Clear as Crystal: Leonardo da Vinci’s Walnut Oil

By Marjolijn Bol

In order to create their colorful palette, fifteenth-century panel painters had to produce most paint supplies from scratch. Unable to walk into a shop of artist’s supplies as we can today, they obtained color from different kinds of earths, minerals, metals, flowers, roots and insectsSlide1A binding medium was required to transform all these pigments into paint. By the end of the fifteenth century, both North and South of the Alps, panel painters mostly used oil for this purpose. Painter’s oil is not just any type of oil, however; it needs to have drying properties. If fifteenth-century painters would have used olive oil for instance, their paintings would still not be dry today. Unlike olive oil, a drying oil, when exposed to oxygen, goes through a complex process of polymerization which causes it to harden over time. Research into historical recipes together with scientific examination of paint samples has shown that painters in the North preferred linseed oil, whereas in the South, particularly in Italy, walnut oil seems to have been more common.

Because oils were important in various other areas of life, including cookery and medicine, painters most likely bought theirs ready-made. Various contracts in which orders for painter’s materials are listed, show that this was indeed the case. Yet, historical recipes show that in order to make it better-suited to their own practice, painters manipulated oils in various ways. It was bleached and thickened in the sun, or cooked over a fire and mixed with various substances to help it dry faster.

Copyright Marjolijn Bol

While most recipes deal with the preparation of oils for painting, in one extraordinary account, a painter was actually concerned with the impact of the method of oil extraction itself. No one other than Leonardo da Vinci wrote that if walnut oil is not extracted in the right manner, it could potentially ruin the painting. He recorded his ideas on what he considered the right method of walnut oil extraction in the notebook presently known as the Codex Atlanticus

Copyright Marjolijn Bol

In the common procedure of oil extraction, the nuts would have been boiled and then thrown into a press to extract the oil. According to Leonardo this is detrimental for a good painter’s oil because “nuts are cov­ered with a sort of husk or skin, which if you do not remove when you make the oil, the coloring matter of the husk or skin will rise to the surface of your painting and cause it to change.” For this reason, Leonardo discusses an alternative method for oil extraction that does not involve a press. His method appeared so straightforward that I decided to attempt to reconstruct it. Leonardo explains that first the husks of the walnut need to be removed by soaking them in water until it becomes soft enough. This is not an easy task and it took quite a long time to remove the thin rinds from the nuts. Leonardo then writes that with the husks removed, the nuts have to be soaked seven or eight times in water until it no longer becomes turbid. Indeed, after seven soakings the water became very clear, and this remained so. The water in which I put the husks on the other hand, turned completely brown.

Copyright Marjolijn Bol

Leonardo explains that if you put the nuts in a shallow open the vessel they will eventually “dissolve” and become “almost like milk”. At this point the oil will rise to the surface. Around thirty days after I started my experiment, I was excited to see that this indeed happened: a tiny layer of oil had surfaced. I found it equally interesting that the glass with the walnut husks (kept under the same conditions) had developed a layer of fungus, while the glass with the dissolving nuts remained perfectly clear. In Leonardo’s own words, the walnut oil extracted from the milky liquid would be “clear as crystal”.

When I started these reconstructions, I did not expect to be able to follow Leonardo da Vinci’s instructions for the preparation of walnut oil so perfectly. Yet, a few interesting questions still remain: Is Leonardo’s oil actually clearer and better than regular pressed walnut oil? If not removed, would the skins of walnuts indeed rise to the surface and could this possibly be observed in fifteenth century paintings? And, finally, can we find out if Leonardo himself used his own “pure” walnut oil to paint with? The process of producing Leonardo’s walnut oil appeared rather painstaking and time consuming to my modern point of view. Yet, in the fifteenth century, painters generally went out of their way to find and produce the best quality and most durable materials. If the walnut oil thus made was really believed to be better, Leonardo may have considered the effort to make it a worthwhile investment.

Cite this blog post
marjolijnbol (2015, June 2). Clear as Crystal: Leonardo da Vinci’s Walnut Oil. The Recipes Project. Retrieved June 19, 2024, from

14 thoughts on “Clear as Crystal: Leonardo da Vinci’s Walnut Oil”

  1. Leonardo, for as famous as his paintings are, I wouldn’t feel is one of the better people to copy paint mediums from — the bad preservation of his works is notorious.

  2. I found this article very interesting especially as I live near Amboise, France where da Vinci spent the last years of his life. The area is noted for its walnuts and the fabrication of walnut oil.

  3. Typing as a chemist and alchemist, I like to see such posts. I also think you might be able to find chemical differences between the two oils prepared in different ways, if you use proper modern analytical equipment. The trick there is finding a helpful chemistry department with the right equipment and someone with time to carry it out.

    (I’m afraid I am more of an inorganic than an organic chemist and am not at university any more. And don’t have room to keep a 2nd hand GC or HPLC even although it would be nice. Although I would rather get an XRF first)

  4. Hello,

    Thanks for posting this information…very generous. Just a question…have you done any tests to make sure there is not microscopic organic matter (like mucilage) in the oil? I believe if you leave a bit of oil sitting on distilled water (say 2cm of oil on 2 inches of water) if there is any organic matter it will settle between the two after a couple of weeks. A more aggressive test would be to shake the two first and then wait for a longer period.


    Pablo Tapia

  5. I know this is an oil post but do you have a process for this extraction? Step by step. I’d like to give this a try.

  6. I second Timothy Rone’s request for step by step instructions. I tried finding it by searching for the Codex Atlanticus and have not had any luck this far.

    Also, did the oil ever go rancid like normal Walnut oil does? Did it last longer than traditional oil having removed the husks and other organic matter?

  7. Leonardo also used walnut panel to paint on .. curious how the panels were prepared ..

  8. I am at this moment trying to prepare walnut oil according to Leonardo’s recipe. I have only found this fragment of the recipe so far:-
    Since walnuts are enveloped in a thin rind, which partakes of the nature of …, if you do not remove it when you make the oil from them, this skin tinges the oil, and when you work with it this skin separates from the oil and rises to the surface of the painting, and this is what makes it change.

    [The following entry is incomplete but it starts with a method of removing the clear oil from the surface of the mucilage milk using a piece of cloth as a siphon]


    … outside the bowl 2 fingers lower than the level of the oil, and pass it into the neck of a bottle and let it stand and thus all the oil will separate from this milky liquid; it will enter the bottle and be as clear as crystal; and grind your colours with this, and every coarse or viscid part will remain in the liquid. You must know that all the oils that have been created in seads or fruits are quite clear by nature, and the yellow colour you see in them only comes of your not knowing how to draw it out. Fire or heat by its nature has the power to make them acquire colour. See for example the exudation or gums of trees which partake of the nature of rosin; in a short time they harden because there is more heat in them than in oil; and after some time they acquire a certain yellow hue tending to black. But oil, not having so much heat does not do so; although it hardens to some extent into sediment it becomes finer. The change in oil which occurs in painting proceeds from a certain fungus of the nature of a husk which exists in the skin which covers the nut, and this being crushed along with the nuts and being of a nature much resembling oil mixes with it; it is of so subtle a nature that it combines with all colours and then comes to the surface, and this it is which makes them change. And if you want the oil to be good and not to thicken, put into it a little camphor melted over a slow fire and mix it well with the oil and it will never harden. ”

    I am sure there in a more complete recipe which I have read amongst Leonardo’s writings which explains (the difficult bit) how to get the skins off the walnut meats cleanly. I think it involved soaking the meats in a mixture of urine and something. I soaked them in warm water for 3 days yet it still took me 15 minutes to peel one walnut (I have about a thousand to do). Does anyone know this other source?
    I have clarified and de-mucilaged linseed, sunflower, safflower, sacha inchi, grapeseed, walnut and hempseed cold pressed organic oils from healthfood shops to use as oil for painting, mostly using information I found on this website but I think the walnut oil I purchased is always pressed with the skins on. Although it remains sweet after the salt & sand method of refining to remove mucilage, the losses are greater than any of the other oils (40% yield).
    Chez Watts

  9. Hi
    I am new to oil painting and love this information. What is the preferred oil (other than linseed) to use as a thinner now? Did da Vanci use Walnut in the Mona Lisa (does anyone know)?

  10. The complete copy of Leonardo’s Walnut oil recipe is printed on page 140 in Mary Merryfield’s Translation of Cennino Cennini’ book Treatise on Painting. This book is available free on line. The recipe was found in Leonardo’s handwriting and was published in the Milan edition of his Treatise on Painting, by C.Amoretti, 1804.

  11. To Diana B:-
    In the early years of oil painting most Northern Artists used Linseed oil and most Southern artists used Walnut oil. Hempseed oil, Sunflower oil (by Russians), grapeseed and, later, poppy oil were less commonly used in Europe. The speed of drying was (fast to slow) linseed, hempseed, walnut, grapeseed, poppy, sunflower. The faster drying oils yellowed more over time. The oil is the medium for the pigment, whereas the thinner is a solvent that quickly evaporates.

  12. Loads of confusion there. In order to produce walnut oil, nobody but really nobody would think of keeping the husks which fall off of their own accord when you leave the nuts to dry. What you do need to absolutely remove is the shell and the skin that separate the quarters inside the nut.
    since France has been mentioned, a very common wood dye was produced until the last quarter of the 20th century out of the husks by simmering them for several hours. This is called brou de noix and is now replaced by vandyk crystals. Good quality walnut oil can be purchased in France. The quality needed for painting is vierge. Not extra vierge as this precludes heating the nuts whereas for painting, the preferred oil is obtained by pressing slightly toasted nuts( torefiees in French) . The absence of the skin is described as emmondee. The amount of torefaction of the nuts varies from mill to mill and mean that some are too dark to be used for painting. The oil is hardly ever used as is for artistic painting. It is usualy cooked and sunbleached. Some very comprehensive painting treatises describe the process. The catch is that you have to read them Integraly in order to make sens of what it says. This saves one from saying things out of order like for instance, saying that before the 19th century, oils were the principal medium is plain inaccurate , the oils being usually laid on top of another leaner and faster drying medium. As for Leonardo Da Vinci, he did not always have the best ideas and is certainly not viewed as a good source of recipe for durable paints as every time he tried to be clever in that respect, the paintings deteriorated nearly immediately. If your goal is to paint with high quality oil.,you will surely save time by letting the miller do what they took centuries to perfect. After all, walnut oil is only used in cooking since the end of the 19th century. Before that, it was kept for lighting and painting. Lastly, most painting oils are somewhat modified by heat or the adjunction of divers elements, and so, the speed of drying varies not so much from oil to oil but more from preparation to preparation. There again, there is no avoiding the learning from Reliable sources. The only clear distinction between walnut and the other oils mentioned, is that walnut oil dries “en masse” as opposed to most others that dry from the outside in.


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