ÆS Ustum, a chemical preparation, made of thin leaves of copper, sulphur, and nitre, placed stratum super stratum in a crucible, and set in a charcoal fire till all the sulphur is consumed; after which, the copper is taken out of the crucible, and reduced to powder. Some quench the leaves of copper in vinegar, and repeat the calcination.—Its principal use is in colouring glass, to which it gives a beautiful tincture. The surgeons use it as a detersive, and some have given it internally; but it is certainly a very dangerous medicine, and should be avoided.