Black was obtained by calcining the bones of animals. The other substances employed in painting were indigenous to the country. The white is made of gypsum, mixed with albumen or honey; the yellows are ochre, or sulphuret of arsenic, the orpiment of our modern artists; the reds are ochre, cinnabar, or vermilion; the blues are pulverised lapis-lazuli, or silicate of copper. If the substance was rare or costly, a substitute drawn from the products of native industry was found. Lapis-lazuli, for instance, was replaced by blue frit made with an admixture of silicate of copper, and this was reduced to an impalpable powder. The painters kept their colours in tiny bags, and, as required, mixed them with water containing a little gum tragacanth. They laid them on by means of a reed, or a more or less fine hair brush. When well prepared, these pigments are remarkably solid, and have changed but little during the lapse of ages. The reds have darkened, the greens have faded, the blues have turned somewhat green or grey; but this is only on the surface. If that surface is scraped off, the colour underneath is brilliant and unchanged. Before the Theban period, no precautions were taken to protect the painter's work from the action of air and light. About the time of the Twentieth Dynasty, however, it became customary to coat painted surfaces with a transparent varnish which was soluble in water, and which was probably made from the gum of some kind of acacia. It was not always used in the same manner. Some painters varnished the whole surface, while others merely glazed the ornaments and accessories, without touching the flesh-tints or the clothing. This varnish has cracked from the effects of age, or has become so dark as to spoil the work it was intended to preserve. Doubtless, the Egyptians discovered the bad effects produced by it, as we no longer meet with it after the close of the Twentieth Dynasty.