Archaeometry ΠAnalyzing the Cultural Heritage
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ART AND CHEMICAL SCIENCES 915 CHIMIA 2001,55, No. 11 Chimia 55 (2001) 915-922 © Schweizerische Chemische Gesellschaft ISSN 0009-4293 Archaeometry - Analyzing the Cultural Heritage Willem B. Stern* Abstract: Archaeometry is an interdisciplinary field of research, where the studied object stems from archaeological, cultural or historical domains whereas the approach of investigation is based on (natural) science. In the present overview the main focus is on case studies and non-destructive techniques. X-ray fluorescence and diffraction studies on ceramics, coin alloys, glass, man-made and natural gemstones, and objects of fine art prove that material analysis may contribute to insights in questions of authenticity and origin, but also in ancient technologies e.g. of coinage. Keywords: Ancient glass· Archaeometry . Authenticity in art . Non-destructive analysis' Numismatics Archaeometry is a relatively recent term. The objects of investigation cover a the analysis itself cause detectable altera- It came into use around 1950 in England, wide range from alloys, over bone, ce- tions or long-lasting radioactivity of the and soon infiltrated many languages. ramics and coins, paper, pigments and studied object. In general, no (sub-) sam- Periodicals bear the term, like 'Archaeo- postage stamps to teeth and textiles. Be- pling is allowed, nor any polishing or metry' since 1958 in the UK, 'Berliner cause many objects represent not only scratching of the object's surface. Beitriige fiir Archiiometrie' since 1976, cultural, but commercial value as well, The analytical techniques in the fol- or 'Revue d'Archeometrie' in France investigations are preferably performed lowing case studies belong to the group since 1977. Nevertheless, it has not been by non-destructive methods. This was of instrumental analysis and deal with included in many encyclopedias or dic- not possible in the early days of analyti- chemical aspects of solids, but also with tionaries. It is missing e.g. in Collins/ cal chemistry, when e.g. Klapproth exe- their physical-mineralogical aspects. x- Pons (1997), Wahrig (1989), Webster's cuted the first analyses of Roman glass ray fluorescence (XRF) spectrometry (1993). and Greek silver coins in the late 18th performs an elemental analysis of rough- Archaeometry is an interdisciplinary century, a 150 years before the term Ar- ly 95% of all elements in the periodic topic, where the studied object stems chaeometry came into use. system, whilst X-ray diffractometry from historical science or the cultural do- For certain material groups like ce- (XRD) detects elemental assemblages main, but the method of investigation is ramic or glass shards, conventional de- provided that they represent physically from (natural) science. Meaning in the structive methods of chemical and physi- ordered compounds or minerals. beginning merely 'measuring/determin- cal analysis are still used in practice, Both aspects of solid matter are of im- ing the age of a historical object by when the mass of available shards allows portance in archaeometry: a Babylonian physical methods like isotope dating' it consumption of a selected quantity. cylinder seal described as 'white stone' adopted later a more general meaning of The following case studies, however, may chemically consist of calcium as 'performing scientific measurements on deal solely with the challenging task of main component besides carbon and oxy- objects of art, culture and history', or non-destructive analysis. Its aim is not gen, which suggests a compound of shortly 'analyzing the cultural heritage'. only the correct labeling of e.g. museum CaC03, and excludes e.g. gypsum (con- exhibits - alabaster is not onyx, brass not taining also sulfur) or bone (containing bronze, silica not necessarily quartz, a phosphorous). But the chemical analysis white pigment not necessarily chalk, does not tell us whether the sea] is made limestone not marble etc. - but also more of chalk (chemically CaC03, minera]ogi- complex targets like questions of authen- cal]y calcite) or of shell (chemically also ticity, or of ancient trade, to mention only CaC03, but mineralogically aragonite). two. A similar case may be a white pigment 'Correspondence: Prof. Dr. W.B. Stern Geochemical Laboratories The present outline is not intended to consisting chemically of titanium (and Institute for Mineralogy and Petrography be comprehensive. It rather reports ar- oxygen), hence being Ti02. But minera- Department of Earth Sciences University of Basel chaeometric fact-finding as it may occur logically different species with the same Bernouillistrasse 30 in a laboratory specialized in instrumen- composition occur, such as anatase, CH-4056 Basel tal analysis and materia] science. brookite and rutile. Natural Ti02 miner- Tel.: +41612673622 Fax: +41 61 2672881 Analytical methods are non-destruc- als are black or brown due to impurities, E-Mail: [email protected] tive when neither preparation steps nor but two of them have been synthesized ART AND CHEMICAL SCIENCES 916 CHIMIA 2001, 55, NO.l1 and commercialized in known, but differ- Crystallography-Mlnoralogy Chemical Micro· end Bulk-Analysls of Solida ent time spans, and belong today to the most widespread white pigments. Hence, the chemical proof of titanium white may 2000 represent a first hint in a question of au- thenticity, but the mineralogical proof of either anatase or rutile gives further in- sight in the question of dating. 1990 XRF and XRD are based upon the same physical principles, namely Bragg's equation and Moseley's law of 1914. For a deeper insight, the reader 1980 may consult the comprehensive textbook of Potts (1993) [I]. A schematic over- view on X-ray applications used during 1970 the past 100 years is given in Fig. ]. Instrumental analysis is in general based upon an often unspoken, but essen- --- tial prerequisite: the object of investiga- 1960 tion has to be chemically and structurally homogeneous. Analytical data from het- erogeneous objects will always be prob- lematic when the investigated area, vol- 1950 ume or mass is smaller than the entire object. The analytical quality may be out- standing, the data nevertheless meaning- less because of lacking representativity. 1940 The topic seems simple but is a perma- nent point of discussion between analysts and archaeologists. 1930 Case Studies 1. Terra Sigillata - Is the Red Gloss 1920 a Clue to Production Origin? Terra sigillata is one example of in- dustrial large-scale pottery production in 1910 Roman time. Terra sigillata was manu- factured at different sites in Italy but also in what is today France and elsewhere. 1900 The manufacturers can, in favorable cas- es, be identified by stamps of origin on the underside of ceramic bowls, dishes, plates. Sometimes not only the produc- Fig. 1. The application of X-rays in material science over the past 100 years. Although X-ray tion place is identified but also the name diffraction (XRD, blue boxes) and X-ray fluorescence (XFA, red boxes) have a relatively long of the craftsman involved. history, their worldwide application only became important after the second world war. XFA with Terra sigillata is a quite spectacular its wave-length dispersive/sequential (WD-XFA) and its energy dispersive/simultaneous options type of pottery, easy to identify by means (ED-XFA) belong today to the most frequently used instrumental techniques for the analysis of inorganic solids in spite of the relatively high investment of 50'000 to 150'000 € [1]. of its characteristic orange-red gloss ('en- gobe', slip), a very fine grained and thin cover on the ceramic body. This cover is of them - may vary according to the ori- termined non-destructively by means of believed to consist of originally a clay- gin of the clay, and therefore from one Energy-Dispersive X-ray Fluorescence mineral enriched slip which turned by production plant to another geographi- Spectrometry (ED-XFA), and the con- oxidative firing to a glaze- or enamel- cally remote site. centrations of Ti02, MnO, Fe203, and like, quite scratch-resistant coating. One From four different production sites K20 found were plotted as elemental ra- may expect that its chemical composition twelve fragments from museum collec- tios in a XY graph (Fig. 2). Although the reflects roughly 'clay' with silica and tions were chosen, each fragment bearing projection points from the surface, and of alumina as the main detectable compo- a stamp of origin, and some also the mark the different shards from each origin do nents, and additional elements in variable of the potter Senti us, known to have scatter, the individual production centers amounts, like potassium, calcium, titani- worked in Italy and France as well. Arezzo (A, Italy), La Graufesenque um, manganese, iron etc. The relative The chemical composition from the (G, France), Lezoux (X, France) and proportions of these elements - or some pottery surface, i.e. the coating, was de- Lyon (L, France) occupy, however, dis- ART AND CHEMICAL SCIENCES 917 CHI MIA 2001.55. No. 11 tinctively different chemical fields. This less than 1 wt.% silver in the ore to 99% an improvement of the aspect. Depend- means that an individual unknown frag- silver which was used for coinage. Silver ing on different factors, the circulation of ment without a stamp of origin could coins from Greece became tender not coins leads to slower or faster wearing- most probably be attributed correctly to only in Greece, but in many places of the off of the noble layer. one of the four production sites, or possi- Mediterranean where Greek traders were Since blanching transforms an initial- bly to an additional not yet studied origin. active, notably in Spain and in the Rhone ly homogeneous alloy into a heterogene- valley then inhabited by Celtic tribes. ous, onion-shaped body, anyinstrumen- They used Greek coins since the 4th cen- tal analysis of such a coin will lead to Terra Sigillala tury BC and since the 3rd century BC possibly doubtful results.