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

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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

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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

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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.

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 cup, exterior side. Optical microscope image, magnification 100x

1)The painted decoration appears to have crazing and flaking of the edges. To the left of the image, the texture of the silver layer is clear: deterioration is occurring in the manner of flake-type loss, leaving a patchy appearance. Bowl of cup, near rim. Hirox 3D digital microscope, Magnification

2)In this Hirox image, the flaking deterioration of the silver layer is again

observed, but there is also the presence of white areas in some of the spaces which would have previously appeared silver.

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NOM22678-55 - Candlestick Head of candlestick, rim. Dino-lite handheld microscope, magnification 20x

3)Crack in the glass in the top of the candlestick, through which both air and wax have come into contact with the silvering on the inside. Base of candlestick, edge of discoloured area. Dino-lite handheld microscope, magnification 55x

4)This image shows the transitional area at the edge of the discolouration surrounding the large break in the base. Even in the part which still appears silver, there is an increasingly patchy appearance, when compared to the silvering in the image below.

Near base of candlestick. Dino-lite handheld microscope, magnification 51x

5)Bubble in the glass formed during the manufacturing process.

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Body of candlestick, applied decoration. Dino-lite handheld microscope, magnification 175x

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

Body of candlestick. Dino-lite handheld microscope, magnification 70x

7)At the top left and bottom 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 “pin-holes” of oxidation in the silver layer.

Base of candlestick. Dino-lite handheld microscope, magnification 42x

8)Drips of wax on the foot 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 ornamental manner.

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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}

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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

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.}

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]

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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 19}th_{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]

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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

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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}

16_{Thompson, 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}

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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}

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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}

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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}

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Table 5.1: Comparison of historic silvering recipes

Author Date Materials Method

T. Drayton38 _1847/8 _{Silver nitrate}

Water

Hartshorn (carbonate of ammonia)

Oils of cassia and clove Wine spirits

Silver nitrate added to water and hartshorn, left to stand. Silver oxides filtered off, oil of cassia added, left to stand. Solution poured onto clean surface, oil of cloves and wine spirits added. Excess poured off and surface left to dry before varnishing.

Baron J. von Liebig (1884 by F. James)39

c.1850 Distilled water Aldehyde of ammonia Silver nitrate

Mix ammonia with distilled water. Separately mix silver nitrate with distilled water. Mix two solutions together and agitate. Cover the object with the solution and apply heat by means of a water bath

Draper (1884 by F. James)40

unknown Silver nitrate Distilled water Water of ammonia Sodium and potassium tartrates (Rochelle salts)

Dissolve silver nitrate crystals in distilled water. Add water of ammonia until the brown precipitate at first formed is nearly re-dissolved, stirring continuously. Dilute to 6 ounces, Dissolve the 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

of Baron Liebig’s (c.1850) method, 1884.41 1884 Silver nitrate Water of ammonia Distilled water Rochelle salts

Dissolve the silver nitrate in distilled water and precipitate by adding water of ammonia. Add water of ammonia drop by drop, stirring 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}

Nitrate of ammonia in water

Caustic potash

Sugar dissolved in water, with tartaric acid, boiled for 10 minutes then cooled. Alcohol is added to prevent fermentation.

The glass is thoroughly cleaned first, using alcohol and caustic potash.

In a shallow dish place equal parts silver nitrate solution and nitrate of ammonia solution. Add equal parts caustic potash solution and the inverted sugar solution (to make a total of four equal parts). Place the object in the dish and agitate gently. When the liquid becomes greyish with flakes of silver, the object is fully silvered and can be removed and rinsed.

J. Fitzpatrick43 ₁₈₅₆ _{Silver nitrate}

Aqua ammonia Distilled water Alcohol Grape sugar

Mix all of the ingredients and place inside the article to be silvered. The whole is then kept at a temperature of 160 F until silvering is achieved.

FOR COMPARISON: A.M Helmanstine44

2018 Silver nitrate solution Ammonium nitrate solution

Dextrose

Sodium hydroxide Distilled water

Mix the silver nitrate and ammonium nitrate solutions. Pour the dextrose solution into the object to be silvered. Add the silver/ammonia solution into the object, followed immediately by the sodium hydroxide. Swirl the object to coat the 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

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]

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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.

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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

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,}

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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):

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

“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.”

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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):

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}

“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.”

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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):

60_{Tartrates are defined as salts of tartaric acid, an organic acid with the formula C} 4H6O6. 61_{Fitzpatrick, J. Scientific American (1856), 11(46), 363}

“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

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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.

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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.

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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

(plus extra for dilution)

113.40g distilled water

Distilled water (for rinsing)

3.89g silver nitrate 3.11g silver nitrate plus

0.13g silver nitrate

28.35g silver nitrate 5ml 0.5M silver nitrate solution 2.46g aldehyde of ammonia Ammonium hydroxide 28.35g ammonium hydroxide 2.5ml 1.5M ammonium nitrate solution 0.56g Rochelle salts (potassium sodium tartrate) 7.10g grape sugar (dextrose) 5ml 5% dextrose solution 56.70g alcohol (ethanol) 5ml acetone 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.

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