Paint and Varnish Thinners.—The thinners used comprise (I) turpentine from varieties of pinus (P. palustris, U.S.; P. maritima, France, P. sylvestris, Russia). The extraction is by tapping the trees for oleo-resin (gum turpentine) or by steam distillation of the wood in the form of chips (wood turpentine). From the oleo-resin, containing 70% rosin (colophony) and 20% turpentine, the spirit is obtained by steam distillation. All forms of turpentine contain one or more terpene hydrocarbons. Amer ican and French turpentine contain pinenes, C10.1116 (b.p. 175° C), and thicken and partially oxidize on exposure, but tur pentine is not to be considered as a siccative. It is the most reliable solvent or thinner for paints and varnishes. White spirit (b.p. up to 210° C) and kerosene are used to thin paints and varnishes either alone or blended with turpentine. It must be pointed out that the resins or resins and oils will give fluids with petroleum, but addition of excess of the thinner will precipitate the resin-oil as a gel. Benzene and coal-tar naphtha are some times used as thinners. The flash point (closed test) of a thinner must not fall below 73° F, so that it may avoid classification among the highly inflammable liquids of the Petroleum Acts (1871-81). The thinners for oil varnishes and paints play no part in the chemical changes during drying, but they may modify the physical properties of the linoxyn film.
Constituents of Varnishes.—The simplest form of varnish are the spirit varnishes which are a solution of a resin in a volatile solvent; as a class they are generally brittle. Oil varnishes are free from this defect, since the drying oils which they contain bind and soften the resin. The resins employed include copals (Kauri, Manila, Congo), rosin (colophony), and, more recently, synthetic resins (q.v.). For black varnishes, the solid components
are asphaltum, or other forms of pitch (petroleum and stearin), together with carbon black. Under the German term Firniss is included a drying oil with metallic driers without a resin, and the film is merely an elastic gel of oxidised linseed oil (linoxyn). Litho varnishes are essentially of the same character. In the manufacture of a resin oil varnish, the copal resins must be heated to C with a loss of 10-25% in weight before they can be taken up by drying oils. The temperature to which the resins must be heated before they become soluble in oil is higher in the case of the hard fossil resins. The drying oils used in oil varnishes are linseed oil with or without tung oil. The resin oil mixture is thinned down with turpentine or white spirit. Driers in the form of lead and manganese compounds (cf. drying oils) are incorporated in the oil before or after the addition of the thinners. The mixture is left to clarify and to mature. The pro portions of resin, oil and thinners vary with the requirements; for coach-work the proportions are 1:2, and for hard coatings A varnish for exterior work is generally richer in oil than an inside varnish. Recently, soluble forms of cellulose nitrate and acetate in appropriate solvents have been largely used with pig ments and plasticisers in the form of enamels for protective coat ings of good elasticity and durability. The most durable form of varnish known is Japan lacquer, obtained from Thus vernicifera (Tsi-chou, varnish tree). It gives a black film on stoving in a moist atmosphere with the formation of an oxidation product of urushiol, the reduction product of which has been synthesised by Japanese chemists. (See also Ons, FATS AND WAXES.)