A Cool Oven: Boerhaave’s Little Furnace, part II

By Ruben Verwaal and Marieke Hendriksen

Ruben Verwaal is curator of the historical collections at Erasmus Medical Centre, Rotterdam, and at the Museum for Communication in The Hague. He obtained his PhD in June 2018 with a thesis on the role of bodily fluids in eighteenth-century chemistry. Marieke Hendriksen is a researcher on the Artechne Project and PI at the Art DATIS Project at Utrecht University and a long-time contributor to The Recipes Project. She specializes in the material culture of science and art in the long eighteenth century. Ruben and Marieke share an obsession with an eighteenth-century object that has since disappeared: a small chemical furnace. In a previous post, they wrote about reconstructing Boerhaave’s little furnace. Now they have two…

The newly build oven, August 2018

In August of this year, we wrote about our first attempt to recreate Boerhaave’s little furnace from old coal stoves. Meanwhile, Marieke’s dad, André, who is a skilled carpenter, was building a furnace from scratch, using Boerhaave’s description and a nineteenth-century example of a Boerhaave furnace in the collection of Museum Gouda as his guidelines. This resulted in a sturdy furnace of solid dried oak, much larger than the furnace we created from coal stoves.

The interesting thing about Boerhaave’s furnace is that many of the experiments that he described in his chemistry book, the Elementa Chemiae, for which the furnace can be used, required a very moderate degree of heat – one could say a cool rather than a hot oven. Two examples we mentioned previously were the distillation of rosemary, and the hatching of eggs, which Boerhaave said he believed his furnace could be used for too. The kind of egg is not specified, but for chicken eggs, the ideal temperature for hatching is 37,6 Celsius. Could we attain that temperature with our furnaces? 

Boerhaave advised to use glowing coals or Dutch turf as fuel, with which a constant and moderate heat should be achieved that could be kept up to 24 hours. As turf is no longer won in the Netherlands, we started with some ordinary barbeque coals – and indeed managed to establish a fairly constant heat of around 30 Celsius in the large oven for an hour or so. But coals did not hatch any chicks.

Coals: a stable 30 Celsius

Suspecting that turf may give better results, we set out to buy turf, which is still won in regions in Germany and Ireland. It turned out to be surprisingly difficult to buy in the Netherlands though. Eventually we managed to purchase a box of Irish turf through the American website of the online retailer we love to hate – but it took eight weeks (!) to arrive.  Though our cool oven still hasn’t incubated a chicken, the first results look promising.

Irish peat via the US
Irish peat via the US

Meanwhile, we started thinking about the experiments we’d like to recreate once we had all necessary materials. Since Ruben wrote his PhD thesis about bodily fluids, he is keen on reconstructing an experiment with milk from different mammals. Preferably, we’d compare the effects of prolonged mild heat on cow’s milk and human breast milk. Raw cow’s milk can be purchased at some farms, so Ruben cycled out to get some, while Marieke hesitantly contacted a friend who was pumping to feed her infant daughter to ask if she was willing to donate some of her leftovers to science. Note for future generations: Marieke has the coolest friends – she instantly said yes! For weeks, she gathered the left overs that her daughter did not drink in freezer bags.

Suddenly, it is December, and we have two furnaces, a box of Irish peat, and milk in two freezers. Now we ‘only’ have to make time for this reconstruction experiment… We live an hour apart and this is our pet project, so we’re desperately searching for a couple of days when we can take time of work. It turns out that the most difficult aspect of this reconstruction project is not the building of the furnaces or the sourcing of the necessary materials, but the absence of what Boerhaave obviously did have: cheap labour in the form of young assistants, who could take turns keeping the furnaces going day and night. We can only hope that once we do manage to take those days off, the Dutch winter is still as mild as it has been up till now! 

How to Prevent the Cooling of the Earth: A Page from God’s Cookbook

By Jean-Olivier Richard

Image from Athanasius Kircher’s Mundus Subterraneus (1678 edn.) vol. 1, p. 194.

Historians studying the relationship between climate and recipes (and yes, historians have good reasons to do so; see Jennifer A. Munroe’s post on seasonality and Katherine Allen’s articles on springtime in recipe books and the common cold) usually frame the question in terms of seasonality. In the pre-modern world, ingredients for recipes could often only be obtained in certain seasons or particular climates – the so-called seasonality of recipes. But the concept of recipe, provided one is willing to stretch its bounds, can also help us understand how our ancestors envisioned the role of God and Man in climate change and cosmic history. What if, for instance, one were to think of the Creation account of Genesis as a recipe? The metaphor is not as far-fetched as it may sound. Recipes entail not only ingredients and instructions, but also agency: isn’t God said to have made everything in measure, number, and weight (Wisdom 11:21)? Many early modern philosophers cherished this notion. Let us imagine, with one such philosopher, a page from God’s cookbook:

In the beginning, create the heaven and the earth. Everything should be in a state of chaos. Then, let there be light. Light will bring the world into sight by causing the formless abyss to sort itself out. Over a period of six days, let the sun, the moon, and the planets coalesce like bubbles, as celestial and terrestrial matter separate; on earth, concentric layers of air, water, and dry land will arise around a pulsing core of fire. In the process, bring forth minerals, plants, and animals. This is good, but not good enough. If the primordial light is left unchecked, all mixed and organic bodies will soon break down and dissolve into their elementary constituents; even beings endowed with seeds will stop propagating their kind. To prevent the cosmos from congealing, add Man to the mixture. Let him stir it, and it will be very good.

A caricature of Louis-Bertrand Castel’s “ocular organ” by Charles Germain de Saint Aubin (1721-1786).

The physics treatise that inspires my thought experiment appeared in 1724, when the boundaries between physical, chemical, and spiritual processes were still porous. Its author was the French Jesuit mathematician and natural philosopher Louis-Bertrand Castel (1688-1757), best remembered today for his ocular harpsichord (a musical instrument that played colors) and his quarrels with the likes of Voltaire and Rousseau. To be clear, Castel did not couch his interpretation of Genesis in culinary terms; but he did argue that God’s primordial “light” must refer to the fundamental, mechanical principle of nature, the force causing elementary particles to weigh against one another and to regroup according to their kind — like mercury, water, and oil mixed together end up settling into layers. What Moses called “light,” ancient philosophers like Empedocles, Parmenides, and Epicurus had known confusedly as “love and strife”, “sympathies and antipathies,” “attractions and repulsions.” Since Newton, moderns recognized it as “universal gravitation” — or as Castel would have it, “universal weighing” (pesanteur). Natural philosophy, just like the original chaos, was sorting itself out.

Yet universal weighing could only be half the story. Castel’s main contribution to science, as he saw it, was to demonstrate the need for another principle: a universal lightness, a kind of spiritual leaven or ferment that would counter the weight of nature and sprinkle a little chaos into the world’s regular march toward equilibrium. This principle had to be spiritual, as opposed to mechanical, because a constant mechanical counterweight would cancel out rather than interrupt the course of nature as needed. Now, God could intervene directly to do just that, but that would be beneath His dignity. A popular alternative — giving nature its own spiritual, vital powers — raised the specter of materialism; out of question for a Jesuit. Castel’s solution? God must have delegated the task of disrupting the world to Man, his troublesome steward. Endowed with free will, humans could bring about everything universal pesanteur could not. Local trouble, Castel reckoned, added up to all the meteorological, climatic, and geological changes needed to prevent the world from grinding to a halt and freezing over.

While this might seem to be a recipe for disaster, Castel trusted that God knew what He was doing. The curse of mortality, for one, ensured humans would not slack off, nor overreach their bounds. Since the Fall, Adam and his descendants felt the weight of nature; they had to sweat and toil to delay the hour of their death (death by gravity, that is). On the upside, tilling the land, breeding animals, cutting down trees, draining fens, building canals, mining the earth, manufacturing goods and transporting them for commerce—all this labor, along with the consumption and excretion of food, renewed the mixtures that nature constantly unraveled. Multiplied by the millions, local actions caused ripples: rain, winds, and waves to which Castel attributed weather pattern and climate change. When well concerted, human efforts made the earth compliant; when excessive or deficient, corrective backlashes ensued — storms, earthquakes, volcanic eruptions. On the whole, the world remained hospitable because man-made mixtures, combined with nature’s weighing and sorting action, preserved the planet’s internal circulation mechanism. Carried down by rivers, oceanic currents, and subterranean channels, the by-products of human activity fueled the central fire of the earth, the reservoir of heat for all living beings on the surface. Castel estimated that the sun only contributed a minute portion of heat compared to the earth’s inner furnace, hence the importance of keeping it alive.

Reproduced with kind permission from http://www.reverendfun.com/toon/20070907/

Fifty years after Castel published his treatise, Georges-Louis Leclerc, Comte de Buffon (1707-1788) hypothesized that the earth was once a globe of molten rocks, whose age he ingeniously estimated by measuring the cooling rate of molten metal spheres. Revisiting the notion of a “central fire” keeping the planet warm from within, Buffon warned his readers about the inevitable freezing of the world, but also suggested that human industry might counter the process. Framed by debates about climate, gravity, progress, and the place of divine and human agency in nature, the Enlightenment saw scores of new ideas about the history of the earth — some of which presaging in surprising ways today’s anxieties about climate change. Yet Castel his contemporaries also expressed more confidence in human stewardship. Humans were God’s special ingredient in a perfectly well-balanced recipe; their 19th- and 20th-century successors would not be so serene.


Richard, Jean-Olivier. “The Art of Making Rain and Fair Weather: Life and World System of Louis-Bertrand Castel, SJ (1688-1757).” Ph.D. dissertation. Baltimore, Johns Hopkins University, 2016.

Castel, Louis-Bertrand. Traité de Physique sur la pesanteur universelle. Paris: Cailleau, 1724.

Buffon, Georges Louis-Leclerc, Comte de. Les Époques de la nature. Paris: Imprimerie royale, 1780.


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


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.

Food, Magic, Art, Science, and Medicine