varnish, homogeneous solution of gum or of natural or synthetic resins in oil (oil varnish) or in a volatile solvent (spirit varnish), which dries on exposure to air, forming a thin, hard, usually glossy film. It is used for the protection or decoration of surfaces and may be transparent, translucent, or tinted. For oil varnishes a hard gum or resin, often a fossilized plant exudation such as kauri or copal, is dissolved in oil (commonly linseed oil or tung oil) and is diluted with a volatile solvent such as turpentine. Spirit varnishes are commonly made of soft resins or gums, such as shellac, dammer, mastic, or sandarac, dissolved in a volatile solvent, e.g., alcohol, benzene, acetone, or turpentine. Enamel is varnish with added pigments. Lacquer may be a cellulose derivative dissolved in a volatile solvent, or it may be a natural varnish made in the East from the sap of trees. Among the varnishes named either for their constituents or for the proposed use are japanner's gold size, cabinet, carriage, bookbinder's, patent-leather, insulating, photographic, shellac, and copal picture varnish. Varnish has been known from antiquity; the Egyptians coated mummy cases with a pastelike form made of soft resins dissolved in oil and applied when warm. Another early use was for coating oil paintings. Stradivarius and other violinmakers used a slow-drying linseed oil varnish on their instruments.
See Oil & Colour Chemists' Assoc., Paint Technology Manual (2 vol., 1961, 1962); C. R. Martens, Technology of Paints, Varnishes, and Lacquers (1968).