Resin is a secretion formed in special resin canals or passages of plants, from many of which, such as, for example, coniferous trees, it is exuded in soft drops, hardening into solid masses in the air. Otherwise it may be obtained by making incisions in the bark or wood of the secreting plant. It can also be extracted from almost all plants by treatment of the tissue with alcohol.
Plants produce resins for various reasons whose relative importances are debated. It is known that resins heal the plant's wounds, kill insects and fungi, and allow the plant to eliminate excess acetates.
Certain resins are obtained in a fossilized condition, amber being the most notable instance of this class; African copal and the kauri gum of New Zealand are also procured in a semi-fossil condition. The resins which are obtained as natural exudations are in general mixtures of different, peculiar acids, named the resin acids, which dissolve in alkalis to form resin soaps, from which the resin acids are regenerated by treatment with acids. They are closely related to the terpenes, with which they occur in plants and of which they are oxidation products. Examples of resin acids are abietic (sylvic) acid, C20H30O2, occurring in colophony, and pimaric acid, C20H35O2, a constituent of gallipot resin. Abietic acid can be extracted from colophony by means of hot alcohol; it crystallizes in leaflets, and on oxidation yields trimellitic acid, isophthalic acid and terebic acid. Pimaric acid closely resembles abietic acid into which it passes when distilled in a vacuum; it has been supposed to consist of three isomers.
Resins when soft are known as oleo-resins, and when containing benzoic acid or cinnamic acid they are called balsams. Other resinous products are in their natural condition mixed with gum or mucilaginous substances and known as gum resins.
The general conception of a resin is a noncrystalline body, insoluble in water, mostly soluble in alcohol, essential oils, ether and hot fatty oils, softening and melting under the influence of heat, not capable of sublimation, and burning with a bright but smoky flame. A typical resin is a transparent or translucent mass, with a vitreous fracture and a faintly yellow or brown colour, non-odorous or having only a slight turpentine odor and taste. Many compound resins, however, from their admixture with essential oils, have distinct and characteristic odours.
The hard transparent resins, such as the copals, dammars, mastic and sandarach, are principally used for varnishes and cement, while the softer odoriferous oleo-resins (frankincense, turpentine, copaiba) and gum resins containing essential oils (ammoniacum, asafoetida, gamboge, myrrh, and scammony) are more largely used for therapeutic purposes and incense. Amber is a fossil resin.