I. NATURAL RESINS The natural resins are mostly obtained by collecting the gummy substance which exudes from cuts made in the bark of the tree; some are fossil in origin, being found in a hard condition buried in the ground; shellac is distinct in being formed as a secretion by insects feeding on certain trees. There are also other sticky secretions of trees known particularly as gums. True gums, as distinct from resins, are soluble in water, as, for example, gum arabic (q.v.) used for making adhesives. The term resin is tech nically restricted to products which are insoluble in water but which will dissolve in liquids like methylated spirit, or which by special treatment will dissolve in oils, like linseed oil and turpen tine, to form varnishes. To differentiate clearly between such resins (which are also spoken of as "gums" by the varnish maker) and the water-soluble gums, the former are often referred to more definitely as varnish resins.
Properties and Classification.—Varnish resins are to be recognized by their transparency and translucency, their brittle ness and glass-like fracture, and the brown or yellow colour. They possess as a rule no taste or smell in the solid condition ; on heat ing, they melt or soften and finally burn with a smoky flame giving an aromatic odour. Solutions of such resins can be made in methylated spirit, turpentine and other essential oils. In using resins with oils, such as linseed oil, a special treatment of the resin, known as "running," is usually necessary in order to make it soluble; this involves heating in the molten condition for some time, when some decomposition and loss of weight occurs, and the resin is considered to assume a simpler molecular structure, under going what is known as a depolymerizing process, thus becoming more readily soluble in the oil.
Solubility and hardness are the chief criteria used technically in classifying a resin, and on these lines resins may be divided into Spirit-soluble resins which are (I) soft, such as the balsams and Burgundy pitch; (2) medium, such as mastic ; (3) hard, such as damar, sandarac and shellac.
Oil-soluble resins which are (I) soft, such as some Manila copals; (2) medium, such as Kauri copal; (3) hard, such as Zan zibar copal and amber.
Much overlapping necessarily occurs in such a classification.
The the name balsam, or balm, suggests, these resins, of a fluid character, are of chief use as healing preparations in pharmacy. They are also called oleo resins, containing a large proportion of volatile essential oil which usually consists of esters of cinnamic or benzoic acid. A hard resin can as a rule be obtained from balsams by evaporating the essential oil. Turpentine, for example, as obtained from the tree, is an oleo-resin which yields solid rosin by evaporating or distilling away the essential oil of turpentine. Burgundy pitch, from Bor deaux turpentine, is used in medical plasters. Elemi is a fluid or semi-solid balsam, in a condition approaching that of an ordinary solid resin ; it is sometimes used in special spirit varnishes.
A medium resin, such as mastic, is still of a soft nature but not fluid like the oleo-resins. It is mainly used in the preparation of high-grade varnishes of pale colour for the protection of paintings. It is obtained from one of the islands in the Grecian archipelago, from a tree of the Pistachia genus. A solution of mastic in turpen tine, with linseed oil added, produces the artist's medium known as "megilp." Mastic is soluble in alcohol to the extent of about 9o%, and melts at about 95° C. Acaroid or gum accroides, melting at about I00° C, is obtained from the Australian grass tree as a yellow or red resin. It contains phenolic compounds which give it medicinal value as a tincture suitable for treating affections of the mucous membrane. The crimson-red resin known as Dragon's Blood exudes from the fruits of the rattan palm, growing in the East Indies. It was used as a varnish by Italian violin makers.