PAINTS AND VARNISHES 373 



pentine exudations. They are hydrocarbons and in composi- 

 tion closely related to turpentine. The varieties of resins 

 are amber, copals, lac, rosin, and asphaltum. Amber is the 

 best resin, but is too costly for ordinary purposes, so that 

 copals are generally used in its place. Rosin is hard, but 

 too brittle to be desirable. Lac is soluble in water and is thus 

 readily distinguished from the other resins. It is formed by 

 the action of insects upon the sap of certain Indian trees. 

 The commonest form of lac is known as shellac, and is dis- 

 solved in wood alcohol or benzine to increase its rate of 

 drying. Asphaltum is a mineral product and is used as the 

 solid ingredient of japan. 



418. Composition of Varnishes. Varnishes are solutions 

 of natural resins in oils and spirits. They are applied as a 

 final coating to painted or stained work and are oftentimes 

 mixed with paints before application. This is done in the 

 case of enamels. A good varnish depends upon both the 

 solvent and the resin, and it is necessary to use a solvent that 

 will oxidize and bind the film of resin. Spirit varnishes, 

 made wholly of volatile solvents, will not dissolve the harder 

 resin, but are desirable because they evaporate quickly. The 

 best varnishes are those consisting of a resin dissolved in lin- 

 seed oil. The surface to which a varnish is to be applied should 

 be free from dust and non-absorbent. A gelatin size will 

 produce this effect. Varnish should be put on in thin layers. 



The qualities looked for in an ordinary varnish are tough- 

 ness, hardness, transparency, body, and freedom from color. 

 A good varnish should be dry and hard enough to touch 

 in ten hours. It should resist a moderate blow without 

 cracking, and the finger should leave no mark when rubbed 

 over its surface. 



