while during the tests at 90° and 170 °C surface gel layers up to several hundred pm in thickness had formed on the glasses. Selective depletion as well as congruent dissolution had occurred and different zones in the leached layers could be distinguished characterised by depletion and/or intrusion of various elements [9-13]. Both the Wareham/Ballidon experiments with archaeological and modern glass and the nuclear waste glass experiment showed that glass is only seriously attacked in alkaline environment and that under these circumstances the glass composition, surface quality and exposure time determine the final physical and chemical state of the affected surface layers. However, neither experiment allows to explain the difference in corrosion rate and degree of deterioration observed even with the naked eye between Roman glass excavated in the Middle East (Qumran samples) and the glasses found in various Western-European sites. The above-mentioned experiments indicate that several factors influence the corrosion rate and physical and chemical changes occurring during the glass-soil interaction. Therefore, a series of controlled corrosion experiments were conducted using fast-corroding glass types with the ultimate aim of obtaining a clearer insight into the various corrosion processes. It was decided to start a systematic study together with Dr. D.R. Fuchs and Dr. H. Romich from the Fraunhofer-lnstitut fur Silicatforschung in Bronnbach, Germany on the corrosion of glass when it is brought in contact with damp soil. In this Institute model glasses with known composition and controllable case history (with the different steps in the production process known) are produced. These model glasses were previously used for weathering experiments in both outdoor conditions and gas pollutant chambers in the laboratory [14, 15]. The model glasses were used to establish a correlation between specific environmental parameters, such as humidity and concentration of S 0 2 and oxidising gases in the atmosphere, and the corrosion process of different glass types. On the one hand, the locations for weathering outdoors reflected a spectrum of different environmental conditions, especially with regard to factors that stimulate corrosion such as moisture and pollutants. On the other hand, the simulation of environmental conditions in weathering chambers had decisive advantages such as ease of reproduction (irrespective of unrepeatable weather conditions) and the possibility of time-lapse experiments by increasing the dosage. Next to these experiments, the model glasses also have been used as environmental sensors to study the combined 165 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses impact of climatic stresses, pollution and microbiological effects on monuments and art objects since the corrosive change in the sensors is a direct measure of local stress factors [14, 15]. Although many weathering experiments were performed, such sensors were never employed in burial experiments. The aim of the present study was to investigate the importance of parameters such as the composition of the soil the model glasses were buried in, the pH, the concentration of water present and the temperature in indoor as well as in outdoor environments. On the one hand the glasses were subjected to a series of controlled environments in the laboratory and on the other hand they were buried in five different natural environments. After exposure periods of three and six months the glasses were recovered and examined with a number of microanalytical techniques in order to obtain a better insight into the migration behaviour of various elements into and out of the glass and to measure the corrosion rate. The underlying assumptions of such experiments are that (1) in a corrosion-prone glass, in a relatively short exposure time, alteration layers will be produced that are similar to those occurring over long periods of time on archaeological glass that corroded in natural soils and that ( 2 ) the corrosion rate will be similarly dependent on the above-mentioned parameters in model and ‘real’ glasses. 2. PRODUCTION PROCESS AND COMPOSITION OF MODEL GLASSES 2.1 Production process The glasses which were produced in the Fraunhofer-lnstitut were molten from analytically pure oxides and carbonates using platinum crucibles in an electric kiln at 1450 C for about two hours. Afterwards the glass melt was poured into blocks, tempered for 30 minutes at a temperature of 30 °C above the transformation temperature and gradually cooled to room temperature over a period of 16 hours. All batches yielded clear glasses. Glass blocks (2.5 x 3 x 5 cm) were cut into slices of 0.7 mm in thickness with a low speed saw using a diamond blade with oil-bath cooling. The slices were polished by fire treatment in a gas furnace (Speedburn II F). The specimens were quickly heated to a temperature of 700 to 800 °C and after a period of 5 to 8 minutes they were slowly cooled to room temperature. The time and temperature were adjusted for each glass type to give the best polishing effect with the lowest level of surface damage. Before and after the polishing process, the samples were cleaned with petroleum ether (50/70) and ethanol [14, 15]. 166 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses 2.2 Composition of the model glasses The selected model glasses (types M1.0, M2.0 and Mill) are all restricted to the three component system Si0 2-K 20 -C a 0 , which is very sensitive to attack by water and humidity. M1.0, which is the most sensitive one, contains the highest K 2O /Si0 2 ratio; for glass Mill, the least sensitive, the reverse is the case while glass M2.0, has intermediate sensitivity and composition ratio. The nominal composition of the model glasses is given in Table 2 in molar %. Table 2. Composition of the model glasses used in the burial experiments (in mol%). Oxide M1.0 M2.0 Mill S i0 2 59.8 61.2 62.3 CaO 2 0 . 0 23.4 27.8 k 2o 2 0 . 2 15.4 9.9 3. BURIAL CONDITIONS 3.1 Natural environments In analogy with the burial experiments in Wareham and Ballidon, model glasses were buried in some natural environments subject to all temperature and humidity changes taking place during the time of the study. As examples of Western- European burial environments, four different test areas in the northern part of Belgium (Flanders), used by the Department of Forestry of the University of Ghent, namely Brasschaat, Wijnendale, Gontrode and the Zonienforest, were selected as exposure sites. These locations were chosen because they are well documented and are also closed to the public, so that no samples would be accidentally removed during the time of burial. All the soils are acidic with a pH between 3.8 and 4.5 with the exception of the deepest zone in Gontrode which has a pH value of 6.0. Brasschaat has a moderately humid sandy soil with a low moisture retention capacity and the lowest concentration in potassium, calcium and magnesium. In Wijnendale a light sand loamy soil is present, in Gontrode a sand loamy soil and in the Zonienforest a loamy soil. All these sites have a higher moisture retention capability and ion exchange capacity than sandy soils like in Brasschaat. The ion exchange capacity is defined as the sum of the exchangeable ions that a soil can absorb. This soil characteristic might be important for glass corrosion since the chemical composition of the solutions which circulate in the burial site, will be very dependent of its ion exchange 167 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses capacity [16]. The most important soil characteristics of the test areas are summarised in Table 3. Table 3. Summary of the soil characteristics in the test areas. Brasschaat Wijnendale Gontrode Zonien Texture sand light sandy loam sandy loam loam pH 3.8-4.1 3.8-4.3 4.1-4.5 4.1 Moist, retention cap. (mm) 50.50 108.85 206.88 248.32 Ion exch. cap. (meq / 1 0 0 g) 1.58-2.79 4.32-3.51 7.46-13.82 3.29 In the test areas, gauge-pits were made to study the stratification of the soil and these were used to bury the glass samples at different depths. In Brasschaat the glasses were placed at 17 and 70 cm depth, in Gontrode at 35 and 75 cm, in Wijnendale at 30 and 85 cm and in the Zonienforest at 17 cm. The glass sheets were put in special holders to assure that they could be recovered after 3 and 6 months. The way of burying the model glasses in the test areas is illustrated in Figure 1. The first series of model glasses ( 6 months) was buried during fall and winter while the second series (3 months) spent summer underneath the soil. Figure 1. Photograph of one of the gauge-pits in the test areas and some model glasses in their holders after being buried for 3 months. A fifth location, Edegem, Belgium with soil similar to the one in Brasschaat was included and this soil was used for the laboratory experiments. 168 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses No burial experiments in the desert soil at or near Qumran could be planned; however, soil from the Qumran site was collected and used as one of the controlled laboratory environments. This soil (containing a considerable amount of rocks) originates from the contemporary top soil (ca. 2 0 cm deep), immediately adjacent to the Qumran site. The soil was transported to Europe in a closed double-walled polyethylene bag. 3.2 Controlled laboratory environments Next to the natural test areas, the model glasses were also brought into a series of controlled environments in the laboratory. Soil was put into plastic boxes which were hermetically closed from the surroundings to assure that no moisture or other components could enter or leave the containers. One box was kept at room temperature and nothing was added to the soil. To the soil inside the other boxes water or CaO was added or the boxes were kept at other temperatures in order to investigate the influence of a series of parameters on the corrosion of buried glasses. These parameters are ; 1) Humidity/water content 2) pH 3) Temperature 4) Glass-soil exposure time 5) Surface condition of the model glasses In addition, model glasses were also exposed in boxes in the laboratory to distilled water for three and six months and to the very fine Qumran desert soil for six months. 3.2.1 Humidity 200 g of the soil was weighed and dried until constant weight. The weight loss, determined to be 4 g, was assumed to be the water content of the soil (2 ml water/100 g soil). The humidity in other boxes was altered by adding distilled water to the soil. To a first box 2 ml water per 100 g soil was added, to a second 4 ml water per 1 0 0 g soil and to a third 6 ml water per 1 0 0 g soil, resulting in boxes containing 2, 3 and 4 times the original water content. In a last box water was added until the soil was clearly saturated with liquid. Since the soil from the Qumran site was completely dry, it was saturated with water before the experiment which turned it into a yellowish sticky substance. 169 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses 3.2.2 pH The pH of the soil was electrometrically determined using a standard procedure [17]. A 20 g air-dry earth sample was weighed, put into a bottle and 50 ml of distilled water was added, yielding a standard soil:water ratio of 1:2.5. Afterwards the bottle was closed and shaken for about 5 minutes. A pH meter, which was previously calibrated, was then placed into the soil suspension and the average of three readings taken. The electrode was rinsed after each separate determination. The pH of the soil was increased by adding different amounts of CaO which resulted in an increase of the original pH of 5.6 to a value of 8.5, 8 . 8 and 9.1. This turns out to be the pH range where leaching is very low and network dissolution does not occur. Unfortunately no method was found to decrease the pH in a fast and easy way without changing too much of the other parameters. The Qumran soil has a pH of 7.6 which means that it is less acidic than the Western-European soils used in the burial experiments. The model glasses were also exposed to distilled water with a pH of 7.4. The pH was measured again after the experiments and it turned out that it had increased to 7.9 after three months and to 8.3 after six months. 3.2.3 Temperature Room temperature (in the laboratory) around 22 °C was taken as reference and other boxes were placed in environments with both lower and higher temperature. A refrigerator with a temperature of 5-6 °C, a freezer at -10 °C and an oven at 50 °C were used. The temperatures also correspond to the natural range buried archaeological glass might be exposed to in a natural environment. Care was taken not to raise the temperature too high because in that way the corrosion processes can be disturbed and changed [17]. 3.2.4 Exposure time First the model glasses were exposed to the natural and laboratory environments for a period of six months; in a second phase a second series of model glasses was exposed to the same environments but only during a three month period. 3.2.5 Surface condition of the model glasses The second series of M2.0 model glasses (M2.0/2, i.e., the ones which were buried for three months) were made using a slightly different production process. The oven in which the glasses were prepared had been moved to another location, resulting in a small change of position of the temperature sensor inside. The glasses produced 170 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model G/asses after the oven removal showed a clearly different, rougher surface structure as could be observed with the naked eye. These model glasses made it possible to investigate whether, next to the composition of the model glasses, the surface structure also influences the corrosion behaviour. The different outdoor and laboratory experiments are summarised in Table 4. Table 4. Summary of the different experiments in the outdoor test areas and in the laboratory. A OUTDOOR BURIAL Environment Six months, samples Three months, samples 1 Brasschaat, depth 17 cm M1.0, M2.0, Mill M 1.0/2, M2.0/2 2 Brasschaat, depth 70 cm M1.0, M2.0, Mill M 1.0/2, M2.0/2 3 Gontrode, depth 35 cm M1.0, M2.0, Mill M 1.0/2, M2.0/2 4 Gontrode, depth 75 cm M1.0, M2.0, Mill M 1.0/2, M2.0/2 5 Wijnendale, depth 30 cm - M 1.0/2, M2.0/2 6 Wijnendale, depth 85 cm - M 1 .0 /2 , M 2 .0 / 2 7 Zonienforest, depth 17 cm M1.0, M2.0, Mill M 1.0/2, M2.0/2 8 Edegem, depth 28 cm M1.0, M2.0, Mill M 1.0/2, M2.0/2 B CONTROLLED LABORATORY ENVIRONMENTS Environment Six months, samples Three months, samples 1 No addition, room temperature M1.0, M2.0, Mill M 1.0/2, M2.0/2 2 4 ml water/100 g soil M1.0, M2.0, Mill M 1.0/2, M2.0/2 3 6 ml water / 1 0 0 g soil M1.0, M2.0, Mill M 1.0/2, M2.0/2 4 8 ml water / 1 0 0 g soil M1.0, M2.0, Mill M 1.0/2, M2.0/2 5 Saturated with water M1.0, M2.0, Mill M 1.0/2, M2.0/2 6 pH 8.5 M1.0, M2.0, Mill M 1.0/2, M2.0/2 7 pH 8 . 8 M1.0, M2.0, Mill M 1.0/2, M2.0/2 8 pH 9.1 M1.0, M2.0, Mill M1.0/2, M2.0/2 9 In freezer (-10 °C) M1.0, M2.0, Mill M1.0/2, M2.0/2 1 0 In refrigerator (5-6 °C) M1.0.M2.0, Mill M 1.0/2, M2.0/2 11 In oven (50 °C) M1.0, M2.0, Mill M1.0/2, M2.0/2 1 2 In distilled water M1.0, M2.0, Mill M1.0/2, M2.0/2 13 In soil from Qumran M 1.0/2, M2.0/2 - 4. ANALYTICAL TECHNIQUES USED TO STUDY THE CORROSION OF THE MODEL GLASSES A number of analytical techniques were employed to study the corrosion level o f the model glasses. The methods provide partly overlapping and partly complementary information. 171 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses SEM and IR both allow to measure the degree of corrosion of the glasses, the first through inspection of the thickness of the leached layer in secondary electron images on cross-sections of the model glasses embedded in a resin and the latter through measurement of the intensity of the OH-absorption band of the model glasses before and after the experiments. Next to an intermediate region, in which both methods can be used, the IR method allows to compare degrees of corrosion when the corrosion layers are too thin to be measured with the SEM (AE below 0.4, thickness of the leached layer below 1 pm); alternatively, in the case of very thick corrosion layers (AE above 3, when the IR-spectrometer saturates), the SEM data can be used. In addition, secondary electron images, obtained by SEM, were also used to study the outlook of the leached layer and the crust while X-ray maps showed the distribution of the major elements throughout these different areas. Since only information about major elements can be obtained by this technique, linescans were collected by SIMS to study the diffusion behaviour of minor and trace elements into and out of the model glasses during burial. To check the results obtained by SIMS, also p-SRXRF linescans were collected but only from the model glasses with the thickest leached layer because of the poorer lateral resolution of this method. Finally, an IR-spectrometer coupled to a microscope was employed to try to characterise the products which had formed in the surface crust. 5. RESULTS FROM IR AND SEM MEASUREMENTS 5.1 Infrared measurements For the characterisation of the corrosion process, the most important part of the IR absorption spectra ranges from 4000 to 2000 cm'1. The increase in the intensity (AE = E - E0) of the OH-absorption band, which is situated at 3350 cm'1, represents a direct measurement of the degree of corrosion of the glass surface. IR spectra (200 scans in the mid-IR region with a resolution of 4 cm"1) were recorded in transmission on a Nicolet 20DXB FTIR spectrometer before and after burial of the model glasses as shown in Figure 2, providing the absorbance of the OH-band, E 0 and E respectively. The water content of the leached gel layer is measured as well as the water content of the corrosion products in the crust. This method cannot be used for highly corroded samples. If the crust is too thick, there will not be enough transmission of the signal; on the other hand, if the leached layer is too extended the spectrometer will become saturated. Figure 3a and b show examples of the IR spectra of model glass M1.0 before and after burial for six months in the outdoor test area in Gontrode. The OH-band intensity is clearly increased in the spectrum obtained after burial. 172 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6: Burial Experiments with Model Glasses 3350 cm' 3350 cm’ Wavenumber Wavenumber measurement E measurement E, ► ► IR spectrometer IR - - -spectrometer IR m m m m 0 Measurement (after experiment) Zero measurement (before experiment) \ \ \ \ AE = E - E Measure for the corrosive effect IR IR ' 1 r + CD 3 3 « d o. CD o H i rr CD jo 3 CD 03 W c CD (D ■o 3 c -t !° T ■o (5' 1 -n co Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses on on cn co cvi on 3 6 0 0 3 2 0 0 2 8 0 02 6 DO 1 6 0 0 1200 WAVENUMBER cn in on OJ c_j ^on g e n cn cn 00 to o o 3 6 0 0 3 2 0 0 2 8 0 0 2 f 0 0 2000 160 0 1200 WAVENUMBER Figure 3. IR-spectra of model glass M1.0 before (a) and after (b) burial for six months in the test area in Gontrode. 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6: Burial Experiments with Model Glasses 5.2 Infrared results before burial IR spectra were recorded from the model glasses before they were buried. The average and standard deviation of these measurements for the different model glasses are shown in Table 5. They were used as zero values (E0) and were subtracted from the results obtained after the glasses were excavated from the soil. The fact that the values are not equal to zero indicates that, although the glasses were kept in vacuum in a desiccator, a very thin layer of the surface was already attacked by water or moisture before the experiments were carried out. This is not surprising since the model glasses, especially M1.0, have a composition that is very sensitive to corrosion. Two series of model glasses were used for the six months and three months experiments respectively. A lower E0-value was found in the second series of model glasses (M1.0/2 and M2.0/2) which means that they were attacked to a lesser degree by moisture before the experiments, probably because they were used faster after production or they were better protected from the environment. Table 5. Results of the infrared measurements of the model glasses before burial. Model glass Average E0 and standard deviation M1.0 (7 glasses) 0.20 ± 0.02 M 1 .0 / 2 ( 6 glasses) 0.07 ± 0.02 M2.0 ( 6 glasses) 0.20 ± 0.04 M2.0/2 ( 6 glasses) 0.02 ± 0.01 Mill ( 6 glasses) 0.16 ±0.04 5.3 Degree of corrosion : SEM versus IR measurements In Tables 6 and 7 the results of characterising the corrosion layer on the various model glasses are given. For every model glass, two ways of measuring the degree of corrosion were employed : (a) direct measurement of the leached layer thickness by SEM and (b) OH-band intensity measurement by IR-spectroscopy representing the water content in the leached layer and the crust. The average thickness of the leached layer, which is determined through inspection of the cross section of the model glasses embedded in a resin, is plotted versus the intensity of the OH-signal in the corresponding infrared spectrum. The results for all model glasses M1.0, M1.0/2, M2.0 and M2.0/2 are represented in Figure 4. 175 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses C\J C\J o o o o csi cn< 2 2 2 5 O □ * K 05 in csi o Q. Ow ■o 2 o O o o o o o co r— CO in t j - CO (uiri) aaAei paqoeaj jo ssam pt.m Figure 4. Thickness of the leached layer as obtained by observation with SEM versus the infrared intensity of the OH-absorption signal for all model glasses M1.0 and M2.0, buried for 6 and 3 months. 176 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses A nearly linear relation is obtained between the two measurements which means that both of them can be used as a measure of the corrosion of the glasses. The deviations of the linearity are due to the fact that the thickness of the leached layer is heterogeneous in most of the glasses and an average value was taken for the graph. The infrared intensity of the OH-signal is the result of one single measurement (which covers a quite large area of the model glass) including both the OH in the leached layer as well as the crust. On the other hand, the infrared detector can become saturated when the leached layer is too thick and too much water or silanol bonds are present. 5.4 Results after exposure to soil in the laboratory After excavation of the glasses from the various test environments, pieces were cut from the glass sheets, embedded in a resin and subsequently sanded, polished and carbon coated. First of all secondary electron images were used to investigate the appearance and to determine the thickness of the leached layer and the crust which had formed during the experiments. Figures 5-8 show examples of cross-sections of some of the glasses. From the part of the glass sheet which was not embedded, the transmission infrared spectrum was recorded to determine the increase in the absorbance of the OH-signal. The results are shown in Tables 6 a and b and they are discussed in detail in the following sections. In the first experiments, the glasses were analysed after a six months exposure. To measure the effect of time on corrosion, the same experiments were repeated but only for three months of exposure since some of the glasses had corroded quite intensively after six months and had formed a very thick leached layer resulting in the saturation of the infrared detector. The Mill glass samples were left out from this second series of experiments as they had only shown marked corrosion phenomena in the most corrosive media after six months. The corrosion obtained after six months is more extensive than that after three months but it appeared that the corrosion process does not proceed linearly with time and eventually reaches a maximum. A comparable effect was noted by Fuchs et al. [15], who mentioned that the development of AE is generally marked by a steep increase in the first 1 0 0 days, then quickly levels off and for the less sensitive glass types reaches a maximum. For the more sensitive glasses, after an initial constant increase, AE is generally subject to large fluctuations. Clark and Zoitos [19] also mention that leaching of nuclear waste glasses generally decreases with time due to the formation of a protective surface layer on the glass. What becomes clear by the examination of the results in Tables 6 and 7 is that corrosion occurs much faster when the glass is exposed to damp soil compared to when it is exposed to the atmosphere since even the most sensitive model glass types show leached layers of only a few micrometers after outdoor weathering experiment during three months. 177 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ZJ CD "O—i o CQ. o CD Q. (D a) Six months exposure Glasses with Model Experiments Burial 6: Chapter "O 2< < CD O’ S." Exp. M1.0 M2.0 Mill CD o 0) ® 3 AE Thickness (pm) Remarks AE Thickness (pm) AE Thickness (pm) C/) C/) cpm °q ' B1 1.44 up to 20-30 thick, very inhom.; Ca-dendr.; crust 0.96 5-10 0.17 < 1 B2 1.83 up to 40 thick, very inhom.; Ca-dendr.; crust 1.20 up to 15 0.26 < 1 •1 Q.® to S _1 B3 2.19 up to 50 thick, very inhom.; Ca-dendr.; crust 1.49 up to 20 0.31 < 1 CD CX13 0) ?♦W O CD m B4 2.63 up to 70 thick, very inhom.; crust 1.78 up to 25 0.39 < 1 O ® o ■o cd “• y, B5 3.77 up to 130 thick, very inhom.; crust 2.45 up to 30 0.45 2 ca •D £2. 3 - (P X CD B6 0.83 5-7 0.66 5-7 0.19 < 1 B7 0.07 < 1 - 0.05 < 1 0.10 < 1 j3 ^ Q) 5 i ' B8 0.02 < 1 - 0.07 < 1 0.05 < 1 m 9> 3 B9 0.02 < 1 - 0.01 < 1 0.01 < 1 0) Q. tt B10 0.93 8 - 0.47 3 0.11 < 1 3. (B «-* 3j 3 3* CD CD B11 1.73 30 thick, rather horn.; layers; crust 0.87 5-10 P r 0) 0.19 < 1 § ® CO CD 3 CD c B12 3.31 40-70 crystals on surface 2.53 10-15 0.60 3-4 ■o U 3 o Q. a> ? s | b) Three months exposure C a a g Exp. M l.012 M2.0/2 o Q) 3 rt* 03 0 w AE Thickness (pm) Remarks AE Thickness (pm) Remarks ■o 5* /-*• o B1 1.08 10 thick, rather horn.; crust 0.96 5-10 layers; crust H co a B2 1.26 10-20 thick, rather horn. 1.13 5-15 CT CP 3" crust CD Q. CD * ® B3 1.50 15-30 crust 1.54 15-30 crust -£■ O q: • co d. B4 2.00 30-60 very inhom.; Ca-dendr.; crust 1.93 up to 60 thick, very inhom.; crust C O B5 3.39 40-70 crust 3.36 40-60 CD3 3 *■ crust ■o CD B6 0.86 5-10 some crust 0.74 5-7 some crust CD Otf- toCO A o B7 0.53 3 - 0.49 3 - C/) o u C/) ■Jo rt>-h B8 0.15 < 1 - 0.14 < 1 - q -=r o ® B9 0.19 < 1 - 0.25 < 1 - CD CD B10 0.76 2-3 - 0.64 2-3 - Q. CD O B11 1.22 5-15 thick, rather horn.; crust 1.16 5-15 crust C/3,A “T^ =:o Q. fl> B12 3.67 40-50 layers; crystals on surface 3.24 30-40 layers; crystals on surface Chapter 6 : Burial Experiments with Model Glasses 5.5 Results after burial in test areas It is difficult to compare the results of the burial experiments with those obtained from the laboratory experiments since the parameters such as humidity and temperature were different and changing in the outdoor test areas. Results show that the glasses are rather heavily corroded and they all have quite thick leached layers. This means that the humidity in the test areas must have been high during the time of burial or that changing conditions between dry and humid conditions are more corrosive than a constant level of humidity. Even the results obtained after six months do not show the same trend as the ones obtained after three months; the model glasses do not show the lowest degree of corrosion in the same test area after three and six months. However, the loamy soil in the Zonienforest, which has the highest moisture retention coefficient, is always the most corrosive medium. 179 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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T--- CO T— 1/2 CL •X o X o CL o X co CD j z CO o in r~- o CO f z CM i n 02 CO r~- CO •"It" in .c 0 2 CM co p ▼- o UJ CO CO CO lO in in N- c CO CM M" CO E < CM t — CM CM CM CO CM o CD E CD X CM CO ■«d" h - CO 9- CM CO ■•O' i n CD CO CO < < < < < < UJ < < < < < < < < 'ro' Table 7. Results of the infrared measurements and the thickness of the leached layers formed after six (a) and three (b) months burial in the test areas. Experiments are numbered as in Table 4. 180 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses 6. THE EFFECT OF DIFFERENT PARAMETERS ON THE CORROSION OF THE BURIED MODEL GLASSES 6.1 Humidity According to the results obtained for model glasses M1.0/2 exposed for three months in the laboratory to soil with different degrees of humidity (Table 8 ), the water concentration and the pH appear to be important parameters affecting glass corrosion. Corrosion increased with humidity, which can be explained by the fact that water is the attacking solution during corrosion and formation of the leached layer. Table 8 . Infrared intensity of the OH-signal and thickness of the leached layer of M 1.0/2 model glasses after exposure for 3 months in the laboratory to soil with different water content. 2ml H2O/100g soil 6ml HzO/1 OOg soil 8 m/ HzO/100g soil sat. HzO AE 1.08 1.50 2 . 0 0 3.39 Thickness of 1 0 15-30 30-60 40-70 leached layer (pm) 6.2 pH As mentioned in Chapter 1, Section 5.2, the pH of the environment is a very important factor affecting the corrosion of glass. In Table 9 the results obtained for model glasses M1.0 after exposure during six months to soil at different pH are given. A decrease in corrosion with increasing pH can be observed. Two types of water induced corrosion processes must be considered, leaching, especially of alkali and earth alkalines, and network dissolution. The latter is normally insignificant below pH 9 but becomes increasingly important above this critical value. According to Clark et at. [20], leaching is the predominant process in acid environment where it is more or less constant and independent of pH until around pH 9 from where it drastically starts to diminish. Table 9 shows that the thickness of the gel layer which is formed during the leaching process decreases in the pH range 8.5 to 9.1. Although network dissolution should become important around this degree of alkalinity, the glass surfaces did not show any sign of flaking off and the thickness of the glass remained the same. 181 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Buriat Experiments with Model Glasses Since the pH of the soil was increased by adding CaO, not only the pH but also the concentration of Ca-ions in the soil was increased. When the leaching reaction of calcium is considered : ~Si-0-Ca-0-Si~ (glass) 2H (solution) 2~Si-OH(g|ass) + Ca (solution) This reaction has a fixed equilibrium constant: [~ Si - OH]2 [Ca2* ] [~ Si - O - Ca - O - Si ~] [H +]2 The extent of ion-exchange depends on the pH and the Ca2* concentration in the solution. This means that the equilibrium will be shifted to the left when the pH increases but also when more Ca-ions are present in the solution. The decrease in leaching is the result of the joint effect of these two parameters. Table 9. Infrared intensity of the OH-signal and thickness of the leached layer of M1.0 model glasses after exposure during 6 months to soil with different pH values. 5.6 8.5 8.8 9.1 AE 1.44 0.83 0.07 0 . 0 2 Thickness of 20-30 5-7 < 1 < 1 leached layer (pm) 6.3 Temperature The deterioration of glass and the extraction of alkali metals are accelerated by a raise of the temperature. Paul [21] states that, for most silicate glasses, the quantity leached is doubled for every 8 to 15 °C rise in temperature, depending upon the composition of the glass and the type of alkali ion in question. Lyle [20] derived the following expression for leaching kinetics with inclusion of a temperature function : logQ = logt - b/T + c in which Q is the quantity of alkali ions released from the glass, T theabsolute temperature and b and c are empirically determined constants. Attempts have been made to express the temperature dependence of alkali extraction in terms of the Arrhenius equation. K = A exp~E"/ST where K is the rate of mass loss, A a constant, Ea the activation energy, R the gas constant and T the absolute temperature. It was found that in the ideal case, in the absence of precipitation reactions, the influence of the temperature could be described by this equation [21, 22-24]. The reaction temperature not only alters the kinetics of the reaction but also dictates the rate controlling mechanism of corrosion. At temperatures below 30 °C the ion exchange process predominates whereas at temperatures above 80 °C network dissolution is predominant. An explanation might be that the solubility of hydrated 182 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses silica (monosilicic acid) in neutral solution is low at room temperature but increases with temperature [17, 25]. This implies that one must take care not to carry out experiments at very high temperatures since the danger exists that reactions will occur at these temperatures which hardly occur at all at ambient temperatures. In addition, the thickness of the protective high-silica surface layer formed on heating is far less than that formed at ambient temperature [26]. According to Clark and Zoitos [19], various crystalline phases which can increase the leachability of the systems and also alter the corrosion mechanisms have been observed to form for a variety of nuclear glass systems between 200 and 600 °C. Model glasses were exposed to soil and held at four different temperatures, the infrared intensity and thickness of the leached layer of model glasses M1.0 which have been buried for six months are given in Table 10. Raising the temperature to 50 °C gives a slight increase in the corrosion whereas a decrease in temperature obviously decreases the corrosion. The fact that almost no corrosion took place in the freezer can be explained by the fact that water at a temperature of -10 °C is frozen and therefore is not able to diffuse into the glass. Table 10. Infrared intensity of the OH-signal and thickness of the leached layer of M1.0 model glasses exposed to soil for 6 months and kept at different temperatures. - 10 C 5 - 6 C Room temp. 50 C AE 0 . 0 2 0.93 1.44 1.73 Thickness of < 1 8 20-30 30 leached layer (pm) 6.4 Composition of the model glass Besides the environmental factors, the corrosion mechanism is also affected by the glass itself. The three-component model glasses used in the experiments all have a composition which is very sensitive to corrosion since their silica content lies below the critical 66.67 mol% (see Chapter 1, Section 5.1) and the alkali present is potassium resulting in glasses with very poor durability. Table 11, showing corrosion data for the 3 different model glasses after a 6 month exposure to the reference soil, clearly confirms the different susceptibility to corrosion that can be anticipated on the basis of the chemical composition of the glasses. Model glass M1.0 having the highest potassium concentration is the most sensitive one. This can be explained by the fact that the corrosion rate of glasses increases with the alkali content. In addition, M1.0 also contains the lowest concentration of CaO which is added to enhance the corrosion resistance. Similar results were found by Fuchs et al. [14, 15]. 183 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with ModeI Glasses The free energy of hydration was calculated from the composition of the model glasses given in Table 2. A value of -43.98, -37.26 and -30.14 kJ/mole was found for model glass M1.0, M2.0 and Mill respectively which corresponds to a corrosion rate of 10, 5 and 0.6 pm/year when plotted in the linear relation diagram of Cox and Ford [27] (see also Chapters 3 and 5). When these results are compared to the thickness of the leached layers in the experiments described in this chapter, it turns out that the experimental corrosion rate is much higher than what the empirical relation predicts. The reason might be that the model glasses are produced with only three components which makes them extra corrosion sensitive. Table 11. Infrared intensity of the OH-signal and thickness of the leached layer of the 3 model glasses after exposure for 6 months in the laboratory to the reference soil (Experiment B1). M1.0 M2.0 Mill AE 1.44 0.96 0.17 Thickness of 20-30 5-10 < 1 leached layer (pm) 6.5 Surface condition of the model glass Not only the composition of the bulk glass but also the composition and the structure of the surface has its effect on the durability and corrosion behaviour of glass. The asymmetry of a surface, like that of any other defect, causes abnormal interatomic distances which in turn can influence the reactions taking place during corrosion [28]. It is known that commercial processing of glass alters the intrinsic durability of the glass. Water, for example, reacts more rapidly with quenched glass than with annealed glass. This can be understood from the higher ionic mobility in the quenched glass, which has a lower density and a more open structure [29], Chemical and structural alterations in the glass surface can occur during processing of the material. The relationship between processing conditions and durability is not completely understood yet but the surface modification is definitely influenced by thermal history, external environment and in some cases intentional surface treatment. High temperatures which can lead to segregation (diffusion) or depletion (volatilisation) of glass components in the surface layers, fire polishing and intentional surface treatments such as tin floating all affect surface composition and hence the ultimate chemical durability [25]. In Table 12 the intensity of the OH-signal and the average thickness of the leached layer is given for model glasses M 2 . 0 which had been exposed in the laboratory for three and six months to the soil with 4 ml water/100 g soil. The second series of 184 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses model glass M2.0 (M2.0/2) has a slightly different thermal history resulting in a more open surface structure. The results for these glasses show that they are much more reactive than the first series since their degree of corrosion after three months is as high as the one for the first series M2.0 glasses after six months but what is even more striking is the fact that the M2.0/2 glasses are as corrosion sensitive as the M1.0/2 glasses since their results are comparable (see results in Tables 6 and 7). Table 12. Infrared intensity of the OH-signal and thickness of the leached layer of model glass M2.0/2 and M2.0 after exposure during 3 and 6 months respectively in the laboratory to soil with 4 ml water/100 g soil (Experiment B2). M2.0 (6 months) M2.0/2 (3 months) AE 1 . 2 0 1.13 Thickness of leached layer (pm) up to 15 5 - 15 6.6 Composition of the soil The model glasses were buried for six and three months in five different outdoor test areas : Edegem (the soil which was also used for the experiments in the laboratory), Brasschaat, Gontrode, Wijnendale and the Zonienforest, all situated in the northern part of Belgium. Next to these, the model glasses were also exposed in the laboratory to distilled water for three and six months and to soil from Qumran for six months. The results of these experiments are shown in Table 13. The leached layers formed during the outdoor experiments are all quite thick but it is very difficult to compare the results from these experiments with the results from the experiments in the laboratory due to the changes in different parameters such as humidity and temperature during the time of burial. It is also difficult to distinguish between the effects of the different composition of the soil to the corrosion of the glass. It is, however, clear that in the Zonienforest, where the soil was most humid due to its high moisture retention coefficient, the thickest leached layer was formed. The degree of leaching is comparable to the one in distilled water. The fine particle desert soil from Qumran which was saturated with water also turned out to be a very corrosive medium, the glasses were totally covered with a thick, brown crust (more than 1 0 0 pm), firmly adhering to the surface and partly entering the underlying leached layer, when they were removed from the soil after six months. The crust layer prevented transmission infrared measurements. The thickness of the leached layers underneath this crust was determined with SEM and has been found to be between 60 and 70 pm, which is comparable to the leached layer obtained for the Zonienforest and distilled water. 185 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses Table 13. Infrared intensity of the OH-signal and thickness of the leached layer of model glasses M 1.0/2 which have been buried for six months in the different test areas and have been exposed in the laboratory to distilled water and Qumran soil, n.m. : not measurable with IR. Edegem Brasschaat Gontrode Zonien Qumran Dist. H?0 AE 3.18 3.71 2.25 4.00 n.m. 3.31 Thickness (pm) 40-50 40 25 80 60-70 40-70 7. EXAMINATION OF CROSS-SECTIONS OF THE GLASSES 7.1 Scanning electron microscopy Elemental X-ray maps (silicon, potassium and calcium) were collected from cross- sections of embedded model glasses with a scanning electron microscope. The results for model glass M 1 .0 which has been exposed for six months to the reference soil in the laboratory are shown in Figure 5 as an example. The different areas in the glass (embedding resin, crust, leached layer and bulk glass) can be easily distinguished. Since the model glasses all belong to the three-component system and only major elements can be determined, it appears that the alkali, potassium and the alkaline earth, calcium have leached out of the glass during contact with damp soil, leaving behind a silica rich gel layer which is similar to the leached layers formed in the archaeological glass objects (see Chapter 3, section 7). On the surface a calcium rich crust has formed. No differences were found between the model glasses which had been buried in the outdoor test areas and the ones which had been exposed to the different soil environments in the laboratory except for the thickness of the leached layer and the crust. The model glasses which had been exposed to distilled water also showed this leached layer. 186 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses Figure 5. Elemental X-ray maps taken from model glass M1.0 from experiment B1 in Table 4. From left to right: embedding resin, crust, leached layer and bulk glass. The leached layers of other glasses were not at all similar to the one represented in Figure 5. Some of them (M2.0/2 from experiment B2 and M1.0/2 and M2.0/2 from experiment B12 in Table 4) showed a layered structure (see Chapter 3, section 7.2), even after this relatively short period of time. This proves that the formation of these sublayers is not an annual (season-driven) process nor is it dependent on other 187 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with ModeI Glasses cyclic changes in temperature or humidity. The backscattered electron image represented in Figure 6 is the leached layer which has formed in model glass M2.0/2 which has been exposed to the reference soil in the laboratory for only three months. It seems that the reason for the formation of these layers must be related to physical (or chemical) cyclic changes taking place in the glass itself as suggested by Douglas [30]. Figure 6 . Layered morphology in the leached layer of model glass M2.0/2 from experiment B1. From left to right : embedding resin, crust, leached layer and bulk glass. In Figure 7 elemental maps and a backscattered electron image are shown taken from model glass M 1 . 0 which has been exposed for six months in the laboratory to soil with 6 ml water/100 g soil. The images show that calcium dendrites have formed in the leached layer but only until a certain depth (50 pm), they have not invaded the complete leached layer. The reason for these structures is unknown, it might be a partial redeposition of calcium in cracks in the leached layer during diffusion. A correlation between calcium, sulphur and phosphorus can be noticed in the dendritic areas, pointing to the formation of calcium salts. These calcium dendrites were encountered in model glass M1.0 from experiment B1, B2 and B3 in Table 4) 188 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with ModeI Glasses Figure 7. Elemental X-ray maps and a backscattered electron image showing the calcium dendrites formed in the leached layer of model glass M 1 . 0 from experiment B3 in Table 4. left lower corner : resin, right upper corner : original glass and leached layer with dendrites in between. 189 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses On the surface of the model glasses which had been exposed to the soil from Qumran a thick brown crust (up to more than 100 p.m) had formed. Figures 8a and b respectively show a secondary electron and backscattered electron image taken from the cross-section of model glass M1.0 which has been exposed to this soil for six months. The distinction can be easily made between the bulk glass, the leached layer (60-70 pm in thickness) and the crust, partly penetrated into this gel layer. The elemental distribution of the major elements across the different areas was similar to that found for the other model glasses, a) Figure 8. Secondary electron image (a) and backscattered electron image (b) of the cross-section of model glass M1.0, exposed to soil from the Qumran site for six months. From left to right: bulk glass, leached layer, crust and embedding resin. 190 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses 7.2 Secondary ion mass spectrometry To obtain information about the leaching behaviour and distribution of minor and trace elements a technique with a higher sensitivity, secondary ion mass spectrometry (SIMS), was used. SIMS lineprofiles provide information on leaching : the thickness of the leached layers can also be determined, the distribution of elements can be related to different leaching mechanisms and the leach-out and influx tendencies of different elements can be determined. Line scans were obtained with a Cameca 4F instrument across cross-sections of the embedded glass fragments. In these measurements, the bombarding ion beam (O', 12.5 kV, 600 nA) is moved across the samples with a step size of 1 pm starting from the embedding resin through the leached layer up to the bulk glass. No SIMS lineprofiles were collected from the model glasses which had been exposed to soil with CaO addition since the leached layers were too thin; neither from the ones exposed to the Qumran soil or distilled water SIMS data could be collected since these leached layers were either too porous (especially the crust on the model glasses exposed to the Qumran soil) or split off the bulk glass, leading to strong surface charging during SIMS measurements. The results obtained for model glass M1.0 which has been exposed to soil saturated with water for six months are represented in Figure 9 and 10 in which the signals for the major/minor and minor/trace elements respectively are plotted as a function of the distance. For the major elements the results are similar to those obtained with SEM, again indicating that both potassium and calcium have leached out of the glass. The minor and trace elements show a fairly complicated behaviour, manganese and rubidium are depleted in the leached layer while the other elements entered the glass from the environment. They are enriched in the leached layer but they do not show their maximum at the same depth. Moreover, the leached layer does not appear to be homogeneous. This might also have led to the formation of the sublayers encountered in the leached layer of some of the glasses. The linescans are representative for the entire corrosion layer of this model glass. Some small differences in the location of the maximum in the signal of a certain element (e.g., copper) were encountered but the elemental tendencies for the various elements (leach-out or influx) are always the same. The same can be said for the different model glasses (M1.0, M2.0 and Mill) exposed to the same environment, apart from the thickness of the leached layer, no differences in elemental distributions were found. Line profiles collected from model glasses which had been buried in different test areas only showed small differences : calcium is slightly less leached out in the soil in Gontrode (which has the highest calcium concentration) while manganese remains constant in glass buried in Brasschaat and Gontrode and leached out in the other environments. The other elements showed similar tendencies in all the test areas. 191 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses CO __ O CO ^ U_ 03 TOC/3 03 3 Distance ((im) Distance TO CD TOO o o o O © ©o o o o o (s/s;o) (euBis Figure 9. SIMS line profile of major/minor elements in model glass M1.0 from experiment B5 in Table 4. 192 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses i > N O > co O) Distance {pm) Distance a> J2 “OQ> o Q>CO o o o o o o o o o o o o (S/SJO)ibu Bis Figure 10. SIMS line profile of minor/trace elements in model glass M1.0 from experiment B5 in Table 4. 193 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses In Figures 9 and 10 only raw line profiles (i.e., signal of the various elements in counts/second against distance in micron) are shown. When a leaching mechanism is considered, the atomic amount of silicon remains constant in the leached layer, therefore the higher signal of silicon in this layer (see Figure 9) can only be explained by a higher sputtering rate in this region [31]. However, when the intensity of the elements in Figure 9 were normalised to the intensity of silicon, which is considered to remain constant (signal of element/signal of silicon), the same elemental tendencies (leach-out or influx) were found. This means that the raw profiles can be used as an indication for the behaviour of the various elements. Quantitative SIMS analysis is very complicated, especially when insulating materials such as glass are considered. The most serious problem is sample charging which was quite severe eventhough samples had been coated with a gold layer prior to the measurements. To compensate for this effect, the specimen potential should be monitored and kept constant by a specially developed electronic device as was done at the Study Centre for Nuclear Energy in Mol, Belgium [13]. Another difficulty in quantitation is the conversion of the measured secondary ion signals into concentrations by means of relative sensitivity factors. These factors, however, are strongly influenced by matrix effects. Therefore, no quantitative SIMS analysis was attempted on the model glasses. Quantitative SIMS analysis on historical glass was performed by Schreiner et al. [31- 35], Quantitative SIMS depth profiling of the main and trace components of dark green coloured lead oxide medieval glass panes as well as the depth distribution of hydrogen was carried out to characterise the surface layers and leaching zones of this naturally weathered glass. They concluded that the leached layer (a few pm thick) was depleted in K, Ca, Pb, Ba, Na and Al and enriched in H and Cu while an outermost surface layer consisting of reaction products of the leached elements and atmospheric components (especially S 0 2 and C 0 2) had formed. Another example, the SIMS depth profiles collected by the Study Centre for Nuclear Energy in Mol, Belgium on the high level waste glasses which had been buried in the Boom clay show comparable results to what we found. The leached layers on the high-level waste glasses, which could be divided into different zones, were characterised by a strong depletion in B, Li, Na and Ca and a compensatory influx of H, Mg, K, Sr, Ba, Ti, Fe, Zr and Cr, depending on the composition of the soil and the glass. In brine a high influx of Mg could be noticed while in clay Mg and K, which are very abundant in this soil, entered the glass when this does not contain a high concentration of these elements. Sometimes even a lowering in the levels of the network builders Si and Al could be seen, pointing to a gradual replacement of the selective-substitutional mechanism by congruent dissolution, the latter being dominant at prolonged leaching times and high temperatures [9-14]. In the SIMS linescans collected on our model glasses, a lowering in the level of silicon was never noticed which means that corrosion only occurred by way of the selective- substitutional mechanism. Elements such as iron, copper, strontium and zirconium could probably diffuse into the model glasses from the environment since they are 194 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses present in the soil in a much higher concentration than in the model glass. Although an influx from potassium was noticed in the Roman glasses (see Chapter 3) which had been buried in the Qumran soil, no such effect was ascertained in the model glasses. This can most probably be explained by the fact that the concentration of potassium in the model glasses, which is much higher than in the Roman glass objects, prevents this element to enter the glass. 7.3 Micro synchrotron radiation induced X-ray fluorescence Another sensitive technique, namely Micro synchrotron radiation induced X-ray fluorescence was used to check the results obtained by SIMS. Because the lateral resolution of this technique is worse than that of SIMS, linescans could only be collected from the model glasses with the thickest leached layers. The resulting line profile with a step size of 15 pm taken across the whole cross-section of the same model glass as shown in Figures 9 and 10, is represented in Figure 11. This technique demonstrates that elements like iron, nickel, copper, strontium and zirconium are enriched in the leached layer and therefore probably have entered the glass from the environment. Because the entire cross-section of the model glass was scanned using this technique, the two leached layers formed on both sides of the glass could be compared. Figure 11 points out that the two surfaces of the model glasses are different since, although elemental tendencies are similar, various elements show a different pattern in the leached layer on either side. This could be expected because the two surfaces of the model glasses have been treated in a slightly different way during fire polishing. 195 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Mode! Glasses A / V V'- A v r v-A~./\ Cl-Ka ______\ A . ^ , max: 657 cts min: 88 cts 1 \ 1 \ K-Ka 1 \ max: 41522 cts / \ min: 0 cts Ca-Ka • max: 97175 cts min: 44 cts ' v \ /W X ^ Mn-Ka / \ m ax: 59 0 cts / V _ min: 0 cts f\ Fe-Ka max: 11349 cts i v ------min: 0 cts ------v N" w - ^ . x . n CO 1 i \ Co-Ka > s \ max: 1466 cts ' \ j \ ■' min: 0 cts ‘c/D ^ ' ------^ • C 1 \ CD Ni-Ka 1 \ m ax: 2138 cts 1 V. _____ min: 0 cts - >% Cu-Ka CO \ max: 2356 cts DC min: 0 cts ■ X •A Zn-Ka "D 1 \ max: 3473 cts CD - A 1 V ______- s __m in:0cts M > W J \ A/w X VA • *%/ '• • *-\/ A- ' To V- v V Rb-Ka * \ max: 3 8 8 cts E t min: 0 cts : _ j\ r \ i Sr-Ka \ max: 6 090 cts / \ min: 0 cts s ^ ! s " / * — — V v ^ \ / \ Zr-Ka max: 3 318 cts / V min: 0 cts V ^ _ Ba-Ka max: 718 cts min: 0 cts i i | i i i | i i i | i i i | i i r t i — i— r t 285 585 885 1185 1485 1785 Distance (jxm) Figure 11. p-SF?XRF line profile of some elements in model glass M1.0 from experiment B5 in Table 4. 196 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses 8. MODEL GLASS EXPOSED TO IRON AND COPPER SOLUTIONS To determine that it is indeed possible for elements to enter glass from the environment within fairly short periods of time, two model glasses of type M 1.0 were exposed to a C uS04 and a FeCI3 solution. Only a trace of these elements was present in the solutions, their exact concentration was not determined since the aim of the experiment was only to find out whether these elements could diffuse into the glass or not. The pH value of the solutions was respectively 3.5 and 0.0 and did not change during the experiment. After six hours the samples were removed from the solution and washed with distilled water. The glass from the copper solution had become totally blue during the experiment and the one from the iron solution showed the typical rust-brown colour caused by iron. The latter glass surface was very corroded and parts of the surface were starting to flake off; the other one showed a clear, apparently unattacked surface. The results are summarised in Table 14. Table 14. Description of the model glasses M1.0 which have been exposed to a copper and iron solution. C u S 0 4 FeCI3 pH 3.5 0 Colour after exposure blue rust-brown Surface state after exposure flaking, seriously corroded clear, intact surface A SIMS depth profile was taken from the model glass which had been brought into the copper solution and the result is represented in Figure 12 in which the signal for silicon, calcium, potassium and copper is shown as a function of sputtering time (depth) starting from the surface, going through the leached layer (3 pm in thickness) into the bulk glass. From the signal of copper which is slowly decreasing throughout the leached layer it becomes clear that this element has indeed entered the glass from the environment and is responsible for its blue colour. 197 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced oprslto o hours. 6 for solution copper iue 2 SM dph rfl o mdl ls M. wih a be epsd o a to exposed been has which M1.0 glass model of profile depth SIMS 12. Figure Chapter 6 : Burial Experiments with Model Glasses Model with Experiments Burial : 6 Chapter 10000 (s/sjo)jeuBis O O O ^ CO 198 92 Time (s) Chapter 6 : Burial Experiments with Model Glasses 9. CRUST FORMED ON THE MODEL GLASSES At the surface of the glasses which had been buried in the test areas or had been exposed to damp soil in the laboratory and which showed a leached layer, a crust had also formed which yielded a strong calcium signal in elemental X-ray maps (See Figure 5). The secondary electron image of the surface of model glass M2.0 which has been exposed to the reference soil for six months (Figure 13) shows the irregular structures which were found. Figure 13. Secondary electron image of the crust formed on the surface of model glass M2.0 from experiment B1 in Table 4. Parts of the crust could be removed and an infrared reflection spectrum was recorded with a FTIR Microscope (Spectra Tech) coupled to a Nicolet 5DXB spectrometer in the mid-IR range (400 - 4000 cm'1) with a resolution of 4 cm'1. The result is shown in Figure 14. The most striking signals were found to correspond to reference spectra of C a(H C 03)2 [36] which explains why a calcium signal predominates in the X-ray maps. This is not the same as the crust formed on the model glasses during weathering experiments which was determined to be gypsum or syngenite, depending on the K/Ca ratio in the glass and the atmospheric conditions [14, 15]. 199 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses £101 soss- WAVENUMBER ■iOOO ■iOOO 3620 32t0 2860 2t80 2100 1720 13^0 9 6 0 Figure 14. Infrared spectrum of the crust formed on model glass M1.0 from experiment B5 showing the typical signals for C a(H C 03)2. 200 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses The difference between the glasses which have been exposed to the soil and the ones which have been exposed to distilled water was that on the surface of the latter crystals had formed during the experiments, the largest having a side of about 0.5 mm (See Figure 15). The X-ray spectrum taking from one of these crystals only showed a calcium signal, again pointing to the presence of calcium carbonate or calcium bicarbonate. Figure 15. Secondary electron image showing the crystals formed on the surface of the model glass M1.0 which has been exposed to distilled water for six months. 10. CONCLUSIONS Experiments were carried out using model glasses from the Fraunhofer-lnstitut fur Silicatforschung to investigate the effect of different parameters (humidity, pH, temperature, composition and production process of the glasses) on the corrosion of glass during burial. These three component glasses which have a composition very sensitive to corrosion were chosen in order to obtain results after a relatively short period of time (three and six months). The model glasses were buried in five outdoor test areas in the northern part of Belgium and they were exposed to different controlled environments in the laboratory, including soil with a variation in a series of parameters, distilled water and soil from the Qumran site. Different analytical techniques (IR, SEM, SIMS and p.-SRXRF), each providing particular information, were used to study the measure and characteristics of the corrosion. When the results are compared to the ones from weathering experiments (glass exposed to the atmosphere), it becomes immediately clear that the model glasses had formed much thicker leached layers during burial. When the influence of the different parameters is considered, next to the composition of the glass, also the structure of its surface, affected by the production 201 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses Different analytical techniques (IR, SEM, SIMS and p-SfRXRF), each providing particular information, were used to study the measure and characteristics of the corrosion. When the results are compared to the ones from weathering experiments (glass exposed to the atmosphere), it becomes immediately clear that the model glasses had formed much thicker leached layers during burial. When the influence of the different parameters is considered, next to the composition of the glass, also the structure of its surface, affected by the production process, turned out to be very important. From the environmental factors, the amount of water in the soil appears to be one of the most important ones, next to the pH. The corrosion of the glass was increased when the humidity was increased and the temperature was raised while a decrease in corrosion with increasing pH could be observed. The pH value was very critical, a minimum in corrosion was found between pH 8 and 9 where network dissolution is still almost non-existent and leaching is very low. Examination of cross-sections revealed that elements such as calcium and potassium had diffused out of the glass resulting in a leached layer, formed by the selective-substitutional mechanism, which sometimes showed the layered morphology. On the other hand, elements from the environment like copper, iron and strontium had diffused into the glass which was confirmed by putting a model glass in a copper solution for six hours. At the surface a calcium rich crust had formed which was determined to be calcium bicarbonate. 202 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses REFERENCES [1] Evans J.G. and Limbrey S., Proc. Prehist. Soc., 40, pp. 170-202 (1974). [2] Holden E.W., Excavations at the Deserted Mediaeval Village of Hangleton. Part I, Sussex Archaeological Collections, 101, p. 101 (1963). [3] Fletcher W., The Chemical Durability of Glass. A Burial Experiment at Ballidon in Derbyshire, J. Glass Studies, 14, pp. 149-151 (1972). [4] Fletcher W „ The Chemical Durability of Glass. Progress Report on Burial Experiments at Ballidon and Wareham, England, British Glass industry Research Association Internal Document (1974). [5] Fletcher W., The Chemical Durability of Glasses. Burial Experiment at Wareham, Dorsetshire. Progress Report no. 1, British Glass Industry Research Association Internal Document (1964). [6] Fletcher W., The Chemical Durability of Glass. Second Progress Report on the Burial Experiment at Wareham, Dorsetshire, British Glass Industry Research Association Internal Document (1968). [7] Fletcher W., The Chemical Durability of Glasses. Third Progress Report on the Wareham Experimental Earthwork, British Glass Industry Research Association Internal Report (1973). [8] Newton R.G., A Summary of the Progress of the Ballidon Glass Burial Experiment, Glass Techn., 22, pp. 42-45 (1981). [9] Van Iseghem P., Lemmens K., Aertsens M. and Put M., Interaction between HLW Glass and Clay : Experiment versus Model, Geoval ‘94, Publ. OECD, pp. 202-217(1995). [10] Van Iseghem P. and Lemmens K., The Interaction between HLW Glass and Boom Clay Host Rock, IAEA-SM, 326(36), pp. 209-223 (1995). [11] Van Iseghem P., Performance of Vitreous Waste Forms and Engineered Barriers under Clay Repository Conditions, Report prepared for the Research Agreement No 6795/CF with the International Atomic Energy Agency (1995). [12] Lemmens K., Aertsen M., De Canniere and Van Iseghem P., The Corrosion of Nuclear Waste Glasses in a Clay Environment: Mechanisms and Modelling, Final Report on the work performed as part of the Europen Atomic Energy Community’s shared cost-programme on ‘Management and Storage of Radioactive Waste’ (1996). [13] Lodding A. and Vand Iseghem P., Corrosion of Waste Glasses in Boom Clay : Studies of Element Concentrations by SIMS, Mat Res. Soc. Symp. Proc., 412, pp. 229-238 (1996). [14] Fuchs D.R., Romich H. and Schmidt H., Glass-sensors : Assessment of Complex Corrosive Stresses in Conservation Research, Mat. Res. Soc. Proc. G : Art and Archaeology, San Francisco (1990). [15] Fuchs D.R., Romich H., Tur P. and Leissner J, Conservation of Historic Stained Glass Windows. International Examination of New Methods, Part 1, Research Report 108 07 005/03 from The Fraunhofer-lnstitute fur Silicatforschung, pp. 26-143 (1991). [16] Macquet C., Contribution a I’Etude des Surfaces Vitreuses Anciennes : Application a /' Amelioration des Techniques de Conservation, p. 26, PhD- 203 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6 : Burial Experiments with Model Glasses thesis, University of Poitiers (1994). [17] Scholze H., Chemical Durability of Glasses, J. Non-Cryst. Solids, 52, pp. 91- 103 (1982). [18] Smith R.T. and Atkinson K., Techniques in Pedology. A Handbook for Environmental and Resource Studies, p. 148, Elek Science, London (1975). [19] Clark D.E. and Zoitos B.K., Corrosion of Glass, Ceramics and Ceramic Superconductors. Principles, Testing, Characterization and Applications, pp. 235-238, Noyes Publications, New Jersey (1991). [20] Lyle A.K., Theoretical Aspects of Chemical Attack of Glasses by Water, J. Amer. Ceram. Soc., 26(6), pp. 201-204 (1943). [21] Paul A., Chemical Durability of Glasses; a Thermodynamic Approach, J. Mat. Sc., 12, pp. 2246-2268 (1977). [22] Adams P.B., Glass Corrosion. A Record of the Past? A Predictor of the Future?, J. Non-Cryst. Sol., 67, pp. 193-205 (1984). [23] Bates J.K., Jardine L.J. and Steindler M.J., Hydration Aging of Nuclear Waste Glass, Science, 218, pp. 51-54 (1982). [24] Cooper G.l and Cox G.A., The Aqueous Corrosion of Potash-lime-silca Glass in the Range 10-250 °C, Appl. Geochem., 11, pp. 511-521 (1996). [25] Clark D.E., Pantano C.G. Jr. and Hench L.L., Corrosion of Glass, Chapter 5, Magazines for Industry Inc., New York (1979). [26] Newton R. and Davison S., Conservation of Glass, Chapter 4, Butterworths, London (1989). [27] Cox G.A. and Ford B.A., The Long-term Corrosion of Glass by Groundwater, J. Mat. Sc., 28, pp. 5637-5647 (1993). [28] Weyl W.A., Structure of Subsurface Layers and Their Role in Glass Technology, J. Non-Cryst. Solids, 19, pp. 1-25 (1975). [29] Bansal N.P. and Doremus R.H., Handbook of Glass Properties, Chapter 18, Academic Press Inc., London (1986). [30] Newton R.G., Archaeometry, 13, pp. 1-9 (1971). [31] Schreiner M., Stingeder G. and Grasserbauer M., Quantitative Characterization of Surface Layers on Corroded Medieval Window Glass with SIMS, Fres. Z. An. Chemie, 319, pp. 600-605 (1984). [32] Schreiner M., Verwitterungserscheinungen an Mittelalterlichen Glasgemaiden Osterreichischer Provenienz, Wiener Berichte uber Naturwissenschaft in der Kunst, 1, pp. 96-117 (1984). [33] Schreiner M., Deterioration of Stained Medieval Glass by Atmospheric Attack. Part 2. Secondary Ion Mass Spectrometry Analysis of the Naturally Weathered Glass Surfaces, Glas. Ber., 61(8), pp. 223-230 (1988). [34] Schreiner M., Grasserbauer M. and March P., Quantitative NRA and SIMS Depth Profiling of Hydrogen in Naturally Weathered Medieval Glass, Fres. Z. An. Chemie, 331, pp. 428-432 (1988). [35] Schreiner M., Piplits K., March P., Stingeder G., Rauch F. and Grasserbauer M., Surface Analytical Investigations of Leached Potash-lime-silica Glass, Fres. Z. An. Chemie, 333, pp. 386-387 (1989). [36] Nakanishi K and Solomon P.H., Infrared Absorption Spectroscopy, Chapter 2, Holden-Day, Inc., Oakland (1977). 204 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Summary SUMMARY Glass can be regarded as a “supercooled” liquid meaning that it is a solid material with the internal structure of a liquid. The major constituent of glass is silica (S i02), which forms a random network by Si-O bondings between the basic S i04 tetrahedra. Addition of alkali metal oxides breaks up this continuous network resulting in a decrease of strength and viscosity. To improve the chemical resistance of the glass again, divalent alkaline earth ions are added which form new links in the network. Components which make up glass can be divided into four categories : network formers, network modifiers, which are subdivided into network breakers and network stabilisers, intermediates and colouring and decolouring oxides. Environmental attack of glasses depends on the presence of moisture. Reactions between glass surfaces and water can be considered to happen in two stages; the initial one involves an exchange process between alkali and earth alkaline ions from the glass and hydrogen ions from the solution and during the second stage the silica network is attacked resulting in total glass dissolution. The rate of degradation of glass objects is related to the properties of the material itself (e.g., composition, homogeneity and surface structure), to environmental parameters (e.g., solution pH and composition, temperature and relative humidity) and to physical factors (e.g., weathering versus aqueous corrosion and exposed surface area-to-solution volume ratio). In this work different microanalytical techniques, scanning electron microscopy (SEM), micro synchrotron radiation induced X-ray fluorescence (p-SRXRF), micro particle induced X-ray emission (p-PIXE), secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectrometry (FTIR), were used to study archaeological glass objects and model glasses. These techniques were selected based on availability, on whether or not they could supply additional information and on the fact that they are non-destructive or only need very small samples. Major, minor and trace composition of the glass samples was determined using SEM (major and minor) and p-SRXRF (minor and trace). The homogeneity of standards (NIST standard reference materials and Corning Museum of Glass standards) was checked, optimal measuring conditions were stipulated and quantitative procedures were compared and chosen. Using a standardless ZAF program, major (and some minor) elements could be determined with an accuracy in the order of a few percent; for heavier elements such as Zn and Ba and for concentrations between 0.1 and 1 % (i.e., close to the detection limit of the method) larger errors were found. By means of a fundamental parameter method which quantifies p-SRXRF spectra, trace element abundances were determined. The archaeological site of Khirbet Qumran which is situated at the north-western shore of the Dead Sea near the Judean desert in Israel used to be a Roman settlement in ancient times, probably inhabited by a religious community, named the Essenes. During excavations of this site in the 1950s, a collection of about 90 glass vessels such as bottles, pearls, goblets and cups, was recovered. These objects, 205 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Summary which are dated to the first half of the first century AD, are almost all in a state of advanced decomposition and show corrosion phenomena on their surface due to the contact with groundwater during 1900 years of burial. The major, minor and trace composition of these glass objects was studied using two non-destructive and complementary techniques, namely scanning electron microscopy and micro synchrotron radiation induced X-ray fluorescence to get to a better insight into the origin of the objects. The composition, durability and rate of corrosion of the unaffected bulk glass turned out to be similar to those of Roman glass objects found at other sites in Europe and the Near East. This series of objects could be divided into three compositional categories which only significantly differ in a relatively limited number of elements such as calcium, manganese, copper, tin, antimony and lead. This suggests that the glass objects were produced from closely related batches of bulk glass or that they were manufactured using slightly different procedures. Anyway, large quantities of glass vessels of various types were being used in Qumran which were either made locally in a glass workshop or were imported ready-made from elsewhere. This is an indication that the hypothesis about some industrial activity in the field of perfume manufacture being located at this site might be true. Next to the composition of the bulk glass, also the corrosion phenomena which had occurred during burial were studied. The thickness of the leached layer varied between a few pm and 0.5 mm, these layers are characterised by a depletion of elements such as Na and Ca and an enrichment of K, Mg and Fe relative to the original glass. The minor and trace constituents showed a complex behaviour. In a number of cases this leached layer was built up from stacks of thin sub-layers which had developed parallel to each other or showed hemi-spherical morphologies. Sometimes a precipitation of a brown/black substance rich in manganese was observed inside the leached layer. The surface crust consisted of salt precipitates of elements which had leached out of the glass. Roman vessel glass shows the remarkable property that its major composition is very similar. In order to obtain a more detailed picture of the differences and similarities of this glass from different periods and geographical locations, an extensive set of samples was analysed for its major, minor and trace composition. The glass collections originate from Cologne and Trier (Germany), Rouen (France), Tongeren, Oudenburg, Grobbendonk and Wallonia (Belgium) and Maastricht (The Netherlands) and cover a time range from the first to sixth century AD. The glass collections contained colourless, naturally coloured and intentionally coloured transparent as well as opaque sample material. If the chemical compositions obtained for all the Roman glasses dated to the fourth century or before are compared, it becomes clear that they are indeed very similar, especially in major elements, as was mentioned in the literature. All the objects show the low magnesium/low potassium soda-lime-silica composition, which is typical for this period. Sometimes slight differences can be seen which are not 206 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Summary sufficient to differentiate between various locations or age. Somewhere during the second half of the fourth century a drastic change in glass production appears to have occurred; this is reflected in stylistic characterisations, colour as well as composition of the glass vessels. The number of forms is strongly reduced to only common tableware, most of the glass shows a dark green or even brown colour and the concentration of iron, manganese, titanium and some trace elements like copper, zinc and zirconium is significantly higher. The reason for this change in composition could be explained by recycling of older glass introducing impurities to the glass during this process and/or the use of less pure or unpurified raw materials. When minor and/or trace elements are considered, differences, which are mainly related to the colour of the glass, can be seen between the various objects. One of the elements covering the largest range of concentration levels is definitely antimony, which in ancient times was not only used as decolouriser but also as opacifying agent and as means to remove air-bubbles from the glass melt. Its use as decolourant seems to have been largely taken over by manganese during later centuries except for the most western part of the Roman territory where it is present in relatively high concentrations in almost all glasses, even the ones dated to the third century. The raw materials used for antimony were on the one hand stibina, a Sb-S mineral, and on the other hand a Sb-Pb mineral or alloy. The most common colouring elements are iron, manganese, copper and cobalt. Iron is present in all the glasses and has probably entered the glass batch as an impurity present in the raw materials used. In its reduced state it is responsible for the blue-green aqua colour which is very typical for Roman glass vessels. An excess of manganese, which was also used as decolourant, results in a purple colour. Depending on the atmosphere in the glass furnace, the addition of copper, in the form of bronze or brass, can lead to a red, blue or green glass while colouring with cobalt, added to the glass batch as either a cobalt ore or a blue glass cullet high in cobalt, results in a brightly blue glass. Since the major composition of all these glass objects is so similar, it is likely that only a few workshops produced glass from raw materials, which was then transported as ingots throughout the Roman Empire. At different sites these were remelted, colourants, decolourants and/or opacifiers were added, introducing different amounts of the various trace elements, and objects were formed. During later centuries older glass seems to have been recycled. What can also be stated is that the glass-makers during that time appear to have had a quite good understanding about how to produce a durable glass and which elements or compounds to use as colourants, decolourants and/or opacifiers. Next to the study of the composition of archaeological glass samples, a systematic study of the corrosion of buried glasses was carried out to investigate the effect of different parameters (humidity, pH, temperature, composition and production of the glasses) on the corrosion of glass during burial. Model glasses M1.0, M2.0 and Mill which are produced in the Fraunhofer-lnstitut fur Silicatforschung, Bronnbach, Germany were used in the experiments. Their composition is restricted to the three 207 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Summary component system SiO2-K2O-Ca0 which means that they are very sensitive to attack by water and humidity in general. The glasses were buried in five outdoor test areas in the northern part of Belgium and they were brought in a series of controlled environments in hermetically closed boxes in the laboratory. After three and six months the glasses were dug up and examined with a series of microanalytical techniques including IR-spectroscopy, SEM and p.-SRXRF. Considering the influence of the different factors affecting glass corrosion, the amount of water present appeared to be one of the most important parameters next to the pH value of the environment. A clear increase in corrosion could be noticed when the humidity is raised while a decrease in corrosion with increasing pH occurred. The pH range used in the experiments was very critical, a minimum in corrosion was found between pH 8 and 9 where leaching was very low and network dissolution still almost non-existent. The deterioration of glass was also accelerated by a raise in the temperature of its environment. Besides the environmental factors, the corrosion was also affected by the composition and surface state of the glass itself. Examination of cross-sections revealed that elements like calcium and potassium had diffused out of the glass resulting in a leached layer which sometimes showed a layered morphology. Minor and trace elements showed a fairly complicated behaviour, manganese and rubidium are depleted in the leached layer while other elements such as copper, iron and strontium seemed to have entered the glass from the environment which was confirmed by exposing a model glass to a copper solution for six hours. At the surface of the glass samples a calcium rich crust had formed which was determined to be calciumcarbonate. Finally, two series of potash based medieval glass objects from Namen and Fagnolle, Belgium, which had also been buried for a considerable amount of time, were studied to compare their composition and corrosion behaviour with the potash based model glasses on the one hand and the soda based archaeological Roman glass objects on the other hand. 208 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Samenvatting SAMENVATTING Glas kan beschouwd worden als een “onderkoelde” vloeistof, wat betekent dat het een vaste stof is met de interne structuur van een vloeistof. Het hoofdbestanddel van glas is silica (S i02) dat een random netwek vormt door Si-O bindingen tussen de Si04-tetrahedra. Toevoeging van alkalimetaal oxiden breekt dit continue netwerk open wat resulteert in een verlaging van de sterkte en de viscositeit. Om de chemische weerstand van het glas terug te verhogen, worden tweewaardige aardalkali ionen toegevoegd die nieuwe verbindingen in het netwerk vormen. De componenten die deel uitmaken van glas, kunnen ingedeeld worden in vier categorieen : netwerkvormers, netwerkwijzigers, welke zelf onderverdeeld worden in netwerkbrekers en -stabilisators, intermediairen en kleurende en ontkleurende oxiden. Aantasting van glas hangt af van de aanwezigheid van vocht in de omgeving. Reacties tussen glasoppervlakken en water kunnen verondersteld worden in twee stappen plaats te vinden; de initiele stap bestaat uit een uitwisselingsproces tussen de alkali en aardalkali ion van het glas en de waterstofionen van de oplossing en tijdens de tweede stap wordt het silica netwerk aangevallen wat leidt tot een volledig oplossen van het glas. De snelheid van degradatie van glasvoorwerpen staat in verband met de eigenschappen van het materiaal zelf (bv. de samenstelling, de homogeniteit en de structuur van het oppervlak), omgevingsparameters (bv. pH en samenstelling van de oplossing, temperatuur en relatieve vochtigheid) en fysische factoren (bv. verwering versus waterige corrosie en de verhouding van blootgesteld oppervlak tot volume van de oplossing). In dit werk werden verschillende microanalytische technieken gebruikt, raster electronenmicroscopie (SEM), micro synchrotronstraling gei'nduceerde X-straal fluorescentie (p-SRXRF), micro deeltjes geinduceerde X-straal emissie (p-PIXE), secundaire ionenmassaspectrometrie (SIMS) en Fourier getransformeerde infrarood spectrometry (FTIR), voor de bestudering van archaeologische glasvoorwerpen en modelglazen. Deze technieken werden geselecteerd op basis van beschikbaarheid, het feit of ze al dan niet bijkomende informatie konden leveren en hun niet- destructieve aard of het feit dat ze slecht heel kleine monsters vereisten. Hoofd- neven- en sporensamenstelling van de glasmonsters werd bepaald met behulp van SEM (hoofd en neven) en p-SRXRF (neven en sporen). De homogeniteit van standaarden (NIST standaard referentie materialen en Corning Museum of Glass standaarden) werd nagegaan, optimale meetomstandigheden werden bepaald en kwantitatieve procedures werden vergeleken en gekozen. Met behulp van een standaardloos ZAF programma konden hoofdelementen (en sommige nevenelementen) bepaald worden met een accuraatheid in de orde van enkele procenten; voor zwaardere elementen zoals Zn en Ba en voor concentraties tussen 0.1 en 1 % (d.i., dicht tegen de detectielimiet van de methode) werden grotere fouten gevonden. Sporenelementen werden bepaald met een fundamentele parameter methode die p-SRXRF spectra kwantificeert. 209 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Samenvatting De archaeologische site Khirbet Qumran die gesitueerd is op de noord-westelijke oever van de Dode Zee nabij de Judeese woestijn in Israel, was een Romeinse nederzetting in de oudheid die waarschijnlijk bewoond werd door een religieuze bevolkingsgroep, genaamd de Essenen. Gedurende opgravingen van deze site in de jaren 1950, werd een collectie van ongeveer 90 glasvoorwerpen, zoals flessen, parels, bekers en kelken teruggevonden. Deze objecten, die gedateerd zijn in de eerste helft van de eerste eeuw na Christus, bevinden zich bijna allemaal in een toestand van vergevorderde decompositie en vertonen corrosieverschijnselen op hun oppervlak ten gevolge van het contact met grondwater gedurende de 1900 jaar dat ze begraven waren. De hoofd-, neven- en sporensamenstelling van deze glasvoorwerpen werd bestudeerd met behulp van twee niet-destructieve en complementaire technieken, namelijk raster electronenmicroscopie en micro synchrotron straling geinduceerde X-straal fluorescentie, om tot een beter inzicht te komen in de oorsprong van deze voorwerpen. De samenstelling, duurzaamheid en corrosiesnelheid van het onaangetaste glas bleken gelijkaardig te zijn aan die van Romeinse glazen voorwerpen afkomstig van andere sites in Europa en het Nabije Oosten. Gebaseerd op de samenstelling, kon deze verzameling objecten onderverdeeld worden in drie groepen, die slechts significant verschillen in een relatief beperkt aantal elementen zoals calcium, mangaan, koper, tin, antimoon and lood. Dit suggereert dat de glasvoorwerpen werden geproduceerd van verwante glassmelten of dat ze vervaardigd werden door enigzins verschillende procedures. In ieder geval werden grote hoeveelheden glasvoorwerpen van verscheidene types gebruikt in Qumran die ofwel ter plaatse werden gemaakt in een glas werkplaats ofwel kant en klaar werden ge'importeerd van ergens anders. Dit is een aanwijzing dat de hypothesis als zou er enige industriele activiteit op gebied van parfum vervaardiging op deze plaats gesitueerd geweest zijn, waar kan zijn. Buiten de samenstelling van het originele glas werden ook de corrosieverschijnselen bestudeerd die zich hadden voorgedaan terwijl het glas begraven was. De dikte van de uitgeloogde laag varieerde tussen enkele pm en 0,5 mm, deze lagen zijn gekarakteriseerd door een vermindering in concentratie van elementen zoals Na en Ca en een aanrijking van K, Mg and Fe relatief ten opzichte van het originele glas. De neven- en sporenbestanddelen vertoonden een complex gedrag. In een aantal gevallen was deze uitgeloogde laag opgebouwd uit een opeenstapeling van dunne sublagen die zich parallel ten opzichte van elkaar hadden ontwikkeld of zij vertoonden hemisfere morfologieen. Soms was een neerslag van een bruin-zwarte substantie rijk aan mangaan aanwezig in de uitgeloogde laag. De oppervlaktekorst bestond uit zouten van elementen die uit het glas waren uitgeloogd. Romeinse glasvoorwerpen vertonen de opmerkelijke eigenschap dat hun hoofdsamenstelling zeer gelijkend is. De hoofd-, neven- en sporensamenstelling van een uitgebreide reeks glasmonsters werd bepaald met het doel een meer gedetailleerd beeld te krijgen over de verschillen en overeenkomsten tussen glasvoorwerpen van verschillende perioden en afkomstig van verschillende plaatsen 210 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Samenvatting in het Romeinse Rijk. De glascollecties zijn afkomstig uit Keulen en Trier (Duitsland), Rouen (Frankrijk), Tongeren, Oudenburg, Grobbendonk en Wallonie (Belgie) en Maastricht (Nederland), dateren van de eerste tot zesde eeuw na Christus en bevatten kleurloos, natuurlijk gekleurd en intensioneel gekleurd transparant en opaak glas Wanneer de chemische samenstelling, bekomen voor al de Romeinse glasvoorwerpen gedateerd in de vierde eeuw of vroeger, wordt vergeleken, blijkt duidelijk dat deze zeer vergelijkbaar is, vooral wat betreft hoofdelementen, zoals ook reeds werd vermeld in de literatuur. Al de glasvoorwerpen vertonen de lage magnesium/lage kalium natrium-calcium-silicium samenstelling die typisch is voor de Romeinse periode. Soms kunnen kleine verschillen waargenomen worden, welke echter niet voldoende zijn om een onderscheid te kunnen maken tussen verscheidene plaatsen of verschillen in ouderdom. Ergens gedurende de tweede helft van de vierde eeuw heeft zich blijkbaar een drastische verandering in glasproductie voorgedaan, die zowel gereflecteerd wordt in stylistische karakteristieken, kleur, als samenstelling van de glasvoorwerpen. Het aantal vormen is sterk gereduceerd tot uitsluitend alledaags tafelgerei, het meeste glas vertoont een donkergroene of zelfs bruine kleur en de concentratie van ijzer, mangaan, titaan en enkele sporenelementen zoals koper, zink en zirkoon is significant hoger. De reden voor deze verandering in samenstelling kan verklaard worden door het feit dat ouder glas werd hersmolten en gerecycleerd waardoor onzuiverheden werden gei'ntroduceerd gedurende dit proces en/of door het gebruik van minder zuivere of ongezuiverde grondstoffen. Wanneer neven- of sporenelementen worden beschouwd, kunnen verschillen in samenstelling vooral gerelateerd worden met de kleur van het glas. Een van de elementen die het grootste concentratiebereik beslaat, is antimoon dat in de oudheid niet alleen werd gebruikt als ontkleurder maar ook als opaakmaker en als middel om luchtbellen te verwijderen uit de glassmelt. In latere eeuwen nam mangaan de rol van ontkleurder voor het grootste gedeelte over behalve in het meest westelijke deel van het Romeinse grondgebied waar antimoon in relatief hoge concentratie aanwezig blijft in alle glazen, zelfs diegene die gedateerd zijn in de derde eeuw. De grondstoffen die gebruikt werden om antimoon toe te voegen zijn aan de ene kant stibina, een Sb-S mineraal en aan de andere kant een Sb-Pb mineraal of legering. De meest voorkomende kleurende elementen zijn ijzer, mangaan, koper en cobalt. Ijzer is aanwezig in alle glazen en is waarschijnlijk in het glas gekomen als een onzuiverheid in de gebruikte grondstoffen. In zijn gereduceerde vorm is het verantwoordelijk voor de blauw-groene aqua kleur die typisch is voor Romeins glaswerk. Mangaan, dat ook gebruikt werd als ontkleurder, geeft een paarse kleur aan het glas wanneer het in overmaat wordt toegevoegd. Afhankelijk van de atmosfeer in de glasoven kan toevoeging van koper, onder de vorm van brons of messing, leiden tot een rode, blauwe of groene kleur terwijl cobalt, toegevoegd als cobalt erts of als glas met een hoog cobaltgehalte, zorgt voor een felblauwe kleur. 211 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Samenvatting Vermits de hoofdsamenstelling van al deze glasvoorwerpen zo gelijkaardig is, is het waarschijnlijk dat slechts enkele werkplaatsen glas produceerden van grondstoffen. Dat werd dan doorheen het Romeinse Rijk getransporteerd als blokken, welke daarna op verschillende plaatsen werden hersmolten en tot objecten werden gevormd. Kleurders, ontkleurders en opaakmakers werden dan ook toegevoegd, wat leidde tot de verschillende hoeveelheden van de sporenelementen. Gedurende latere eeuwen blijkt dit glas dan gerecycleerd te zijn. Wat ook kan worden geconcludeerd is dat de Romeinen blijkbaar reeds over een zeer goed inzicht beschikten over hoe een duurzaam glas moest gefabriceerd worden en welke elementen of materialen konden gebruikt worden als kleurder, ontkleurder of opaakmaker. Naast de bestudering van de samenstelling van archaeologische glasmonsters, werd een systematische studie uitgevoerd over de corrsie van begraven glas om het effect van verschillende parameters (vochtigheid, pH, temperatuur, samenstelling en productieproces van het glas) te onderzoeken op de corrosie van glass wanneer het in contact is met vochtige grond. Model glazen M1.0, M2.0 en Mill, gemaakt in het Fraunhofer-lnstitut fur Silicatforschung, Bronnbach, Duitsland, werden gebruikt in de experimenten. Hun samenstelling is beperkt tot het drie componenten systeem Si02-K20-C a0 wat betekent dat ze zeer gevoelig zijn voor aanval van water en vochtigheid in het algemeen. De glaasjes werden begraven in vijf testzones in het noordelijke deel van Belgie en zij werden in een reeks gecontroleerde omgevingen in hermetisch afgesloten dozen in het laboratorium geplaatst. Na drie en zes maanden werden de testglaasjes terug opgegraven en bestudeerd met een aantal microanalytische technieken, o.a. IR-spectroscopie, SEM en p-SRXRF. Wanneer de verschillende factoren die een invloed hebben op glascorrosie werden beschouwd, bleek de hoeveelheid water die aanwezig was een van de belangrijkste parameters te zijn naast de pH van de omgeving. Een duidelijke stijging in corrosie kon opgemerkt worden wanneer de vochtigheid werd verhoogd terwijl een daling in corrosie plaatsvond bij stijgende pH. Het pH-bereik dat werd gebruikt in de experimenten was zeer kritiek, een minimum in corrosie werd gevonden tussen pH 8 en 9 waar uitloging zeer laag was en netwerkontbinding nog bijna niet gebeurde. De aantasting van het glass werd ook versneld bij een stijging van de omgevingstemperatuur. Naast de omgevingsfactoren werd de corrosie ook beTnvloed door de samenstelling en de toestand van het oppervlak van de testglaasjes zelf. Onderzoek van dwarse doorsneden toonden aan dat elementen zoals calcium en kalium uit het glas waren gedifundeerd wat resulteerde in de vorming van een uitgeloogde laag die soms een gelaagde structuur vertoonde. Neven-en sporenelementen vertoonden een vrij gecomplceerd gedrag, mangaan en rubidium waren verminderd in de uitgeloogde laag terwijl andere elementen zoals koper, ijzer en strontium het glas bleken binnengedrongen te zijn vanuit de omgeving. Dit werd bevestigd door de testglaasjes bloot te stellen aan een koperoplossing gedurende 212 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Samenvatting zes uur. Op het oppervlak van de glasmonsters had zich een calciumrijke korst gevormd die bepaald werd calciumcarbonaat te zijn. Tenslotte werden twee reeksen kaliumrijke middeleeuwse glazen voorwerpen bestudeerd van Namen en Fagnolle in Belgie die eveneens aanzienlijke tijd begraven geweest waren en dit om een vergelijking te kunnen maken in samenstelling zowel als corrosiegedrag met enerzijds de kaliumrijke modelglaasjes en anderzijds de natriumrijke archaeologische Romeinse glasvoorwerpen. 213 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Publications LIST OF PUBLICATIONS [1] J. Swerts, A. Aerts, N. De Biscop, F. Adams and P. Van Espen Age determination of Chinese porcelain by X-ray fluorescence and multivariate analysis Chemometrics and Intelligent Laboratory Systems, 22 (1994) 97-105 [2] A. Aerts, K. Janssens, F. Adams and H. Wouters Microanalysis of leaching phenomena in Roman glass from Qumran, Jordan Proc. 28th Microbeam Analysis Conference 1994, New Orleans, LA, USA, 31 juli-5 augustus 1994, VCH Publishers, 28 (1994) 45-46 [3] A. Aerts, K. Janssens and F. Adams A chemical investigation of altered jade art objects Orientations, november (1995) 79-80 [4] A. Aerts, K. Janssens and F. Adams Chemical analysis of the powder deposit on Chinese jade objects In : Chinese jade and paintings from the Dongxi collection, N. De Biscop [edit.], 1995, 12-13 [5] K. Janssens, A. Aerts, L. Vincze, F. Adams, C. Yang, R. Utui, K. Malmqvist, K.W. Jones, M. Radtke, S. Garbe, F. Lechtenberg, A. Knochel and H. Wouters Corrosion phenomena in electron, proton and synchrotron X-ray microprobe analysis of Roman glass from Qumran, Jordan Nuclear Instruments and Methods in Physics Research B 109/110 (1996) 690-695. [6] K. Janssens, L. Vincze, B. Vekemans, A. Aerts, F. Adams, K.W. Jones and A. Knochel Synchrotron radiation induced X-ray microfluorescence analysis Michrochimica Acta [Suppl.] 13 (1996) 87-115. [7] A. Aerts, K. Janssens and F. Adams A chemical investigation of altered jade art objects In : Chinese Jade, Selected Articles from Orientations 1983-1996 (1995) 170- 171. [8] F. Adams, A. Adriaens, A. Aerts, I. De Raedt, K. Janssens and O. Schalm Micro and surface analysis in art and archaeology Journal of Analytical Atomic Spectrometry, 12 (1997) 257-265. [9] B. Vekemans, K. Janssens, L. Vincze, A. Aerts, F. Adams and J. Hertogen Automated segmentation of p-XRF image sets X-Ray Spectometry, in press [10] A. Aerts, K. Janssens, F. Adams and H. Wouters Microanalysis of Roman glass from Khirbet Qumran Journal of Archaeological Science, Proceedings of the Illinois Urbana Archaeometry Conference, submitted [11] A. Adriaens, A. Aerts, J.P. Candelone and F. Adams SIMS studies of archaeological objects and materials in simulation experiments Argon National Laboratory Monograph, in press [12] A. Aerts, K. Janssens, F. Adams, C. Fontaine-Hodiamont, H. Wouters, E. De 215 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Publications Witte and R. Donceel Antique glass from Khirbet Qumran. Archaeological context and chemical determination Bulletin of the KIK/IRPA, submitted [12] H. Romich, A. Aerts, K. Janssens and F. Adams Simulation of corrosion phenomena of glass objects on model glasses Proceedings o f the International Conference on Glass XVIII, The American Ceramic Society, submitted 216 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Conference Contributions and Study Visits CONFERENCE CONTRIBUTIONS [1] A. Aerts, K. Janssens, H. Wouters and F. Adams Microanalytical investigation of ageing phenomena in Roman glass artifacts Deauville Conference 1994, Montreux, Switzerland, 16-20 May 1994, poster presentation [2] A. Aerts, K. Janssens, H. Wouters and F. Adams investigation of ageing phenomena in Roman glass artifacts by means of SEM and SIMS European Conference on Energy Dispersive X-ray Spectrometry, Budapest, Hongary, 30 May-3 June 1994, poster presentation [3] A. Aerts, K. Janssens, F. Adams and H. Wouters Microanalytical analyses of a glass collection from Khirbet Qumran, Israel International Symposium on Archaeometry, Urbana, Illinois, USA, 20-24 May 1996, poster presentation [4] A. Aerts Migration of Ionic Species Inside Corroded Glass Workshop Archaeological Glass, University of Antwerp, Wilrijk, Belgium, 19 March 1997, oral presentation [5] A. Aerts, K. Janssens, B. Velde and F. Adams Analyse de la composition des verres de Qumran et des verres Romains d ’Europe occidentale Xlleme Rencontres de I’Association Francaise pour I’Archeologie du Verre, Lyon, France, 25-26 October 1997, oral presentation STUDY VISITS [1] Technische Universitat Wien, Institut fur Analytische Chemie, Austria 7-21 November 1993, study visit scanning electron microscopy [2] PGT, Peterborough, Great-Britain 12-18 December 1993, Software course PGT [3] National Institute of Standards and Technology, Gaithersburg, Maryland, USA 2 September-3 December 1995, study visit image analysis and secundary ion mass spectrometry 217 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IMAGE EVALUATION TEST TARGET (Q A -3 ) ■- IHH h im 1.0 |5 0 IIIM’nrrn I- |I!S [2.2 !l S ii £ 2.0 l.l 1.8 1.25 1.4 150mm /4 R R L IE D _== IIW1C3E . Inc 1653 East Main Street - = = r - Rochester, NY 14609 USA Phone: 716/482-0300 Fax: 716/288-5989 © 1993. Applied Image. 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