Page:The American Cyclopædia (1879) Volume XVI.djvu/288

 268 VARNISH a shining appearance, and to protect them, against the air and moisture. The principal substances which enter into the composition of varnishes are : as solid constituents, rosin, am- ber, mastic, sandarach, lac, elemi, dammar, ben- zoin, copal, asphalt, and caoutchouc; as sol- vents, oil of turpentine, linseed, poppy, and a few other vegetable oils, wood naphtha, ben- zine, and some other constituents of petro- leum, alcohol, and ether. Aa coloring mate- rials, gamboge, dragon's blood, aloes, saffron, annatto, and turmeric are used ; to which may be added small quantities of cochineal, French yellow, and other colors, to impart increased brilliancy. Varnishes may be divided into four classes: 1, ether varnishes; 2, spirit varnishes; 3, volatile-oil varnishes; and 4, fixed-oil var- nishes. Of the first kind the only one much used is that for repairing the glazing of the colored enamels used in jewelry. It is made by gradually adding five parts of pulverized copal to two parts of sulphuric ether, in a flask which is corked and occasionally shaken for 24 hours or longer. Spirit varnishes made with alcohol are conveniently prepared, and, on ac- count of their rapid drying and leaving no dis- agreeable smell, are in frequent use in the household for covering various articles of art or vertu made by amateurs. When large quan- tities of spirit varnish are made, a still, with a capital and worm, is usually employed. A stirring rod passes through a stuffing box in the capital. The still is heated by a steam or water bath. The materials being placed in the still, the heat is raised until the alcohol be- gins to distil. The heat is then lowered, and the solution continued with stirring in an equable manner with as little evaporation as possible. "When the stirring rod by its easy turning gives evidence of complete solution, the varnish is passed through a silk sieve and then through porous paper, or allowed to clarify in stone jars. The alcohol which may have distilled over should be added to the varnish. Spirit varnishes are liable to scale off or crack, and are incapable of resisting friction or blows. To diminish this tendency, small quantities of oil of turpentine are often added to them, or some of the softer and more adhesive resins are employed in their composition. The alco- hol should not be of less strength than 40 to 36 Baume (sp. gr. 0-830 to 0'849), as other- wise the resins dissolve with much more diffi- culty, and the varnish will neither be so bril- liant nor so drying. Throe ways of making the solution are employed, viz. : 1, by simply digesting the resins, &c., in the proper quantity of alcohol, either in the shade or exposed to the sun, occasionally shaking the bottle ; this takes a long time, and many substances cannot be dissolved in this way, but the product has the least possible amount of color ; 2, by heat- ing over a water bath, which is much more rapid, but produces more highly colored var- nishes ; 3, by heating over an open fire, which still more changes the color of the resins, and consequently of the varnish, but it is so much more rapid that it is employed almost exclu- sively for manufacturing purposes. Oil of turpentine is almost the only volatile oil em- ployed in varnishes, and the most important one into which it enters is copal varnish. (See COPAL.) The ingredients and modes of prepa- ration are nearly the same as for spirit var- nishes. An important difference between the two kinds is, that spirit varnishes are injured in quality by keeping, while those with oil of turpentine are very much improved by it, from the more intimate union which takes place be- tween the resins and the oil. Fixed-oil var- nishes are almost entirely made with linseed oil, for which poppy oil is sometimes substi- tuted, and they generally contain also a large proportion of oil of turpentine. The resins used are almost exclusively the different kinds of copal, and amber. In consequence of the slow evaporation of the solvent, and the large amount of residue from it, they are, of all var- nishes, the slowest in drying, but the most du- rable. (See LINSEED OIL.) For articles of sheet iron, tin, copper, or brass, and for all articles exposed to constant wear and frequent rubbing, they are particularly adapted. (See JAPANNING.) In consequence of the difficult fusibility of the copal, a very different process from that used for spirit and turpentine var- nishes must bo employed in their preparation. The resin is first melted over an open fire ; and when perfectly liquid, the linseed oil, heated to 300 or 400 F., is incorporated with it, and finally the oil of turpentine. It is indeed pos- sible to melt copal directly in boiling oil, and the subsequent addition of a proper quantity of oil of turpentine will bring it to a proper consistence ; but as in this case the oil is al- ways more or less burnt, the varnish is both more highly colored and less drying, and this method has been generally abandoned. Great care should be taken that the resins used are of equal fusibility, for if the heat is much raised or long continued after complete fusion of a part, there will be much more color than even if the more infusible parts alone were used. Besides the four classes of varnishes above described, others are employed for special pur- poses which can scarcely be included under any of them. Dr. Bolley describes a caoutchouc varnish, which appears to possess valuable, properties. The caoutchouc is cut into small pieces and digested in sulphuret of carbon, which forms a jelly with it ; and by treating this with benzole, a large proportion will be dissolved. The liquid must be strained through a woollen cloth, and the sulphuret of carbon removed by evaporation in a water bath, after which the liquid may be diluted with an addi- tional quantity of benzole. A gutta percha varnish is made by dissolving one part of gntta percha in 4 or 6 parts of oil of turpentine, and adding 8 or 10 parts of linseed-oil varnish, boiling hot. It is well adapted for varnishing maps and prints, as it does not affect the