PAINTS AND WHITEWASHES 231 



and impervious to air and water. After the varnish is 

 applied, the solvent evaporates, leaving the resin, or the oil 

 oxidizes and dries. 



Spirit varnish consists of resin dissolved in alcohol, 

 petroleum spirits or some other volatile solvent. Turpentine 

 varnishes are those in which turpentine is the solvent used. 

 Linseed oil varnishes may consist of linseed oil alone, or 

 resin and turpentine may be added. The addition of tur- 

 pentine tends to overcome the tendency to scale off. The 

 most important varnishes are made with shellac, while 

 mastic, sandarac, and dammar are som'etimes used. 



Oil Varnish. The greater part of the varnishes made are 

 compounded of linseed oil, resin and turpentine. Tur- 

 pentine varnishes dry slowly but they are tough and flexible. 

 Linseed varnishes are the most important, but they do not 

 show the surface brilliancy that some other varnishes show. 



180. Shellac. Shellac is a form of the resin lac which is 

 produced by the bite of certain insects on the small twigs 

 of several species of trees which grow in the East Indies. 

 The insects feed on the plant sap and exude the lac, which 

 finally covers the insect and her eggs. The twigs bearing 

 these exudations are collected and appear commercially as 

 stick lac. The crude material is first treated by macerating 

 in warm water to remove a red dye-stuff that it carries. This 

 material is sold as lac-dye, and the residue from the macera- 

 tion is known as seed-lac. This is refined by melting and 

 straining through muslin bags. The melted lac is poured 

 in thin films over cold surfaces, to which it will not adhere, 

 and is allowed to cool. These flakes are sold as shellac. 

 Shellac is completely soluble in caustic alkalies and in borax 

 solutions. It is partly soluble in alcohol, turpentine, chloro- 

 form or ether. 



181. Glue. Glue is a product of the decomposition of 

 animal connective and elastic tissues. It contains two essen- 

 tial constituents, gluten and chondrin. The former has 

 great adhesive properties and the latter is adhesive to a less 



