Boiling Milk: Experimenting with Boerhaave’s Little Furnace, Part III

By Ruben Verwaal and Marieke Hendriksen

Fig. 1. Ruben keeps an eye on the temperature.
Fig. 1. Ruben keeps an eye on the temperature.

It has been exactly 350 years since Herman Boerhaave’s birthday. What better way to honour the renowned professor than to redo some of his old experiments? 

On Monday 31st of December, in the year 1668, Herman was born. And already as a kid, he and his brother James probed the curiosities of nature: plants, minerals, liquids and bodily fluids. As Herman recalled some 30 years later, “how many whole days and nights we have spent successively together in the chemical examination of natural bodies” [1]. It must have been around this time that Herman invented his little furnace.

“I’ll put an alarm to take the milk out of the freezer,” Marieke texted Ruben the week before New Years’. Between all the Christmas dinners, the 31st was the only day still free to meet up over the holiday. Weeks before we had bought Irish turf online and collected raw milk from a farm near Delft, as well as from a breastfeeding friend . Having finally found the time, we gathered their materials together and started experimenting.

Why Milk?

As a physician, Boerhaave was fascinated with the human body. How does it work? What is it made of? Boerhaave soon realised that a newborn solely grows on breastmilk. Mothers eat their food and digest it with juices from their intestines; after circulating in their bodies, the fluid concocts into chyle and develops into the maternal sustenance in their breasts. Not only human babies, Boerhaave reasoned, but all mammals are nourished by milk and can grow solely on it. “Milk, therefore, appeared to be the first thing to be examined.” [2]

Making Curd

Fig. 2. 4PM: Raw milk heated with vinegar gives you cheese - well, sort of.
Fig. 2. 4PM: Raw milk heated with vinegar gives you cheese – well, sort of.

We set out to replicate the first experiment, titled “fresh cow’s milk coagulates with acids, even in a boiling heat.” We lit the turf in the fireplace. Once it was hot, glowing, and smelling, Marieke put some in an earthenware bowl and placed it in our wooden furnace to let it heat up. Meanwhile Ruben added vinegar to fresh milk in a glass vessel. As the fluid was gradually heating up in our furnace, parts of the mixture were slowly coagulating into curd.

We were basically imitating the cheese-making proces – a more than common practice in the early modern Dutch Republic. Boerhaave, however, assigned physiological significance to this process. For the cheese could be hardened and burned, smelling like bone – proving that even the hardest parts of a baby’s body could have its origin in milk. “This is a strange change of so fluid a matter as milk, but is, perhaps, the origin of all the solids in the body.” [3]

Red Milk

The second experiment was to show how “recent cow’s milk coagulates, turns yellow, and red, by boiling over the fire with fixed alcali.” We basically repeated the previous steps, but instead of using vinegar we added ammonia. Slowly but surely, the white fluid indeed turned yellow, then a dark orange – and was about to turn red. Here we had to stop, unfortunately, because the turf was cooling down, and it was getting dark outside.

Yet via this relatively simple process, Boerhaave confirmed a common illness: milk fever. The milk from mothers suffering from fever “becomes yellow, saline, thin and sanious.” [4] It also clarified why Dutch cows gave yellow milk during the 1714 outbreak of cow’s fever.

Fig. 3. 6PM: Raw breast milk heated with ammonia: 'bloody' milk?
Fig. 3. 6PM: Raw breast milk heated with ammonia: ‘bloody’ milk?

So What Have we Learned?

First, turf smells! We can only surmise that our early modern colleagues were simply oblivious to the smell due to its omnipresence. Second, our apparatus passed the test. Boerhaave’s little furnace successfully kept the heat inside at an evenly distributed yet high temperature (around 60℃). This is an important feat, especially when working with milk. Anyone who has ever boiled milk knows how easily it becomes a big mess when you don’t pay attention for just two seconds. Yet we were able to have 15-minute glühwein and oliebollen breaks without any problem. 

Third, our experiments have shown us how relatively easily some of Boerhaave’s experiments can be replicated – as opposed to some of his contemporaries who made secret potions or applied intricate and dangerous procedures with metals and minerals. Historical reproduction, reconstruction, and re-enactment are methodologically complex and potentially problematic because of the impossibility of repeating history and reliving the experiences of historical actors. Yet our experiments do enhance our understanding of the past; they make our historical understanding more holistic, less linear and text-based. [5] For example, these experiments help us to understand why Boerhaave was such a popular teacher; with the help of a small oven based on his design, students could learn by doing. 

Fourth, with more time and patience we could have gained better results. This is the case with everything, of course. Yet some of Boerhaave’s experiments with milk – for example the milk turning sour by digestion (i.e. at 37℃) – is described as taking twelve days! Lastly, replicating early modern experiments is fun. We won’t deny that working on your object of study outside the library is refreshing. The photos and videos of the process have a public appeal too. We hope you enjoyed it.



[1] ‘Dedication’ in Herman Boerhaave, Elements of Chemistry (London, 1735), A3r.

[2] Herman Boerhaave, A New Method of Chemistry (London, 1741), 2, 185.

[3] Ibid., 187–188.

[4] Ibid., 188–189.

[5] Pamela H. Smith and Tonny Beentjes, “Nature and Art: Making and Knowing: Reconstructing Sixteenth-Century Life-Casting Technniques,” Renaissance Quarterly 63 (2010): 128–79. Marieke M.A. Hendriksen, Elegant Anatomy. The Eighteenth-Century Leiden Anatomical Collections (Leiden & Boston: Brill, 2015), Chapter 1. Donna Bilak et al., “The Making and Knowing Project: Reflections, Methods, and New Directions,” West 86th 23, no. 1 (2016): 35–55. Hjalmar Fors, Lawrence M. Principe, and H. Otto Sibum, “From the Library to the Laboratory and Back Again : Experiment as a Tool for the History of Science,” Ambix 63, no. 2 (2016): 85–97.

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! 

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.

Boerhaave’s contemporary fame: a letter from China to recipe books

By Marieke Hendriksen

Herman Boerhaave, J. Chapman, 1798. Source credit: via Wikimedia Commons.
Herman Boerhaave, J. Chapman, 1798. Source credit: via Wikimedia Commons.

My current research project focuses on how Herman Boerhaave’s (1668-1738) medical and chemical ideas, particularly those on metals, influenced the theories and practices of his students and other followers. The longer I work on this topic, the more I notice Boerhaave’s general influence on his contemporaries and the next generations. Already during his lifetime, Boerhaave was famous far beyond the borders of the Netherlands, even though he hardly left Leiden and the furthest journey he made during his life was to Harderwijk, a Dutch town about 100km from Leiden, where he gained his doctorate. A popular (alhough never documented) story told that a letter sent from China, addressed simply to ‘the illustrious prof. Boerhaave, physician in Europe,’ reached him without delay.

Then there was Boerhaave’s stove, a small wooden box-like incubator Boerhaave described in one of his text books and which students, apothecaries and amateurs constructed at least till the early nineteenth century to conduct chemical experiments, prepare ingredients for drugs, and even hatch eggs. Another trace of Boerhaave’s influence on Dutch eighteenth-century culture was the continuing description of dark candy sugar as ‘Boerhaave’s sugar,’ because he prescribed it as an ingredient in cough syrups. Yet most clearly of all can his influence be seen in both professional and private eighteenth-century manuscript recipe books.

Brown candy sugar, also known as 'Boerhaave's sugar' in the eighteenth century
Brown candy sugar, also known as ‘Boerhaave’s sugar’ in the eighteenth century. Image credit: © Alice Wiegand / CC-BY-SA-3.0 via Wikimedia Commons.

Although it may seem obvious that Boerhaave’s medicine influenced that of other medical men, it is interesting to see how diverse his influence was. Lately I compared a number of eighteenth-century Dutch manuscript recipe books and was pleasantly surprised by the different ways in which Boerhaave’s medicine influenced both medical men and others. In an anonymous recipe book that was probably compiled by a medical man of some sort, four recipes are attributed to Boerhaave, and three to his direct successor at Leiden University, Jerome Gaub (1705-1780). That this book was most likely used by a medical professional can be told from the fact that most of the recipes were written down in Latin, and measurements were given in apothecary shorthand, i.e. weights were indicated in drams.[1] Moreover, recipes for analgesics and purges to be used in persistent illnesses such as venereal disease and epilepsy, often accompanied by a note that they are only to be used if all else fails, are dominant, and ingredients like mercury, antimony, and sulphur are frequently listed.

This forms a stark contrast with household recipe books from the same period, like the anonymous ‘Medicamentboek’ that contains recipes attributed to ‘Bourhavi’ [sic] against fever and coughing. The most remarkable difference with the first recipe book is that almost all recipes are in Dutch, and contain primarily readily available ingredients, such as beer, bread, wine, honey, candy sugar, herbs, spices, rhubarb, tongue of veal, red cabbage, liquorice, and vinegar. Moreover, instead of cures for persistent and grave illnesses such as advanced venereal disease, this recipe book lists cures for more common ailments such as dandruff, irritated gums, coughing and winter hands.  Although some recipes in this book have been written in apothecary shorthand, in Latin, or contain more exotic ingredients such as red coral and boiled puppies, these entries are all in a different hand than the bulk of the recipes, suggesting the recipe collector occasionally asked a medical professional to add a recipe to his or her household medical recipes book.[2]

Rather than reading all these attributions to Boerhaave as direct evidence of medical networks, which is problematic, as they do not prove that the compiler had any affiliation with the names source, they can be read as proof of the diverse yet widely dispersed influence of Boerhaave’s medicine in eighteenth-century Dutch society.[3] For medical men, he was a professional example whose recipes, especially the more complicated ones that contained potentially dangerous ingredients such as metals, were collected as a resource for extreme cases. For laymen, Boerhaave’s name and simpler recipes, based on more readily available ingredients and aimed at more common ailments, carried equal authority.

[1] Anonymous. “Receptenboekje”, ca. 1750. Museum Boerhaave Library, Leiden: BOERH a 313.

[2] “Medicament boek : met een recept van Boerhaave tegen koorts”, 17XX. Museum Boerhaave Library, Leiden: BOERH a 308.

[3] For more on interpreting early modern recipe books, see Michelle DiMeo and Sara Pennell, eds. Reading and Writing Recipe Books 1550-1800. Manchester and New York: Manchester University Press, 2013.