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.

True Colors, or the Revelatory Nature of Cold

By Thijs Hagendijk

Heat is transformative, brings about change, separates substances or bring them together. Every student of chemistry knows how to enable or enhance a chemical reaction by applying energy to a system, usually in the form of heat. Early modern practitioners did not think otherwise. Fire was the transformative element and key to the production of all kinds of different materials, ranging from the philosopher’s stone to artisanal products such as glass, porcelain or pigments. Applying heat to bring about change is publicly ingrained thermodynamics, but one thing is even more obvious. Once heated, things have to cool down again.

Figure 1: Eikelenberg’s notes on the art of painting, comprising five different manuscripts. Photograph: Regionaal Archief Alkmaar.

When the request came to write a blogpost on cold and recipes, I was somewhat hesitant. Heat seems to elicit the most interesting stories and anecdotes, but interesting cases with respect to cold failed to come to mind immediately. Hence, I tried a different approach and looked at how cold featured in a collection of overtly practical notes on the preparation of paint materials collected by the Dutch polymath and painter Simon Eikelenberg (1663-1738). Intended for publication, he promised his readers an “accurate descriptions of the origin of making, preparation and general use of paint materials, oils, mix-fluids and varnishes.”[1]  It was within the confines of this manuscript that I began to discern two themes with respect to cold in practices of making.

Figure 2: Reconstruction of one of Eikelenberg’s varnish recipes. The varnish was prepared in a glazed pot, placed in a sand bath and heated on fire. Photograph: Thijs Hagendijk.

It is only when things have cooled down that the transformative work of heat can really be judged. Eikelenberg describes for instance how he experimented with minium, a red lead-based pigment, which he heated in a crucible and placed in a fire. “The more it glowed, the more the minium turned yellow near the sides of the crucible, the lowest parts alike; which, when it was cold, appeared to be nothing else but yellow massicot.” [2] Eikelenberg also describes the preparation of various varnishes. Here too, quality and properties of substances are explicitly observed after the varnishes have cooled down. “When the varnish was cold I found that it was rather thin and that it did not cover well.” [3]  Another varnish was prepared on a hot sand bath, after which Eikelenberg “filtered it through a cloth and let it cool: it appeared then as a thickish and yellowish varnish.” [4]  Pay attention to the word “then”: there is a clear order of things that speaks through Eikelenberg’s notes. Being cold is a condition that precedes testing and Eikelenberg makes that rather explicit.

Figure 3: It is hard to achieve a homogeneous mixture when preparing varnishes. A whitish sediment is developing in this varnish, which is in coherence with Eikelenberg’s notes. Photograph: Thijs Hagendijk.

Whereas heat is transformative, it is only in the absence of heat that things can be trusted to stay the same. Continuing with the varnishes, Eikelenberg was well aware that their preparation does not stop after the ingredients have been heated and combined. As long as it is still hot, the apparently homogeneous concoction can easily coagulate and fall apart. Eikelenberg wrote in his notes: “We can conclude that to prevent curdling it is necessary not to stop stirring before the mixture is cold.” [5] Indeed, each time he made varnishes, Eikelenberg made sure to keep stirring until everything was cooled down: “stirring steadily until all was cold” or “having stirred until it became cold”.[6]

Figure 4: Eikelenberg mentions that: “[w]e can conclude that to prevent curdling it is necessary not to stop stirring before the mixture is cold.” Passage marked in red. Photograph: Regionaal Archief Alkmaar.

For Eikelenberg, heat was both friend and foe and until his varnishes reached firm, cool ground, they required careful guidance and attention. Cooling down was thus as arduous a process as heating the mixture was in the first place. Yet, once cooled down, true colors are revealed – deprived from heat and stabilized by the cold.

[1] Simon Eikelenberg, “Aantekeningen betreffende schilderen,” MS 391, Collectie Aanwinsten, Regionaal Archief Alkmaar: fol. 1. “Naukeurige beschrijving van de oorsprong of making, bereiding en ’t algemeen gebruik der verfstoffen, olijen, mengvogten en vernissen.”
[2] Simon Eikelenberg, “Aantekeningen betreffende schilderen,” MS 390, Collectie Aanwinsten, Regionaal Archief Alkmaar, fol. 806. Original: “na mate dat het gloejend wierd, veranderde de menij die naast tegen de zijden van de kroes aan-zat en wierd geel, gelyk ook ’t onderdtste; ‘t welk doe ‘t kout was niet anders dan gele masticot geleek”.
[3] Eikelenberg, “Aantekeningen betreffende schilderen,” MS 390, fol. 827. Original: “Doe de vernis koud was bevond ik ze wat dun en datze niet genoeg dekte.” Translation from: A. van Schendel, “Simon Eikelenberg’s Experiments on the Preparation of Varnishes,” Studies in Conservation 3 (1958), 130.
[4] Eikelenberg, “Aantekeningen betreffende schilderen,” MS 390, fol. 802. Original: “Doe ‘t wel vermengt was, kleijnsde ik ‘t door een doek en liet het kout worden, wanneer ‘tzelve een dikagtige en geelagtige vernis vertoonde” Translation from: Schendel, “Simon Eikelenberg’s Experiments,” 128.
[5] Eikelenberg, “Aantekeningen betreffende schilderen,” MS 390, fol. 824. Original: “Hieruijt kan men afnemen dat om ’t schiften voor te komen, men niet moet op-houden met roeren voordat se kout is.” Translation from: Schendel, “Simon Eikelenberg’s Experiments,” 129.
[6] Eikelenberg, “Aantekeningen betreffende schilderen,” MS 390, fol. 827. Original: “gestadig omroerende totdat het gantschelijk koud was.” Translation from: Schendel, “Simon Eikelenberg’s Experiments,” 130. Eikelenberg, “Aantekeningen betreffende schilderen,” MS 390, fol. 832. Original: “tot koutwordens toe geroert te hebben”.

 

Cold! A Recipe Project Thematic Series

Hiroshige, Two men by a gate in the mountains. Image from Wikimedia Commons.

– it’s cold! A dreary chill and rain have just descended across Europe and perhaps most of you are also cranking up the heat and bringing out winter scarves and hats. December has arrived and it seems apt for us to follow our fun and successful series on “Heat!” with a thematic series on “Cold!”. Within medical conceptions of the human body across a number of cultures, notions of hot and cold are hardly be separated. Within kitchens, craft and artisanal workshops, although heat played a crucial role in production processes, cold was also essential occasionally – especially if ingredients had to be preserved for a period of time, or if heat had to be tempered in some way.

To get ready for the long winter, our contributors have explored the notion of “Cold!” in a number of areas. Thijs Hagendijk returns to the RP with a post on the Dutch polymath and painter Simon Eikelenberg (1663-1738), detailing how cold features in the practices of his paint making with surprising insights.  Jean-Olivier Richard, a historian with interests in early modern natural philosophy, alchemy and environmental history, invites us reflect upon mankind’s impact on our planet by offering a reading of “divine recipes for a cooling earth”.

Having written about how to “treat the heat in 1793 Beijing”, Marta Hanson returns to the RP this month with a post titled “Treating the Deadly Cold in 1918 China”, co-authored with Michael Shiyung Liu. Returning to another theme explored in the Heat! Series – fertility recipes – Yi-Li Wu will tell us about Chinese formulas dealing with cold genitals, the standard historical explanation for male and female infertility.

Finally, as we move closer to the holidays, we offer a few posts to “warm” you up. Marieke Hendriksen and Ruben Verwaal return with more adventures with Boerhaave’s “little furnace” (go here for part 1 of their explorations). New contributor historian Reinhild Kreis will tell us about Christmas Cookies in 20th century Germany and our Tales from the Archives will feature the wonderful post on “snowballs” by Rachel Snell.

“Christmas Dessert of layers of fruit, arranged for color effect. ‘Snowball’ is one of the most attractive Christmas Desserts” from American Homes and Gardens, 1911.

We can’t do much about the chilly weather outside but we hope that this wide-ranging edition of the Recipes Project might distract you from the weather and inspire you to think about the cold and chills in different ways.

Enjoy and happy holidays!

Marieke Hendriksen and Elaine Leong

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Ps. This is my last edition for a little while as I’m taking a tiny break from editing the Recipes Project in 2019. Things have been all-go at the RP headquarters over the past few months, and we have some really exciting news to share with you after the holidays. So, watch this space and see you all soon, Elaine.

Tales from the Archives – Gumpowder? A strange little recipe for sensitive teeth…

In September 2017, The Recipes Project celebrated its fifth birthday. We now have over 600 posts in our archives and over 150 pages for readers to sift through. That’s a lot of material! (And thank you so much to our contributors for sharing such a wealth of knowledge on recipes.) But with so much material on the site, it’s easy for earlier pieces to be forgotten. So, the editors have decided that, every now and then, we’ll pull something out of the archives to share with our readers anew.

November in the UK is marked by fireworks, which commemorate the failed Gunpowder Plot, orchestrated by Guy Fawkes in 1605. When I first moved to the UK in 2001, I was a little surprised to see firework displays in the Autumn – in Belgium and France they are much more common in the Summer. However, I quickly got used to wrapping up warm to go and enjoy sparkling nights.

I have trailed the Recipes Project archive for a firework-related post, and have found this post from 2012 by Katherine Foxhall on the therapeutic uses of gunpowder. Certainly not one to try at home!


By Katherine Foxhall

If you go to your bathroom and check the ingredients in your well-known brand of sensitive toothpaste, you may well find that the recipe contains the active ingredient potassium nitrate. Also known as saltpetre or nitre, this naturally occurring mineral is found in foods as a preservative (e.g. corned beef), and used in fertilizer, cigarettes, blood pressure medicines and fireworks. Since medieval times it has formed one of the main ingredients in gunpowder, and it is this connection that has also given potassium nitrate a long association with teeth and gums.

Many of the seventeenth and eighteenth century recipe books in the Wellcome Library’s manuscripts include treatments for gunpowder burns, but some also proposed that gunpowder could be therapeutic. Katherine Jones, Lady Ranelagh (sister to the famous chemist Robert Boyle), recommended a ‘little gunpowder’ applied in a linen cloth to ease toothache. On one page of Anne Brumwich’s recipe book (Wellcome MS 160, p.83) we can find nine recipes for toothache remedies written in two different hands. One, ‘An aproved medecine for ye toothake’ (approved meant that it worked) required gunpowder, aniseed water and lint, mingled together to ‘make a litell thing’.

Once the sufferer had picked their tooth very clean, the recipe instructed them to push the preparation into the tooth, taking care not to allow any of the mixture down the throat.

A century later, in A Treatise on the Scurvy (1795) David Paterson introduced his fellow naval surgeons to a wonderful, and apparently unknown remedy for scurvy: during a voyage in 1784, he claimed, he had restored the health of eighty sick seamen not with lemon juice, fresh fruit or vegetables, but with the potassium nitrate extracted from the gunpowder in his ship’s stores. Paterson’s remedy was soon forgotten, until in 1828, a desperate surgeon named Charles Cameron, having used up all his supplies of lemon juice, remembered Paterson’s recipe. Cameron was stranded in the calms near the equator and he was faced with a ship’s hospital full of scorbutic convicts, less than half way through the voyage to Australia. He extracted the nitre from the powder, dissolved some of it in vinegar, and mixed some more with vinegar and lime juice. He also added a little sugar (to taste?!) The effects were ‘miraculous’.

For the Navy, if Cameron was right, this was a money-saving opportunity; nitre was cheap and did not decompose over time. In the following decades surgeons continued to experiment with different remedies for scurvy until, in 1840, the Admiralty decided to perform a large-scale experiment to determine once and for all the best scurvy remedy. Over the next four years the surgeons of sixty ships transporting fifteen thousand convict men from Britain and Ireland to Australia received crystallised citric acid, potassium nitrate, and lemon juice. Their instructions clearly forbade the surgeons from trying to cause scurvy during the voyage but if the disease did appear, the patients were to be divided into three groups, each group receiving one of the remedies. Of course, the surgeons often had their own ideas, and often altered, combined and varyed the doses according to their own personal favoured recipe. So, while Surgeon Deas mixed some nitre with lime juice and some with citric acid, and felt that both mixtures were useful, Alexander Bryson gave each group the remedies mixed in a glass of wine, water and sugar. After many of the convicts developed severe scurvy, Bryson finally decided that potassium nitrate was ‘objectionable’. The surgeons had come to very different conclusions about the value of potassium nitrate but the results of the experiment were clear; potassium nitrate was abandoned as useless, lemon juice was in for good.

In the mid 1970s, dental researchers – in laboratories this time, rather than on ships – began to report a strange occurrence: mixing potassium nitrate with toothpaste seemed to reduce dental sensitivity in sufferers.  More work confirmed the compound’s beneficial effects, but the scientists still admitted that they were unclear why it should work; being soluble, it seemed that it should simply dissolve in water and wash out of the teeth at first rinse.

Jump forward again to the present, and potassium nitrate is often used as the active ingredient in products for sensitive teeth. So we have come a long way in medical understanding since women like Anne Brumwich stuffed aching teeth with gunpowder soaked lint, or Victorian naval surgeons dosed their convicts with nitre in the certainty that it helped with scurvy, and yet nitre has proved persistent: these earlier ideas about potassium nitrate’s ability to reduce not only the pain of toothache, but the symptoms of scurvy – a disease so commonly experienced in the mouth and gums – are worth wondering about.