The Diamond Mines of South Africa 



355 



Automatic Diamond Sorter, called the Greaser, the new method of 

 separating the diamonds. See text below 



the water. After this was determined 

 by 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 dis- 

 covery of the affinity of grease for dia- 

 monds, 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 picture) 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. 



Strange to relate, the descending dia- 



monds stick on the face of the grease, 

 while all other minerals pass over it. 

 Only about one-third of i 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 marvelous. Native work- 

 ers, although experienced in the handling 

 of diamonds, often pick out small crys- 

 tals of zircon, or Dutch boart,by mistake; 

 but the senseless machine is practically 

 unerring. It will catch rubies, sapphires, 

 and emeralds 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. 



