An annotation on influence of the tree species on the chemical composition of turpentine -  BnF Ms Fr 640

Sanne Berbers                                                                The Art Technical Source

Student number 10193030                                                 Dr. M.A.H. Bol

1st  of April, 2016                                                         Faculty of Humanities                                                                                University of Amsterdam

To conduct a historical reconstruction experiment always entails much more than expected when starting preparations. Research into the recipe, the maker and the materials and methods available at the time of the recipe are important subjects to consider. This thought process usually brings about even more questions and research. It can often, depending on the research question, be more important than the actual reconstruction. Such was the case with the reconstruction a varnish recipe from the French late sixteenth century manuscript BnF Ms. Fr. 640. One of the questions which arose from the material sourcing research is, what are the chemical differences between turpentine’s originating from different trees and is this a factor which can influence the final reconstruction?

        When sourcing for materials it is essential to first clearly state what the research objective is. When the objective is the creation process, the materials become less important. But if the objective is to learn about the materials historically used in the recipe and the mechanical properties of the resulting varnish, the materials are important to extensively consider. The next question is, how historically accurate do the materials need to be, and can they realistically be, and what does historical accuracy  mean? When preparing and conducting the reconstruction of a collection of varnishes from the BnF Ms. Fr. 640, this was no different. What kind of turpentine does the author-practitioner mean in his recipe, what turpentine would be available in his time and location, of what quality, type of tree, from where, how was it processed. And do the materials influence the outcome of your historical reconstruction.

Within the varnish recipe fol. 3r_a2, two different types of Venice turpentine were used, made from Larch trees and one clear turpentine made from pine trees. Within our reconstruction we used two different types of Venice turpentine, one from Kremer Pigmente and one from Verfmolen de kat. The latter one with added colophony and turpentine from Kremer without. They both appeared different regarding colour and handling properties. Contacting the manufacturers did not shed much light on the difference as the pines from the Kremer turpentine originated ‘from the north of the pole’. This lead to the question, what is the difference between the different turpentines, and does it make a difference which kind of tree the turpentine comes from? In the 1950s there were over 70 different kinds of pine species known to us, all of which can be used to make turpentine.^[1] Within this annotation the general composition of turpentine and turpentine oil will be discussed first, which will then be correlated to the different turpentines used within the BnF Ms. Fr. 640 and other historical sources.

General chemical composition

Turpentine is the collective name for two different things, turpentine itself and turpentine oil. Turpentine is the sap obtained from coniferous trees, such as pine and larch. It is an oleoresin, meaning a  mixture of essential oils and resin. Oleoresins can be obtained from many different organic sources, when it is from trees the term balsam is also used^[2]. The turpentine is extracted from the tree by means of a tap or from dead wood.^[3] This is then processed by means of steam distillation which results in the volatile distillate known as turpentine oil, which contains essential oils and is quick evaporating clear liquid. The semi solid distillate residue is called rosin or colophony and is composed of different resin acids. The distillation process has only been in uses since the fifteenth century. ^[4] It is therefore safe to assume that all references to turpentine from before that period refer to the crude oleoresin and not the distillate turpentine oil. As the manuscript is dated to the late sixteenth century, the use of turpentine oil is an indication that these recipes had not been in use a long time before they were written down.

Within Europe different species of pine and larch from the coniferous family Pinaceae have been mainly used as a source of turpentine.^[5] These trees produce an oleoresin which is a mixture of around 80% resin acids and 20% essential oil.^[6] Essential oils are mixtures of terpenes and terpenoids, both which are aromatic hydrocarbons, the latter having the addition of functional groups. Resin acids are carboxylic acids, such as abietic acid, which consist of three fused carbon six rings and the general chemical formula C19H29COOH. These resin acids are formed when small carbon molecules bond with isoprene to form monoterpenes (volatile, C10H16), sesquiterpenes (volatile, C15H24), and diterpenes (non-volatile, C20H32), which then oxidise to form carboxylate acids. In their crystalline form the resin acid diterpenes give the turpentine colour and add to the viscosity, the monoterpenes and sesquiterpenes to the fluidity.

 The main terpenes which can be found in turpentine oil are the monoterpenes  α-pinene and β-pinene (two isomers) of which the α variation is more abundantly present in north American coniferous trees and β-pinene in European species.^[7] But also the monoterpenes carene, camphene, dipentene and terpinolene are be present.^[8] In general it can be said that turpentine oil  from tapped turpentine contains around 60-65% α-pinene, 25-35% β-pinene and 5-8% other terpenes.8 Colophony contains the remaining resin acids (C20H30O2)  such as abietic acid and in smaller amounts its isomers neoabietic acid, palustric acid, dehydorabietc acid, primaric acid, isopimaric acid and sandarac copimaric acid,  which make up 90%. The other 10% is composed neutral components such as anhydrides, phenolics, diterpene aldehydes, alcohols, stilbenes and steroids^[9]. Colophony can differ in colour from light yellow to dark red which is caused by iron contamination and oxidation of the resin acids. But there are many factors which can influence the composition of the turpentine, the type of tree, tapping method, collection season and the life of the tree. This makes it very complicated to define the exact chemical composition of different turpentines. An overview of all these chemicals mentioned and their chemical structure can be found in Table I. The differences in the chemical composition are important, because they change the mechanical properties and optical properties of the turpentine. Viscosity, colour, drying time and chemical reactions occurring are just a few of the characteristics of turpentine which will change with a different chemical composition.

Different turpentine’s within BnF Ms Fr 640

Within the BnF Ms Fr 640 manuscript the main use of turpentine is within varnish recipes. However, historically the main production of turpentine was not for the art market. Turpentine was used in very large quantities within the naval and shipping industry to create a water tight layer on wood, this is called the navel stores industry.  The production for the art market was therefore only a small fraction of the total produce.^[10] The large market also enabled a wide range of turpentines with different qualities to be available.^[11] The author-practitioners refer mostly to grades of turpentine, and not specific types of turpentine within the recipe. The only mention of a specific type of turpentine is when Venice turpentine is needed. This is the case in: fol. 3r_a2, fol. 56r_a2, fol. 66v_1, fol. 73v_3, fol. 80r_2. Venice turpentine is a larch oleoresin. It is very viscous and specifically referenced as Venice turpentine because of its production method. The oleoresin is obtained from the heart of the tree rather than from the fresher outer layers of wood, as is the case with all other turpentines. This causes the turpentine to contain less resin acid crystals (colophony in its crystalline form) and more resin acids in non-crystalline form. The resulting Venice turpentine is less intense in colour than other turpentines and more fluid. However often Venice turpentine is sold with some crystalline colophony added to the pure oleoresin. This makes the oleoresin more viscous and changes the colour. When used for varnishes, the resulting varnish layer becomes harder due to crystalline colophony and more yellow. When sourcing materials for the reconstruction of the recipe of fol. 3r_a2 it was difficult to find Venice turpentine without added colophony. Something of which one should be wary off as it considerably changes the mechanical an optical properties of the turpentine.

When referring to the other turpentines, the author practitioner is not specific. In some recipes there is the mention of different grades of turpentine, but not specific types. The properties, manufacturing method and price are mentioned as an indication of a kind of turpentine. The viscosity of the turpentine to be used is often mentioned. An overview of the different terms used by the author-practitioner to describe turpentine within the manuscript can be found in Table II. Within other literature, references to specific types of turpentine can be found. What is called Strasbourg turpentine is the oleoresin from silver fir trees. ^[12] They were grown in the Vosges and Tyrol region of the alps around the border of France with Germany and the main trading centre of this turpentine was Strasbourg. This turpentine is less viscous and therefore needs less turpentine oil to make it suitable for use as a varnish. It is often mentioned in connection to the protection of pigments which decompose easily (such as Verdigris) and to dissolve amber for use in a varnish. The turpentine usually referred to as Burgundy or French turpentine comes from the oleoresin of the Pinus Martimimus pine, and was grown in France.

With the discovery of the Americas new families of coniferous trees were available which could be used as a source for turpentine, and of which the turpentine was imported into Europe. An example of this is copaiba balsam, which is obtained from the copaifera landsdorfi tree from south America and was introduced in Europe in the 17th century. It is very viscous and has a high content of essential oil. Canada balsam is obtained from the Canadian Abies balsamea. It is a very viscous transparent pale yellow balsam with a high refractive index. 

The manuscript of Filippo Bonnani, as translated to Dutch by Jacobus Willeke in 1742, on the reproduction of Chinese varnishes with materials available within Europe is an interesting source concerning turpentine.^[13] Within the book several different types of turpentine are compared and their mechanical properties are discussed in order to find the turpentine from which to make something similar to Chinese turpentine. This also illustrates the awareness of the different types of turpentine available, their mechanical properties and how this influences the properties of the end result. In this manuscript the author mentions a lot of different types of turpentine, the trees they originate from and sometimes the location and collection method.^[14] The author also mentions that where he lives the Venetian turpentine cannot be distinguished from Chiofe turpentine (turpentine oil). And which turpentine resembles the turpentine found in China most closely. ^[15],^[16] An overview of some of the different turpentines which can be encountered within literature, what is indicated with them and from which three they would originate from can be found in Table III.

Conclusion

        The precise chemical composition of turpentine is difficult to determine. There are no definite numbers to be given, as there are many contributing factors such as the tree type, age, location, history, time of oleoresin collection and method of oleoresin collection. Within ecology and naval stores industry there is research into the chemical composition of oleoresins. Most of it dates from before the world wars, when the naval stores industry was much larger than it currently is. However, this research does not focussed on coniferous tree types interesting for art historical research, but on northern American coniferous trees, north America being the largest manufacturer of turpentines at that time.^[17] Modern ecology research uses oleoresin composition to differentiate between different species within the same family. Which indicates the small changes even within trees of the same family that make it difficult to draw broad conclusions. Research  into the chemical analysis of turpentines and other resins within works of art has been published by John Mills in 1977.^[18] However, the differentiation within turpentine composition together with the changing composition due to evaporation, oxidation, polymerisation and other chemical components makes the originally used turpentine hard to distinguish.

        Yet, the type of turpentine used in historical reconstruction can influence the physical and mechanical properties of the final product. To gain more understanding in the distribution and availability of different turpentines across Europe it would be interesting to investigate the production centres and their distribution. When starting a reconstruction it is essential to realise that the type of turpentine used can be an influencing factor.

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References
        

“Blog Balams kremer pigments,” last modified January 15, 2011, http://kremerpigments.com/blog/balsams

http://www.faculty.ucr.edu/~legneref/botany/gumresin.htm#copaiba published by the University of California, accessed March 15 2016  

Filippo Bonnani, translation Jacobus Willeke, “Verhandeling over de vernissen waar in de wyze opgegeeven wordt, om 'er een toe te stellen, dat 't Chineesch vernis volmaakt gelykt : benevens verscheyde andere zaken rakende de schilderkunst, 't vergulden, 't etsen, etc”. Leiden, 1742.

Eloise Gerry, “Oleoresin Production, a microscopic study of the effects produced on the woody tissues of southern pines by different methods of turpentining” United States Department of Agriculture Bulletin 1064 (1922) 1-40

James Kent, Handbook of Industrial Chemistry and Biotechnology vol. I, Springer, 2007, 11th  ed, chapter 28

Ralph Mayer, The Artist's Handbook of Materials and Techniques, 5th  ed. New York: Viking ,1991 

John Mills, Raymond White, “Natural resins of art and archaeology their sources, chemistry and identification”. Studies in Conservation, 22.1 (1977) p 12-13

Mills, John S. and Raymond White. The Organic Chemistry of Museum Objects. Oxford: Butterworth-Heinemann, 1999.

N. T. Mirov, “Composition of Gum Turpentine of Coulter Pine,” Industrial and Engineering Chemist 28-4 (1946): 405-7

Rutherford J. Gettens and George L. Stout. Painting Materials: A Short Encyclopaedia, New York: Dover Publications, 1962, 72.

Tables

Table I

Table II

Table III