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22 THE DIAMOND MINES OF SOUTH AFRICA
aids
as well as diamonds, but so far as it has been tested, it will not
cling to anything but a precious stone. The grease which is used loses
its power to catch diamonds after a few hours' work, owing to its
becoming more or less mixed with particles of water. It is then scraped
off the tables, together with the diamonds adhering to it, placed in a
kettle made of finely perforated steel plates, and steamed. The grease
passes away to tanks of water, where it is cooled and is again fit for
use. The diamonds, together with small bits of iron pyrites, brass
nails from
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the
miners' boots, pieces of copper from the detonator used in blasting,
which remain on the tables owing to their high specific gravity, and a
very small admixture of worthless deposit which has become mechanically
mixed with the grease, are then boiled in a solution containing caustic
soda, where they are freed from all grease. The quantity of deposit,
from the size of five-eighths of an inch downwards, which now reaches
the sorting table, does not exceed one cubic foot for every 12,000
loads (192,000 cubic feet) of blue ground washed. As already stated,
one-twelfth of one per cent of the whole mass of blue formerly passed
to the
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