The Old Masters' painting grounds were formed of calcined sheep's-trotters, pounded and mixed with glue or fish size; and the remarkable brilliance of many of the Venetian paintings after several centuries testifies to the excellence of this preparation. In truth it is chemically more permanent than the modern "prepared canvas", because the ground of the latter material is composed of a mixture of linseed oil, white lead, and driers (litharge, or "sugar of lead," &c.) These consist of a very pernicious compound to bring into contact with oil colours, because the lead base is liable to react chemically on paints spread over it, and also because it absorbs sulphurous fumes and darkens thereby, which darkening cannot fail to dim the brilliancy of light colours and delicate hues.,