a 
470 
are two methods of producing the desired ^result, known as the 
heat cure and cold cure, respectively. 
IN THE HEAT CURE. 
2 3- T* 1 ? raw rubber and finely powdered sulphur are mixed 
together intimately on a mixing or a masticating machine. If other 
ingredients are to be added to the rubber it is done at the same 
time that the sulphur is incorporated. Chemical union between 
the sulphur and the rubber takes place neither during this mixing 
nor afterwards, as long as the mixture is kept cold. If hoWever, 
it be heated to about 300° F. chemical union takes place slowly 
and the new product, vulcanised rubber, is formed. By far the 
greater bulk of rubber is vulcanised in this way. The hot cham- 
bers in which the actual heating and vulcanisation are carried out 
are of several types, and differ in the way in which the heat is 
applied. Where pressure has to be exerted on the rubber during 
vulcanisation the goods are vulcanised in moulds, between large 
plates of iron, which are hollow and heated by steam. In other 
cases, large chambers heated by steam are used and into these the 
rubber goods, placed on trays and smothered in French chalk, are 
taken. Fabrics coated with rubber — such as sheeting and mack- 
intosh cloth — are wound round a large iron drum and immersed in 
water, which under pressure is heated to the required temperature. 
Long tunnels, 50 or 60 feet long, dry heated by steam, are used 
for vulcanising hose pipe and lengths of tubing which cannot be 
coiled. The temperature is regulated so as to slowly rise to about 
300° F., and after maintenance at that point for a period varying 
from half to three hours, it is slowly allowed to drop again. 
During vulcanisation a portion of the sulphur combines with the 
rubber and forms the new addition compound, which is quite 
distinct from raw india-rubber, and from which the sulphur 
cannot be removed by any known process. Although the whole 
of the rubber is acted upon by the sulphur to greater or lesser 
degree, the action is slow and the whole of the sulphur present 
is not used up during the short period that the vulcanisation lasts, 
and free uncombined sulphur remains disseminated throughout the 
vulcanised product. A prolonged period of heating during 
vulcanisation diminishes this excess of sulphur, and leads to the 
production of more highly vulcanised rubber. The more sulphur 
which vulcanised rubber has used and actually combined with, the 
darker and harder the product until the extremes of vulcanite and 
ebonite are reached. From partially vulcanised goods the excess 
of free sulphur can be Chemically extracted, and this is one of the 
operations in “ recovered ” vulcanised rubber : the combined 
sulphur, however, remains always in the recovered rubber. The 
recovery of rubber, therefore, is an operation by which the mecha- 
nically mixed substances, such as the excess of sulphur and the 
fillings with which the rubber was mixed in manufacture, are 
wholly or partially removed, and the residue resulting is worked 
up into a form in which it can be blended with new rubber, and 
act as a substitute for a portion. 
