MACHINERY AND MANUFACTURE 191 



closer together and press the sheets of rubber into a very 

 compact web of lace-like rubber with as few holes in it as 

 possible. The smooth, chilled rollers generally supplied are 

 too much polished and do not grip the sheets of rubber properly, 

 and complaints are made that the rubber " jumps." The 

 action of these rollers is too much that of polishing the rubber. 

 Hard, cast-iron rollers, owing to the slight grain on them, are 

 said to grip the rubber much better in this mill, and are much 

 more effective in pressing the crepe, so that small holes are filled 

 up. In practice it is found that with such rollers the crepe can 

 be finished by passing them through four or five times, whereas 

 with polished rollers the crepe has to be passed through seven 

 times, and the final result is not so good, the crepe having more 

 small holes left in it. The result of careful observations on 

 Kuala Lumpur, Langkat Sumatra and other estates go to 

 establish this fact, although it is disputed by some others. 

 Chilled rollers are harder than hard cast-iron ones, and there- 

 fore wear longer, and are thought to resist the action of any 

 superfluous acid in the coagulated latex rather better. As, 

 however, of recent years more care is being taken to add a 

 smaller excess of acetic acid to effect a complete coagulation 

 than was formerly the case, complaints on this score are of 

 rare occurrence nowadays. The cost of chilled-steel rollers, 

 18 in. by 12 in., is about 6 higher than of what are known as 

 hard-cast or hard-grain rollers. This higher price quite dis- 

 counts the saving of the, presumably, longer life of the chilled 

 rollers. 



Lump-rubber, by which is meant the lumps of rubber self- 

 coagulated in the latex cans, passes through the same mills in 

 the same sequence as crepe-rubber. 



Sheet-rubber for smoking is usually simply coagulated and 

 run through a mill with plain or slightly-grooved rollers. This 

 process, while pressing the coagulated rubber from a wet, 

 spongy mass into sheets, does not subject it to the severe shred- 

 ding, tearing and washing that crepe-rubber undergoes, and the 

 sheet-rubber therefore retains a lot of the protein elements 

 originally present in the latex, which are more or less liable to 

 set up fermentation actions. It is for this reason that sheet- 

 rubber is usually smoked, smoke being of an antiseptic 

 character. 



