varnishes turpentine spirit resin oils
VARNISH. A varnish is a fluid preparation which, when spread out in thin layers, dries either by evaporation or by chemical action into a hard, transparent, and glossy film. Varnishes are used to communicate lustre and brilliance to many different kinds of dressed surfaces, - metal-work, wood, paint, paper, leather, &e., - and to protect such surfaces from the influence of air and damp. The chief requisites of a good varnish are that it forms a firmly-adherent layer on the surface over which it is spread, that it dries hard, yet with sufficient elasticity and tenacity not to crack with changes of temperature, that it forms a glossy durable surface, and that it dries quickly. The materials which almost exclusively form the permanent body of varnishes are the drying oils and resinous substances, the chief of which are the copals, lac, dammar, elemi, amber, sandarac, mastic, and rosin. For certain forms of varnish the drying oils themselves act as the solvent for the resins, but in other cases volatile solvents are employed. The solvents chiefly used are methylated spirit, wood spirit, ether, benzin, and turpentine and other essential oils. Soluble colouring ingredients are also, in some cases, used in varnishes and lacquers, those principally available being gamboge, dragon's blood, aloes, cochineal, turmeric, and coal-tar dyes.
According to the solvents employed, the ordinary kinds of varnish are divided into three classes, - (1) spirit, (2) turpentine, and (3) oil varnishes. Spirit varnishes dry with great rapidity owing to the volatilization of the solvent spirit, leaving a coating of pure resin of great hardness and brilliance, but the film is deficient in tenacity, cracking and scaling readily on exposure. The resin lac, either as grain, shell, or bleached lac, is the basis of most spirit varnishes ; but sandarac is also largely used, and to these are added in varying proportions the softer resins, - elemi, Venice turpentine, Canada balsam, mastic, &c., - which give elasticity and tenacity to the varnish. The solvent is almost exclusively methylated spirit. The resins are ground and mixed with powdered glass, which prevents the resinous particles from agglutinating, and thus facilitates the solvent action of the spirit. The solution is effected by agitation in closed vessels with the aid of heat, and the varnish when strained off cutest be kept tightly closed from the air. Spirit varnishes are used principally for cabinet-work and turnery, stationery, gilding, and metal-work. Coloured spirit varnishes and lacquers are largely employed for metal-work, for imitation gilding and bronzing, for toys, &c. Turpentine is the solvent principally used for making dammar varnish, the solution being effected by powdering the resin and boiling it with a proportion of spirit of turpentine, after which more turpentine is added in the cold state to bring the preparation to a proper consistency. To increase the tenacity of such dammar varnish some proportion of boiled linseed oil or of oil copal varnish may be added. In place of oil of turpentine other essential oils may be used as solvents, and in practice oil of spike is largely utilized in preparing fine varnishes for oil paintings. Turpentine varnishes are also made in which the principal resinous bodies are sandarac and common rosin ; and, moreover, turpentine is largely employed to reduce the consistency-and to improve the drying properties of copal varnishes. The basis or solvent of oil or fatty varnishes consists principally of linseed oil ; but the other drying oils - poppy and walnut, &c. - may also be used. These oils, without the addition of resins, themselves form varnishes which on exposure in thin layers dry by a process of oxidation into tough glossy films ; but the drying proceeds very slowly unless the oils are previously boiled with the red oxide of lead or otherwise treated to increase their 'power of absorbing oxygen (see LINSEED OIL, VOL XIV. p. 677). It is in the form of boiled oil or of oil prepared with driers that these oils are used in varnish-making. Oil varnishes thus differ from the other classes in the circumstance that the principal solvent is not volatile and dissipated on exposure, but in itself forms an essential and permanent ingredient in the preparation. The resin principally used in oil varnishes is copal, and its varieties differ very much in hardness, that is, in the temperature at which they melt and distil. Hard and semi-hard copals can only be made to mix with and dissolve in oils at the temperature at. which they distil, which ranges from 230° to 360° C. The copal in varnish-making is melted and brought to the requisite temperature in a copper vessel. Simultaneously the oil is heated to the boiling-point in a separate copper vessel, and at the proper moment a measured quantity of the boiling oil is added to the liquefied resin. They are then boiled together till the mixture becomes perfectly clear, and by a series of alternate additions of oil and resin at proper temperatures the solution is brought to the desired consistency. After the mixture of oil and resin has sufficiently cooled, oil of turpentine in certain proportion is added. The making of copal varnish is attended with great risk of burning, and special precautions have to be observed for the extinction of the fire that frequently bursts out in these highly-heated and most inflammable bodies. Copal varnish is also made by boiling together the requisite proportions of resin and oil under pressure in a closed vessel, and subsequently adding turpentine, or by dissolving the resin and turpentine at the high heat and adding the oil afterwards. Amber varnish is prepared by the same methods as those followed in preparing copal, but, the resin being still more refractory and insoluble, a higher heat is required. A peculiar varnish forms the basis of the celebrated. Japanese lacquer. The substance is a resinous exudation, which is obtained by making incisions in the bark of the Japanese urushi or lacquer-tree, _Rims verizieVera. The resinous juice on settling in vats separates into two layers, the upper and thinner of which, on mixture with a drying oil, forms a transparent varnish, having a rich yellow colour, which dries into coatings of remarkable tenacity, durability, and lustre (see JAPANNING, VOL xiii. p. 592).