All posts by marjolijnbol

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.

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):

Illuminating the ‘elusive’: reconstructing mediaeval recipes for anthocyanin pigments

By Sylvie Neven

Due to their trade value, a huge range of artistic materials is documented and recorded in historical written sources, such as guild regulations, contracts between artists and patrons or pharmacy price lists. However, certain substances are not recorded in these types of archives because they did not have market value. This is notably the case for plant-based materials that artists could easily find in nature and use for their colouring properties, such as the anthocyanin colourants. As these colourants are highly unstable in ordinary daylight, it was generally thought that they were not used in the dyeing of quality materials or to make paint but rather employed in a domestic context, as dyes for everyday clothes. To complicate the issue, anthocyanin colourants are difficult to identify through currently available analytical methods. For all these reasons, these substances have long remained ‘elusive’.

Nevertheless, research on mediaeval artists’ recipe collections has highlighted that a number of these books not only describe these organic colourants, but also indicate their use in illumination[2]. Based on a corpus of more than 400 mediaeval artists’ recipe books, this project intends to analyse the significance of these organic substances and to (re)define their use in an artistic context. In order to find citations of anthocyanin colourants throughout a vast amount of recipes, a specific database of artistic recipes was used (‘Colour ConText Database’). This database enables easy identification of recipes dedicated to anthocyanin colourants and allows researchers to deduce the availability of these colourants in a chronologically and geographically defined area. The database also makes it possible to establish and compare the different ways in which these colourants were prepared.

Thanks to this tool, I have found that the use of anthocyanin colourants is particularly frequent in the artists’ recipe books of the so-called ‘Strasbourg tradition’. Within this group, the recipes describe the use of corn poppies (Papaver rhoeas L.), blue cornflowers (Centaurea cyanus L.) bilberries (Vaccinium myrtillus L.), and elderberries (Sambucus sp.) for producing colours.

sylvie flowers 1
Papaver Rhoeas L. & Centaurea Cyanus L.
sylvie flowers 2
Vaccinium myrtillus L.
sylvie flowers 3
Sambucus sp.

From these sources, it becomes clear that anthocyanin colourants were used for modelling and shading in the realization of flesh tints, draperies and landscapes. Furthermore, they also appear to have been applied as glazes on layers of gold or silver and served for making the inks that were used to underline rubrics or elements of decoration in illuminated manuscripts.

The basic operation is relatively simple and similar from one recipe to another. The petals or the berries are ground and crushed in a mortar in order to produce a foam. The juice is then extracted by filtering the foam through a clean (linen) cloth.sylvie pestle

sylvie foam

sylvie juice

The recipes mention several sorts of additional ingredients. For example, alum (potash alum or aluminium sulphate) is mentioned frequently and could have various functions. Its addition can modify the pH and provide aluminium cations that have an impact on the final hue and improve the stability of the colorant. 

In order to be used in painting and illuminating, anthocyanin colorants were preserved by setting them on a small piece of cloth, mentioned in the recipes as tüchlein or pezette, usually known in English as a ‘clothlet’. These pieces of cloth were first steeped in the prepared juice until they were saturated by the dye solution. After drying, the pieces of cloth were then wrapped in clean paper and kept between the pages of a book or in a wooden box, and stored in an area free of humidity. To use a stored colorant, the artist would cut off a small piece of the dyed cloth, place it in a shell or horn and pour some gum water or other sort of binding medium, such as egg white, onto it.

With the help of Dr Sanyova (KIK-IRPA), I have reconstructed a number of recipes of the so-called ‘Strasbourg tradition’ in the laboratory. Our purpose was to establish the different ways in which these colourants were prepared and explore how they were used in illumination techniques. We focused on comparing different procedures to make colourants from anthocyanins. The possibility of collating different manuscripts of the same textual tradition allowed us to study the evolution of recipes for the preparation of anthocyanin colourants both through time and through different copies. The numerous recipes recommended the use of different plant species and ingredients in varying quantities.

Dummy samples before and after artificial ageing process
Dummy samples before and after artificial ageing process

Over the period of several weeks, we prepared series of dummy samples of red and violet tüchlein from the juice of petals of Papaver rhoeas L. After that, the samples were evaluated by visible spectrophotometry and high performance liquid chromatography (HPLC), before and after the artificial ageing process. We found that the different recipes created a number of saturated hues providing artists with various colours.  However, interestingly, the products of the different recipes all responded similarly to the artificial ageing process.

[1] Anthocyanins are colourants that provide most of the blue, red and violet colours in numerous plants and fruits.

[2] Notably : Neven, S., ‘The Strasbourg Family Texts: Originality and Survival. A Survey of Illuminating techniques in Medieval South Germany’, in Revista de Historia da Arte, n° especial, 2011, pp. 65-77; Neven, S., ‘Describing the “elusive”: a new perception of the practices and the resources of illuminators in the North of Europe from the fourteenth to the sixteenth century, in Renaissance Workshop, London, pp. 188-190.

Art Technological Source Research

By Ad Stijnman[1]

On March 20th, 2002, a group of ten professional researchers with backgrounds in the fields of art history, conservation science and bibliography for historical art technology met for a one day meeting at the Netherlands Institute for Cultural Heritage in Amsterdam. The meeting was convened by Christoph Krekel and Ad Stijnman and the participants discussed what they would term ‘art technological source research’.[2] The term ‘sources’ then covered such various materials as technical treatises and books with recipes, imposts and artist’s inventories, studio interiors and depictions of tools, construction drawings of machines and plans of workshops, while the art objects themselves were seen as primary sources.

16th century recipe book
Fig. 1: Kunstbüchlin, Frankfurt aM: Egenolff, 1535, Bibliotheek Rijkmuseum, Amsterdam.

As a result, they founded the Art Technological Source Research (ATSR) working group, because the one thing the participants had in common was a close interest in and study of coeval documentation of the technology of any type of art or craft discipline through all times. It was then considered that – although most participants had a certain interest in the conservation of art objects – research of art technological sources could be linked to many other disciplines, for example: art history, the histories of economy, science and technique, and to the modern practices of artists and craftsmen.[3] The working group’s main objective became ‘to be a platform for the exchange and dissemination of information on the subject of sources of art materials and techniques’.

The definition of what these ‘sources’ could be – and consequently what ‘art technological source research’ should comprise – was defined rather in a general manner at the time, as it also included examination of objects and contextual art historical study. This approach caused some confusion about what ATSR stood for and often enough – although many abstracts were promising – resulted in presentations at ATSR meetings that showcased the results of successful art conservation projects that would have been better suited to conferences in the fields of Conservation, Conservation Science or Technical Art History. The actual research of ‘sources’ was then usually limited to a short reference to a historic text, sometimes an image, but often without critical apparatus (no consideration of the source’s deeper meaning and context, or evaluation of the validity of the published transcriptions or translations).

The sixth ATSR symposium Sources on Art Technology: Back to Basics (Amsterdam 2014) therefore focused on the working group’s core business: the art technological sources themselves and their research, less so what could be done with the research results, as was a common theme in preceding symposia. These, on the other hand, had demonstrated that the profound disclosure of sources relates coeval information on art technology to the style and the material condition of art objects, which is important for all further study of such objects, in whatever field.

Instead of an all-encompassing definition of the working group’s activities, it may be better to describe ‘art technological source research’ as: the close study of contemporaneous textual and visual documents which communicate information on art technology of the period under study. Understanding these documents requires variously: language skills, knowledge of palaeography and codicology, an art technological training, some background in science, an interest in art history, an eye for political, religious and social aspects, an understanding of historical weights and measures, etc. In some instances, understanding of sources is greatly enhanced by carrying out reconstructions, ranging from preparing basic recipes to re-enactments and historically accurate reconstructions.

The need for Art Technological Source Research can be illustrated by the progress that has been made in the last decades in the understanding of technical terminology in historical texts. Due to the interdisciplinary perspective and focus that researchers have brought to this field, a growing amount of source material that had long been neglected can now be appreciated and understood, as its specialist vocabulary whose meaning had been lost over time, has been deciphered for modern readers. Without the backgrounds in science, craft, materials and the technology of contemporary objects that is typical of ATSR researchers, the historical terminology for art materials and processes remains opaque, ignored as it is by etymological dictionaries and art historical discussions due to its highly specialised nature. Without testing and reconstruction, the validity and meaning of many sources remained hidden; after recent researches, we can now better distinguish between the practical and the alchemical, the intellectual construct and the workshop note in many cases.

Art Technological Source Research of visual sources is largely pending, however. Although initial progress is made with some dedicated projects researching historic studio interiors any standardisation of this kind of research and definition of visual elements is still in its infancy.

Fig. 2: Catalogue of Charles Roberson & Co, London (c.1900), private collection.

This leaves the question where Art Technological Source Research stands in relation to Art History, Conservation, Conservation Science, Technical Art History and a range of other historical disciplines, as well as modern art making practice. It can be a discipline of its own, but can also form a cornerstone of one of these disciplines, while otherwise one of these can be a cornerstone for ATSR. It will depend on the initial research question and the background of the researcher. While ATSR concentrates on textual and visual sources, the information obtained from it can be applied in yet larger ways, contextualising it to allow us to better understand the materiality of our artistic heritage.

Further Reading

Proceedings of the ATSR working group’s symposia:

M. Clarke, J.H. Townsend and A. Stijnman (eds), Art of the Past: Sources and Reconstructions, London: Archetype Publications, 2005.

S. Kroustallis, J.H. Townsend, E. Cenalmor Bruquetas, A. Stijnman and M. San Andrés Moya (eds), Art Technology: Sources and Methods, London: Archetype Publications, 2008.

E. Hermens and J.H. Townsend (eds), Sources and Serendipity: Testimonies of Artists’ Practice, London: Archetype Publications, 2009.

S. Eyb-Green, J.H. Townsend, M. Clarke, J. Nadolny and S. Kroustallis (eds), The Artist’s Process: Technology and Interpretation, London: Archetype Publications, 2012.

H. Dubois, J.H. Townsend, J.Nadolny, S. Eyb-Green and S. Neven (eds), Making and Transforming Art: Technology and Interpretation, London: Archetype Publications, 2014.

ATSR became an official working group under ICOM-CC in 2008:, and published papers in the proceedings of its Triennials:

J. Bridgland (ed.), Preprints of the ICOM-CC 15th Triennial Meeting, New Delhi: Allied Publishers, 2008, 3–24.

J. Bridgland (ed.), 16th ICOM Committee for Conservation Triennial Meeting, Lisbon, Paris: ICOM-CC, 2011. [CD-ROM]

[1] This text represents my personal view on ATSR and its relation to other disciplines, but I want to acknowledge Jilleen Nadolny for her additional suggestions and discussions.

[2] ‘Kunsttechnologisch bronnenonderzoek’ (Dutch), ‘Kunsttechnologische Quellenforschung’ (German).

[3] A. Stijnman, M. Clarke and J.H. Townsend, ‘Foreword’, in Art of the Past: Sources and Reconstructions, M. Clarke, J.H. Townsend and A. Stijnman (eds), London: Archetype Publications, 2005, v-vii.