Historical Account. 9 



rubber is produced. Although the principles involved are well known, the 

 precise steps are preserved secret. The process, which is based on meth- 

 ods of organic analysis, is not widely used, and only a small part of the 

 total manufacture is carried on in this way. 



Of greater interest, not only in itself, but for the future economic 

 development of the rubber industry, is the mechanical method. This 

 may be described only in its broader outlines, since the steps employed 

 by various manufacturers are changed from time to time as experience 

 indicates. 



The shrub is first washed so as to free it from dust and other foreign 

 matters which affect the specific gravity of the " worm " rubber by becom- 

 ing attached to the agglomerated particles. It is then passed between 

 rolls which comminute it while it is being sprinkled with water. 1 The 

 rolls used have been supplied with knives, or have been adapted to 

 pulverize the material, or, as now used, the shrub may be run twice be- 

 tween corrugated rollers, running differentially, for the sake of even and 

 fairly fine grinding. The resulting mass is then placed in a pebble-mill, 

 which is a short cylinder of steel, lined with Belgian flint bricks, such as 

 is used for grinding cement, paint, charcoal, and the like (plate 3, figs. 

 A, B). The grinding is accomplished by means of Norwegian or Medi- 

 terranean flint shore-pebbles. 2 



The pebble-mill charge consists of one-third its volume of pebbles, 

 one-half of water, together with 6 to 8 bushels of shrub. The mill is 

 revolved at the rate of about 30 times a minute for a period lasting 90 

 minutes to 2 hours, at the expiration of which time there results a finely 

 ground pulp consisting of minute agglomerations of rubber mixed with 

 fine particles of bagasse. This is separated from the dirty water in which 

 it was ground and is then run into tanks, where a skimming process sepa- 

 rates the rubber, which floats, from the bagasse, which sinks. A part 

 of the bagasse, however, does not sink at this time, namely, that con- 

 sisting of flakes of light yellow cork. 



Nor is the rubber free from particles of wood fiber, imprisoning more 

 or less air, and this interferes with the complete separation of rubber and 

 bagasse. The complete water-logging of the bagasse may be attained 

 by means of a compressor, in which the skimmed rubber, with its adhe- 

 rent fiber, is subjected under water to a pressure of about 225 pounds 

 for a period of 15 minutes to 2 hours, according to the kind of shrub 

 being treated. Subsequent treatment in a beater-washer, an elliptical 

 tank, supplied with a paddle-wheel of half its transverse diameter, 

 prepares for the final separation of rubber and bagasse in settling-tanks. 



1 It has been suggested (Whittelsey, 1908) that decortication, previous to 

 grinding, would be an economy. It is interesting to recall that this was done 

 on an experimental scale, albeit a generous one in 1895 (India Rubber World, 

 April 1895). 



An alternative method, recently proposed by Chute and Randel (India Rubber 

 World, vol. 42, p. 360, 1 9 10), consists in grinding the shrub dry and then deresinat- 

 ing (the solvent to be recovered by distillation) . The ground shrub, now supposedly 

 free from resin, is then treated as here described, beginning with the pebble-mill. 



2 The internal structure of this mill has been the subject of numerous patents. 

 Thus, steel balls, associated with various forms of projections from the interior 

 surface of the cylinder, have been used, but without supplanting the " pebble-mill." 



