468 
and depths according to the nature of the work. Cold water is 
always playing over the surfaces of the rollers in use, and it is 
directed usually above the rollers on to the rubber in the hopper, 
but in some cases at the sides, with the object of washing the dirt 
away from the rubber as the rubber emerges from between the 
rollers. The rubber usually goes through two sets of rolls, the first 
two break down the big lumps and roughly sheet the rubber, the 
second pair of rolls is smoother and in closer contact, and the final 
washing and working into a fine-grain uniform sheet is performed 
on these. In preparing rubber for the very finest work, such as 
cut sheet, a third set of rollers with smooth chilled steel surfaces 
is used. The object of these is to crush any particles of sand or 
grit which might otherwise be left in the rubber and damage the 
knives and spoil the sheet rubber when being cut. The rollers are 
always provided with metallic guides to keep the rubber away 
from the ends of the rolls and prevent contamination with grease 
and oil from the bearings. There is great uniformity of pattern 
and general arrangemeut of these washing mills in all the factories, 
the differences are in the form of grooving and roughening of the 
surfaces. The rollers which produce the smoothest and most uni- 
form sheets are those in which the grooves are nearly obliterated, 
and in which the surface has become rough with the natural wear 
of the metal : rolls in this condition w ould, I think, be most 
effective with freshly coagulated latex on an estate. 
DRYING. 
19. The rubber in the washed or crepe form is wet not only 
with surface moisture but with water held in the substance of the 
rubber itself. It is usually dried by hanging up the strips in dark 
rooms warmed to about 90° F., an operation taking about a week 
or ten days. In no case did I notice any artificial circulation of 
the air to accelerate the drying. A few manufactories have adopt- 
ed vacuum drying, which I have already described and discussed. 
There is no sign, however, of this process ousting the older fa- 
shioned method of simple air drying. 
MASTICATING RUBBER. 
20. The next process through which the washed and dryed rub- 
ber passes is that of mastication, during which the rubber is torn, 
stretched, heated, and generally kneaded about until the toughness 
and elasticity, so characteristic of it hitherto, are destroyed, and 
the rubber becomes plastic. 
The masticating machine consists of two steel rollers with 
smooth polished faces, which revolve on horizontal axes in the same 
horizontal plane. The distance between the two rolls can be adjust- 
ed until they are brought into contact with one another. The rolls 
may be of any convenient size, and are usually about 3 feet in 
length and 12 to 18 inches in diameter. They are hollow and 
heated by injected steam, and may be driven at even or differential 
speeds. The machine, in fact, is in many respects similar to a 
rubber-washing machine, but differs in the rollers being smooth 
