Tag Archives: Marjolijn Bol

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.

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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.

Topazes, Emeralds, and Crystal Rubies. The Faking and Making of Precious Stones

Marjolijn Bol

Today the making and illegal selling of factitious stones has reached an unseen level of sophistication. Advanced technologies allow man to produce synthetic versions of the most precious of stones – diamonds, emeralds, sapphires and rubies (fig. 1). So convincing are these synthetic gems they can only be distinguished from natural precious stones in laboratories with advanced spectroscopic devices.

Fig. 1 Synthetic gemstones
Fig. 1 Synthetic gemstones

The making of imitations of precious stones is not just typical of our modern age. In fact it dates back to at least Egyptian times, as graves from this period show that glass was used to substitute for jewels. Seneca (AD 1 – AD 65) and Pliny the Elder (AD 23- AD 79) were the first authors to write about the practice. Whereas Seneca only mentions that ‘sometimes stones are boiled to resemble emerald (smaragdus)’, Pliny provides us with a rather lengthy account in which he explains the various ways in which gems were imitated. He mentions that glass pastes were used for the imitation of seals and that in some cases stones were cemented together to imitate sardonyx. By a third method, ‘Indian crystals’ were colored with certain dyes to make them look like more expensive minerals. These ‘gems’ were apparently so convincing that ‘(…) there is considerable difficulty in distinguishing genuine stones from false; the more so, as there has been discovered a method of transforming genuine stones of one kind into false stones of another.’ While some physical evidence still survives of the first two practices, the factitious gems made by the third manner, has barely, if at all, come down to us. The many recipes that explain how to make such imitation stones (surviving from the fourth well into the sixteenth century) nevertheless suggest that this last method must have also been practiced, and perhaps even on a large scale. I therefore hoped that a physical reconstruction of the imitation gems might give some further insight into the appearance of these imitation gems. Would it be possible, as the recipes suggest, to make a convincing imitation of a precious stone? Could such a ‘fake’ potentially fool the innocent eye into thinking it was real?

The earliest examples of recipes for imitating stones can be found in the so-called Stockholm Papyrus, a recipe collection written in Greek at about 200-300 AD. The Papyrus includes no less than 71 how-to’s for the imitation of precious stones with lesser materials, including ruby, beryl, amethyst, sunstone and emerald. In subsequent centuries, numerous other recipe books include similar instructions for making counterfeit stones. While the recipes are various, they are almost all based upon two basic operations that I have attempted to reconstruct (fig. 2).

Fig. 2 Opening up of stones
Fig. 2 The opening up of stones

First, a transparent mineral, such as rock crystal, selenite, or topaz, is roughened or ‘opened up’. Following the recipes instructions, I investigated this by cooking the minerals in potash alum (potassium aluminium sulphate) dissolved in vinegar. As soon as I removed the heat from this mixture, it rapidly crystalized into a hard crust around the stones. In accordance with the recipes I left the stones overnight.  Nevertheless, when I inspected them the next day (after having chiseled them out) I could not see any changes. Regardless of whether the stones were successfully ‘opened up’, the coloring of the minerals did produce some surprising results.

The recipes instruct that the ‘opened up’ mineral should be colored using a mixture of a colorant ground with oil or resin to make it assume the appearance of a specific gemstone. The dyes and pigments used varied according to the type of gemstone that was to be imitated. A red dye made from alkanet root (used by ancient cloth dyers) was advised for the imitation of rubies and the green copper pigment verdigris was used to transform transparent minerals into emeralds. For my first reconstructions I choose to make a ‘fake’ emerald. It was one of the precious stones imitated the most frequently and the instructions for making it remained almost the same through (post-) classical and pre-modern times (fig. 3).

Fig. 3 Coloring of stones
Fig. 3 The coloring of stones

I first ground the pigment verdigris with linseed oil and used this substance to cover the base of a rock crystal, topaz and piece of selenite. This resulted in a beautiful, translucent green stone that, due to the instability of the pigment verdigris, in time assumed the saturated forest green color of a real emerald. Whereas more reconstructions are certainly required to investigate the nature of this method of gemstone imitation, these first experiments show that, when ancient sources insist how visually convincing the imitations of precious stones could be, they are probably not exaggerating.

Fig. 4 Emerald imitation
Fig. 4 Emerald imitation

Recommended reading:

Marjolijn Bol, ‘Coloring Topazes, Crystals and Moonstones: The making and meaning of factitious gems, 300-1500’, in Marco Beretta and Maria Conforti (eds.), F for Fakes: Hoaxes, Counterfeits and Deception in Early Modern Science, 2nd Watson Seminar in the History of Material and Visual Science, Museo Galileo, Florence, June 7, 2013, Science History Publications (Brill Publishers: 2014), pp. 108-129.

Earle R. Caley, “The Stockholm Papyrus: An English Translation with Brief Notes”, Journal of Chemical Education 4 (1927), pp. 979-1002.

Pliny, The Natural History, books 33-37 (book 36 includes Pliny’s natural history of stones): http://www.perseus.tufts.edu/hopper/text?doc=Plin.+Nat.+toc

Medieval Makeup ‘Artists’. Painting Wood and Skin

by Marjolijn Bol

What there is stays the same. That she can never change.                                                                        Jan van Boendale (ca. 1280 – ca. 1351)

For art historians, one of the most important and best-known ‘recipe books’ is  Cennino Cennini’s (c. 1370 – c. 1440) Il Libro dell’arte (The Craftman’s Handbook). The Italian treatise is famous because it offers the reader many detailed recipes that explain how to make a panel painting. These include the preparation of wood so that one can paint on it, how to make paint from egg and pigments, how to make brushes, how to paint draperies, faces and beards, and, finally, how to varnish the finished work with a mixture of oil and resin (the ‘tears’ from trees such as amber) so that it becomes it shiny and is protected from dust and dirt (fig. 1).

Fig. 1. Giovanni Boccaccio, Marcia painting her self-portrait from De Claris Mulieribus (Of famous women, written 1361), early fifteenth century illumination, Biblioteque Nationale, Paris, ms. 13420, f.101v
Fig. 1. Giovanni Boccaccio, Marcia painting her self-portrait from De Claris Mulieribus (Of famous women, written 1361), early fifteenth century illumination, Biblioteque Nationale, Paris, ms. 13420, f.101v

Little known, however, is the fact that Cennini also writes that painters were sometimes asked to be ‘makeup artists’:

In excercise of your profession, you will sometimes have to stain or paint on flesh, chiefly to paint the face of a man or woman.1

Cennini explains that there are three methods for painting faces. You can have the colors or pigments tempered with egg, or, when you want to make the face more brilliant, with linseed oil or varnish (the oil-resin mixture). Cennini writes furthermore that in order to clean the face, one takes egg yolks and, gradually rubbing them on the face with the hands, this removes the face paint. The fact that panel painters were asked to be make-up artists, begins to make more sense when we consider the fact that for painting on wood and the face many similar materials were used (figs. 2 and 3).

Fig. 2. Shared materials panel painter and face painter, left to right: Egg, linseed oil, resin tears, dried resin (mastic) and below: two pigments: white lead and vermillion.
Fig. 2. Shared materials panel painter and face painter, left to right: Egg, linseed oil, resin tears, dried resin (mastic) and below, two pigments: white lead and vermillion.
Fig. 3. Bernardo Daddi, detail Virgin Mary's face, 14th century. Egg-based paint on panel Gemäldegalerie, Berlin (inv. nr. 1064)
Fig. 3. Bernardo Daddi, detail Virgin Mary’s face, 14th century, egg-based paint on panel, Gemäldegalerie, Berlin, inv. nr. 1064.

There are in fact many more painter’s recipe books that include instructions for making face paint. The fifteenth century German Strasburg manuscript for instance, a collection of recipes for painting in books and on panel, also provides us with an instruction for making a face paint from the resin mastic.2

Another interesting  source that reflects the similarities between panel painting practice and face painting practice is the fourteenth century poem Jan Teesteye (ca. 1330-1334) written in Middle Dutch by Jan van Boendale (ca. 1280 – ca. 1351). In the section of his verse that deals with ‘women and their bad habits’ Van Boendale compares the crèmes and ointments used by women to cover up facial irregularities to the painter’s varnish:

They (the women) grease and anoint their faces, to appear beautiful, and admired by many;but as a painter varnishes an image with all its deceiving decoration, shining beautifully as if though it were solid gold, it is, on the inside, still wood; So a woman, has varnished her skin to make it look beautiful and shining, it is, however, still a futile thing; What there is stays the same. That she can never change.3

Thus, for Van Boendale, women who varnish their faces are like the painter who has varnished his work, embellished with fake jewels, glistening beautifully like gold but, ultimately, on the inside still made from wood (fig. 4). This way, Van Boendale uses the practice of make-believe in painting to compare it to another aspect of fourteenth century material culture; how women sometimes fool us with their counterfeit beauty created with face paint. What makes van Boendale’s comparison even more interesting is that he synonymously uses the Middle Dutch word ‘vernis’ for both the cosmetics used by ladies to smooth their faces and the final varnish layer applied by painters to make a painting look smooth and shiny (and protect it).

Cennino Cennini, like Van Boendale, also had some critique on the practice of painting faces. Contrary, however, to Van Boendale’s moral critique on the trickery involved in face painting, Cennini, as a painter, was well aware of the health risks involved in using some of the painter’s materials for painting the skin:

But I will tell you that if you wish to keep your complexion for a long time, you must make a practice of washing in water – spring or well or river: warning you that if you adopt any artificial preparations your countenance soon becomes withered, and your teeth black; and in the end ladies grow old before the course of time; they come out the most hideous old women imaginable.4 

Cennini was certainly right to warn against face paint. At least since antiquity many of the materials used on the face included toxic ingredients, such as the pigment lead white for applying a ‘foundation’, or vermillion for making one’s cheeks look red and blushing. Such pigments poisoned the person using it, and, eventually, could even lead to death. Here the old Dutch proverb – that one has to suffer for beauty – applies a little too well.


1. Daniel Varney Thompson (ed.), The Craftsman’s Handbook: the Italian “Il libro dell’ arte”. Cennino Cennini, New York, 1960, p. 123 and for the Italian see: Fabio Frezzato (ed.), Il libro dell’arte. Cennino Cennini, Vicenza, 2004, p. 203 [nr. CLXXIX].

2. Rosamund Borradaile and Viola Borradaile, The Strasburg manuscript: A medieval painter’s handbook/Das Strassburger Manuskript: Handbuch für Maler des Mittelalters, München, 1976, p. 83.

3. Gerrit Komrij, De Nederlandse poëzie van de twaalfde tot en met de zestiende eeuw in duizend en enige bladzijden, Amsterdam, 1994, p. 103, English translation by author

4. Thompson (ed.), The Craftsman’s Handbook, p. 123.