Revisiting Marieke Hendriksen’s Indigo or no indigo?

Today we revisit a post written in pre-Covid-19 times, when borders were open, planes were flying and we used to travel the world. In this post from 2018, Marieke Hendriksen recounts how her holiday in Laos offered opportunities to learn more about indigo and the local dyeing processes. Elaine Leong

Marieke Hendriksen

Fermenting indigo at Ock Pop Tock, Laos. January 2018.

When you say indigo, the first thing many people will think of is blue – jeans blue. (Or if you’re me, you’ll think first of a seventeenth-century recipe to make decorative blue prunes from wax with indigo. Occupational deformation.) But historically, indigo has been used in many more ways, and to make more dye colours than just blue, as I recently discovered. Today, most jeans are died using a synthetic blue dye, but indigo dyes, made from some of the over 750 species of the genus Indigofera as well as from some other plants, have been used to dye textiles for at least 6,000 years, while other subspecies of Indigofera were traditionally used as analgesics with anti-inflammatory properties.

The term ‘indigo’ according to the OED started to occur from the sixteenth century onwards in various European languages to denote blue dyes from India (or east Asia more generally), but can now also refer more generally to dyes, violet-blue light, or blue hues. It might be argued that the term only really applies to dyes created from Indigofera subspecies, while it could also be said that indigo is any dye created from plants through the decomposition of the glucoside indican, which exists not merely in the indigo-plant, but in woad and various other plants too.

Wash the freshly picked leaves…

While on holiday in Luang Prabang, Laos, I took a weaving and dying workshop with Ock Pop Tock, an organization that was established to preserve the traditional Laotian craft of making hand-loomed textiles. There, I discovered that there is more indigo besides Indigofera, and that one indigo plant can give many more dyes than just blue. They also have a wonderful informative website on natural dyes. At Ock Pop Tock, the plant species used to create indigo dyes is Persicaria tinctoria, or long leaf Japanese indigo, a plant indigenous not to Japan but to China, Vietnam, and Laos. Depending on how the leaves are treated, it can be used to create blue, green, black, and mauve.

…give them a good pounding to create a dye.

Using the fresh leaves creates a green dye, fermenting them for at least five days and adding limestone as a mordant gives a blue dye. Traditionally, the Lao believed that the dye was female, and that it fermented because it attracted a male spirit. To coax the spirit, the pots containing the dye would be dressed in a skirt, and a knife placed on top of the lid to ward off evil spirits that could ruin the dye. The fermentation is actually a naturally occurring oxidation process, with atmospheric oxygen as the oxidant. Regular stirring ensures the process continues. The longer the indigo mixture is left to ferment, the darker it turns. If this mixture is boiled, it turns black. Alternatively, a rare indigenous plant, mak bow or bow vine, can be added to the blue dye to create mauve.

The end result: a beautiful scarf

As part of the half-day workshop, I got to dye a silk scarf with a dye of my choice. I love green hues and wanted to make a dye from start to finish, so I chose to dye my scarf ‘indigo’ green. This was, apart from some pretty intense pounding of leaves, surprisingly easy. I got to pick fresh Persicaria tinctoria leaves in the beautiful garden, washed them, and mashed them vigorously in a mortar for about five minutes. Then I transferred the mashed leaves into a tub, added some cold water and then the raw silk scarf. After kneading the dye into the fabric for a couple of minutes, I could rinse my scarf and hang it to dry. The end result is a beautiful soft green scarf, that is not just a souvenir, but a tangible reminder of the traditional Laotian knowledge about natural dyes preserved and shared at Ock Pop Tock.

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.

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.

HEAT! A Recipes Project Thematic Series

Astronomy- the Earth and the sun during summer in the Northern hemisphere. Wellcome Collection.

As humans, we want to control heat. We want to create heat, temper or even extinguish it, depending on context and purpose. We have a very limited temperature range at which we are comfortable (some microbes and bacteria can survive temperatures as low as -20C and as high as 130C), so we spend an incredible amount of time managing heat, whether it is our body temperature or that of our homes, offices, laboratories, cars, or food.

When we started planning this series of blog posts in early May, we could not have suspected how appropriate the theme would feel by now. While much of northwestern Europe has not seen rain in weeks and is sighing under a heatwave with temperatures rising to 37C/98F, we have fled to airconditioned spaces to edit the posts. Climate-wise, heat is seasonal in these regions, but it has always played an important role in recipes year-round, all around the world, from antiquity to the present day, in fields as diverse as alchemy, chemistry, art, cooking, medicine, and personal grooming.

The common denominators of heat in all these realms are that it is either internal – emanating from the human body – or external, naturally occurring from the sun, thunder, or lava, or man-made through friction or fire. In the posts in this series, we will see a wide variety of attempts to control these different kinds of heat through recipes and instructions.

Managing natural heat: Venice turpentine is a thick paste (aka sticky mess) at room temperature, but becomes fluid around 25C, and nicely mixes into the rest of the varnish ingredients when left out in the sun. Photo: Marieke Hendriksen

Some of our authors recreate such attempts by reconstructing experiments outlined in historical art technical and chemical sources. Indra Kneepkens recounts how she discovered through reconstruction research that sometimes for a recipe to make sense, it should not only be followed to the letter, but also be read between the lines. Ruben Verwaal and Marieke Hendriksen use their experiments with two reconstructions of small chemical ovens to reflect on the role of experimental heat in the development of theories on the nature of life in the eighteenth century. Working in a similar vein but with culinary recipes, Marissa Nicosia, from Cooking the Archive, examines the problem of heat control in her recipe recreation adventures, outlining the challenges of translating cooking instructions meant for the early modern hearth to the modern gas stove.

The four elements, four qualities, four humours, four seasons, and four ages of man. Airbrush by Lois Hague, 1991. Wellcome Collection.

Other authors in the series took our invitation as an opportunity to investigate heat in medical theories across time and place. Aileen Das kicks off this series by arguing that heat occupied a central place in ancient Greek, Roman and medieval Islamicate theories about the human body and its care. Taking on the smelly problem of perspiration and body odor, Cari Casteel reminds us that this issue is as old as mankind and offers several remedies from Roman authors.  Catherine Rider, on the other hand, examines notions of heat in fertility remedies in medieval England, noting that, whilst popular, heat-based treatments (to either increase or reduce heat in the body) were not the only kind of fertility aid available to English couples. The series concludes with two posts focused on fever and disease. Fittingly in this current heatwave, Laurence Totelin offers a post on fever and dog days of summer in antiquity. Finally, writing on in Late Imperial China, Marta Hanson introduces us to ideas of fever in Chinese medicine.

We’ve had a great time editing this series – hope you enjoy reading the posts wherever you are. Happy summer and stay cool!

Marieke Hendriksen and Elaine Leong

P.s. We were so inspired by our contributors’ posts that we’ve decided to dedicate the December issue to (you guessed it) COLD!. If you’re interested in joining the conversation, send a short pitch to