Resins terpenoid varnishes

Py-GC/MS Characterization of Terpenoid Resins and Varnishes 12.2.1 Chemical Composition of Terpenoid Resins... 

Terpenoids are also put to uses for which their physical or chemical properties suit them but which are not the uses for which nature originally intended them. Rubber is a polymer of isoprene which is produced in the rubber tree as a defensive secretion but is widely used by humans because of its elastic properties. Turpentine has a long history of use as a solvent, particularly for paints and, similarly, lac resin as varnish. Nowadays, turpentine is also used as a feedstock for the synthesis of other materials of commercial interest, in addition to a wide variety of fragrance ingredients. 

The terpenoid resins commonly used as artists materials were colophony, Venice turpentine, mastic, dammar, copals and sandarac, and most of the scientific literature concerning the identification of Old Master varnishes and the study of their degradation focuses on these resins. 

The earliest synthetic resin to be used in commerce seems to have been a polyester then termed ester gum, which was made by esterifying rosin (essentially an unsaturated monocarboxylic terpenoid acid, abietic acid) with glycerol. When cooked with tung oil (a glycerol ester of 9,11,13-octadecatrienoic acid), this provided varnishes that dried overnight. In this case, the polymer is formed by an addition copolymerisation process, but the product is nevertheless also a polyester. 

Until chemistry became a science in the nineteenth century, the history of terpenoids in coatings was not one of steady transformations and improvements but rather a series of uneven lurches and trade-offs. The composition of oleo-resin was a mystery except for the art of separating it into tar, pitch, and turpentine. Practically nothing was known about terpenes, resin acids, and their derivatives. Rosin and other natural resins, e.g., amber, copal, dauri, and Congo, were used to manufacture varnishes through the eighteenth and nineteenth centuries by cooking them plus a drier with linseed oil (Mattiello, 1941-1945). 

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