VARNISH. A homogeneous liquid, which when thinly applied, dries, on exposure to air, to a hard film giving decorative effect and protective action to the surface to which it is applied.
The use of varnish dates back to great antiquity; the ancient Egyptians were acquainted with the sof ter resins, such as sandarac, mastic, etc., which they melted up in oil to form a varnish which was applied warm with the finger or a knife. On the mummy cases in the British Museum the varnish still retains its lustre and its surface is untracked and undamaged. But it is not until the middle of the i8th century that any description can be traced of linseed oil varnish thinned with oil of turpentine, and incorporating lith arge as a drier.
Varnishes may be classified broadly into two varieties, (t) oil varnishes and (2) spirit varnishes (including cellulose varnishes). Oil varnishes are composed of hard gum resins, a drying oil— usually linseed oil—and a volatile solvent such as turpentine ; on application the volatile solvent first evaporates leaving a soft oil resin film which subsequently dries by absorption of oxygen from the air. Spirit varnishes are solutions of soft resins dissolved in a volatile solvent and on evaporation leave hard dry resinous films.
The first process in the manufacture of an oil varnish is the selection and grading of the gums according to colour, hardness, etc., only the palest gums being used for the highest grade var nishes. These gums are put through a crusher so as to reduce them to'a uniform size. The crushed gums are placed in a copper or aluminium varnish pot, provided with a hood or cover which is connected to a flue to take away the fumes given off during the "running" or melting process. The cover has a hole in the centre to allow the introduction of a copper or aluminium stirrer. The charge of gum—which varies from 50 lb. to 120 lb.—is put into the pot which is then placed over a hot coke or gas fire and gradually heated up to a temperature of about 300° C. When the gums are all melted and in a thin liquid condition, linseed oil—previously heated up to 300° C—is gradually added and the mixture thoroughly stirred ; the heating is continued until com plete amalgamation has taken place ; this is indicated when a por tion on being taken out and put on a piece of glass appears bright and transparent. The driers which consist of lead and manganese salts are next introduced and the mixture boiled until they are thoroughly incorporated. The pot is then removed from the fire
to the thinning shop and when it has cooled down sufficiently, turpentine or white spirit is gradually added, the mixture being continually stirred so as to reduce the mass to a suitable con sistency. The varnish is then strained to remove any dirt and pumped into storage tanks where it is left to mature.
A large number of varieties of oil varnishes are made, each having its own special use. The most important varieties are, coach-builders' varnishes, including finishing and hard drying body varnish, elastic carriage varnish and flatting or rubbing varnish, decorators' varnishes, such as copal oak varnish for inside and outside work, church oak varnish and French oil varnish. A special type of varnish known as japan goldsize is made for gilders' use and as a binder for coach colours ; it dries in two hours or less. Other types of varnish made for specialized uses include, egg shell flat varnish which dries with a mat surface, stoving or baking varnishes for tin printing work, and insulating varnishes for electrical purposes. Oil varnishes are sometimes classified as long and short oil varnishes based upon the relative amounts of gum and oil used; that is on the number of gallons of oil per ioo lb. of gum. The general formulae for oil varnishes may be sum marized as follows :— Spirit Varnishes.—These are simple solutions of soft recent resins—such as shellac, sandarac, dammar, mastic, etc., in a vola tile solvent or mixtures of solvents such as methylated spirit, turpentine or acetone. When a spirit varnish is applied the vola tile solvent rapidly evaporates leaving a thin coating of resin covering the object. The manufacture of a spirit varnish is a comparatively simple matter; the process consists in placing the resin and solvent in a barrel or churn which is rotated at a slow speed for a period of about 12 to 48 hours until the resin is com pletely dissolved. The varnish is then strained to remove any dirt or other insoluble matter, into suitable containers where it is left to stand for a few days, and it is then ready for use. The most important spirit varnish is French polish (q.v.). White polish which is used for polishing very light woods such as satin wood is made by dissolving bleached shellac in methylated spirit— usually 2 lb. of shellac tor gallon of methylated spirit. White and brown hard spirit varnish which is largely used as a quick drying varnish for woodwork, basket work, chairs and other articles of furniture is made by dissolving 4 to 5 lb. of soluble manila gum in one gallon of methylated spirit. By the addition of various aniline dyestuffs to these varnishes, a large range of col oured varnish stains may be obtained which are used on wood work to imitate mahogany, oak, walnut, etc.