[b-hebrew] silver-refining cupellation Britannica DVD

Ethel Jean (Kowan) Saltz macnietspingal at airmail.net
Mon Oct 30 12:32:52 EST 2006


       Introduction


      silver processing


      preparation of the ore for use in various products.

      Silver has long been valued for its white metallic lustre, its ability
to be readily worked, and its resistance to the corrosive effects of
moisture and oxygen. The lustre of the pure metal is due to its electron
configuration, which results in its reflecting all electromagnetic radiation
of wavelengths shorter than 3000 angstroms. Since 3000 angstroms is in the
ultraviolet range, all visible light is effectively reflected, thus
conferring the white colour.

      Silver (Ag), like gold, crystallizes in the face-centred cubic system.
It melts when heated to 962° C (1,764° F). With a density of 10.49 grams per
cubic centimetre, it is the lightest of the precious metals. It is also the
least noble of the precious metals, reacting readily with many common
reagents such as nitric acid and sulfuric acid. Metallic silver can be
dissolved from gold alloys of less than 30 percent gold by boiling with
30-percent-strength nitric acid in a process referred to as parting. Boiling
with concentrated sulfuric acid to separate silver and gold is called
affination. Both these processes are used on a commercial scale for
separating silver and gold.


      History
      Silver was discovered after gold and copper about 4000 BC, when it was
used in jewelry and as a medium of exchange. The earliest known workings of
significant size were those of the pre-Hittites of Cappadocia in eastern
Anatolia. Silver is generally found in the combined state in nature, usually
in copper or lead mineralization, and by 2000 BC mining and smelting of
silver-bearing lead ores was under way. Lead ores were smelted to obtain an
impure lead-silver alloy, which was then fire refined by cupellation. The
best-known of the ancient mines were located at the Laurium silver-lead
deposit in Greece; this was actively mined from 500 BC to AD 100. Spanish
mines were also a major source.


 cupellation

      separation of gold or silver from impurities by melting the impure
metal in a cupel (a flat, porous dish made of a refractory, or
high-temperature-resistant, material) and then directing a blast of hot air
on it in a special furnace. The impurities, including lead, copper, tin, and
other unwanted metals, are oxidized and partly vaporized and partly absorbed
into the pores of the cupel.







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