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 insectsA 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.
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.
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.
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.