(1) Vegetable-derived, amorphous mixture of carboxylic acids, essential oils, and terpenes occurring as exudations on the bark of many varieties of trees and shrubs. They are combustible, electrically nonconductive, hard and glassy with conchoidal fracture when cold, and soft and sticky below the glass transition point. Most are soluble in alcohols, ethers and carbon disulfide, and insoluble in water. The best known of these are rosin and balsam, obtained from coniferous trees; these have a high acid content. Of more remote origin are such resins as kauri, congo, dammar, mastic, sandrac, and copal. Their use in varnishes, adhesives, and printing inks is still considerable, though diminishing in favor of synthetic products. (2) Miscellaneous types: shellac, obtained from the secretion of an Indian insect, is still in general use as a transparent coating; amber is a hard, polymerized resin that occurs as a fossil; ester gum is a modified rosin; amorphous sulfur is considered an inorganic natural resin; liquid resins, sometimes called resinoids, are represented by linseed and similar drying oils.See Gum, Gasoline; NaturalResinSynthetic Natural Gas.
