Pound (mass) a legal unit for trade (it was abolished in the United Kingdom on 6 January 1879 by the Weights and Measures Act of 1878), but the troy ounce, of a troy pound, is still used for measurements of gems such as opals, and precious metals such as silver, platinum and particularly gold. The system called Tower weight was the more general name for King Offa's pound. This dates to 757 AD and was based on the silver penny. This in turn was struck over Arabic dirhams (2d). The pound was based on the weight of 120 Arabic silver dirhams, which
Metallurgical assay on the composition of the rock in which the precious metals are concentrated, and in many laboratories an empirical approach based on long experience is used. A complex reaction takes place, whereby the carbon source reduces the lead oxide to lead, which alloys with the precious metals: at the same time, the fluxes combine with the crushed rock, reducing its melting point and forming a glassy slag. When fusion is complete, the sample is tipped into a mold (usually iron) where the slag floats to the top, and the lead, now alloyed with the precious metals, sinks to the bottom,