A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s3.3

v1.c18.s3.3

But the case is different with copper, the oxide of which is formed at temperatures which are ordinarily attained by the goldsmith in the course of his operations. Hence, if a mass of copper is heated in the goldsmith's fire, it is divested of its bright red colour and acquires a dirty black appearance due to the formation of a coating of cupric oxide. Now, an alloy is nothing but an intimate mechanical mixture of its constituent metals—the molecules of the several metals lying promiscuously as it were. In an alloy of gold accordingly in which copper forms a constituent part, the particles of the latter are freely interspersed between those of gold. When such a substance is heated in air, the gold particles retain their primitive yellow, but the particles of copper, which are exposed to air, are oxidised into CuO. This substance is, as we know, a black amorphous powder and it gives to the whole mass a black appearance, which becomes darker as the metal is hammered over and the powder is thoroughly spread over the surface of the metal.