By Umberto Veronesi

The product of human ingenuity, glass perfectly embodies the alchemical power to imitate nature by art and since the Bronze Age it has proved an incredibly hard substance to classify. Although glass only requires sand, salts and the action of fire, a quick look at any recipe collection will reveal that glassmakers have used a vast array of ingredients depending on what materials were available to them and on the physico-chemical characteristics desired. Colours and opacity were provided by the addition of the right metallic oxides, but even a perfectly colourless glass required specific reagents.[i]

Here, I am going to explore three 17^th-century recipes for white enamels, what Venetians called lattimo. Enamels are glass pastes that could be coloured according to the need and then used as paint or to counterfeit gems. There are plenty of recipes out there, many are listed in Antonio Neri’ L’Arte Vetraria. However, in this post I am going to take my start from a different set of “primary” sources, namely the very crucibles used to manufacture white enamel at one of Europe’s leading chymical laboratories, the Old Ashmolean in Oxford. The residues found stuck to the walls of the vessels (Fig. 1) contain the chemical fingerprint of the ingredients used. The analysis of small cross-sections of such residues with a scanning electron microscope (SEM) are therefore a way to explore the recipes.

The chemical composition of the three residues shows both similarities and important differences. All of them have high levels of silica, corresponding to sand, the main component of glass. To melt silica a fondant is essential, and it needs to be added to the crucible. Here, two residues (B and C) bear the traces of a potassium-based fondant, probably saltpetre or even salt of tartar. Residue A has sodium oxide instead, which means that a different fondant was, pure soda most likely. Recipe-wise, this is the first relevant difference. Next, a reagent must also be added in order to render the glass paste white and opaque. A look at the microstructure of the residues (Fig. 2-4) helps identify what such reagents were and what different choices were made[ii].A (Fig. 2). The white aggregates visible in cross-section are the remnant of a mixture made of lead and tin calcined and then added to the crucible. This, together with somewhat large grains of sand, would produce the required colour and opacity.

B (Fig. 3). Here too crystals can be seen scattered throughout the glass and, like before, these are responsible for an opaque white enamel. However, these are made of tin oxide only, indicating that in this case the calx did not contain lead.

C (Fig. 4). There seems to be a third lattimo recipe being tested at the Old Ashmolean. This is more than a simple variant because it used a wholly different type of reagent, the antimony ore stibnite. The glass is indeed rich in antimony oxide while the microstructure reveals small white opacifying particles. These are a compound made of calcium and antimony that form when stibnite is added to the glass and heated. Such recipe is less common in technical writings, but it is reported in Christopher Merret’s commentary to Antonio Neri’s glassmaking treatise.[iii]

From this necessarily brief survey we can see that there is more than one way to make an opaque white glass paste. What is interesting is that such diversity happened at one of the leading chymical laboratories of its time, giving us an idea of the experimental nature of this enterprise. Making glasses was certainly a way of investigating nature, of looking at how transformations come about. At the same time, it was a way to test recipes for the industry. In this sense, artifacts can become a powerful tool for the history of recipes, another way to enter the arena of artisanal knowledge.

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[i] Cable Michael 2001, p. 307.

[ii] Neri’s recipes for white enamel can be found in: Cable Michael. The world’s most famous book on glassmaking. The Art of glass by Antonio Neri, translated into English by Christopher Merrett (The Society of Glass Technology, 2001), Book 3.

[iii] For a general survey on glassmaking I suggest chapters from: Janssens Koen (Ed.). Modern Methods for Analysing Archaeological and Historical Glass, 2013.

Umberto Veronesi  is a Ph.D. candidate at the Institute of Archaeology, University College London. His dissertation entitled, “The archaeology of laboratory experiments and early chemistry: Oxford to Jamestown and back” focuses on exploring the practice of alchemy through the lenses of the archaeological materials coming from early chemical laboratories and uses scientific archaeology as a means to inform historical research and questions. Veronesi received his BA in Archaeology from the Sapeinza Universita di Roma in 2013, and his MSc Technology and Analysis of Archaeological Materials from the Institute of Archaeology, University College London in 2014.

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