VARNISH (FR., Vernis ; GER.. Firniss).
A " varnish" is any substance which, when applied to the surface of au object, leaves on that surface a continuous, smooth, imperviona coating, whose functions are :—(1) To increase the lustre snd polish of that surface, and, hy obliterating its asperities and filling up its pores, prevent its easy soiling, and facilitate washing (e. g. paint, papier-mache, glazing of pottery, &c.); (2) by its high refractive power t,o increase the transparency of surfaces which are by their porosity rendered too opaque, and thus to bring into view any "grain " or interior structure mhich it i8 desirable to show up (e. g. oiling, varnishing, or waxing wood, transparent blinds, tracing-paper, &c.); (3) by its insolubility in water and oils, to protect perishable structures from the effect of damp, decay, mould, &c., metuls from rust and corrosion by acids, and porous bodies from grease spots ; (4) by hardening the surface of softer objects to save them from abrasion (e.g. varnishing deal and other soft woo)s, pasteboard, book-covers. leather, &c.).
Varniahes group themselves naturally into 5 well-marked divisions :—I. Natural lacquers of the Indian and Chinese type, produced liy several genera of trees of the natural order Anacardiacem. These are liquid at first, but dry slimly (by oxidation ?) on exposure to the air (see p. 1692). II. Drying oils, which indurate or resinify by oxidation in the air, either in their natural state, ur made inore drying by various chemical treatment. III. Oil-varnishes proper, composed of an intimate combination of a drying oil with a fitsed resin, and hardening hy the oxidation of the oil. These may either be applied in their undiluted state, or may be thinned with essential oils, or other liquid volatile hydrocarbons, t,o sueh a consiatence :is will enable them to be applied with a brush. IV. Varnishes composed of a volatile liquid holding in solution resins, or gums, or other solid amorphous (non-crystalline) substances, which, on the evaporation of the solvent, are left behind as a vitreous coating on the surface varnished. V. Olazea and enamels, applied
to the work in a solid state, in powder or otherwise, and attached to its surface by raising it to a heat sufficient to melt the glaze, &c.; or formed by the fused surface of the object itself, when vitrified at a high temperature by appropriate fluxes (e. g. etching-ground, glazing of porce lain and pottery, salt-glazed stoneware, &c., the last two not coming within the scope of this article, but described on pp. 1571-2, 1594-6).
MATEnkus rsEn IN VARNISH-MAKING.—These may he conveniently arranged under 6 separate heads, according to the part they play in the finished varnish :—(1) The drying oils. (2) Those resins which have sufficient hardness and transparency for the purpose, and whose melting-point is high enough to prevent their becoming softened and sticky in summer, in heated rooms, or by the heat of the hand. (3) " Colloid " bodies soluble in water, such as albumen, gum aiabic, dextrine, gelatine, &c. (4) " Solvents," i. e. volatile liquids used to dissolve resins or gums, or to dilute uil-varnishes, but which evaporate entirely during the drying of the varnish, thus contributing nothing to the thickness of the coatiog. (5) " Tougheners," or substances added t,o varnishes, of Class IV., to make them more flexible, and less liable to crack, or be scratched. (6) " Driers," which, by giving up oxygen, or by acting as " carriers " of atmospheric oxygen, hasten the hardening of the drying oils. (7) Soluble transparent colouring matters, resinous or otherwise, used in lae,quers and ehanging-varnishes.
Drying-alls.—The principal oils in use at the present day for paints and varnishes are those of the poppy, walnut, hemp-seed, and linseed.
Poppy-oil (p. 1404—The chief use of this oil in England seems to be for the grinding of the fine colours used by artists in picture-painting. Being generally considered (and probably correctly so) a much slower drier than either linseed- or walnut-oil (commonly called uut-oil "), it is seldom if ever used in varnish-making.