All posts by mariekehendriksen

Marieke Hendriksen is a historian of science, art, and ideas, based at Utrecht University , The Netherlands. She is particularly interested in the material culture of eigtheenth-century medicine and art, and the transmission of instructions through text. She currently is a postdoc with the ARTECHNE project (

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

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.


The “Gentle Heat” of Boerhaave’s Little Furnace

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 postdoctoral researcher on the Artechne 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.

With the introduction of chemistry into the university curriculum in the late seventeenth century, new practical needs arose for students  such as being able to perform experiments. Would it be possible to build a chemical furnace that provides a gentle heat, yields no smoke, and is safe for students to use? Herman Boerhaave (1668–1738) believed he found the perfect solution in, what came to be called, Boerhaave’s little furnace.

Portrait of Herman Boerhaave by Cornelis Troost, c. 1730.

Boerhaave was professor of medicine, botany and chemistry at Leiden University in the early 18th century.[1] Instead of starting with the most difficult experiments with metals and minerals, he was convinced that students were better off when they learned the techniques of through simpler processes, such as distilling leaves and flowers, and fermenting bodily fluids. But most chemical laboratories were equipped with elaborate devices too complicated for freshmen students, who in the eighteenth century could be as young as fourteen. Moreover, the brick-build furnaces were designed to create high temperatures, in which small and delicate materials like rosemary leaves would burn instantly.[2] Boerhaave hence needed a device that was low-cost, user-friendly, and would provide a gentle heat.

The plan for the oven, • H. Boerhaave, Elementa Chemiae, Quae Anniversario Labore Docuit in Publicis, Privatisque Scholis, (Leiden 1732).

A small wooden oven was the answer. Boerhaave claimed he had designed this type of furnace when he himself was studying chemistry in the 1690s. He opened the chapter on instruments in his chemistry textbook with the words: “I shall begin with my simplest furnace; which I invented forty years ago, when I practiced chemistry in no large study, where there was only one little chimney, and where I required several furnaces at once.”[3]

Woman at the Virginal and stove under her feet, by Jan Miense Molenaer, 1630-1640. Rijksmuseum, Amsterdam.

This kind of device was probably inspired by ordinary foot stoves. These little stoves, also known as foot warmers, were very popular in the Dutch Republic. Coming in a wide variety of shapes (square, octagonal, cylinder), these stoves often feature in books and paintings. Filled with glowing coals or peat, women placed the little stoves under their robes or blankets to keep warm.[4] Many foot stoves were equipped with a wire bail handle for lifting and easy transportation. Such stoves were used in carriages, sleighs, at home and in church to keep one’s feet warm. This ordinary foot warmer got new applications too, namely as tea and coffee stove,   and we suspect it was the model for the ‘simplest furnace’ in the Leiden chemical laboratory.

Woman carrying a little stove, Harmen ter Borch, 1648–1677. Rijksmuseum, Amsterdam.

The gentle heat produced by Boerhaave’s small oven proved very useful in performing all kinds of chemical experiments. Take rosemary, for example, the evergreen aromatic shrub. Distilled atop a “violent fire”, it would have been turned to flame, smoke, and ashes. But when rosemary instead was distilled at “summer-heat” (approx. 85º F), the mild operation would instead reveal the most volatile, fragrant and aromatic part of the plant ordinarily exhaled in summer. The same process could be applied to Angelica, basil, and all other aromatic plants.

Students in the Leiden laboratory, in Herman Boerhaave, Institutiones et experimenta chemiae (‘Paris’, 1724). Ghent University Library.

Boerhaave, in other words, attributed the success of his device to one’s control over gentle heat. Whenever the wooden oven was filled with hot pieces of coal or Dutch turf that was no longer smoking, it established a constant and moderate heat that could be kept up to 24 hours. As such, the instrument was perfect for students to perform all kinds of heating processes and distillations. In fact, he was so excited about this apparatus, that he claimed that “I believe eggs may be hatched by it”.[5]

Was Boerhaave’s little furnace really that user-friendly and effective as he claimed it was? We checked it out by recreating Boerhaave’s stove and performing experiments with it. Check out our next blog to entry to find out whether we succeeded!

Creating an oven from two old stoves… to be continued!


[1] More on Boerhaave, see Marieke Hendriksen, “Boerhaave’s Mineral Chemistry and Its Influence on Eighteenth-Century Pharmacy in the Netherlands and England”, Ambix(2018) and Ruben Verwaal, “The Nature of Blood: Debating Haematology and Blood Chemistry in the Eighteenth-Century Dutch Republic”, Early Science and Medicine(2017).

[2] Boerhaave, Elementa Chemiae (Leiden:  Isaac Severinus, 1732), vol 2, experiment 1.

[3] Ibid., vol 1.

[4] Le Francq van Berkhey,Natuurlyke historie van Holland (Amsterdam: Yntema and Tieboel, 1769–1778), vol. 3, 706-707, 1200.

[5] Boerhaave, Elementa Chemiae, vol 1.

Boiling hot oil: on the assessment of temperature in late medieval processing of linseed oil

By Indra Kneepkens

Indra Kneepkens is a technical art historian, specializing in the materials and techniques of late medieval panel painting. She is currently finalizing her dissertation, which is focused on the use of processed linseed oils and paint additives in the painting practice of the fifteenth and early sixteenth century.

Raw linseed – the basis for linseed oil

From art technological sources, such as recipes and manuals for the preparation of paints, and from the analysis of paint samples, we know that late medieval and early modern craftsmen heated oils for use as a binding medium in paints, as well as for the preparation of varnishes. As a technical art historian, I research and reconstruct these oils and varnishes, to be able to establish the effects they have on paints. My aim is to understand how the development and use of these materials influenced the painting process and the final appearance of art works. Unfortunately, 14th-  to 17th-century art technological sources that include heat-treatment of oil typically do not give absolute temperature indications. This is no surprise, because thermometers were developed in different stages between the late 16th and early 18th century. Therefore, craftsmen had to rely on their senses for the assessment of temperature.

Cennini’s handbook has been transcribed an translated repeatedly, most recently by Lara Broecke in 2015

That controlling temperature was important to late medieval craftsmen is illustrated by a remark in the Libro dell’arte (ca. 1390), that was written by the Italian painter Cennino Cennini. In his recipe for a heat-treated oil the writer almost personifies fire, warning us that it would willingly burn down the house if it got a chance to reach into the pan.[1] And indeed, it seems likely that the temperature of oil was sometimes raised pretty high, to the point where potentially explosive gasses form. So how did pre-thermometer craftsmen determine the temperature of their oils?

Even if not in absolute terms, early modern sources do contain clues about the kind of temperatures that were considered useful in oil processing. In most cases, oils were heated over fire, in ceramic, bronze or copper vessels. As wood or peat fires can easily reach temperatures over 500°C, and the vessels were able to withstand these, it was clearly possible to reach the auto-ignition point of linseed oil (ca. 343°C). Most likely, temperature would have varied from case to case, depending on factors like fuel type and the distance between the vessel and the heating source.[2]

Sometimes sources mention a strong reduction of the oil, or they describe the texture of the end result, comparing it to fluid honey for example. In varnish recipes a string-test reoccurs, in which a finger is dipped into, and then lifted from a cooled drop of the mixture of boiled oil and resins, to see if a thread is formed.[3]My own experiments have shown that to reach a sufficient thickening of an oil or varnish, and to dissolve most resins, the materials must be heated for quite some time and at a fairly high temperature. Finally, empirical tests that remind us of everyday cooking may have been used to keep an eye on the temperature of oils. In Arte de la Pintura (1649), Pacheco describes how bread, garlic and feathers were stuck in boiling oil; if they appeared browned or scorched, the cooking process was completed.[4]

During my experiments over the past few years, it has become evident that one can make a reasonably accurate temperature assessment based on how the oil behaves during heat-treatment. A swirling movement for example, was noticed in oils from a temperature of ca. 90°C, while repugnant fumes typically started to develop around 200°C.

Figure 1. Foam developing on oil heated in a ceramic vessel

I also realized that the development of foam, which is commonly mentioned in recipes, may have been a clear indication of temperature. But it is also closely related to the type of cooking vessel that was used. I did not see any foam until I exchanged my glass laboratory beakers for a glazed ceramic pot. Foam appeared when the oil reached a temperature of 100°C. It must have been caused by water evaporating from the clay body, as I had rinsed the vessel before use. Bubbles also formed when garlic and bread were boiled in oil, from ca. 61 and 80°C respectively, with a high point again around 100°C. Although the garlic started to darken earlier, both bread and garlic clearly browned around 180°C, and completely blackened between 230 and 240°C.

In another experiment thirteen feathers, of different size and from various birds, were dipped in hot linseed oil until they started to curl. Surprisingly, all feathers curled within a range of 27°C, between 237 and 264°C. These experiments not only support the relative accuracy of these empirical methods, but also indicates that temperatures of ca. 100 and 200-250°C were meaningful to early practitioners.

Figure 2. Raw linseed oil (far left) heated at 150 (1-4 hours, jars 2-5 from the left), 200 (8 hours, larger jar) and 300 degrees C (1-4 hours, jars 7-10 from the left).

In more systematic tests, linseed oil was heated to 150 and 300°C, for one, two, three, and four hours. Interestingly, the color and thickness of the oils that were heated at 150°C appeared more or less unaltered after the experiment. In tests where these oils were mixed into paints, they behaved very similar to raw linseed oil. At 300°C the oils thickened and darkened considerably. They affected paints in a very significant way, causing them to level, and making it possible to create smooth glossy films without any visible imprint of the brush. Paints with these oils were also less prone to yellowing. An oil that was heated at 200°C for eight hours however, still made paints level perfectly, but it also caused extreme yellowing.

So knowing how to assess temperature and balancing it over time must have been crucial skills for those who prepared heat-treated oils and varnishes. Experiments have shown that knowledgeable individuals would have been able to make a fairly accurate assessment of temperature, using their senses. They could note changes in the appearance and behavior of oil and indicator materials, and manually test its viscosity. Although there are several indications that temperatures of 200°C and higher were preferred, it makes sense that craftspeople would adapt the temperature to the materials at hand and the desired end result. The lack of unambiguous temperature indications in oil processing recipes reflects this adaptive use of temperature and a reliance on the senses that was expected of craftspeople before the invention of thermometers.

[1]Broecke, Lara. Cennino Cennini’s Il Libro dell’ Arte: A new English translation and commentary with Italian transcription.London: Archetype, 2015, 127, chapter 91.

[2]Aldeias, Vera, Harold L. Dibble, Dennis Sandgathe , Paul Goldberg, and Shannon J.P. McPherron . “ How heat alters underlying deposits and implications for archaeological fire features: A controlled experiment” Journal of Archaeological Science67 (2016): 66.

[3]Broecke 2015, 127. Neven, Sylvie. The Strasbourg Manuscript. A Medieval Tradition of Artists’ Recipe Collections (1400-1570). London: Archetype, 2016: 132-135, no. 92-94.