The chief spirit varnish in which turpentine is the volatile sol vent is crystal paper varnish, made by dissolving 7 to 8 lb. of white dammar gum in one gallon of turpentine. This varnish is water-white in colour and is used for varnishing wallpapers and delicate interior work. Other spirit varnishes include mastic var nish used for varnishing oil paintings and sandarac used as bdok binders and negative varnish.
Cellulose Varnish.—This consists of a base, usually cellulose nitrate, dissolved in suitable solvents. A large number of solvents may be used for dissolving cellulose nitrate, but those most com monly employed are acetone, amyl acetate, butyl alcohol and ethyl lactate. As these cellulose nitrate solvents are very expen sive it is usual to substitute as far as possible cheaper diluents such as benzol, alcohol and light petroleum ether; these diluents while not in themselves solvents for nitro-cellulose nevertheless when mixed with the other solvents yield a good solution. When cellu lose varnish is applied to a surface the solvents evaporate leaving the cellulose nitrate behind as a thin film. These films are very brittle and do not adhere well to the surface and it is therefore necessary to add to the varnish a proportion of softening agent, or "plasticizer" in order to prevent shrinking and to toughen them ; an addition of certain gums such as kauri, resin, ester gum, dammar, etc., is also made to promote adhesion of the film.
The process of manufacture of cellulose varnish or lacquer is comparatively simple and consists in adding the cellulose-nitrate to the mixed solvents and stirring until complete solution has taken place, the gums and plasticizers being incorporated during the mixing process. The finished varnish is then strained off into suitable airtight containers and is ready for use.
Cellulose varnishes are perfectly clear and transparent, dry quickly leaving tough, hard films, which wear well and are re markably resistant to alcohol, petrol and the effects of heat. They are largely used as wood finishes on tables, pianos and furniture generally; they have the advantage over French polish in that they can be more quickly applied and the resultant finish is not damaged when in contact with hot articles such as tea-pots, hot plates, etc. These varnishes may be coloured with aniline dye
stuffs giving coloured lacquers which are used for the colouring and protection of metal and other surfaces.
By the addition of strong staining pigments to the transparent cellulose varnishes the so-called opaque cellulose enamels are ob tained which are largely used in the painting of motor-cars, etc. The pigments are ground in the varnish to a very fine state of division, but the amount which can be safely used without injuring the film is very limited. The finish obtained from these cellulose enamels has neither the full gloss nor the opacity given by oil enamels, and if a high gloss finish is required it is necessary to use a final oil or wax polishing process. The rapid rate of drying, hard ness of finish and speedy rate of application by the spray process has resulted in their being extensively used.