Natural resins are sticky, water-insoluble substances that exude from a wide variety of trees. Most consist of compounds known as terpenoids. The resin that exudes from some types of trees contains copious amounts of natural solvents. One example is pine resin, from which turpentine, a natural solvent is distilled. Resins were known in ancient Egypt, and quite probably earlier. In Egyptian times, they were used in varnishes and figured in mummification procedures, among other uses.
A bewildering variety of resins are known. In Egyptian times, perhaps pine resin and Chios turpentine, a type of resin that came from trees growing in northern Africa and on the island of Chios, were the best known. During the early Middle Ages, treatises mention many other varieties, including sandarac, Venice turpentine (larch resin), and amber. Resins that became popular in furniture varnishes and other applications during the nineteenth century included copals (of which there are many varieties), mastic, and damar.
Many of these resins can be dissolved, usually with heating, in oil. They can also be put into solution in organic solvents such as turpentine, although such simple resin solutions were probably not used until the eighteenth or nineteenth century. Some resins are only partially soluble in oil or solvents, but can be made more soluble by first heating them, which partially breaks down their structures. Amber and some copals need to be treated in this fashion in order to be made into varnishes.
Aside from picture varnishes, the major use of resins in oil paintings was in glazes. An oil-resin medium produces a rich, shiny paint layer, well suited to glazes. One popular color in European medieval and later periods is known as copper resinate. It was often made by dissolving a green pigment (verdigris) in a mixture of oil and resin. Pine may have been the major resin used for this purpose.
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