to
the floors or sent to the crushing rollers. The gravel which is left
from the washing is worth about £150 per load. A load of the blue as it
went in would be worth about 30s. The gravel is then sent to the
pulsators: steel sieves with holes from one-sixteenth to five-eighths
of an inch in diameter, which separate the sizes. The small sizes are
conveyed to a washing pan and the larger ones to revolving picking
tables, where the large diamonds are taken out. All the remaining
stones then go to the grease-shaking tables. In 1897 an employee of the
De Beers named Fred Kirsten noticed that of all the minerals contained
in the blue, only diamonds would adhere to grease. This resulted in a
machine which not only separates the diamonds mechanically much more
rapidly than it could be done by hand, but surely secures all that are
present, however small they may be, and prevents opportunity for theft.
It consists of a series of sloping corrugated iron tables which are
coated with grease and shaken by percussion as the gravel goes over
them. Everything but the diamonds passes on, they alone adhere to the
grease. The tables are then cleaned, and the process repeated. It is
said the grease, after being used awhile, loses its power over the
diamonds, but regains it after being melted. The crystals are then
cleaned in a mixture of acids, assorted, weighed, registered, and added
to the stock in readiness for shipment. As a precaution, the grease
tables are arranged in series, though a diamond seldom escapes the
first table; about one-third of one per cent. only. It is said none
ever yet got past the second.
Perhaps nothing illustrates more strikingly the
