thorough
experiments, more suitable shaking tables were constructed at the
Company's workshops. These were from time to time improved upon, until
now all the sorting (except for the very coarse size) is done by these
machines, whose power of distinction is far superior to the keenest
eye of the native. Since the discovery of the affinity of grease for
diamonds, experiments have been made with rubies and sapphires from
Burma, and it was found that grease caught these gems with the same
certainty that it catches diamonds.
After
a thorough trial a number of these unique diamond-catching tables (see
cut, p. 20) were constructed, and are now working on De Beers
concentrates. Each shaking table is made of corrugated cast-iron plates
in five sections, with a drop of about an inch from one division to
another. Thick grease is spread on the plates to cover them to the top
of the corrugations.
The
concentrates are conveyed from the jigs upon a conveyor belt and
deposited into hoppers, where the load is elevated to revolving
cylinders covered with perforated steel plates. Through the graded
screens of these cylinders the concentrates pass into small hoppers,
one above each table, fitted with automatic feeders, — cast-iron
cylinders with grooves corresponding to the graded sizes of the
concentrates, — and are distributed evenly across the upper portion of
the shaking tables, and carried down by a flow of water from a trough
fixed behind the feeders. During the time the table is working it is
rapidly shaken from side to side by an eccentric placed on a shaft
under the table.
Strange
to relate, the descending diamonds stick on the face of the grease
while all other minerals pass over it. Only about one-third of one per
cent of diamonds is lost by the first table, and these are recovered
almost to a stone when the concentrates are passed over the second
table. The discrimination of this sorter is surely marvellous. Native
workers, although experienced in the handling of diamonds, often pick
out small crystals of zircon, or Dutch boart, by mistake, but the
senseless machine is practically unerring. It will catch rubies,
sapphires, and emer-
