Varnish and its use has been known since ancient times. Altho the knowledge of varnish has been attributed to the Japanese as early as 500-600 B.C., Tschirsch and A.D. Stevens say that the Japs did not possess this art originally, but that they probably acquired it from the continent during the third century. Nevertheless, varnish did not come into general use until the middle ages. In the 12th century, a monk by the name of Theophilus published the first directions for making an oil varnish, but it was not until 1790 in England, and between 1820 and 1830 in France and Germany, that a factory was established for the commercial production of varnish. In general, a varnish may be defied as a homogeneous, sticky, viscous solution of resins, of a colloidal nature, which when applied to a surface in a thin coat, dries to a hard, smooth, glossy surface. The color of varnish varies considerably, depending on the grade and on its use. Pale varnishes are generally sought, yet there are many dark varnishes of excellent quality. The following scale gives an idea of the degree of color of varnish. A solution of 0.25g K2Cr2O7 in 100g pure concentrated H2SO4 is equivalent to a very light varnish. A solution of 2.00g K2Cr2O7 in 100g pure concentrated H2SO4 is equivalent to a medium varnish. A solution of 4.00g K2Cr2O7 in 100g pure concentrated H2SO4 is equivalent to a dark varnish. Varnishes may be divided into two main classes, according to the manner in which the film dries. The varnishes of the first class, which is the largest and most important, are known as oil varnishes. These varnishes dry mainly by the oxidation of the oil, which forms a tough, elastic film. The secondary drying in these oil varnishes is caused by the evaporation of the volatile vehicle, and by the polymerization of the constituents. On the whole, the drying of an oil varnish is of a chemical nature. The varnishes of the second class are called spirit varnishes. These dry merely by the evaporation of the solvent, and there is practically no chemical action. The gum is transferred from a lump form into a thin sheet by means of the solvent. The most important example of this class is shellac, which is the hardened secretion of the lac insect, dissolved in alcohol.