The Fire and the Furnace: Making Recipes Work

By Thijs Hagendijk

While working on the Ars Vitraria Experimentalis (1678), the principle book on seventeenth-century glass, I came a across a peculiar remark. The author of the book, the German alchemist and glassmaker Johann Kunckel (1630-1703) composed a commentary on a series of Italian glaze recipes, and somewhere along he dropped the following line: “reproducing this glaze requires as much Art as inventing it” (p. 193). I was struck by this remark and have been thinking about it ever since. What is it that Kunckel tried to tell us here? What does it tell us about recipes, these miraculously concise pieces of text? And how do these written instructions fit into workshop practices? After all, when reproducing a recipe becomes an investment that equals its invention from scratch – why bother with books at all?

That people bothered is in fact beyond dispute. Recipe research from the past years has come up with ample reasons why people engaged with practical texts. People were working on their reputation, tried to organize experiential knowledge, or were concerned about the epistemic status of their crafts. And indeed, all these elements instantly return in Kunckel’s Ars Vitraria Experimentalis. Kunckel is particularly keen on stressing that everything in his book has been vetted through experience. His book reads as an attempt to apply for a good position at the Brandenburg court, and he openly runs down his main competitor, Friedrich Geißler, who worked on a similar book project. “Lieber Herr Geißler, I am sorry that you are so utterly unfortunate in your judgments and comments” (p. 188). In the end, however, we should not forget that the Ars Vitraria Experimentalis was born out of practice. It finds its basis in the workshop, amidst the radiant heat of furnaces, brightly glowing glass, skilled labor and the stinging smell of smoke.

Back to the glaze recipes. While the Italian glassmaker Antonio Neri (1576-1614) originally conceived the glass recipes some seventy years earlier, Kunckel now presented a German translation of these recipes, lavishly annotated with his comments. In an attempt to better understand how this concoction of recipes and commentaries would fit into actual workshop practices, I teamed up with Márcia Vilargiues (Universidade NOVA de Lisboa) and Sven Dupré (Utrecht University) to rework a couple of recipes. We gathered and prepared the ingredients and spent days around furnaces trying to reproduce the recipes.

Figure 1. One of the wood-fired furnaces we used to reproduce the glaze recipes. Note that the fire is stoked from the sides. Location: Telheiro da Encosta do Castelo, Montemor-o-Novo, Portugal.

We dragged wood, stoked furnaces and patiently waited for the heat to come. We tried to control the smoke and played games guessing temperatures from different shades of bright orange. Showers at the end of the day invariably turned my white bathtub into a deep brown. During these days, our main occupation was fire. Ingredients for the glaze, their quantities and the order in which they had to be combined became mere side issues.

Figure 2. The furnace opening was closed with bricks to keep in the heat. Bricks were only temporarily removed to move samples.

It was in this smoky atmosphere that we began to see a fundamental characteristic of Kunckel’s commentary. The focus of commentaries, annotations and marginalia in practical texts had traditionally been the testing, correcting and improving of recipes, but Kunckel took a slightly different approach. He started to dress up Neri’s recipes in his commentary and added new and previously unarticulated layers to the original recipes. What layers? Well, think for instance about the role of fire. While Neri straightforwardly communicated ingredients, ratios and the different steps in his glaze recipes, nothing prepared us for the important task that fire management turned out to be, which went far beyond sending some wood up in flames. Kunckel, on the contrary, emphasizes this very issue in his commentary, and repeatedly argues that “the Fire is the principle thing to observe” (p. 194).

Figure 3. To work efficiently, we reproduced several recipes at once.

Indeed, while working with different furnaces, we learned that furnaces are more than simple and inert providers of heat. Instead, wood-fired furnaces become a tool by which the quality of the glass is shaped in addition to the ingredients and the quantities prescribed by the recipe. Temperature, atmosphere and position in the furnace are all partly responsible for the end result. In the end, reworking the same recipe in three different furnaces left us with three sets of glazes in different qualities. See how Kunckel cleverly shifted the perspective in Neri’s recipes? Kunckel made his readers aware of the circumstances they had to navigate when putting a recipe into practice, something for which Neri left them unprepared.

Figure 4. Two samples of the red roischiero glaze.

Making processes can be understood as processes of growth, the anthropologist Tim Ingold tells us. And this stance on making has significant consequences for how we understand the position of texts in these processes. Makers stand in an ecological relation with their environment, their materials, tools, and the forces they work with. To make a glaze means that the molten glass, the furnace, the smoke, etc. act in correspondence with an observant and anticipating glassmaker. Crucially important here is that ideas, designs or written instructions cannot simply be imposed onto reality. Recipes need to become part of it instead. To make something means to adapt and respond to the local and unique joining and intertwining of forces, materials and elements – and written instructions form just one strand in this creative process. Seen in this light, Kunckel’s remark was not so much a pessimistic note on the possibly superfluous nature of glaze recipes, but rather a reminder that reading and reproducing a recipe is an all-encompassing effort to make the recipe work in the unique constellation of the individual workshop. Reproducing a recipe is reinventing the wheel.

Thijs Hagendijk is a lecturer at Utrecht University. Earlier this year, he defended his dissertation on reading and writing practices in the early modern arts, with a specific focus on text usage in historical glassmaking, painting and metalworking. He works on the intersection of technical art history and the history of chemistry, and is interested in performative methods, such as reworking, re-enacting and reproducing historical techniques, materials and processes.

*You can read more about this project in Thijs’ recent article in Ambix.

Archaeology and early modern glassmaking recipes: The case of Oxford’s Old Ashmolean laboratory.

By Umberto Veronesi

Crystal blown bottle decorated with milk glass festoon (festoni di lattimo), c. 17th c., Venice.  Image courtesy of Wikimedia Commons.
Crystal blown bottle decorated with milk glass festoon (festoni di lattimo), c. 17th c., Venice. Image courtesy of Wikimedia Commons.

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 17th-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.

Figure 2. Crucible fragments analysed with glassmaking residues.
Figure 2. Crucible fragments analysed with glassmaking residues.

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.

Figure 3. SEM image of residue A showing dark sand grains and remains of lead-tin calx used as opacifying agent.
Figure 3. SEM image of residue A showing dark sand grains and remains of lead-tin calx used as opacifying agent.

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.

Figure 4. SEM image of residue B showing the tin oxide crystals as opacifying agent.
Figure 4. SEM image of residue B showing the tin oxide crystals as opacifying agent.

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]

Figure 5. SEM image of residue C, showing the small opacifying crystals of calcium antimonate.
Figure 5. SEM image of residue C, showing the small opacifying crystals of calcium antimonate.

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.

[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.