Silvered Glass Deterioration & Manufacture

Conservation and Restoration (Glass, Ceramics and Stone)

An investigation into the influence of composition and manufacture on the differences in deterioration between two mid-19th century silvered glass objects from the Nederlands Openluchtmuseum, Arnhem MA Thesis

Tegen J. Symons Student Number: 11723351

Supervisors: Kate van Lookeren Campagne (Primary supervisor) Dr. Bas van Velzen (2nd Reader) Professor Dr. Ella Hendriks Professor Dr. Maarten van Bommel

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Contents 1 English Summary ...... 4 2 Dutch Summary ...... 5 3 Introduction ...... 6 4 The Case-study Objects ...... 7 4.1 Object Description and Condition ...... 7 Description and Examination of Deterioration ...... 8 4.2 Photographic documentation ...... 9 Optical Microscopic Examination ...... 11 4.3 Object Biographies ...... 14 Similar Objects in Other Collections ...... 15 5 Current (Scientific) Knowledge ...... 18 5.1 Historical Knowledge ...... 18 Glass for Silvering ...... 18 History of the silvering of glass ...... 20 Historical silvering recipes ...... 22 5.2 Scientific discussion of the historical recipes ...... 24 6 Recipe Reconstructions ...... 25 6.1 Recipe Interpretation ...... 25 Recipe 1: Liebig (c.1850) ...... 25 Recipe 2: James (1884)...... 27 Recipe 3: Fitzpatrick (1856) ...... 28 Recipe 4: Helmanstine (2018) ...... 29 6.2 Methodology ...... 30 Test Methodology and Observations ...... 31 6.3 Results and Discussion ...... 35 Results Recipe 1: Liebig ...... 35 Results Recipe 2: James ...... 35 Results Recipe 3: Fitzpatrick ...... 35 Results Recipe 4: Helmanstine ...... 36 Discussion ...... 38 Relation to case-study objects ...... 41 7 Scientific Analysis ...... 43 7.1 Methodology ...... 43 Optical Microscopy ...... 43

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SEM/EDX ...... 43 7.2 Results and Discussion: Case-study objects ...... 46 Optical Microscopy ...... 46 SEM/EDX ...... 47 7.3 Instrumental analysis of the recipe reconstructions ...... 54 Methodology ...... 54 Results and Discussion ...... 55 Comparison with case-study objects ...... 59 8 Conclusion ...... 62 9 Table of Figures ...... 63 10 Table of Tables ...... 64 11 Acknowledgements ...... 65 12 Bibliography ...... 66 13 Appendix I: The Objects...... 69 14 Appendix II: Reconstructions ...... 70 15 Appendix III: Scientific Analysis ...... 75

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1 English Summary

This thesis was written as part of the Master for the Conservation and Restoration of Cultural Heritage (Glass, Ceramic and Stone specialisation, Spring/Summer 2019) on the topic of two silvered glass objects exhibiting different forms of deterioration. Contrary to much literature on the subject, silvered glass objects are produced using a chemical method which requires the combination of ammonia and silver nitrate to produce a fine layer of silver metal upon the surface of glass and does not require the use of mercury or tin, despite the persistence of the term “mercury glass” to describe these objects. Silvered glass, often considered as part of the category of objects known as “folk art” in museum collections, has been overlooked within the field of glass (and metal) conservation. Two objects, in the collection of the Openluchtmuseum, Arnhem, since the 1950’s, were found to have experienced deterioration of the silver layers that had been applied to their interiors. The two hollow-blown glass objects, a vase and candlestick thought to have been manufactured in Bohemia in the mid-19th century, exhibited different forms of deterioration that may have resulted from differences in their morphology, object history or manufacturing process. This thesis was undertaken to conduct research into the history and science behind the silvering of three-dimensional glass objects and conduct reconstructions using historic recipes, in order to increase our knowledge of this technique and to better inform conservation attempts of objects produced in this manner. SEM/EDX analysis has shown that the metallic silver layer present upon these objects is extremely thin and thus susceptible to physical damage and that the exposure to sulphur in the atmosphere is the likely cause of the deterioration present in the candlestick. The deterioration and detachment of the silver layer on the glass vase presented a more complex diagnostic challenge, but it seems probable that the deterioration can be related to the morphology of the object. The reconstruction of the historic (and one modern) recipes has helped to increase our understanding of the chemical process of silvering on glass and identified possible mechanisms by which silvered glass objects could be rendered susceptible to deterioration resulting from their manufacturing methods. It is hoped that the reconstructions and the in-depth investigation centred on these two case-study objects will provide a foundation for the conservation and diagnosis of silvered glass and shed light on a process that has historically been misunderstood.

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2 Dutch Summary

Deze scriptie is geschreven als onderdeel van de Master voor de Conservering en Restauratie van Cultureel Erfgoed (specialisatie Glas, Keramiek en Steen, lente / zomer 2019) over het onderwerp van twee objecten van verzilverd glas, met verschillende vormen van verslechtering. In tegenstelling tot veel literatuur over het onderwerp, worden objecten van verzilverd glas geproduceerd met behulp van een chemische methode die vereist dat de combinatie van ammoniak en zilvernitraat een fijne laag zilvermetaal op het glasoppervlak vormt en geen gebruik van kwik of tin vereist, ondanks het voortbestaan van de term "mercury glass" om deze objecten te beschrijven. Verzilverd glas, vaak beschouwd als onderdeel van de categorie objecten die bekend staat als "volkskunst" in museumcollecties, is over het hoofd gezien op het gebied van glas (en metaal) conservering. Bij twee objecten, in de collectie van het Openluchtmuseum, Arnhem sinds de jaren 1950, bleken de zilver lagen, die op hun binnenkant waren aangebracht, te zijn gedegradeerd. De twee holle objecten van geblazen glas, een vaas en een kandelaar waarvan men aanneemt dat ze halverwege de 19e eeuw in Bohemen zijn vervaardigd, vertoonden verschillende vormden van achteruitgang die het gevolg waren van verschillen in hun morfologie, objectgeschiedenis of productieproces. Deze scriptie is opgesteld om onderzoek te doen naar de geschiedenis en de wetenschap achter het verzilveren van driedimensionale glazen voorwerpen en om reconstructies uit te voeren met behulp van historische recepten, om onze kennis van deze techniek te vergroten en om behoudspogingen van op deze manier geproduceerde objecten beter te informeren. SEM / EDX-analyse heeft aangetoond dat de metalen zilverlaag die op deze voorwerpen aanwezig is extreem dun is en dus vatbaar is voor fysieke schade en dat de blootstelling aan zwavel in de atmosfeer de waarschijnlijke oorzaak is van de in de kandelaar aanwezige verslechtering. De verslechtering en het loslaten van de zilverlaag op de glazen vaas vormde een complexere diagnostische uitdaging, maar het lijkt waarschijnlijk dat de verslechtering kan worden gerelateerd aan de morfologie van het object. De reconstructie van de historische (en een modern) recepten heeft bijgedragen tot een beter begrip van het chemische proces van verzilvering op glas en identificeerde mogelijke mechanismen waardoor objecten van verzilverd glas gevoelig zouden kunnen worden voor degradatie als gevolg van hun productiemethoden. Gehoopt wordt dat de reconstructies en het diepgaande onderzoek gericht op deze twee casestudy-objecten een basis zullen vormen voor het behoud en de diagnose van verzilverd glas en licht werpen op een proces dat in het verleden verkeerd is begrepen.

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3 Introduction

Silvered glass, produced predominantly during the 19th century in Germany, Bohemia, England and the United States, is often described as “kitsch”. With its negative implications, this description suggests that silvered glass is therefore not worth preserving or understanding. This is a mistake that this thesis will aim to rectify by exploring in depth the processes by which silvered glass has historically been manufactured. As such, it will attempt to broaden our understanding of how the production processes could influence the deterioration of two case-study objects provided by the Nederlands Openluchtmuseum in Arnhem. The silvered glass objects under discussion are made of mould-blown glass, which is hollow, allowing a silver nitrate-based solution to be poured into their interiors through a hole in the base and creating a thin layer of silver on the interior surface of the glass. Little previous research has been conducted into the different recipes and methods used for silvering glass in this way and even less conservation-focused literature has been published on the stability of these objects in museum collections. The two objects used as case-studies for this thesis, both held in collection storage since the 1950’s, were found to show signs of the deterioration of their silver layers but this deterioration was manifesting in very different ways. One of the objects, a candlestick with significant physical damage to the glass at the base and head, was shown to have severe discolouration of the silver layer surrounding these broken areas. The other object, a vase, did not experience damage to the glass or discolouration, but the silver layer was almost entirely lost from parts of the object’s interior, especially near the narrow stem and appeared to have detached from the glass in flakes. This thesis focusses on the relationship between deterioration and production, namely: To what extent can the deterioration of the silver-coloured coating on 19th century ‘Poor Man’s silver’ glass objects be attributed to their composition and manufacture? To answer this question, the history of the objects and of the wider production of silvered glass will be considered, as well as instrumental analysis of the deterioration of the two case study objects. Recipe reconstructions will also be made, using historical silvering recipes, to further investigate the influence of production procedure and recipe composition upon silvered glass objects.

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4 The Case-study Objects

In this section, the objects will be described, and their known history given, since both will inform later explorations of their deterioration.

4.1 Object Description and Condition NOM 11931-51 (vase) The object is a glass vase with a foot. The bowl is 16cm high and 9cm wide at the rim. The foot is 8 cm wide. It is mould-blown and has a double wall with approximately 3 mm between the sides at the rim, where the gap is narrowest and approximately 3cm from the bottom of the bowl to the base, where the gap is widest. It has a closed form with a hole under the base, with a diameter of 1.4 cm. The bowl of the cup has a gently scalloped or facetted shape, with 16 facets. Both walls of the object are silvered on the interior. On the outside are the remains of what appears to be a floral decoration, possibly painted.

The glass is in good condition, with some wear that is commensurate with its age, for example scratches on the base where the glass touches a surface. There are no visible losses or damage to the glass elsewhere. There is some dust/ surface deposition of dirt in the bowl of the cup and possibly inside the foot of the object, particularly in the protruding scalloped areas (Figures. 4.1, 4.2). The museum catalogue number of the object is painted on the glass underneath the base in a green paint medium. The painted decoration on the exterior of the object is degraded and has large areas of loss, making it difficult to determine the original appearance and design. The colour of the painted decoration is pale yellow, but this may be a result of deterioration.

NOM: 22678-5 (candlestick) The object is a glass candlestick, 21 cm in height. It is made of mould-blown glass and is silvered on the interior, which is hollow and accessed through a hole (diameter 1cm) in the base (approximately 9.2 cm in diameter). The stem of the candlestick is decorated on the exterior of the glass with a painted decoration, probably floral in design. The object is broken in two parts.

The glass is in poor condition when compared to the vase. There is significant physical damage, including the loss of a large section of the side of the object’s base, the separation of the object into two parts, a hole near the rim of the head of the candlestick and a large crack down the side of the broken off top section (Figures. 4.1, 4.3). As with the vase, there is some wear around the underside of the object’s base, resulting from contact with the surface it has been standing on, and features resulting from the manufacturing process, such as bubbles. The museum catalogue number has been painted on the base and this has been sealed with a clear varnish-like medium which has undergone considerable yellowing. The top section of the object has a large piece of wax in the hollow interior, probably as a result of hot wax from a candle leaking through a crack in the head of the candlestick. There are also deposits of wax near the base of the object, indicating that liquid wax dripped from a burning candle onto the foot of the object. Both the head and base of the object have significant build-up of dust and dirt in any concave areas. The base of the candlestick, which exhibits the large loss mentioned above, is open

7 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture to the air and as a result there is dust build up in the interior of the object, contributing to the discolouration in this part of the candlestick. Similar to the vase, the candlestick has the remains of an applied decoration on the exterior surface, also in a creamy yellow medium that appears painted onto the object. This decoration is deteriorated and missing some areas, making it difficult to determine the original design and colour.

Figure 4.1: Object NOM 11931-51 (vase) and NOM 22678-55 (candlestick) in current condition

Description and Examination of Deterioration This research has focused on one aspect of the condition of these objects, namely the deterioration of their respective silvered layers. This deterioration is different for each object. In the vase, the deterioration of the silver layer manifests as complete loss of the silvering toward the base of the object. The stem of the object, the narrowest part, is the worst affected area and there is little to no silver layer remaining. The rim of the cup is in the best condition and silvering in this area is still uniform in texture and colour tone. Throughout the bowl of the cup, the silvering appears to be flaking and is cracked. In the Hirox digital microscope image (Table 4.1, Image 2), a white opaque rim appears around the flaking areas; since this phenomenon is not visible to the naked eye, it seems likely that the white appearance is caused by differences in the angle of reflection, due to changes in the surface of the silver layer, as it detaches from the object. The deterioration of the silver layer of the candlestick provides a stark visual contrast. Here, there is a dark discolouration and loss of reflectance of the silver layer at the base of the object, in particular surrounding the large break in the base, which leaves the hollow interior (and therefore the silvered surface) open to the air. This discolouration, which is brown to grey-black in colour, also surrounds the hole in the head of the candlestick, again where a loss has left the silvering exposed. The rest of the body of the object is unaffected by this discolouration and the silver layer appears in good condition.

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4.2 Photographic documentation The condition of the object at the point of receiving it into the Ateliergebouw was recorded using a Nikon D750 digital SLR camera.

Figure 4.2: Object NOM 11 (vase) – A.) The vase. B.) The applied decoration. C.) The base of the vase. D.) The foot of the vase. E.) The rim of the vase.

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Figure 4.3: Object NOM 22(candlestick) – A.) The candlestick. B.) The neck of the candlestick where the head has broken off. C.) Plan view of the break in the neck, showing the candle wax on the interior. D.) The base of the candlestick. E.) The foot of the candlestick.

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Optical Microscopic Examination A closer examination of the objects and their deterioration was undertaken using optical microscopy using a Dino-Lite handheld digital microscope and Hirox 3D digital microscope (Hirox KH-7700 3-D with Photonic Optics lights). Obtaining images was problematic. The highly reflective surface of the case-study objects meant that the microscope images were disrupted by reflected light and the focusing of the image was complicated by the multi-layered, variable transparency of the hollow objects. Attempts were made to resolve this difficulty by using white reflectors and varying the angle of the light, but future research could use a white box tent to remedy this issue further. Table 4.1, below, provides the images of specific details taken using different levels of magnification. See Appendix I: Figure 13.1 and 13.2 for diagrams showing the locations of the magnified images.

Table 4.1: Optical microscopic images of the case-study objects in their present condition

Image Location Description NOM11931-51 - Vase Bowl of the 1)The painted decoration cup, exterior appears to have crazing and side. flaking of the edges. To the left of the image, the texture of the silver layer is clear: deterioration is occurring in Optical the manner of flake-type microscope loss, leaving a patchy image, appearance. magnification 100x

Bowl of cup, 2)In this Hirox image, the near rim. flaking deterioration of the silver layer is again Hirox 3D observed, but there is also digital the presence of white areas microscope, in some of the spaces which Magnification would have previously appeared silver.

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NOM22678-55 - Candlestick Head of 3)Crack in the glass in the candlestick, top of the candlestick, rim. through which both air and wax have come into contact with the silvering on the inside.

Dino-lite handheld microscope, magnification 20x Base of 4)This image shows the candlestick, transitional area at the edge edge of of the discolouration discoloured surrounding the large break area. in the base. Even in the part which still appears silver, there is an increasingly patchy appearance, when compared to the silvering in Dino-lite the image below. handheld microscope, magnification 55x Near base of 5)Bubble in the glass candlestick. formed during the manufacturing process.

Dino-lite handheld microscope, magnification 51x

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Body of 6)The painted decoration candlestick, shows crazing and flaking applied when examined under decoration. magnification and there is deposition of dirt or dust in the cracks in the surface.

Dino-lite handheld microscope, magnification 175x

Body of 7)At the top left and bottom candlestick. right, there are the remains of painted decorations, which appear to be flaking away from the glass substrate. Dark spots are primarily on the inside of the glass and are likely Dino-lite “pin-holes” of oxidation in handheld the silver layer. microscope, magnification 70x Base of 8)Drips of wax on the foot candlestick. of the object encourage the accumulation of dust and dirt, as well as reinforcing that the object was considered functional and not used in only an Dino-lite ornamental manner. handheld microscope, magnification 42x

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As seen in the images in Table 4.1, the deterioration of the silvering in both objects is very different in appearance and location. However, the deterioration of the painted decoration on both objects is very similar, exhibiting crazing and flaking away in both cases (Table 4.1, Images 1 and 6). This suggests that the objects were probably decorated using similar methods and material and could suggest a common date and/or location for their manufacture.

The discolouration of the silver layer in the candlestick is concentrated around the physical damage – the large loss to the base and the crack in the candlestick head. If this deterioration were due only to air ingress through the damaged glass, it could be expected that similar discolouration would occur in the vase, since this object is also missing a stopper in the base and is therefore open to the air and had been exposed for many years to the same atmospheric conditions in the museum storage. As such, it seems likely that the discolouration of the candlestick is the result of a particular susceptibility of its silver layer, perhaps exacerbated or enabled by the physical damage. The flaking loss in the vase occurs throughout the object, though it is more concentrated in some areas (for example, the stem), which perhaps indicates that the morphology of the object is contributing to an issue with the adhesion of the silver layer – aiding protection of the layer in some areas (for example the rim of the cup, where the interior space is tightly enclosed) and encouraging loss in others (the stem). It could also be posited that, had the objects been silvered using the same method and recipe, their morphology could have contributed to differences in their susceptibility to deterioration, since the candlestick has a much larger amount of open space within its hollow body, when compared to the more tightly closed interior of the vase.

4.3 Object Biographies Both of the case-study objects have been part of the collection of the Openluchtmuseum, Arnhem since the 1950’s. The vase was acquired in 1951 as part of a collection of 30 – 40 objects, donated to the museum by Mr de Vree van Gelder, of Maurik, near Tiel.1 The documents kept by the museum regarding this acquisition, namely a letter notifying the donor of the arrival of the objects into the collection and an attached inventory summarising the items, do not provide any insight into the object’s history. Due to the large number of objects, items are only listed, with no further description. It is likely that “zilvervaas”2 refers to this object. The other items are predominantly household goods, such as lanterns, walking sticks, baskets and cooking pots and some, more decorative items such as a “bloemversiering voor schoorsteen”.3 The nature of the assembled donation, all of which is often classed as “folk art” suggests that no special value was attached to the silvered glass object. It was donated along with other items that may have outlived their usefulness or fallen out of fashion, but it is significant that it was felt necessary to donate rather than discard these items, suggesting that the owner believed that they should be saved or that they represented part of a culture or period that ought to be preserved.

1 Het Nederlands Openluchtmuseum, “11826 11935”, Letter to J.H. de Vree van Gelder (1951) 2 English translation: “Silver vase” 3 Het Nederlands Openluchtmuseum, “11826 11935”, Letter to J.H. de Vree van Gelder (1951), English translation: “floral decoration for chimney”

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The candlestick is furnished with even less documentation than the vase; a single short entry from December 1954 indicates that the object formed part of a collection of objects offered by the widow Meeuwsen-Kuipers, from Budel and does not mention the object specifically.4 The catalogue entry for the object does provide an image of the object’s appearance before it broke into two pieces (Figure 4.4).5

Figure 4.4: The candlestick before it was broken, Nederlands Openluchtmuseum, catalogue entry for NOM22678-55, accessed 27/04/2019 [No date provided for image]

Similar Objects in Other Collections To ascertain how common the presence of silvered glass items in museum collections is, enquiries were sent to several institutions both in the Netherlands and further afield. The Zuiderzee Museum in Enkhuizen has several silvered glass objects in its collection. These objects, including two sets of matching candlesticks, are present in variable condition and a range of forms and decorative motifs (see Figure 4.5). The first candlestick pair (catalogue no. 002675)6 are of similar dimensions to the case study candlestick; both 17cm tall, they are decorated with a cold painted design of yellow butterflies. The holes in their bases are filled with pieces of cardboard, indicating that at some point in their history, they too lost their original stoppers (or that objects such as these were never provided with them). The Zuiderzee museum has dated these objects from “1850 to 1940”, which correlates with the

4 Nederlands Openluchtmuseum, “22669 22684”, Letters from Thijs Mol to donors (Dec 1954) 5 Nederlands Openluchtmuseum, catalogue entries for NOM11931-51 and NOM22678-55, accessed 27/04/2019. 6 Images of the objects from Zuiderzee Museum reproduced here with permission from L. Roscam-Abbing. 15 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture period in which the case-study objects were also produced but suggests that it is again difficult to date them with any certainty.7 The second pair of candlesticks in the Zuiderzee collection are undated, but similar in design and shape to both the first set and NOM22678- 55. The other objects possessed by the Zuiderzee museum include a 3-part “kaststel” (011579) in which the objects are described thus “…veel van de zilverlaag en decor is weggesleten…”.8 This set is dated to the end of the 19th century and though the forms are not similar to that of the case study objects, the loss of the silvering towards the base of the kaststel objects is similar to the flaking loss of the vase, again without the presence of discolouration.

Though it does not provide information on the deterioration of the silver layer, an object in the collection of the Pitt Rivers Museum in Oxford, United Kingdom, also shed some light upon the uses of silvering on glass objects. Object 1926.6.1 is a “witch bottle”; a small glass bottle, probably from the late 19th century, silvered on the inside and sealed with a waxed cork, purportedly to contain a witch (Figure. 4.6).9 In this case, the glass was silvered for the purpose of rendering the glass opaque and reflective (presumably to trap the witch or to prevent others from looking upon it). The silvering was most likely carried out upon the re-purposed bottle, rather than creating the vessel for use in its current manner.10 This example is relevant because it shows that silvered objects in museum collections were not only of decorative function and that silvering was used upon small objects to impart both practical purpose and extra meaning or value. From a conservation perspective, the thorough sealing (using both a cork and then a layer of wax) has ensured that the silver layer has been completely protected from atmospheric exposure.

Figure 4.5: The silvered glass objects in the collection of the Zuiderzee Museum, Enkhuizen. LEFT: catalogue no. 002675, candlestick pair with yellow butterflies. RIGHT: catalogue number 011579, three-part “kaststel”.

7 Laura Roscam-Abbing (Zuiderzee Museum: Conservator Wooncultur), pers. comm. (24/04/2019) 8 Zuiderzee Museum, Catalogue Entry: 011579, courtesy of L. Roscam-Abbing (2019), English translation: “much of the silver layer is worn away”. 9 Pitt Rivers Museum, Catalogue Entry: 1926.6.1, retrieved from: http://objects.prm.ox.ac.uk/pages/PRMUID25731.html [Accessed 17/01/2019] 10 Jeremy Uden (Pitt Rivers Museum, Oxford: Head Conservator), pers. comm. (16/01/2019) 16 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Figure 4.6: The “witch-bottle” in the collection of the Pitt Rivers Museum, Oxford (2013).

In summary, while the two case-study objects are both suffering from deterioration of the silver layer, the deterioration appears to be very different. The vase exhibits flaking detachment of the silver layer from the inside of the glass, while the silver layer on the candlestick appears well adhered but has significant discolouration surrounding the large break in the base and other physical damage. Microscopic examination of the deterioration and instrumental analysis of the silvered layer should help to determine the nature of the deterioration and differences in the composition of the silvering in order to better understand in how far production may have influenced the different deterioration seen in the objects. The next chapter will turn to the history of silvered glass objects and consider the chemical processes by which the silver layer is formed.

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5 Current (Scientific) Knowledge

This chapter will explore the history of production of silvered glass objects and put forward scientific explanations for the choices made in selection of materials and processes by which silvering was conducted, to form a basis from which to investigate causes of deterioration.

5.1 Historical Knowledge To develop an understanding of the deterioration of the silver-coloured coatings on the vase and candlestick, it is important to consider the history of the production of these and similar objects and what influence these processes may exert on the objects’ condition through time. As such, the history of the glassmaking and silvering processes will be discussed, before moving on to a consideration of several historical recipes for the silvering of glass. As is noted in the definitive work on German and Bohemian silvered glass by Endres et al., very little historical research has been conducted into silvered glasswares and their makers. This is in part due, they surmise, to the lack of clear information regarding an industry that seems never to have been documented in any detail.11

Glass for Silvering The case-study objects are considered by the Openluchtmuseum to have been produced in Bohemia during the mid 19th century. Discussions of Bohemian glass of the 18th to 20th century are often dominated by the vibrant red glass of Hyalith vessels (c.1820) or the wheel engraving which had cemented the reputation of Bohemian glass workshops among elite collectors since the 17th century, and the silvering of glass has tended to form a mere footnote in this rich history of production.12 Accordingly, there is almost no available information regarding the process of glass making for objects intended for silvering, though it can be assumed that a primary requirement was that the glass be as transparent as possible, in order that the reflective silver layer could be fully appreciated. The composition of the glass used is likely to have varied by region; it is known that many English pieces were produced using lead glass, for example, which would not have been the case elsewhere in Europe, where soda and potash glasses were more commonly used.13

Though there are few sources which discuss the exact glasses which were most often silvered, the history of the forming of glass vessels for silvering can be examined. Drayton’s patent, the first patent issued for the silvering of glass in 1848, refers to the possibility of use upon three-dimensional objects thus “The invention is applicable for the manufacture of looking glasses, and every other description of glass, either hollow or with a flat surface…”.14 As Endres et al. note, only in a very few sources is the double-

11 Endres, W., E. Voithenberg and G. Voithenburg, Silberglas: Bauernsilber : Formen, Technik und Geschichte. München: Callwey (1983), p.12 12 Tait, Hugh, and British Museum. Five Thousand Years of Glass. London: Published for the Trustees of the British Museum by British Museum Press, (1991), p.190-3 13 Lytwyn, D. Pictorial Guide to Silvered Mercury Glass: Identification & Values, Collector Books: Kentucky (2005), p.8 14 Drayton, T. Patent No. 12358: Silvering Glass and Other Surfaces, (1848), p.2 18 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture walled characteristic of the objects suggested, though it is of course necessary, to prevent contact between the silver layer and the atmosphere or physical damage.15 Thompson and Varnish’s 1849 patent states that their invention “consists of blowing or forming glass vessels so as to leave hollow spaces between the sides, so that the effect of silvering will be seen interiorly, and exteriorly…”.16 The existence of this patent, concerning the silvering of ornamental objects and referring to the ability to blow double-walled glass vessels specifically for this purpose, confirms that the technique must already have been known in Western Europe (the patent was filed in London) at this time, despite the very recent development of the silvering process, by scientists such as J. von Liebig (discussed below, Table 5.1).

In any case, Czech glassmakers were by the 18th century familiar with the production of double walled vessels, using the original Roman method of slotting one vessel inside another to create a space between them, and painted decorations on these glass objects were common, often taking the form of religious or floral motifs.17 It is not difficult then to trace the progression of these objects, in Bohemia, to the creation of blown double-walled hollow objects during the mid-19th century and indeed, moulds were already a popular method to create early Christmas decorations, the insides of which were silvered using the methods discussed below. In Randau’s 1905 account of German glass manufacturing methods, silvering glass in the manner of Liebig is mentioned explicitly, in connection with the silvering of hollow glass objects.18 Though this source is half a century more recent than Varnish’s patent, it helps to illuminate the commonality of silvering glass and provide a rare insight into early 20th century German glass production, especially since it describes that by this point, even cheap glasses were regularly silvered, so familiar and affordable was the technique.19

A 1912 account of the production of Christmas decorations in the town of Lauscha in Germany (which still produces blown glass ornaments today) describes how the ornaments are manufactured using pre-made glass tubes, reheated over a flame and blown to the desired size before shaping by pulling and twisting,20 while other designs, such as the popular glass insects, were created by placing the hot tube into a mould and then blowing the glass to expand into the shape of the mould.21 As such, mould-making was a

15 Endres, W. et al. (1983) p.24 16Thompson, F.H and E. Varnish. Patent No.12905: Inkstands, Mustard Pots and Other Vessels of Glass, (1849), p.2 17 Poche, Emanuel. České Sklo 17. a 18. Stoleti : Súvodní Expozicí Středověkého Skla : Katalog Výstavy Pořádané Ministerstvem Kultury ČSR a Uměleckopru°myslovým Muzeem v Praze U Příležitosti Mezinárodního Kongresu Assotiation Internationale Pour L'Historie Du Verre v červenci Až Srpnu 1970 v Královském Letohrádku Na Hradě Pražském. Praha: UPM, Obelisk, (1970), p.69-70 18 Randau, P. Die farbigen, bunten und verzierten Gläser: eine umfassende Anleitung zur Darstellung alter Arten farbiger und verzierter Gläser, der vielfarbigen irisierenden und metallisch schimmernden Mode und Luxusgläser, ferner d. Schmückung d. Gläser durch Metalle, Emaille und Bemalung, sowie durch Ätzen, Sandblasearbeit, Gravieren und Schleifen. Wien & Leipzig: Hartleben (1905), p.233 19 Randau, P. (1905), p.233 20 Journal of the Royal Society of Arts, The Manufacture of Glass Christmas Tree Ornaments in Germany, Vol. 60, No. 3129 (NOVEMBER 8, 1912), pp.1126-1127, p.1126 21 Ghidiu, L.W., and Ghidiu, G.M. “Is Your Christmas Tree Bugged? A History of Glass Insect Ornaments.” American Entomologist 52, no. 4 (2006): 240–242, p.240 19 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture familiar process to the manufacturers of these silvered objects and the bright and shiny designs became popular as far away as in the United States, increasing the demand for silvered glassware and possibly guiding the styles of the objects produced. The case-study candlestick, though thought to originate from Bohemia, has a shape that was fashionable in England during the late 18th and early 19th centuries, with a smooth baluster shaft that also appears in paktong, silver and brass examples.22 In this way, it could be suggested that moulds were created based upon forms that were in fashion, and silvered glass objects created for trade throughout Europe.

History of the silvering of glass Tin-Mercury The earliest method for the silvering of glass was the tin-mercury amalgam method. This method was used to produce flat mirrors from the 14th and 15th centuries to the beginning of the 20th century, when it was finally superseded by silver-backed mirroring techniques.23 The tin-mercury amalgam mirror was produced by rolling a fine sheet of metallic tin onto a smooth stone slab, over which liquid mercury was poured to a thickness of up to 5mm. The resulting tin oxide rose to the surface of the mercury, from which it was skimmed away and a polished sheet of glass was laid horizontally onto the mercury layer and weights were placed on the back of the glass, compressing excess mercury from the sides. Over the course of approximately 1 month, the tin-mercury amalgam (Sn8Hg) was formed and some liquid mercury volatilised, leaving behind a semi-solid layer.24 The amalgam layer is composed of crystals of solid amalgam which grow larger over time and a liquid, mostly mercury, phase which penetrates between the crystals via capillary action.25 In condition assessments forming part of a deterioration study of amalgam mirrors, Hadsund found that for mirrors produced in the 18th and 19th centuries, the amalgam was still partially fluid and drops of mercury were sometimes found in the frames.26 The excess of mercury used in the production process ensured continuous contact between the amalgam layer and the glass, allowing for a perfectly reflective surface, since the solid phase alone does not exhibit entirely even contact.27 Further, the effect of gravity drawing liquid mercury to the bottom of the mirror (and indeed evaporation) is mitigated somewhat by the capillary action of the growing amalgam crystals, ensuring that some of the liquid phase remains throughout the length of the mirror and in this way giving significant resistance to deterioration.28 Where deterioration occurs, it is commonly the result of the production of tin oxides and tin monoxide as corrosion products due to atmospheric conditions and manifests as darkened spots or patches, sometimes with paler concentric bands, occurring in the areas most exposed to the surrounding environment, for example the edges where the mirror meets its frame (Figure. 5.1).29 In summary, amalgam mirrors can be considered generally to be

22 Michaelis, R.F., Old domestic base-metal candlesticks from the 13th to 19th century, produced in bronze, brass, pakton and pewter , Woodbridge: Antique Collectors’ Club, (1978), p.127 23 Hadsund, P. "The Tin-mercury Mirror: Its Manufacturing Technique and Deterioration Processes." Studies in Conservation 38, no. 1(1993) 24 Hadsund, P. (1993), p.4-5 25 Hadsund, P. (1993), p.7 26 Hadsund, P. (1993). P.8 27 Hadsund, P. (1993), p.8 28 Hadsund, P. (1993), p.10 29 Hadsund, P. (1993), p. 12 and Herrera, Duran, Franquelo, Justo, and Perez-Rodriguez. "Hg/Sn Amalgam Degradation of Ancient Glass Mirrors." Journal of Non-Crystalline Solids 355, no. 37 (2009) p.1982 20 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture quite stable, provided they are kept under constant conditions and this stability contributed to their continued production even after patents such as Drayton’s (1848) highlighted the possibilities of mirroring using silver.30

Figure 5.1: (left) Dino-lite image (55x magnification) of the deterioration on object NOM 22678-55 (candlestick). (right) image taken from Hadsund, Per. "The Tin-mercury Mirror: Its Manufacturing Technique and Deterioration Processes." Studies in Conservation 38, no. 1 (1993), p.11 (Fig.12), showing corrosion of a mercury amalgam mirror.

Silver Nitrate Hadsund focuses on the production of flat mirrors (looking-glasses) using mercury amalgams, and it seems likely that silvering using the nitrate method was undertaken for the production of smaller silvered 3D forms prior to its widespread use in household mirrors. Though it is difficult to corroborate, James states that proponents of silver-nitrate silvering, such as Baron Liebig (Table 5.1) were motivated to develop their techniques for humanitarian reasons, namely the eradication of whole communities poisoned by the amalgam trade, and that such efforts were strongly rebuffed, due to the high wages available to those willing to work in such toxic conditions.31 Thus the transition to silver mirrors may have been set back by some years but the eventual triumph of silver over amalgam methods can be attributed to, as mentioned, its reduction in the need for toxic raw materials and also in the volumes of materials (and expense) required to create the mirror layer.32 James points also to the difference in the quality of the light reflected, in that silver mirrors provide a, supposedly, more flattering white-yellow bright reflectance compared to the cooler, grey-blue shine of amalgam.33

To relate these discussions back to the objects under consideration, their estimated date of production and origin indicate that they were likely produced using a form of silver-nitrate silvering, although this does not suggest that similar objects were not produced using the amalgam process. As Hadsund describes, through the addition of bismuth and lead, a liquid amalgam is created which could be used for coating the interior of hollow objects before pouring out the excess, similarly to methods using a liquid silver

30 Hadsund, P. (1993), p.3 31 James, F. L. (1884). The Deposition of Silver on Glass and other Non-Metallic Surfaces. Proceedings of the American Society of Microscopists, 6, p.72 32 James, F.L (1884), p.73 33 James, F.L (1884), p.74 and Hadsund, Per. 1993, p.4 21 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture solution.34 Sources relating to the amalgam coating of hollow objects are limited and certain authors maintain that mercury was never used as a liquid material for the silvering of interior surfaces,35 but it would be logical to expect that, should a hollow-blown glass object have been coated with amalgam, over time it would exhibit similar deterioration as that of an upright mirror, chiefly that liquid mercury would accumulate at the base unless a perfect seal prevented evaporation. In any case, the lasting connection between amalgam and silver coated items is the persistence of the term “mercury glass” to describe hollow-blown, double walled objects which have a silvered interior, both in descriptions contemporary to its manufacture in the 19th century and in present day in collector’s guides, auctions and common parlance.36 To prevent confusion, this thesis will use the term “silvered glass” throughout to refer to these objects.

Historical silvering recipes There are many extant written methods and recipes for the production of silvered-glass using silver nitrate but relatively few true patents, indicating that manufacturers competed and experimented to create improved results leading to a proliferation of silvered-glass simultaneously in England, the USA, Germany and Bohemia.37 The earliest patent was registered in England in 1848 by Thomas Drayton of London (see below for a summary of materials and method) and from this, many subsequent methods were developed, although the basic materials – silver nitrate and ammonia – remain consistent. The silvering of glass has been increasingly viewed as a decorative method that can be attempted at home, as the wealth of online articles attests, meaning that there is no shortage of modern iterations of silvering recipes. Unfortunately, these recipes often fall short in explaining the chemistry of the reaction, so it is necessary to make a comparison of the methods and materials in order to extrapolate an idea of the chemical processes by which the silver coating was formed. Table 5.1 attempts to make this comparison.

34 Hadsund, Per. (1993), p.5 35 Lytwyn, D. (2005), p.7 36 Lytwyn, D. (2005), p6 37 Lytwyn, D. (2005), p.8 22 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Table 5.1: Comparison of historic silvering recipes

Author Date Materials Method T. Drayton38 1847/8 Silver nitrate Silver nitrate added to water and hartshorn, left to Water stand. Silver oxides filtered off, oil of cassia Hartshorn (carbonate of added, left to stand. Solution poured onto clean ammonia) surface, oil of cloves and wine spirits added. Oils of cassia and clove Excess poured off and surface left to dry before Wine spirits varnishing. Baron J. von Liebig c.1850 Distilled water Mix ammonia with distilled water. Separately mix (1884 by F. James)39 Aldehyde of ammonia silver nitrate with distilled water. Mix two Silver nitrate solutions together and agitate. Cover the object with the solution and apply heat by means of a water bath Draper (1884 by F. unknown Silver nitrate Dissolve silver nitrate crystals in distilled water. James)40 Distilled water Add water of ammonia until the brown precipitate Water of ammonia at first formed is nearly re-dissolved, stirring Sodium and potassium continuously. Dilute to 6 ounces, Dissolve the tartrates (Rochelle salts) Rochelle salts in distilled water and make up to six ounces. To use, mix the solutions in equal parts and proceed as with Liebig’s method. F. James own version 1884 Silver nitrate Dissolve the silver nitrate in distilled water and of Baron Liebig’s Water of ammonia precipitate by adding water of ammonia. Add (c.1850) method, Distilled water water of ammonia drop by drop, stirring 1884.41 Rochelle salts continuously until the precipitate is almost dissolved. Dissolve the Rochelle salts in distilled water and boil this solution while adding two grains of silver nitrate (previously dissolved in distilled water). Boil for 3-4 minutes, cool and dilute with distilled water. To use, mix the two solutions in equal portions, heat to 100-120 F. M.A. Martin42 1875 Silver nitrate in water The glass is thoroughly cleaned first, using Nitrate of ammonia in alcohol and caustic potash. water In a shallow dish place equal parts silver nitrate Caustic potash solution and nitrate of ammonia solution. Add Sugar dissolved in water, equal parts caustic potash solution and the with tartaric acid, boiled inverted sugar solution (to make a total of four for 10 minutes then equal parts). Place the object in the dish and cooled. Alcohol is added agitate gently. When the liquid becomes greyish to prevent fermentation. with flakes of silver, the object is fully silvered and can be removed and rinsed. J. Fitzpatrick43 1856 Silver nitrate Mix all of the ingredients and place inside the Aqua ammonia article to be silvered. The whole is then kept at a Distilled water temperature of 160 F until silvering is achieved. Alcohol Grape sugar FOR COMPARISON: 2018 Silver nitrate solution Mix the silver nitrate and ammonium nitrate A.M Helmanstine44 Ammonium nitrate solutions. Pour the dextrose solution into the solution object to be silvered. Add the silver/ammonia Dextrose solution into the object, followed immediately by Sodium hydroxide the sodium hydroxide. Swirl the object to coat the Distilled water inside and then rinse the object with distilled water.

38 “Drayton's process for silvering glass”. (1847). Journal of the Franklin Institute, 44(4), 248-254 and The Magazine of Science, Oxford University, p.245 39 James, F. L. (1884). The Deposition of Silver on Glass and other Non-Metallic Surfaces. Proceedings of the American Society of Microscopists, 6, p.75 40 James, F. L (1884), p.75 41 James, F. L (1884), p.76 42 Martin, M. A.“On the Silvering of Glass by Inverted Sugar, for Optical Instruments and Experiments”. Monthly Notices of the Royal Astronomical Society, 36(2) (1875), 76–78 43 Fitzpatrick, J., Scientific American (1856), 11(46), 363 44 A.M. Helmanstine (2018), “Silver Ornaments: A Holiday Chemistry Project”, ThoughtCo: https://www.thoughtco.com/silver-ornaments-christmas-project-606131 [Accessed: 25/02/2019] 23 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

5.2 Scientific discussion of the historical recipes A comparison of this small selection of recipes provides several common ingredients: Silver nitrate, (distilled) water, ammonia and sometimes a form of sugar. Tollen’s test, which is used to demonstrate the presence of aldehyde groups, follows a similar procedure to the methods provided above and can be used, to some degree, to illuminate the science of the silvering reaction. Here the components are: silver nitrate solution (AgNO3), aqueous ammonia (NH4OH), sodium hydroxide (NaOH), nitric acid (HNO3) and dextrose solution (C6H12O6). The flask to be silvered is rinsed with nitric acid and then water and pre-warmed using hot water. In a separate beaker, a solution is prepared using silver nitrate, aqueous ammonia and sodium hydroxide. The hot water is poured away and the dextrose is added to the flask, the silver and ammonia solution is then added to the dextrose and swirled to coat the inside of the flask with a silver layer.45 In this case, because the dextrose contains an aldehyde group (a functional group with structure -CHO where the C has a double bond with O), the aldehyde reduces the silver-ammonia complex and a metallic silver layer forms on the glass. Only reducing sugars (with an aldehyde group [-CHO], rather than a ketone group [RC(=O)R], as in sucrose) are able to achieve this, which perhaps explains why “grape sugar” is used in some recipes, since this infers glucose/dextrose, rather than sucrose.46 The role of sodium hydroxide (NaOH) or potassium hydroxide (KOH), or tartaric acids, in the other recipes is less clear, but these compounds likely perform a catalytic function or ensure the full reduction of the silver nitrate to metallic silver.

Summary To conclude this section, there has so far been little research into the methods used historically to produce silvered glass objects. This chapter has aimed to provide a basic background into the production techniques and history of silvered glass, in order to inform the discussions in later chapters regarding composition and potential factors contributing to deterioration. In the next chapter, some of the silvering recipes discussed above have been selected for reconstruction to explore the implications of different silvering recipes for the long-term stability of the objects.

45 Method from: “Chemistry Lecture Demonstration Facility - Demos Formation of a Silver Mirror on a Glass Surface”, Rutgers School of Arts and Sciences https://rutchem.rutgers.edu/cldf-demos/1032-cldf-demo-silver-mirror, [Accessed 24/02/2019] 46 Both ketones and aldehydes have a carbonyl group (C=O) where there is a double bond between carbon and oxygen, but the two groups are structured differently, allowing for different properties and reactions with other compounds. 24 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

6 Recipe Reconstructions

Since historical and literature research indicated that the case-study objects were produced by the deposition of a fine layer of silver, reconstructions were created using 19th century recipes to replicate these techniques. The reconstructions were performed in order to investigate what influence production methods could have had upon the susceptibility of the objects to deterioration and to see whether any variations resulting from the different recipes could account for the different patterns of deterioration shown by the case-study objects.

The selected recipes presented a variety of techniques and materials and each required only a short duration to completion, allowing for the conduction of repeat testing. Four recipes for silvering glass were selected from the literature, based upon the following criteria: the materials needed to be accessible within the time constraints of this project; the materials did not present a serious health hazard (when used with the required protective clothing and fume extraction). A larger selection of recipes that were considered can be found in Chapter 5: Current Scientific Knowledge, Table 5.1. Since the reconstructions were performed as part of this project in the form of a limited pilot study, a more detailed investigation could incorporate more of these recipes.

The 4 recipes selected for reconstruction were: 1. Method of Baron Liebig (c.1850), as presented in 1884 by F. James.47 2. F. James method, as published in the Journal of Microscopy (1884).48 3. Recipe provided by Joe Fitzpatrick, Scientific American (1856).49 4. A.M Helmanstine (2018), “Silver Ornaments: A Holiday Chemistry Project.50

6.1 Recipe Interpretation

Recipe 1: Liebig (c.1850) Liebig’s method was selected for reconstruction for its simplicity (later methods increasingly use sugar or salts in addition to the basic components of silver nitrate and ammonia-based chemicals). The instructions make clear that the method is intended primarily for the silvering of flat objects, such as mirrors. As such, this method was reconstructed in order to examine the relationship between methods designed for flat, open-formed objects and closed forms, such as the case-study objects and to determine whether they could be applied in this manner. This recipe, attributed to Liebig, was published by James in 1884, but the original, German-language method was published in 1856.51 In it, Liebig states “Die Versilberung von kleineren hohlen oder erhabenen Spiegelglasern bietet keine Schwierigkeit dar.”, indicating that the method could be used

47 James, F. L. (1884). The Deposition of Silver on Glass and other Non-Metallic Surfaces. Proceedings of the American Society of Microscopists, 6, p.75 48 James, F. L (1884), p.76 49 Fitzpatrick, J., Scientific American (1856), 11(46), 363 50 Helmanstine, A.M. (2018), “Silver Ornaments: A Holiday Chemistry Project”, ThoughtCo: https://www.thoughtco.com/silver-ornaments-christmas-project-606131 [Accessed: 25/02/2019] 51 Liebig, J. (1856). Ueber Versilberung und Vergoldung von Glas. Annalen Der Chemie Und Pharmacie, 98(1), 132–139. 25 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture upon 3D objects with little trouble.52 The version published by James was selected for its concise instruction and its formulation as a more workable method than the original, which required laboratory conditions, limiting its use outside of academic circles.53 The use of “aldehyde of ammonia” presented some difficulty in interpretation, since the term is no longer used in modern chemical nomenclature. Liebig himself is credited with the classification of aldehydes within organic chemistry and through communication with J. W Dӧbereiner, he discovered that acetaldehyde (CH3CHO) would produce a crystalline compound in reaction with ammonia. This compound was termed “aldehyde ammonia” and the reaction with acetaldehyde formed the basis for Liebig’s 1835 research into the classification of aldehydes in relation to the other functional groups.54 However, the chemical structure of “aldehyde ammonia(s)” was never well defined (various compounds can be formed, dependent on the aldehyde that is reacted, Liebig having only defined the reaction with acetaldehyde) and confusion persisted as to how these compounds should be classified within contemporary organic chemistry. Nielsen et al have synthesised “aldehyde ammonias” from aldehydes and ammonia and discovered that the resulting compounds are in fact 2,4,6-trialkyl-1,3,5-hexahydrotriazines, or hydrates of such.55 Therefore, for the purposes of this reconstruction, acetaldehyde ammonia trimer (IUPAC name: Hexahydro-2,4,6-trimethyl-1,3,5-triazine) was used. The recipe as written (Table 6.1):

“In one pint of distilled water dissolve thirty eight grains of aldehyde ammonia, and in an equal quantity of distilled water sixty grains of nitrate of silver. For use mix the two solutions in equal parts, agitate and filter. The object to be silvered having been previously cleaned is placed in a suitable vessel and covered with the filtered solution. A gentle heat is now applied by means of a water-bath or otherwise, and the temperature raised to 130Fahr. Silver commences to deposit at 122Fahr, and the operation is soon completed. Some little ingenuity may be exercised by the operator in each individual instance as to the best methods of immersing the object. If flat it may be laid in a saucer or suspended on the surface of the fluid; if more than one object is to be silvered, and the metal is to be deposited on one side only (as is most generally the case), they may be placed face to face and a narrow rubber band sprung around the edges.”

Table 6.1: Liebig’s recipe

52 Liebig, J. (1856), p. 133 53 James, F. L (1884), p. 74-5 54 Walker, F. "Early History of Acetaldehyde and Formaldehyde. A Chapter in the History of Organic Chemistry." Journal of Chemical Education 10, no. 9 (1933): 546, p.549 55 Nielsen, Arnold T., Ronald L. Atkins, Donald W. Moore, Robert Scott, Daniel Mallory, and Jeanne M. Laberge. "Structure and Chemistry of the Aldehyde Ammonias. 1-Amino-1-alkanols, 2,4,6-trialkyl-1,3,5- hexahydrotriazines, and N,N-dialkylidene-1,1-diaminoalkanes." The Journal of Organic Chemistry 38, no. 19 (1973): 3288-295, p.3289 26 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Recipe 2: James (1884) This method was chosen because of its connection to the recipe by Liebig, above. F. James studied under the tutelage of Baron Liebig at the University of Munich and observed Liebig’s work on silvering and organic chemistry.56 This method bears similarities to that of Liebig, in its use of heat, ammonia and silver nitrate, but differs in the addition of Rochelle salts (potassium-sodium tartrate, KNaC4H4O6·4H2O). While James himself lists a recipe by Draper that includes Rochelle salts,57 in 1911 James’ method was being referred to as “The Rochelle Salts Process” by Curtis, who includes a recipe by Draper in which caustic potash (potassium hydroxide, KOH) is used instead.58 In any case, James’ method is useful for reconstruction since it draws clearly upon the older method produced by Liebig but presents a modification that is intended to make the method “the most practical for off-hand use in the laboratory or workshop”.59 The recipe as written (Table 6.2):

“Silvering Solution: In one ounce of distilled water dissolve forty eight grains of crystalized silver nitrate. Precipitate by adding strongest water of ammonia, and continue to add ammonia drop by drop, stirring the solution with a glass rod until the precipitate is nearly, but not quite redissolved. Filter and add distilled water to make twelve fluid drams. Reducing Solution: In one ounce of distilled water dissolve twelve grains of Rochelle salts. Boil in a clean, long-necked flask, and while boiling add two grains of crystalized nitrate of silver previously dissolved in a dram of distilled water. Continue the boiling for three or four minutes, remove from the lamp, let cool, filter and add distilled water to make twelve fluid drams. For use mix the two solutions in equal proportions. While a temperature of from 100 to 120 Fahr. hastens the deposition of silver with this fluid it is by no means necessary, as the metal will separate (though slowly) at a very low temperature.”

Table 6.2: James’ recipe

56 James, F. L (1884), p.73 57 James, F.L (1884), p. 75 58 Curtis, H. D. "Methods Of Silvering Mirrors." Publications of the Astronomical Society of the Pacific 23, no. 135 (1911): 13-32, p. 17 59 James, F.L (1884), p. 76 27 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Recipe 3: Fitzpatrick (1856) This recipe, submitted by J. Fitzpatrick to the Scientific American in 1856, was selected for its simple instructions and limited range of ingredients. It explicitly states that it should be used for the silvering of 3D objects (“[the solution] is placed in the article to be silvered – a bottle for instance”), unlike the more complex recipes provided by Liebig and James, above. Fitzpatrick’s recipe makes use of grape sugar (dextrose), which is found more commonly in later methods and in those in use today (see Helmanstine, below). The term “grape sugar” reinforces the link between silvering and the by-products of wine making, which appear in many recipes in the form of tartrates and tartaric acid.60 An examination of the other submissions to the Scientific American contemporary with that of Fitzpatrick, for example a piece on “Electro-Chemical Baths” and a call for inventors who might provide an improved piano keyboard,61 suggests that the audience of the publication consisted of inventors, hobbyists, craftsmen and engineers, rather than the purely academic chemical scientists targeted by James and Liebig. The recipe is thus designed to be achievable under less than perfect laboratory conditions and it will be seen whether this is reflected in a difference in the quality of the silver layer produced. The recipe as written (Table 6.3):

“The following is a recipe for silvering glass: Take 1 oz. pure nitrate of silver, 1 oz. aqua ammonia, 2 0 z. distilled water. Mix and add 2 oz. of pure alcohol, 2 oz. of distilled water, 1/4 oz. of grape sugar. The above is placed in the article to be silvered (a bottle, for instance,) and kept at a temperature of 160OF till the silvering is effected. The purity of the chemicals influence the result, in fact, all depend upon that. [Those beautiful silverized glass globes seen in the windows of many stores are produced by the above described process. The information communicated by our correspondent is useful and interesting.”

Table 6.3: Fitzpatrick’s recipe

60 Tartrates are defined as salts of tartaric acid, an organic acid with the formula C4H6O6. 61 Fitzpatrick, J. Scientific American (1856), 11(46), 363

28 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Recipe 4: Helmanstine (2018) This modern method, published online by Helmanstine in 2018, was included in the reconstructions to provide a comparison, both in the interpretation of instructions and in the materials used. In contrast to the publication of the previous methods in well-respected journals of the period, this method was produced in an easily accessible internet article aimed at anyone wishing to silver their own Christmas ornaments. In this way, it has been written for a very different audience to that of the other methods, although it bears similarity to the method by Fitzpatrick in that it specifically refers to the silvering of closed, 3D objects, unlike Liebig (flat mirrors) and James (non-specific). This method uses dextrose, as does Fitzpatrick, but does not require heating of the solution or object and requires sodium hydroxide solution (NaOH) instead of potassium-sodium tartrate. Further, it is expected that the reaction should occur instantaneously, rather than the gradual deposition of the silver layer described in the historic recipes. Recipe as written (Table 6.4):

1. Gently and carefully remove the metal ornament holder and set it aside. You should be left with a hollow glass ball with a short neck. 2. Use a pipette to pour acetone into the ball. Swirl the acetone around and then pour it into a waste container. Allow the ornament to dry. The acetone step may be omitted, but it helps to clean the inside of the ornament to produce a better silver finish. 3. Use a graduated cylinder to measure 2.5 ml of silver nitrate solution. Pour the silver nitrate solution into a small beaker. Rinse the graduated cylinder with water, discarding the rinse water. 4. Use the graduated cylinder to measure 2.5 ml of ammonium nitrate solution. Add the ammonium nitrate solution to the silver nitrate solution. Swirl the beaker or use a glass stirring rod to mix the chemicals. Rinse the graduated cylinder with water and discard the rinse water. 5. Use the graduated cylinder to measure 5 ml of dextrose solution. Pour the dextrose solution into the dry glass ornament. Rinse the graduated cylinder with water and discard the rinse water. 6. Use the graduated cylinder to measure 5 ml of sodium hydroxide solution. Pour the silver nitrate and ammonium nitrate solution into the glass ball, followed immediately by the sodium hydroxide solution. 7. Cover the opening of the glass ball with a piece of parafilm and swirl the solution, making certain the entire interior surface of the glass ball is covered. You will see a silver mirror coating from inside the ball. 8. When the ball is evenly coated, remove the parafilm and pour the solution into the waste container. Important: Rinse the inside of the glass ornament with distilled water. Failure to rinse the ornament could result in the formation of a shock sensitive compound. 9. Use a pipette to add about 2 ml of acetone to the inside of the ornament. Swirl the acetone around inside the ornament and then discard it in the waste container. Allow the ornament to air dry. Replace the ornament hanger and enjoy your silver holiday ornament! 10. The waste material should be immediately rinsed away with water to prevent the formation of an unstable (potentially explosive) compound.

Table 6.4: Helmanstine’s recipe

29 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

6.2 Methodology

Materials and Equipment The glass substrates used for this research were glass microscope slides and hollow blown- glass Christmas ornaments, which offered the best comparison with the closed forms of the case-study objects Glass The glass microscope slides used for the reconstructions were British Standard Microscope Slides from Thermo Fisher Scientific and composed of soda-lime glass. Hollow Spherical glass ornaments Due to problems of supply, the ornaments used for the Liebig and Helmanstine recipes were 6cm in diameter, instead of the 4cm used for the James and Fitzpatrick recipes. The composition of the glass is the same – a soda-lime glass very similar to that of the microscope slides (see Chapter 7: Scientific Analysis for SEM/EDX of the glass).

Equipment Glass beakers and long neck flasks Glass measuring cylinder Glass object for silvering (4cm/6cm diameter clear glass, spherical Christmas ornaments, dekwast.nl) Glass pipettes Glass stirring rods Hot plate Paper filter Parafilm Scale Stainless steel tongs Thermometer

Silvering Materials See Table 6.5 below for the materials used for each recipe. The similarities in materials have been highlighted, as well as the differences.

30 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Table 6.5: Comparison of the selected historic silvering recipes

Liebig (c.1850) James (1884) Fitzpatrick (1856) Helmanstine (2018) 946ml distilled water 56.70g distilled water 113.40g distilled Distilled water (for (plus extra for water rinsing) dilution) 3.89g silver nitrate 3.11g silver nitrate 28.35g silver nitrate 5ml 0.5M silver plus nitrate solution 0.13g silver nitrate

2.46g aldehyde of Ammonium 28.35g ammonium 2.5ml 1.5M ammonia hydroxide hydroxide ammonium nitrate solution

0.56g Rochelle salts (potassium sodium tartrate) 7.10g grape sugar 5ml 5% dextrose (dextrose) solution 56.70g alcohol 5ml acetone (ethanol) 5ml 10% sodium hydroxide solution

Test Methodology and Observations

The reconstruction experiments were performed in the Glass, Ceramic and Stone studio of the Ateliergebouw (Rijksmuseum and UvA), Amsterdam. All of the methods were carried out in a fume cupboard under the same conditions (temp +/- 18˚C / RH +/- 50% ) All glassware was washed thoroughly with warm tap water, rinsed with distilled water and allow to dry completely before use. The glass balls were rinsed with ethanol, followed by distilled water and allowed to drain before application. The historic instructions were followed as closely as possible and any alterations or additions (where information was lacking or unclear) are detailed below. All of the test objects were rinsed thoroughly with distilled water after the silvering process was ended. All units were converted into modern metric measures, with conversion taking care to allow for differences between British and American pints, for example. A list of the historic quantities has been provided in Appendix II (Table 14.1). None of the recipes provided specific time-frames within which the silvering should take place, relying upon the user to determine when silvering has been completed. As such, the time required to achieve silvering could not be anticipated before performing the recipes and the tests were either stopped at the point that a uniform silver layer had been deposited, at the point after which it became apparent that further application would decrease the quality of the silver layer or when it became clear that no silvering was going to take place. In this way, the time needed for each recipe could not be standardised, but the results of this pilot study have provided information on time needed that can be used in further research to create a testing protocol.

31 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Test Procedure

Recipe 1: Liebig (c.1850) The two solutions were prepared at half of the volume given in the original recipe. They were mixed together in equal parts (20ml of each) and over the course of 5 seconds, this solution turned from clear and colourless to translucent peach colour with a blue reflectance. Though no visible precipitate was formed, the solution was filtered according to the instructions. The combined solution was poured into the test object and swirled briefly to coat the inside. The object was then immersed in a warm water bath at approximately 54OC. After approximately 2 minutes, there was an increase in the reflectivity of the immersed portion of the glass object and the beginning of formation of a silver-coloured layer at the fill line of the solution inside the object. Over the next few minutes, the solution darkened in colour and a silver layer was formed wherever the glass was in continuous contact with the solution. In subsequent tests, when the two solutions were mixed, they did not produce as strong a colour change, becoming a weak, transparent pink.

Recipe 2: James (1884) This recipe was prepared at half quantities quoted in the original text. The silver nitrate was dissolved in distilled water and the ammonium hydroxide was then added gradually. A brown precipitate formed almost immediately but it was difficult to determine when this precipitate had been “nearly but not quite” re-dissolved. This solution was then filtered, leaving a black, grainy precipitate on the filter paper, while the filtrate was now a transparent brown-grey. This solution was then diluted with distilled water to a volume of 22ml. The Rochelle salts was dissolved in distilled water and in a separate beaker, a smaller amount of silver nitrate was also dissolved. The Rochelle salts solution was brought to the boil and the second silver nitrate solution was added, at which point the colour changed from clear to a light, opaque brown. This was boiled for 3.5 minutes and removed from the heat. During the boiling, a silvery reflective layer was formed on the interior of the boiling flask. After cooling and filtering, the Rochelle salt solution was added to the (first) silver nitrate solution, in equal parts. The colour of the mixture shifted from milky, cream-white initially, before a dark brown-grey precipitate was formed. After approximately 10 seconds, the mixture was entirely black.

Recipe 3: Fitzpatrick The solutions were prepared at 1/10 of the original quantities. The mixture was prepared in two parts, the first containing the silver nitrate, ammonium hydroxide and distilled water and the second containing the alcohol, grape sugar and distilled water. In the first solution, the silver nitrate crystals were dissolved in the distilled water prior to the addition of the ammonium hydroxide, upon which the mixture instantly turned brown. In the second solution, the grape sugar was dissolved before the addition of the alcohol, after which there was no change in colour or transparency. In both solutions, the solid materials were added first to ensure complete dissolution of any solid particulates, which may have reduced the efficacy of the silvering process (since they would be left unreacted). 32 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

The two solutions were then mixed together, forming a brown liquid which produced a brown precipitate when left to stand, and poured into the glass object. The object was immersed in a warm water bath of approximately 70OC. Upon exposure to the heat, the colour of the mixture turns quickly to black and a reflective layer can be seen on the interior of the glass within seconds. If the mixture was heated in the object for longer than approximately 4 minutes, the solution began to turn brown and a brown layer was deposited on the glass. When rinsed with distilled water, this layer was found to be easily damaged by the motion of the liquid and when dry, the glass ornaments exhibited a brown colour, with varying levels of reflectance. As such, it was difficult to determine the point at which “silvering had been effected” and the object could be removed from the water bath and emptied. Accordingly, various tests were performed for different lengths of immersion and the results will be discussed below. The recipe was found to be effective when the solutions were swirled around the inside of the object and then emptied out before heating the object briefly in the water bath (test 7 and 8), contrary to the original text, which suggested the filled object be immersed until silvering was achieved, which almost invariably produced a brown, semi-reflective result.

Recipe 4 Helmanstine All of the solutions were prepared according to the instructions, at greater volumes so that the experiment could be performed multiple times. All four solutions were clear and colourless. When the sodium hydroxide was added as the final step, the solution became brown. During the swirling of the mixture inside the object, the colour changed to black. After 30 to 40 seconds of swirling, the interior of the object also was coated in a black layer, which became increasingly reflective and the colour lighter, until a smooth silver layer was formed.

33 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Repeat experiments After performing each of the recipes as the instructions directed, changes were made on the basis of the observations recorded. As such, no universal protocol was followed for the repeat testing of the recipes, since they were all different in terms of procedure and materials. Alterations in temperature or application time were made for each test on the basis of the preceding tests, in an attempt to improve the results of the recipe or to explore the methodology of the recipe in greater depth. Since this research should be seen as a pilot study, a project of greater scope would include the creation of a testing protocol and allow for a greater number of tests, so that the approach could be more standardised.

Table 6.6 shows the modifications made to each recipe or method for the repeat tests, based upon the observations made during initial testing. Unless otherwise indicated, the time given was the time the object was immersed in the water bath.

Table 6.6: Test procedures for each recipe

Recipe Initial Test Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8 Liebig 54OC bath 54OC 60OC 54OC temperature Immersion 6 mins 7 3 mins 4 mins 43 for 4 mins 43 secs 55 secs secs secs.

James 45OC Object 70OC 45OC 45OC Immersion filled -3 5:1 AgNO3 1:5 for with minutes to Rochelle AgNO3 10 minutes solution salt. to for 10 Rochelle minutes. salt. No 7 heating minutes. Fitzpatrick Microscope 70OC 70OC 70OC 70OC 70OC 70OC As for slide. 30 10 7 minutes 2 10 Mix Test 7. 30 minutes+ seconds. minutes minutes seconds swirled 3 No heating. 30 and repeats. seconds. emptied before immersion in water bath (20 seconds). 1 repeat. Helmanstine Under 2 3 2 Microscope 2 minutes, minutes minutes slide. minutes (no heating) swirling 30 48 3 minutes. 4 continuously. seconds seconds seconds.

34 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

6.3 Results and Discussion

A summary of the results of each test is given below in Table 6.7. Photographs of each test object are provided in Appendix II.

Results Recipe 1: Liebig As shown in Table 6.7, the initial test of Liebig’s recipe produced a good silvering result that was achieved in 4 minutes and 43 seconds. In the second and third tests (see Table 6.6), at which point the two solutions had been made 1 hour previously, it was found that when combined, the solution did not undergo such a strong colour change as in test 1, turning only a weak, transparent pink (with the same blue reflectance). Silvering was effected using this solution, but only considered complete after 6 minutes and 7 seconds of immersion. A third test was made using a higher temperature, again using the original solutions and a silver layer was formed in 3 minutes 55 seconds. In the fourth test, silvering occurred at the same rate as in test 1 and the mixture initially darkened but by the end had become a milky peach-brown, unlike in test 1, which showed a milky grey colour at the end. The silvered layer produced was similar to that of test 1, but slightly warmer in colour and with a hazier reflectance. Once dry, the result appears more pearlescent than the bright metallic reflectance of test 1. All of the tests produced moderate or good results but the silver layer is slightly warm in colour when compared to the case-study objects and the reflectance is generally more pearlescent than mirror-like.

Results Recipe 2: James In tests 1 and 2, both objects gave the same result; no silvering of the glass was achieved, and a black precipitate settled to the bottom of the liquid, with a clear layer above. In test 3, at a higher temperature, a transparent black, slightly reflective layer was formed on the glass, appearing yellow-brown when held to the light. A mixture was then made using 5ml of the silver nitrate solution combined with 1ml of the Rochelle salts solution and heated at 45OC for 7 minutes. A highly reflective, purple-black layer was formed. When these quantities were reversed (1 ml silver nitrate, 5ml Rochelle salts solution), the liquid only became milky-grey and a grey precipitate formed at the bottom of the liquid. The glass was found to have become slightly yellow upon rinsing, but no silver layer was produced.

Results Recipe 3: Fitzpatrick The first test slide (test 1) did achieve a slightly reflective layer, on the portion of the slide that was immersed in the solution, but this was partially covered with a grey-brown, speckled deposit, which could easily be removed with gentle contact. The silvery layer was also found to be vulnerable and easily brushed away. Test 2 gave a reflective surface that was fairly uniform on the part of the object that had been exposed to the solution, but this was very dark in colour, appearing a metallic black. Test 3 gave a brown interior coating, with little reflectivity. Test 4 gave a darker brown result than test 3, with a grainy, uneven texture in some areas, with a clear tideline showing the level of the solution on the interior. Test 5 gave a paler brown and slightly more reflective result, again with a clear tideline. Test 6 gave a fairly reflective, pale coloured result, with a brown tint. Test 7 gave a silvery, reflective result that was slightly uneven in opacity, giving a mottled appearance to the

35 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture interior and with a pale beige colour. Test 8 gave a highly reflective result with an even appearance and slight brown tint. Three days after tests 5, 6 and 7 were performed, it was found that they had undergone a slight colour change, losing some of the warm, brown hue and becoming more grey. In those with higher reflectance, this had the effect of making them appear more silver than previously. In all of the test objects except 6, 7 and 8, a silvery halo was observed towards the top of the object, above the fill-level of the solution on the objects’ interior.

Results Recipe 4: Helmanstine In the initial test, the glass was coated in a black layer which became increasing reflective over the following 45 seconds to 1 minute to produce a uniform silver layer of high reflectance. In the second test, a brown tint was observed in the silver layer formed. Test 3 took 2 minutes and 48 seconds to reach complete silvering but towards the base of the object, irregular losses in the silver layer occurred that could not be remedied by continued swirling. Test 4 was conducted upon a clean glass microscope slide, immersed in a glass petri dish and agitated. Silvering of the slide occurred from the edges into the middle and continued agitation eventually led to the formation of a grainy precipitate. Test 5 was conducted using a Christmas ornament and silvering was complete in 2 minutes and 4 seconds, but again there was some abrasion of the silver layer which could not be improved upon with continued swirling (indeed, further swirling appeared to worsen the damage).

Results Summary As can be seen in Table 6.7, the results of the four different recipes varied significantly. The modern recipe, by Helmanstine, gave the most consistent results and formed a silver layer with the greatest uniformity and best metallic reflectance. The recipe by James gave the most inconsistent results, failed to produce a layer with a silver colour and only a single test object with any metallic reflectivity. Liebig’s recipe produced some good results and a silver layer with high reflectivity, but a generally warmer colour and less mirror-like than those of the Helmanstine recipe. The recipe by Fitzpatrick also produced some good results but this was largely due to the modifications made to the method upon observations of the initial results, indicating a lack of information provided in the original text regarding the time after which the object should be emptied and rinsed.

36 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Table 6.7: Results of the reconstruction tests

Recipe Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8 1. Liebig Silver coloured Warmer Similar to Test Warmer colour layer, high colour than 2, hazy and less reflectance Test 1. Hazy reflectance with reflectance than reflectance. warmer colour Test 1, despite same procedure.

2. James No layer No layer Formation of Reflective Slight yellowing formed. formed. black layer, purple-black of glass. No slightly layer formed. silvering. reflective

3. Fitzpatrick Semi-reflective, Formation of Brown layer, Brown layer, Brown layer, Reflective silver Silver colour Silver colour, fragile brown reflective grey limited limited increasing layer, brown layer, slightly good layer with layer, with reflectance. reflectance. reflectance towards base. patchy in reflectance, chalky texture. silver halo at Silver halo at Silver halo. towards top. appearance. some patchiness fill line. fill line. to texture.

4. Helmanstine Highly Reflective Highly Slide. Highly reflective and silver layer, reflective silver Highly reflective layer, uniform silver slightly layer, with reflective, with pattern of layer. warmer colour pattern of losses chalky texture abrasion. than Test 1. near base. on one side.

No silvering Poor Moderate Good Excellent

37 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Discussion When assessing the results produced by the four recipes, it became clear that there were two main factors that contributed to the success of the resulting silvering. The first of these was the composition of the recipe and the second was the influence of the application procedure, including agitation and time required. The discussion of the results will be undertaken using these two aspects, before moving onto an exploration of how the reconstruction research can inform investigations of the two case-study objects.

Influence of composition As shown in Table 5.1, the recipes tend to require similar ingredients, or different compounds with the same elements or functional groups. As noted above, the Helmanstine and Liebig methods gave the best results and were easily performed without alteration. The recipes for silvering loosely follow the principles of the Tollen’s reagent test (see Chapter 5.2) for which the chemical processes are: sodium hydroxide (NaOH) reacts with silver nitrate (AgNO3) to form silver(I) oxide (Ag2O), a black/brown precipitate. The ammonia + compound forms the diamminesilver(I) complex ([Ag(NH3)2] ) with the silver(I)oxide, which is then reduced to elemental silver (Ag) by glucose (dextrose; C6H12O6). The formula for this process is as follows:

2 AgNO3 + 2 NaOH → Ag2O (s) + 2 NaNO3 + H2O

Ag2O (s) + 4 NH3 + 2 NaNO3 + H2O → 2 [Ag(NH3)2]NO3 + 2 NaOH

2[Ag(NH3)2]NO3 + 9C6H12O6 + 3H2O → 2Ag + 9C6H12O7 + 6NH3

Though there are clear differences, all of the recipes are based upon similar reactions, namely the formation of an ammonia-silver complex which can then be reduced to silver with the addition of a reducing agent. It seems likely that where a brown precipitate is formed or remains, this substance is silver(I)oxide that has not become part of a complex with ammonia and is not reduced. By comparing the resulting silver layers, it can be said that the two recipes requiring dextrose (recipe 3: Fitzpatrick and recipe 4: Helmanstine) were fairly successful but since the Liebig recipe was also able to form a silver layer of similar quality, the presence of dextrose specifically was not essential to the formation of a good silver layer, providing another reducing compound was present. All of the recipes required ammonia in the form of ammonium hydroxide (NH4OH), ammonium nitrate (NH4NO3) or ammonia acetaldehyde trimer (C6H15N3) and the use of the different ammonia containing compounds did not appear to contribute towards the success of particular recipes, for example, both the Liebig recipe (containing ammonia acetaldehyde trimer) and the Helmanstine recipe (ammonium nitrate) were able to produce highly reflective silver layers in a short timeframe. Liebig’s recipe uses acetaldehyde ammonia trimer, silver nitrate and water only and it is likely that this is made possible by the instability of the acetaldehyde ammonia trimer at average room 62 temperature, at which it loses ammonia gas (NH3). The application of heat stipulated by

62 Nielsen et al.(1973), p.3290 38 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture the recipe likely encourages this process. In this way, it can be suggested that the process occurring in the Liebig recipe is: + - NH3 (g) + H2O (l) → NH4 + OH (NH4OH) then 2AgNO3 + 2NH4OH →Ag2O + H2O + 2NH4NO3

At this point, the need for a reducing compound is apparent and it could be suggested that such is provided as acetaldehyde (CH3CHO) by the acetaldehyde ammonia trimer as it loses NH3. It is possible that the volume or concentration of the ammonia compounds had some influence on the results, since the recipe by James (the least successful), only required ammonium hydroxide to be added drop by drop until a precipitate was formed (a few drops of 1M solution, compared to the next lowest volume of 2.5ml of 1.5M ammonium nitrate solution in the Helmanstine recipe). However, this suggestion is contradicted by the high volume of distilled water required by the Liebig recipe, which uses similar amounts of ammonia compound and silver nitrate to the James and Helmanstine recipes, suggesting that the distilled water does not have a diluting effect on the other compounds in the recipe. Fitzpatrick’s recipe requires the highest ratio of silver nitrate and ammonia compound to distilled water yet did not produce results as effective as the much more dilute Liebig and Helmanstine recipes. The tests of the James recipe using a ratio of 5:1 silver nitrate and ammonium hydroxide to Rochelle salts solution (test 4) versus a ratio of 1:5 of the same ingredients (test 5) indicated the importance of the silver content (as would be expected in the formation of a layer intended to be entirely composed of elemental silver). It is possible that the failure of the James’ recipe to produce a silver layer was due to the limited availability of Ag2O, which (when following the instructions) is filtered out of both solutions when it forms as a precipitate. Ultimately, there are many contributing factors and it is difficult to determine with certainty the correlation(s) between the composition of the recipes and their effectiveness at producing a silver layer. It is likely that the method of application also has an important effect on the final layer formed.

Influence of procedure Despite the general similarity of the ingredients, the recipes each require their own application procedure, and this will in turn contribute to the final silver layer produced. Preparation Firstly, the preparation and after treatment of the glass is likely to have an influence on the deposition of the silver layer and in the preservation of reactants on the silvered surface. The Helmanstine recipe is the only one which explicitly recommends rinsing the object prior to use (with acetone) to remove any dirt, debris or chemicals left behind after the manufacturing process. All of the glass ornaments used for all of the recipes were rinsed with distilled water to ensure a clean surface, but Helmanstine was the only one which required acetone. It is possible then that this extra step allowed for the dissolution of compounds which were not removed before the application of the other recipes, contributing to the success of the Helmanstine tests. Rinsing with acetone was also carried out after silvering, perhaps helping to remove any leftover ammonia and thus ensuring the

39 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture stability of the silvered layer since fewer/no reactants were left on the surface. This could indicate why both the Liebig and Fitzpatrick test objects underwent a colour or reflectivity change as they dried, which did not occur in the Helmanstine tests. Agitation It seems that one of the reasons for the dilution of all of the ingredients in large volumes of distilled water is to ensure that the object to be silvered can be completely covered or filled (in the Helmanstine recipe, a small volume is used and the object is agitated to coat the interior) with the solution. In the Fitzpatrick tests, the fill level of the object contributed to the formation of a silvery “halo”, around the circumference of the test objects, at the point at and just above which the object had been filled with the solution (Figure. 6.1). This suggests that deposition of metallic silver has occurred in the portion of the object where contact with the solution is limited to that caused by the movement of the liquid when the object is moved, in opposition to the portion of the object continually exposed because it is below the fill line. Greater silver deposition in this area could be the result of capillary action (due to surface tension) upon silver containing precipitates, as a result of the meniscus formed at the fill line, or perhaps this “halo” of silver may be the result of the greater exposure of the reactants to the air, in contrast to the anaerobic conditions in the filled portion of the object (especially where agitation is limited). It is also possible that the level of moisture is a factor, since the area above the fill line is able to dry faster, perhaps before the reaction can continue to the brown stage, as it does in the lower part (see Figure. 6.1). In contrast to the test objects produced by the Fitzpatrick method, tests of the Liebig recipe only exhibited silver deposition on the parts of the surface continually exposed to the solution, although it was observed that deposition began near the fill line and progressed downwards to the base of the object. Agitation or swirling of the solution around the inside of the objects was not mentioned as part of the procedure of any recipe except Helmanstine’s and yet it was discovered that the best result using the Fitzpatrick recipe could be obtained by swirling the solution around the object, emptying the excess and heating the object with only a thin layer of liquid remaining on the glass. This was repeated to build up a more opaque silver layer. As such, though it was not required by all of the recipes, agitation of the solution appeared to allow better distribution of the silver layer as it was deposited and perhaps the continual mixing of the reactants contributed to a more complete reaction of the ingredients. However, agitation of the solution was also a potential source of damage to the silver layer, since it was observed that continued agitation of the Helmanstine solution after silvering of the glass had occurred resulted in the formation of a grainy precipitate, which could have led to the development of the abraded patination on test objects 3 and 5. No such patterns were observed in any of the other recipe results. Time The time taken for each recipe to produce a silver layer varied, and tests were stopped when silvering was considered sufficient. As such, the time taken tended to be a result of the recipe and procedure, rather than prescribed by the recipe. It is important to note that continued time did not correspond to a better result in most cases, contributing to the production of a brown, chalky layer in the Fitzpatrick tests and abrasion in the Helmanstine tests.

40 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Figure 6.1: Fitzpatrick Test 3, showing the clear silver “halo” formed during the test

Relation to case-study objects The reconstructions were performed in order to gain a greater understanding of the chemical processes by which the case study objects were manufactured and to determine how this may or may not have influenced their deterioration. The durability of the silver layers produced in the reconstructions was low; all of the recipes produced silver layers which could be removed from the glass surface with only light pressure, for example by touching with gloved hands. In attempting to remove a sample from the silver layer of the case study vase (Chapter 7: Scientific Analysis), it was discovered that the silver layer could only be removed from the glass with considerable difficulty. There are a myriad of reasons that there should be such a difference in the durability of the case-study and reconstruction objects, including the potential application of a sealing layer, different chemical processes resulting in a stronger bond, incomplete reaction of the chemicals leading to insufficient bonding and differences in the composition of the glass substrate causing a weaker adhesion. Research by Chitvoranund et al. suggests that the surface treatment of the glass has an effect upon the formation of the silver film and the strength of its bond to the glass.63 In the study, the researchers tested the application of tin chloride (SnCl2) to the glass surface, as well as acid etching and grinding, prior to silvering with a similar recipe to the Helmanstine recipe.64 It was found that the rougher the surface was, the greater the adhesion of the silver layer; thicker silver layers were better adhered and that ultrasonic vibration of the solution did not enhance the silver formation but increased the rate at which it was formed.65 It was further suggested that the mechanism of adhesion of silver to glass is a result of Van der Waals forces, which explains the benefit of silvering

63 N. Chitvoranund et al., "Effects of Surface Treatment on Adhesion of Silver Film on Glass Substrate Fabricated by Electroless Plating", Advanced Materials Research, Vol. 664, pp. 566-573, (2013)

64 Chitvoranund et al. (2013), p.567 65 Chitvoranund et al. (2013), p.572 41 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture on a rougher surface (more surface area allows for greater Van der Waals interactions).66 As such, it is possible that the poor adhesion of the reconstructed samples was due to differences in the glass surface morphology or that, especially in the case of some of the very briefly silvered samples (for example Fitzpatrick: test 6 and 7), the thinness of the layer could have affected its durability. The abrasion of the silver layer during the silvering process, as discussed above, is an interesting point for consideration. Though neither of the case-study objects obviously suffers from this kind of abrasion (though the vase has lost a considerable percentage of its silvering completely), it is possible that this process could have occurred, even at a microscopic level, rendering the layer more susceptible to further damage or chemical degradation. The rinsing of the silvered objects could also have contributed to their degree of vulnerability, perhaps allowing unreacted compounds to remain on the silvered surface has increased the likelihood of colour changes (and future deterioration, although this would require further testing to confirm).

In order to make a more detailed comparison with the case-study objects, further research on the morphology and elemental composition of both was undertaken using SEM/EDX analysis. It was possible to assess cross-sections of the glass and silver layers of the reconstruction objects to determine how representative they are and try to identify any chemical or physical differences that may result from different manufacturing processes.

66 Chitvoranund et al. (2013), p.572 42 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

7 Scientific Analysis

The nature of the deterioration of the silver layers was initially investigated using optical microscopy. Instrumental analysis was then applied to investigate the morphology of the silver layer and the interface with the glass as well search for as physical manifestations of deterioration. This analysis was also conducted on samples taken from the exterior decoration of both case-study objects, to explore the manufacturing technique and materials used, in an effort to aid dating or locating the production of these objects. The same investigation was undertaken on the objects produced during the reconstruction experiment, to allow for comparison of the silver layers formed with those present on the case-study objects, as well as to facilitate comparison of the efficacy of the historic recipes.

7.1 Methodology Optical microscopy and Scanning Electron Microscopy with Energy-Dispersive X-Ray Spectroscopy (SEM/EDX) was applied.

Optical Microscopy Optical microscopy of the samples from the case-study objects was carried out using a Hirox KH-7700 3-D Digital Microscope with Photonic Optics lights. The light used was raking, in order to highlight any texture in the sample surface.

SEM/EDX Further exploration of the morphology and composition of the silver layer was done using SEM/EDX analysis.67 It was anticipated that SEM back-scattered images of cross-sections through the glass and silver layer would shed light on the thickness of the silver coating and potentially illuminate our understanding of the relationship between the silver layer and the glass substrate. EDX analysis was applied to provide semi-quantitative data on the elemental composition of the silver layer and any other inorganic material, in an attempt to determine whether compositional differences could be found in differing parts of the objects that would reflect the visually observed variations in condition. It was also hoped that differences in the elemental composition or morphology of the silver layer observed under SEM may shed light upon the manufacturing processes and materials used to produce the objects. EDX analysis was also performed on samples taken from the applied decoration present on the exterior of both objects. Embedded and adhesive-mounted samples of the objects were analysed using a NovaNanoSEM450 from Thermo Scientific and EDX Ultradry SDD detector with pathfinder software also from Thermo Scientific. Analysis was undertaken under low vacuum and at an accelerating voltage of 15kV, with the exception of some of the adhesive mounted samples, which were analysed at 20kV and with a working distance of between 5.9mm and 8mm. For each sample, points were selected in both the glass substrate and in the silver layer, where applicable.

67 Carried out by Ineke Joosten (Cultural Heritage Agency of the Netherlands) 43 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Sampling Though it would have been possible to place both objects into the NovaNano SEM chamber in their entirety, there were two problems. On the one hand, there was the vulnerability of the thin glass and the potential for the (low) vacuum conditions inside the SEM to exacerbate any undiagnosed glass disease or crizzling, or dislodge any loose flakes of silvering.68 On the other hand, the silver layer was on the inside of the objects making it impossible to analyse the silver layer on the object that was not broken. This meant that it was preferable to take samples of the glass, decoration and silver layers for the purpose of this analysis. It was decided to embed small samples of silvered glass in epoxy resin, since this would also allow for the observation of cross sections, to gain insight into the morphology of the interface between the glass and silver layer.

Working under a stereo microscope, a scalpel was used to scrape away a small flake of the enamel decoration from each object and the resulting flakes were applied to adhesive carbon disks on round metal stubs. Small rubber-tipped pliers were used to remove samples of the glass, with attached silver layer, of approximately 0.5mm2 to 1mm2, from the top of the stem of the candlestick, where the head of the candlestick had been broken off. Samples of the glass and silvering were also taken from the base of the candlestick, where a large loss in the base permitted access to the break edges and the silver layer where it was blackened and degraded (see Figure. 7.1 for locations of samples). Due to the good condition of the glass of the vase, it was not considered acceptable to take glass samples and so only samples of the silver coating were taken from the object. This was undertaken with some difficulty, since the form of the object prevented easy access to the interior surfaces and, interestingly, the silver layer was found to be more strongly adhered to the glass than expected. A folded section of stainless-steel wire and a fine scalpel blade were eventually used to remove some micro-flakes of the silvering from the inside of the object, which were applied to adhesive carbon mounts. Finally, samples of glass with silvering were also taken from two 20th century glass Christmas ornaments with silvered interiors, one of which showed significant blackening of the silver layer, as well as cracks and losses to the glass (XB5) and the other a painted orange colourant on the exterior surface of the glass (XB0).69 These samples were included for comparative purposes. See Appendix III: Scientific Analysis for photographs of the two Christmas ornaments. The silver flakes and glass fragments taken from the objects were embedded in Epofix epoxy resin and wet-polished using silicon carbide abrasive sheets in grades 500, 800, 1000, 1200 and 4000. The embedded samples were then polished further using Micro- Mesh silicon carbide (grey) abrasive sheets in grades 6000, 8000 and 12000, followed by Micro-Mesh aluminium oxide (cream) abrasive sheets in grades 8000 and 12000. See Table 7.1 for information regarding sample numbers and locations from all objects.

68 Craddock, P. (ed.), Scientific Investigation of Copies, Fakes and Forgeries, Taylor Francis (2009), p.222 69 Christmas ornaments provided by Kate van Lookeren Campagne (UvA). 44 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Table 7.1: Sample numbers and locations for samples taken from case-study objects

Object Sample Composition Location Notes/Condition NOM22679-55 22G1 Glass with Break edge, No visible Embedded silver layer neck of deterioration candlestick 22G2 Glass with Break edge, base Visible Embedded silver layer of candlestick blackening/loss of silvering 22G3 Glass with Break edge, base Visible Embedded silver layer of candlestick blackening/loss of silvering 22G4 Glass with Break edge, No visible Embedded silver layer neck of deterioration candlestick 22G5 Glass with Break edge, base Visible blackening/ Adhesive mount silver layer of candlestick loss of silvering 22ED Surface Exterior, side of Losses to Adhesive mount decoration candlestick decoration NOM11931-51 11SF1 Flakes of Interior, side of Patchy loss of Adhesive mount silver coating cup silvering 11SF2 Flakes of Interior, base of Visible loss and Adhesive mount silver coating cup possible darkening of silvering 11ED Surface Exterior, side of Losses to Adhesive mount decoration cup decoration Modern silver XB5 Glass with Break edge, side Visible blackening Christmas Embedded silver layer of object and deterioration of ornament with silvering. Cracks in design of cockerel glass substrate. Modern orange XB0 Glass with Break edge, side No visible Christmas Embedded silver and of object deterioration ornament orange layers

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Figure 7.1: Object Nom 22 showing locations of sampling. 1) top of candlestick, samples 22G1 and 22G4. 2) Sample of decoration 22ED. 3) base of object, samples 22G2, 22G3 and 22G5.

7.2 Results and Discussion: Case-study objects

Optical Microscopy The microscopic (Hirox) images of the case-study samples can be found in Appendix III, Table 15.1. In all of the samples, the Hirox images using raking light show that abrasion of the glass has occurred during the preparation of the embedded samples. It is certain that this is the cause of the scratching visible in the Hirox images, since the scratches often run through the epoxy resin and into the glass in continuous lines. This abrasion is reflected in the SEM back-scattered images, as discussed below. Figure 7.2 (below) shows the scratches in samples 22G3 and XB5.

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Figure 7.2: Sample 22G3 (left) and sample XB5 (right) showing scratches to the resin and glass.

SEM/EDX SEM back-scattered images: results and discussion The SEM back-scattered images, shown in Figure. 7.3, provide information on the structure and condition of the sampled areas, as well as highlighting problems encountered with the use of this analytical technique for this research project. Firstly, with regard to the silvering layer, images B and E (samples 22G4 and 22G2, respectively) demonstrate the difficulty experienced in attempting a thickness measurement of the silver layer, which exhibits significant disruption and in some areas, appears to be lifting away from the glass substrate. It is probable, as discussed above, that this disruption is the result of the preparation process of the samples, but it could also suggest that the silver layer is not, in actuality, the smooth, homogenous layer that could be expected based upon a purely visual examination of these objects. Another back-scattered image, Figure. 7.4, of sample 22G5 (from the discoloured base of the candlestick) indicates that the silver layer here exhibits an uneven texture that cannot be shown by the cross-sectional images seen in Figure. 7.3. This could suggest that the texture of the layer becomes uneven during the process of discolouration, or that the structure of the layer is granular during its formation, although this would require additional research to investigate further and is beyond the scope of this research. A comparison of images A and B (sample 22G4) with image E (sample 22G2) demonstrates that even in parts of the object thought to have little to no silver layer remaining (E), there is little difference in the SEM images between these and the “intact” areas (A and B), in terms of the continuity, thickness and texture of the silver layer. The parts of the candlestick that appear blackened and with significant loss of reflectivity have been found to have a silver layer of equivalence to those of the parts with bright, highly reflective surfaces, indicating a change in chemical composition but little to no change in the thickness and distribution of the metallic layer and demonstrating that even where reflectivity is lost, significant elemental silver remains. Image D shows a flake taken from the interior of the vase (sample 11SF1). The red circle indicates part of the sample that was analysed and determined to be a flake of the glass, while the rest of the sample consists of the silver layer. The bond between the glass and silver layer was such that the sample was removed with great difficulty, and the presence of the glass fragment confirms the strength of the bond between substrate and metallic layer.

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Image C (sample XB5) illustrates a similar texture of the silver layer to that exhibited in images A, B and E, but the point of interest here is the layer clearly visible in the glass substrate, reflected in the EDX results by a significant drop in the weight percentage of Na2O nearer the glass surface (see Table 7.3). Since the silver layer appears very similar to the other samples, this could again indicate a change in composition rather than distribution, but it is clear that this process of chemical change has occurred alongside the deterioration of the glass substrate, unlike the samples taken from the Openluchtmuseum objects. Ultimately, the subject of glass disease is beyond the scope of this project and would require further research to fully diagnose, but Image C has raised interesting questions in this regard. Finally, Image F shows a sample of the applied decoration from the vase, indicating that the decorative motifs were painted on using a pigment (white fragments seen in SEM image) suspended in a primarily organic medium. The presence of zinc (in the EDX results, below) could suggest zinc white.

Figure 7.3: SEM back-scattered images of samples from objects NOM 11 and 22. A) sample 22G4, 2500x magnification. B) sample 22G4, 12,000x magnification. C) sample XB5, 3500x magnification. D) sample 11SF1, 250x magnification. E) sample 22G2, 1200x magnification. F) sample 11ED, 800x magnification.

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Figure 7.4: SEM backscattered image of sample 22G5, magnification 350x

EDX Results In Table 7.4 the results of the EDX elementary analysis can be found. The data has been normalised to take into account the influence of the C and O of the epoxy resin. In all samples, the silver layer was indeed found to be silver and there were no traces of mercury or tin, confirming that the objects were manufactured using a silver reduction method, as opposed to the tin-mercury amalgamation technique. The samples taken from the discoloured base of the candlestick were found to have a high level of sulphur, when compared to samples taken from the top of the object, where the silvering is still bright and intact. This suggests that the darkening and eventual loss of the silver layer is due to reaction between the elemental silver and sulphur in the atmosphere. The silver flakes from the vase (samples 11SF1 and 11SF2) showed high percentages of silver and very low amounts of sulphur, indicating that this type of deterioration is not present in both of the case-study objects. Table 7.4 also provides the results of the EDX analysis of the glass of the case- study objects. One of the silver samples taken from the vase had a tiny sliver of glass attached enabling analysis of the glass. Due to the fragmentary nature of this sample, the analysis of the glass was not as clear as that of the candlestick, but it serves to provide an indication of the composition. The candlestick is shown to be a soda glass; while the vase appears to have a similar composition as the other glass, the percentage of silica detected is considerably lower than that recorded for the candlestick due to the high percentage of silver recorded in the sample, indicating contamination with the silver layer.

EDX Discussion The SEM back-scattered images demonstrate clearly the microscopic morphology of the silver layer and can shed some light upon the physical condition of the glass substrate. However, the relationship between deterioration and elemental composition can only be fully investigated with the application of EDX analysis (Energy Dispersive X-Ray Spectroscopy), to provide elemental data to aid the interpretation of the SEM images. The spectra of two samples in particular (22G1 and 22G3, see Figure. 7.5) show that comparison of the EDX data and SEM back-scattered images is necessary, since, as

49 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture remarked upon above, the silver layers appeared similar for all samples under SEM, yet significant differences were detected by EDX. A comparison of the spectra for points analysed within the silver layer of both samples (see Figure. 7.4) shows a much higher count for sulphur in the silver layer of sample 22G3, taken from the visibly blackened base of the candlestick, than that of sample 22G1, taken from the neck of the object. This difference is made clearer in the numerical data, where the deteriorated silver layer (22G3) exhibits a compound weight percentage of SO3 of 25%, in comparison to the un- deteriorated silver layer in which SO3 only accounts for 1% (see Table 7.4). Both samples gave a relatively high weight percentage value for silver, with only a 10% difference in Ag2O measured in the more sulphur-rich sample 22G3, reinforcing the conclusions drawn from the SEM back-scattered images that elemental silver is still present and that the visible deterioration (blackening and loss of reflectivity) results from a reaction with sulphur to form silver sulphide (Ag2S).

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Figure 7.5: EDX Spectra for samples 22G1 and 22G3. 22G1 was taken from the intact part of the silver layer while 22G3 came from the visibly blackened layer near the base of the candlestick.

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Table 7.2: SEM back-scattered images for the points analysed by EDX

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Table 7.3: EDX Results for the silver layer and glass substrate on objects NOM11(vase) and NOM 22 (candlestick).

Point Location Na2O Al2O3 SiO2 SO3 Cl K2O CaO ZnO Ag2O

Candlestick SILVER 22G1(1)_pt4 Silver layer 2.5 0.0 20.7 1.0 0.0 0.0 2.5 0.0 73.3 22G2(5)_pt1 Silver layer 0.3 0.0 5.7 18.7 0.0 0.5 0.6 0.0 74.2 22G3(2)_pt2 Silver layer 1.7 0.0 9.3 25.4 0.0 0.0 0.0 0.0 63.6 22G4(1)_pt1 Silver layer 3.7 0.6 44.8 0.6 0.3 3.4 4.5 0.0 42.0 22G5(2)_pt2 Silver layer 1.4 0.0 5.4 18.4 0.0 1.4 3.0 0.0 70.3 GLASS 22G1(1)_pt1 glass 7.8 0.0 78.8 0.0 0.3 5.6 7.5 0.0 0.0 22G2(5)_pt2 glass 7.9 0.0 78.0 0.0 0.0 5.6 8.5 0.0 0.0 22G3(1)_pt1 glass 12.5 0.0 77.6 0.0 0.0 3.7 6.2 0.0 0.0 22G4(2)_pt1 glass 8.8 0.8 77.9 0.0 0.1 5.3 7.2 0.0 0.0 22G5(2)_pt1 glass 3.2 0.0 77.0 0.7 0.1 7.6 11.3 0.0 0.0 DECORATION 22ED(1)_pt1 decoration 0.0 0.0 1.3 5.4 3.6 0.2 0.0 89.5 0.0 22ED(1)_pt2 decoration 0.0 0.0 0.6 6.9 3.5 0.3 0.0 88.8 0.0

Vase SILVER 11SF1(1)_pt1 Silver flake 0.0 0.0 0.0 1.5 0.0 0.0 3.9 0.0 94.6 11SF2(1)_pt2 Silver flake 1.8 0.0 6.1 0.0 0.0 0.7 0.5 0.0 90.9 GLASS 11SF1(1)_pt4 Glass fragment 2.3 0.0 54.2 0.0 0.0 11.1 6.9 0.0 25.5 in sample DECORATION 11ED(2)_pt1 decoration 0.0 0.0 0.0 0.8 0.0 0.0 0.0 99.2 0.0 11ED(3)_pt1 decoration 0.0 0.0 0.0 0.0 0.0 0.0 0.0 100.0 0.0

XB0 SILVER XB0(1)_pt2 Silver layer 0.0 0.0 21.5 0.0 0.0 0.0 0.0 0.0 78.5 GLASS XB0(1)_pt1 glass 10.3 3.2 74.8 0.0 0.0 6.0 2.8 0.0 0.0

Silver Christmas SILVER Ornament XB5(1)_pt3 Silver layer 3.7 2.2 31.3 38.1 0.0 0.7 0.7 0.0 23.1 GLASS XB5(1)_pt1 glass 15.4 3.7 75.5 0.0 0.0 3.2 2.2 0.0 0.0 XB5(1)_pt2 Glass (“gel 3.5 4.3 86.6 0.0 0.0 2.9 2.7 0.0 0.0 layer”

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7.3 Instrumental analysis of the recipe reconstructions

SEM/EDX analysis of the reconstructions produced using historic recipes was then performed, so that the samples from the case-study objects could be compared to those taken from the reconstruction test objects and to assess how far the reconstructions could inform our understanding of the silvering processes used to produce these objects.

Methodology The chemical analysis of the reconstruction samples was undertaken using a JEOL JSM5910LV at low vacuum mode of 30 Pa, accelerating voltage 20kV and a working distance of 10mm. Energy-dispersive X-ray Spectroscopy was undertaken using a ThermoFisher Scientific SDD Ultradry detector and NSS (Noran System Seven) software.70 Samples were selected from the reconstruction test samples in the following manner. Two samples were taken from Test 1 and Test 4 of the Liebig recipe (c.1850, recipe 1), to assess whether the difference in the age of the solutions was reflected in the formation of the silver layer. One sample was taken from Test 4 of the James recipe (c.1884, recipe 2), to determine the chemical differences between the purple-brown layer present in the sample to those with a more silver colour. Three samples were taken from objects produced using the Fitzpatrick recipe (c.1856, recipe 3): Test 4 to determine the nature of the brown, non- reflective layer, Test 3 to assess the composition of the silver “halo” and Test 8 because it was considered the most successful result produced by this recipe. Two samples were taken from objects silvered using the Helmanstine recipe (2018, recipe 4); Test 1 represented the best result produced by any of the silvering recipes and Test 4 was conducted on a glass microscope slide, offering an opportunity to compare silver layer formation on a different glass composition. The samples were prepared in the same manner as described above for the case-study samples. Table 7.4 gives an overview of the sample numbers for the reconstructions.

Table 7.4: Sample numbers and “condition” of the reconstruction tests.

Sample Number Recipe and Test No. Silvering Result L1 Liebig Test 1 Good L4 Liebig Test 4 Moderate J James Test 4 Poor F3 Fitzpatrick Test 3 Poor F4 Fitzpatrick Test 4 Poor F8 Fitzpatrick Test 8 Good H1A Helmanstine Test 1 Excellent H1B Helmanstine Test 1 Excellent H4A Helanstine Test 4 Good H4B Helmanstine Test 4 Good

70 Carried out by Kate van Lookeren Campagne (UvA) 54 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture

Results and Discussion

SEM back-scattered images

The SEM back-scattered images of the reconstruction samples show that all recipes were able to produce a silver-containing layer and that the structure of this layer is similar to that exhibited by the case-study objects. The EDX results for the silver layers formed during the reconstruction were complicated by their thinness, which meant that selecting a point for analysis was difficult, even under high levels of magnification.

Figure 7.6 shows some of the back-scattered images produced by SEM analysis of the reconstruction samples. The first image (A) depicts the silver layer of case-study object NOM 22, for comparison. All of the images were taken at 1200x magnification, to enable comparison. The most obvious feature of the reconstruction samples is their similarity to both the layer present on the candlestick (Image A) and each other. Image B, which shows Test 3 using the Fitzpatrick recipe, appears to possess a very similar thickness and structure of the deposited layer to the samples from the other recipes, despite presenting a purple-brown colour in the object, rather than silver. Images C and D (Liebig Test 4 and 1, respectively) show little difference in the appearance of the silver layer, indicating that the increased time since the preparation of the solution did not result in a significant difference in the structure or thickness of the layer formed. Image E shows the silver layer formed in the first test of the Helmanstine recipe (2018), which was assessed as the highest quality result produced by any of the recipes. Though it is not dramatically different from the other samples in thickness, it could be suggested that the SEM image shows that the layer produced by the Helmanstine recipe is smoother and more uniform, which would explain its sharper reflectivity, when compared to the surface of the other reconstructions. Interestingly, Image F shows that the recipe by James was able to form a layer of similar appearance to the others, despite the apparent lack of silver oxide in the silvering solution, which, it was suggested, had contributed to the lack of a visible (to the naked eye) silver layer. As with the samples taken from the case-study objects, the embedded samples from the reconstructions show clear signs of the impact of polishing, in the form of scratches to the glass surface and the ragged nature of the silver layer, clearly visible in all of the SEM images, below. It was decided to prepare the reconstruction samples in the same way as the case-study samples despite this issue, to allow for comparison of the two sets of samples.

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Figure 7.6: SEM back-scattered images at 1200x magnification. A) sample 22G1 from the top of the candlestick. B) Fitzpatrick Test 3. C) Liebig Test 4. D) Liebig Test 1. E) Helmanstine Test 1. F) James Test 4.

EDX results and discussion Table 7.6 shows the results of the EDX analysis. The composition of the glass used for the reconstructions was broadly similar to that of the case-study objects; a soda-glass with similar levels of CaO, although levels of K2O were significantly higher in the case-study objects and there was an increase in the percentage of Na2O in the reconstruction glass. There was very little compositional difference between the glass of the test samples conducted upon the Christmas ornaments and the sample that used a glass microscope slide (see Table 6.6, Helmanstine Test 4).

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Table 7.5: SEM back-scattered images for the point locations analysed by EDX (reconstructions)

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Table 7.6: EDX analysis of the silver layer and glass of the reconstruction test samples

Point Location Na O MgO Al O SiO Cl K O CaO Ag O PbO 2 2 3 2 2 2 Liebig Test 1 L1 x 1200_pt2 Silver layer 10.2 1.6 1.3 43.5 0.0 3.1 40.3 0.0 L1 x 1200_pt3 Silver layer 7.5 0.0 1.6 31.5 0.0 2.0 57.4 0.0

L1 x 1200_pt1 glass 17.7 2.4 2.5 70.7 0.0 0.8 4.4 0.0 1.5

Liebig Test 4 L4 x 1200(1)_pt2 Silver layer 8.5 0.0 1.5 36.5 0.0 2.1 51.4 0.0 L4 x 1200(1)_pt3 Disrupted 7.6 0.0 1.4 26.6 0.0 1.8 62.6 0.0 flake

L4 x 1200(1)_pt1 glass 17.8 3.0 2.1 71.3 0.2 0.9 4.7 0.0 0.0

James Test 4 J(1) x 1200_pt2 Silver layer 11.0 1.8 1.5 46.4 1.3 2.3 34.0 1.7 J(1) x 1200_pt3 Silver layer 10.2 1.3 1.9 44.7 1.1 2.0 38.8 0.0

J(1) x 1200_pt1 glass 17.8 2.1 1.8 73.6 0.2 0.8 3.7 0.0 0.0

Fitzpatrick Test 3 F3 x 1200(1)_pt2 Silver layer 12.8 1.9 1.6 61.6 1.5 3.5 17.2 0.0 F3 x 1200(1)_pt3 Silver layer 3.8 0.0 0.0 17.7 0.0 0.0 78.4 0.1

F3 x 1200(1)_pt1 glass 17.4 2.2 1.6 73.3 0.3 0.8 3.8 0.0 0.6

Fitzpatrick Test 4 F4 x 1200(1)_pt3 interface 13.8 2.4 1.8 59.0 0.0 3.2 19.8 0.0 F4 x 1200(1)_pt4 Silver layer 12.7 2.2 1.5 55.6 0.7 3.2 23.5 0.6

F4 x 1200(1)_pt1 glass 18.2 2.5 1.8 72.6 0.2 0.7 3.9 0.0 0.0

Fitzpatrick Test 8 F8 x 1200(1)_pt2 Silver layer 12.2 2.1 1.5 50.4 1.0 2.5 28.6 1.7 F8 x 1200(1)_pt3 Silver layer 8.8 0.0 1.6 40.6 0.0 0.0 49.1 0.0

F8 x 1200(1)_pt1 glass 17.9 2.8 1.6 73.0 0.3 0.8 3.8 0.0 0.0

Helmanstine Test 1 H1A x 1200(1)_pt3 Silver layer 11.3 1.8 1.7 46.3 0.0 3.0 33.7 2.1

H1A x 1200(1)_pt1 glass 15.9 3.1 2.6 72.6 0.3 0.0 5.3 0.0 0.2

Helmanstine Test 4 H4A x 3700(1)_pt3 Silver layer 10.2 3.2 1.2 58.4 0.0 4.5 21.7 0.0 H4B 2400(1)_pt5 Silver layer 8.8 2.4 1.0 37.4 0.0 3.2 47.3 0.0

H4B 2400(1)_pt1 glass 16.0 4.4 1.4 72.0 0.2 0.7 5.2 0.2 0.0

The EDX results for the analysis of the reconstruction samples indicate that all of the recipes were able to induce the deposition of a silver-containing layer onto the surface of the glass.

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Due to the use of organic compounds as primary ingredients (NH3, C6H12O6), it was not possible to identify reaction products or reactants remaining as part of the silver layers formed. However, a correlation between a low w% of Ag2O where SiO2 is also recorded as being lower could indicate that there is the presence of organic compounds remaining in the silver layer.71 In general, the results showed comparatively high levels of silica at points where the silvering was selected for analysis (for example 43.5 w% detected in Liebig Test 1 compared with an average of 17.18 w% for all of the candlestick silvering samples). This may indicate that there was considerable integration of material from the glass in the deposited layer, possibly as a result of polishing but it could also suggest that the interaction between layer and substrate goes further than simply the deposition of a metallic layer on top of the glass. The EDX results for the Helmanstine samples were not substantially different from those of any of the other samples, showing that the suggested “smoother” layer did not correspond to the presence of a greater percentage of Ag2O, reflected again in the high levels of silica detected. Thus, although there was a visual difference between the results of the modern and historic recipes, the actual layer formed appeared to be very similar and suggested that the basic process for the deposition of the silver layer is the same. A comparison of the results of the analysis on the silver layer of the Fitzpatrick test results indicates that there was not a large difference between the percentages of Ag2O and SiO2 detected for each, despite their very different visual appearances. This highlights again how the results were influenced by the problems of successfully selecting the silver layer for measurement, at least partially as a result of the low-quality SEM back-scattered images due to the necessity of using low vacuum conditions. As suggested by the SEM image in Figure 7.6, the James recipe, in which it was hypothesised in the discussion of the reconstruction results that insufficient Ag2O was left in the solution during preparation, produced a silver layer with very comparable detected Ag2O to that of the other recipes.

Comparison with case-study objects The primary aim of the reconstructions was to investigate how the application and composition of the silver layer could influence its appearance and adhesion and compare these results with the case-study objects, in order to help inform how these processes could have contributed to their current condition. As seen above, the SEM back-scattered images indicate that there appears to be little difference in the distribution and formation of the layers upon the glass between the case- study and reconstruction samples, which confirms that a similar process was used to silver the objects. The reconstructed samples produced more compositionally mixed results under EDX analysis, which could be explained by the fact that the silver layer of the case-study objects is more clearly defined and contains a higher w% of silver and, possibly, that in some cases silvering had not been completely achieved in the reconstructions, with the potential for reactants and by-products to remain behind in the silver layer. It is also possible that the case- study objects contain organic material (in protective coatings or deposits) that was not detected by EDX analysis. Assessment of the condition of the objects, using visual observation, instrumental analysis and the creation of reconstructions of historic silvering recipes, has illustrated how

71 The C and O results were removed when the results of the embedded samples were normalised. It was impossible to know with certainty if the CO w% was from the epoxy or other organic matter. 59 T. Symons UvA 2019 Silvered Glass Deterioration & Manufacture production processes may have contributed to the deterioration of the silver layer of the two objects from the Openluchtmuseum.

The Candlestick EDX analysis of samples taken from the silvered candlestick showed that the discoloured areas of the silver layer, surrounding the breaks in the base and top of the object, were the result of a reaction between sulphur and the silver in the layer. This is presumably due to exposure to sulphur present in the atmosphere and it is possible that this object is particularly susceptible to this form of deterioration due to the physical damage to the glass, which has left the silvered surface open to the atmosphere and also to dirt and debris which can accumulate inside the object. Although there is little documentation that relates to the object’s history before or during its time in the collection of the museum, it is certain that it will have been exposed to levels of atmospheric sulphur that are sufficient to cause tarnishing 72 (sulphidation, resulting in the formation of Ag2S), since it has been found that even normal indoor levels of hydrogen sulphide and carbonyl sulphide (50-100 ppt and 300 – 500 ppt, respectively) can result in black tarnish on silver objects.73 Due to the varied nature of the museum’s collection and storage, it is possible that tarnishing has been exacerbated during the object’s history by proximity to other objects containing known sources of sulphur- containing gases, such as wool, vulcanised rubbers and certain types of paint and glue.74 That this object should be visibly tarnished and the vase not, despite probable storage under the same conditions for almost 60 years, suggests that production processes have contributed to greater sulphur-susceptibility in the candlestick. Although it was not possible to confirm this through SEM/EDX analysis, the colour change experienced by some of the reconstruction recipes over the first few days after silvering indicates that products are either left in the silver layer or that a reaction with the atmosphere is occurring immediately after production. That this occurred in the results of some recipes but not all suggests that the composition or application method has an influence on the chemical reactions that occur in the resultant silver layers, even after the process has been considered complete.

The Vase The nature of the detachment of the silver layer from the vase was more difficult to investigate using the instrumental analysis, predominantly because samples could not be taken. The fact that organic materials cannot be specified using EDX makes it impossible to identify any remaining reactants or chemical by-products left in the silver layer. The EDX confirmed the lack of sulphur-containing compounds, which suggests that the composition or morphology of the silver layer may have reduced its vulnerability to this form of deterioration. The vase has a facetted form with only a narrow internal space in certain areas, such as the rim, and the loss of the silvering is primarily concentrated in the more open parts of the form, such as the stem, which are also more accessible.

72 2Ag + H2S → Ag2S + H2 73 Selwyn, L. Metals and Corrosion: A Handbook for the Conservation Professional, Canadian Conservation Institute: Ottawa (2004), p.137

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The reconstructions showed that the silver layer formed by the silver nitrate process, as written in the recipes, is easily damaged by any contact and though there was some difficulty in removing a sample from the vase, it could be suggested that continued physical contact between the object’s silver layer and another surface (such as that of the missing plug from the base) could have resulted in the wear to the silver layer. Abrasion created during the silvering process, as experienced in the reconstruction of the Helmanstine method, may also create micro-damage to the silver layer which renders it vulnerable to further loss in the future, although this would require further investigation. There is the possibility also that the flaking could be caused again by storage conditions, perhaps fluctuations in relative humidity to which the silver layer would be much more vulnerable to than the glass, but to confirm this it would be necessary to conduct RH ageing tests upon the reconstruction samples.

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8 Conclusion

In conclusion, it is necessary to turn again to the central question: To what extent can the deterioration of the silver-coloured coating on 19th century ‘Poor Man’s silver’ glass objects be attributed to their composition and manufacture?

While it has not been possible to fully diagnose the deterioration experienced by the two case study objects within the scope of this research, significant progress has been made in identifying possible factors influencing deterioration. Although it can be said that the production method must have had an impact upon the susceptibility of the objects to certain forms of degradation the processes of deposition and deterioration of the silver layer have been shown to be very complex. To what extent production method has been the primary contributor will require further investigation. The reconstructions of historical silvering recipes expanded our understanding of the complexity of the effect of variation in the materials and methods used for silvering and how they influence the success of the silver layer produced. This also helped to illuminate possible mechanisms by which deterioration could occur as a result of changes in either of those aspects. Further research could extend the reconstruction of silvering recipes to a wider range of recipes and glass shapes in order to investigate the possibility of a relationship between object shape and preservation or deterioration of the silver layer (for example, whether more closed forms are more resistant to deterioration). Ageing tests could provide valuable information on how the rate of deterioration is affected by storage conditions or how differently composed silver layers respond to changes in storage climate. In summary, the analysis conducted as part of this thesis research has provided important insight in identifying the cause of deterioration in one of the two case-study objects investigated and the analysis conducted has gone some way to show conservators and museum professionals the direction for further research into the origins of the detachment of the silver seen on such objects. Through this research it is hoped that the complexities of silvered glassware as a category of objects are more fully appreciated by the cultural heritage community and that further research into the conservation issues of these intriguing objects can be facilitated by the spread of knowledge regarding their manufacture.

Future Research Due to the limited scope of this reconstruction experiment within the master thesis, a further study would be greatly beneficial to extend our understanding of the chemical silvering process and the effect of ageing (using temperature and humidity). It is important to note that in order for ageing tests to be representative, further analysis of the case-study objects would need to be undertaken, to fully explore the potential for organic coatings having been applied to protect the silver layer or improve adhesion to the glass. Assuming this could be achieved, the results of these tests could be built upon by the creation of a more standardised protocol for the testing of each recipe and perhaps the extension of the testing to a wider range of objects, to examine the effect of the object form on the susceptibility of the silver layer to deterioration. Word Count: 17,551

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9 Table of Figures

Figure 4.1: Object NOM 11931-51 (vase) and NOM 22678-55 (candlestick) in current condition ...... 8 Figure 4.2: Object NOM 11 (vase) – A.) The vase. B.) The applied decoration. C.) The base of the vase. D.) The foot of the vase. E.) The rim of the vase...... 9 Figure 4.3: Object NOM 22(candlestick) – A.) The candlestick. B.) The neck of the candlestick where the head has broken off. C.) Plan view of the break in the neck, showing the candle wax on the interior. D.) The base of the candlestick. E.) The foot of the candlestick...... 10 Figure 4.4: The candlestick before it was broken, Nederlands Openluchtmuseum, catalogue entry for NOM22678-55, accessed 27/04/2019 [No date provided for image] ...... 15 Figure 4.5: The silvered glass objects in the collection of the Zuiderzee Museum, Enkhuizen. LEFT: catalogue no. 002675, candlestick pair with yellow butterflies. RIGHT: catalogue number 011579, three-part “kaststel”...... 16 Figure 4.6: The “witch-bottle” in the collection of the Pitt Rivers Museum, Oxford (2013). . 17 Figure 5.1: (left) Dino-lite image (55x magnification) of the deterioration on object NOM 22678-55 (candlestick). (right) image taken from Hadsund, Per. "The Tin-mercury Mirror: Its Manufacturing Technique and Deterioration Processes." Studies in Conservation 38, no. 1 (1993), p.11 (Fig.12), showing corrosion of a mercury amalgam mirror...... 21 Figure 6.1: Fitzpatrick Test 3, showing the clear silver “halo” formed during the test ...... 41 Figure 7.1: Object Nom 22 showing locations of sampling. 1) top of candlestick, samples 22G1 and 22G4. 2) Sample of decoration 22ED. 3) base of object, samples 22G2, 22G3 and 22G5...... 46 Figure 7.2: Sample 22G3 (left) and sample XB5 (right) showing scratches to the resin and glass...... 47 Figure 7.3: SEM back-scattered images of samples from objects NOM 11 and 22. A) sample 22G4, 2500x magnification. B) sample 22G4, 12,000x magnification. C) sample XB5, 3500x magnification. D) sample 11SF1, 250x magnification. E) sample 22G2, 1200x magnification. F) sample 11ED, 800x magnification...... 48 Figure 7.4: SEM backscattered image of sample 22G5, magnification 350x ...... 49 Figure 7.5: EDX Spectra for samples 22G1 and 22G3. 22G1 was taken from the intact part of the silver layer while 22G3 came from the visibly blackened layer near the base of the candlestick...... 51 Figure 7.6: SEM back-scattered images at 1200x magnification. A) sample 22G1 from the top of the candlestick. B) Fitzpatrick Test 3. C) Liebig Test 4. D) Liebig Test 1. E) Helmanstine Test 1. F) James Test 4...... 56 Figure 13.1: Locations of microscope images (vase) ...... 69 Figure 13.2: Locations of microscope images, candlestick...... 69 Figure 15.1: Case-study (comparison) objects, XB0 (left) and XB5 (right) ...... 75

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10 Table of Tables

Table 4.1: Optical microscopic images of the case-study objects in their present condition .. 11 Table 5.1: Comparison of historic silvering recipes ...... 23 Table 6.1: Liebig’s recipe ...... 26 Table 6.2: James’ recipe ...... 27 Table 6.3: Fitzpatrick’s recipe ...... 28 Table 6.4: Helmanstine’s recipe ...... 29 Table 6.5: Comparison of the selected historic silvering recipes ...... 31 Table 6.6: Test procedures for each recipe ...... 34 Table 6.7: Results of the reconstruction tests ...... 37 Table 7.1: Sample numbers and locations for samples taken from case-study objects ...... 45 Table 7.2: SEM back-scattered images for the points analysed by EDX ...... 52 Table 7.3: EDX Results for the silver layer and glass substrate on objects NOM11(vase) and NOM 22 (candlestick)...... 53 Table 7.4: Sample numbers and “condition” of the reconstruction tests...... 54 Table 7.5: SEM back-scattered images for the point locations analysed by EDX (reconstructions) ...... 57 Table 7.6: EDX analysis of the silver layer and glass of the reconstruction test samples ...... 58 Table 14.1: Converted units of measurement for reconstruction recipes ...... 70 Table 15.1: Hirox digital microscope images of the case-study samples ...... 76

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11 Acknowledgements

I would like to thank the following persons for their contributions to this thesis research:

Universiteit van Amsterdam I would like to thank Kate van Lookeren Campagne, Mandy Slager, Dr. Tonny Beentjes, Dr. Maartje Stols-Witlox, Dr. Rene Peschar, Prof. Dr. Ella Hendriks and Prof. Dr. Maarten van Bommel for their supervision, support and technical advice during this project.

The Dutch Cultural Heritage Agency /Rijksdienst voor het Cultureel Erfgoed (RCE) Dr.Ineke Joosten and Dr. Luc Megens for their expertise in performing the instrumental analysis.

Het Nederlandse Openlucht Museum, Arnhem Hans Piena (Conservator Wooncultur) for the kind loan of the two silvered glass objects and provision of their acquisition documentation.

Zuiderzee Museum, Enkhuizen Laura Roscam-Abbing for her contributions to the research into silvered objects in other museum collections.

Pitt Rivers Museum, Oxford Jeremy Uden (Head of Conservation) for his contribution to the research into silvered objects in other museum collections.

Diane Lytwyn for providing her expert knowledge on the history and variety of silver glass items.

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12 Bibliography

"Mirrors, Silvering of." Journal of the Society of Arts 24 (1875): 784 "The Manufacture Of Glass Christmas-Tree Ornaments In German." Journal of the Royal Society of Arts 60, no. 3129 (1912): 1126-127. “Chemistry Lecture Demonstration Facility – Demos Formation of a Silver Mirror on a Glass Surface”, Rutgers School of Arts and Sciences https://rutchem.rutgers.edu/cldf-demos/1032- cldf-demo-silver-mirror, (2019) [Accessed 24/02/2019] “Drayton’s Process for Silvering Glass.” Journal of the Franklin Institute 44, no. 4 (October 1847): 248–254. “Gilding and Silvering by Immersion.” Journal of the Franklin Institute 37, no. 3 (March 1844): 214–215. “Glycerine, silvering mirrors by means of” Journal of the Society of Arts 31, (Nov 17 1882): 515 “New process for silvering glass”, Journal of the Franklin Institute, Volume 41, Issue 3, (March 1846), Page 209 Andrukh, T., D. Monaenkova, B. Rubin, W. Lee, and K.G. Kornev. "Meniscus Formation in a Capillary and the Role of Contact Line Friction." Soft Matter 10, no. 4 (2013): 609-15. Chitvoranund, N., S. Jiemsirilers, D. Pongkao Kashima "Effects of Surface Treatment on Adhesion of Silver Film on Glass Substrate Fabricated by Electroless Plating", Advanced Materials Research, Vol. 664, (2013), pp. 566-573 Convert-me.com. (2019). Capacity and Volume Conversion (Online Units Converter) [online]. Available at: https://www.convert-me.com/en/convert/volume/ [Accessed 20 Jul. 2019] Convert-me.com. (2019). Convert Weight and Mass Units Instantly. [online] Available at: https://www.convert-me.com/en/convert/weight/ [Accessed 20 Jul. 2019] Craddock, P. (ed.), Scientific Investigation of Copies, Fakes and Forgeries, Taylor Francis (2009) Curtis, H. D. "Methods Of Silvering Mirrors." Publications of the Astronomical Society of the Pacific 23, no. 135 (1911): 13-32. Drayton, T. Patent No. 12358: Silvering Glass and Other Surfaces, London (1848) Endres, W., E. Voithenberg and G. Voithenburg, Silberglas: Bauernsilber : Formen, Technik und Geschichte. München: Callwey (1983) Fitzpatrick, J. “Silvering Glass” Scientific American, 11(46), (1856), 363–363. Ghidiu, L.W., and G.M. Ghidiu "Is Your Christmas Tree Bugged? A History of Glass Insect Ornaments." American Entomologist 52, no. 4 (2006): 240-42.

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Hadsund, P. "The Tin-mercury Mirror: Its Manufacturing Technique and Deterioration Processes." Studies in Conservation 38, no. 1 (1993): 3-16. Helmanstine, M.A. (2018), “Silver Ornaments: A Holiday Chemistry Project”, ThoughtCo: https://www.thoughtco.com/silver-ornaments-christmas-project-606131 [Accessed: 25/02/2019] Henbest, N. "The Universe, Decanted." New Scientist 228, no. 3052-3053 (2015): 73-75. Herrera, Duran, Franquelo, Justo, and Perez-Rodriguez. "Hg/Sn Amalgam Degradation of Ancient Glass Mirrors." Journal of Non-Crystalline Solids 355, no. 37 (2009): 1980-983. James, F. L. “The Deposition of Silver on Glass and other Non-Metallic Surfaces”. Proceedings of the American Society of Microscopists, 6 (1884), 71 Liebig, J. “Ueber Versilberung und Vergoldung von Glas. Annalen Der Chemie Und Pharmacie”, 98(1), (1856), 132–139. Limam, E., V. Maurice, A. Seyeux, S. Zanna, L. H. Klein, G. Chauveau, C. Grèzes-Besset, I. Savin De Larclause, and P. Marcus, “Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis” The Journal of Physical Chemistry B (2018) 122 (2), 578-586 Lytwyn, D. Pictorial Guide to Silvered Mercury Glass: Identification & Values, Collector Books: Kentucky (2005) Martin, M. A. “On the Silvering of Glass by Inverted Sugar, for Optical Instruments and Experiments” Monthly Notices of the Royal Astronomical Society, 36(2), (1875), 76–78. Meeks, N. Historical Technology, Materials and Conservation : SEM and Microanalysis /. London: Archetype Publications, in Association with the British Museum, (2012). Michaelis, R.F., Old domestic base-metal candlesticks from the 13th to 19th century, produced in bronze, brass, pakton and pewter, Woodbridge: Antique Collectors’ Club, (1978) Nielsen, A.T., R.L. Atkins, D.W. Moore, R. Scott, D. Mallory, and J. M. Laberge. "Structure and Chemistry of the Aldehyde Ammonias. 1-Amino-1-alkanols, 2,4,6-trialkyl-1,3,5- hexahydrotriazines, and N,N-dialkylidene-1,1-diaminoalkanes." The Journal of Organic Chemistry 38, no. 19 (1973): 3288-295. Pinn, K. Paktong: The Chinese Alloy in Europe 1680-1820, London: Antique Collectors Club, (1999) Poche, Emanuel. České Sklo 17. a 18. Stoleti : Súvodní Expozicí Středověkého Skla : Katalog Výstavy Pořádané Ministerstvem Kultury ČSR a Uměleckopru°myslovým Muzeem v Praze U Příležitosti Mezinárodního Kongresu Assotiation Internationale Pour L'Historie Du Verre v červenci Až Srpnu 1970 v Královském Letohrádku Na Hradě Pražském. Praha: UPM, Obelisk, (1970). Randau, P. Die farbigen, bunten und verzierten Gläser: eine umfassende Anleitung zur Darstellung alter Arten farbiger und verzierter Gläser, der vielfarbigen irisierenden und metallisch schimmernden Mode und Luxusgläser, ferner d. Schmückung d.

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Gläser durch Metalle, Emaille und Bemalung, sowie durch Ätzen, Sandblasearbeit, Gravieren und Schleifen. Wien & Leipzig: Hartleben (1905)

Sage, B. G "Observations on the Polishing of Glass, and on the Amalgam used for Silvering Mirrors." Philosophical Magazine 22, (1805) 112. Selwyn, L. Metals and Corrosion: A Handbook for the Conservation Professional, Canadian Conservation Institute: Ottawa (2004) Stewart, Davies, and Garside. "The Formation of Particle Clusters near an Interfacial Meniscus." Chemical Engineering Science 48, no. 4 (1993): 771-88. Tait, H. (ed.), Five Thousand Years of Glass. London: British Museum by British Museum Press, (1991). Thompson, F.H and E. Varnish. Patent No.12905: Inkstands, Mustard Pots and Other Vessels of Glass, London (1849) Tobin, W. "Foucault's Invention of the Silvered-glass Reflecting Telescope and the History of His 80-cm Reflector at the Observatoire De Marseille." Vistas in Astronomy 30, no. P2 (1987): 153-84. Walker, F. "Early History of Acetaldehyde and Formaldehyde. A Chapter in the History of Organic Chemistry." Journal of Chemical Education 10, no. 9 (1933): 546. Object Documentation: Nederlands Openluchtmuseum, “11826 11935”, Letter to J.H. de Vree van Gelder (1951) Nederlands Openluchtmuseum, “22669 22684”, Letters from Thijs Mol to donors (Dec 1954) Nederlands Openluchtmuseum, catalogue entries for NOM11931-51 and NOM22678-55, accessed 27/04/2019, courtesy of Hans Piena (Conservator Wooncultur)

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13 Appendix I: The Objects

Diagram showing the locations of the microscope images in Table 4.1.

Location of microscope images 1 and 2.

Figure 13.1: Locations of microscope images (vase)

Image 6 and 7

Image 5

Image 4

Image 8 Image 3

Figure 13.2: Locations of microscope images, candlestick.

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14 Appendix II: Reconstructions

Chemicals and Suppliers for reconstructions: Aldehyde of ammonia (acetaldehyde ammonia trimer, (CH3CHNH)3 ) (Sigma-Aldrich) Acetone (C3H6O, Labshop) Ethanol (C2H5OH, Labshop) Silver nitrate (AgNO3, Merck) Rochelle salts (potassium-sodium tartrate, KNaC4H4O6·4H2O) (Sigma-Aldrich) Ammonium hydroxide (NH4OH, Sigma-Aldrich) Ammonium nitrate (NH4NO3, Sigma-Aldrich) Dextrose (C6H12O6, Holland & Barrett) Sodium hydroxide (NaOH, Sigma-Aldrich)

Table 14.1: Converted units of measurement for reconstruction recipes

Unit from original recipe Modern metric converted unit 1 oz (ounce) 28.35g 1 gr (grain) 0.06g 1 Fluid dram 3.7ml 1 US pint (l) 473ml

Quantities converted using:

Convert-me.com. (2019). Capacity and Volume Conversion (Online Units Converter) [online]. Available at: https://www.convert-me.com/en/convert/volume/ [Accessed 20 Jul. 2019]

Convert-me.com. (2019). Convert Weight and Mass Units Instantly. [online] Available at: https://www.convert-me.com/en/convert/weight/ [Accessed 20 Jul. 2019].

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Reconstruction Results: Photographs

Recipe 1 Results: Liebig Test 1 Test 2 Test 3 Test 4

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Recipe 2 Results: James Test 1 Test 2 Test 3 Test 4 Test 5

No silvering No silvering achieved. achieved.

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Recipe 3 Results: Fitzpatrick Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8

Test 1: Microscope slide.

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Recipe 4 Results: Helmanstine Test 1 Test 2 Test 3 Test 4 Test 5

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15 Appendix III: Scientific Analysis

Images of Christmas Ornaments

Figure 15.1: Case-study (comparison) objects, XB0 (left) and XB5 (right)

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Hirox images of the case-study samples

Table 15.1: Hirox digital microscope images of the case-study samples

Sample 22G1 Magnification: 160x

Sample 22G3 Magnification: 160x

Sample 22G2 Magnification: 160x

22G4 Magnification: 160x

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Sample 22G5 (uncoated) Magnification: 40x

Sample 22ED (uncoated) Magnification: 140x

XB5 Magnification: 160x

Sample XB0 Magnification: 160x

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