PASTE. In gem sculpture a prepara tion of glass, calcined crystal, lead, and other ingredients, for imitating gems. This art was well known to the ancients, and after being long lost, was restored, at the end of the fifteenth century, by a Milanese painter.
Some modern artists have succeeded in obtaining a composition pessessing a hard, fine, and brilliant lustre or appear ance; but pastes, or mock diamonds as they are called, depend most for brilli ancy on the art displayed in setting the foil or tinsel behind them. Several re cipes have been given by M. Fontanier ; but the most useful, and that generally employed for the production of artificial diamonds, is the following ; Take of litharge 20 parts, of silex 12, of nitre 4, of borax 4, and of white arsenic 2 parts. These ingredients are to be well mixed together in a crucible and melted; the fused metal is thrown into water ; and should any of the lead employed be re duced to the metallic state, it becomes separated by this process and the glass is remelted for use. For the finer kinds rock crystal is used instead of flint or sand, as it occurs in a much purer state ; i. e., more free from the admixture of metallic oxides, which give to vitreous compounds their different colors. In
place of the above, Loysel recommends the following ingredients to form a com pound, having the same specific gravity as the oriental diamond, and on this ac count considered superior, as it more nearly approaches the gem with regard to its refractive and dispersive powers; but, like the former, it requires to be kept for some two or three days in a fused state, in order to expel the superabun dant alkali and to perfect the refining. A moderate degree of heat fuses it. The following is its composition : Take of white sand purified by being washed, first in muriatic acid, and afterwards in pure water, until all traces of acid are re moved, 100 parts; red oxide of lead (minium) 150 parts; calcined potash 30 to parts ; calcined borax 10;. and oxide of arsenic 1 part.
The term paste is also applied to the earthy mixture for pottery and porce lain ; also to dough, and to the solution of starch or wheat flour, made by first mixing it with a proper portion of cold water, and then adding water under constant stirring, so as to form an i even solution. Alum is often added to paste to strengthen it.