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History
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The fire assay method of determining the amount of precious
metal in a rock was developed in ancient times (Haffty, J.;
Riley, L.B.; Goss, W.D., A Manual of Fire Assaying and
Determination of the Noble Metals in Geological Materials,
United States Geological Survey Bulletin 1445, US Government
Printing Office, Washington, D.C., 1977; p. 2 - 6).
Archaelogical finds in Troy (about 2600 BC) and other
places show that pure silver was made in the 25th century B.C.
The process of collecting the precious metal from the rock
using lead, and the removal of that lead leaving behind the
precious metal (called cupellation) was invented in Asia Minor
sometime after 2500 B.C. shortly after the discovery of the
manufacture of lead from galena (lead sulfide).
The method was described by Georgius Agricola in 1556 in De Re
Metallica (translated in 1912 by Herbert Clark Hoover and Lou
Henry Hoover) (Hoover, Herbert C. and Lou H. Hoover, De Re
Metallica, Dover Publications, Inc, N.Y., 1950, p. 219-265)
as adding fluxes to separate the metal from the surrounding rock.
These fluxes had to be specific to the type of ore, but
included lead to concentrate the gold or silver. Agricola
then described the melting of the ore and flux mixture in the
assay furnace, then removal of the lead button to the cupel, a
special crucible that abosrbed the lead as it melted in the
furnace. The cupel with the lead button was placed in the
muffle furnace until the fire had consumed all of the lead, then
the bead composed of gold and silver, plus any other noble
metals, was removed from the ashes of the cupel to be weighed and
parted, or reacted with nitric acid to dissolve the silver and
leave the gold and other metals not dissolved by nitric acid
alone. Agricola described the technique in such detail that
it can be readily compared to the method used today, which is
essentially the same as that used reported by Agricola.
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History
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