to
the pulsator, where they are sifted into five sizes, ranging from one
sixth to five eighths of an inch diameter, and passed into a
combination of jigs or pulsators with stationary bottoms covered with
screens with square meshes a little coarser than the perforated plates
of the cylinders that size the concentrate for the jigs. Upon the jig
screens, a layer of leaden bullets for the finer sizes and of iron
bullets for the coarser sizes is spread, forming a bed that prevents
the deposit from passing through the screen too rapidly. The heaviest
part of the deposit, with the diamonds, passes through the screens into
pointed boxes from which the deposit is drawn off and taken to the
sorting tables. The refuse goes to the tailing heap.
But
one per cent, of the total amount of blue ground washed goes to the
pulsator, and fifty-eight per cent, of this flows over the jigs as
waste. Numerous experiments were unsuccessfully made to effect the
separation of the diamonds from the worthless concentrates in a less
tedious and expensive way than sorting them by hand, when a De Beers
employee, Fred Kirsten, suggested coating a shaking or percussion
table with grease; and this resulted in the notable discovery that
diamonds only, of all
