BLOCK RUBBER.--This mode of preparation is employed only in comparatively few instances. The block is prepared from crepe rubber, which has been dried either in a hot-air drier or in a vacuum chamber.
There is another type of block which is made by placing layers of dry crepe under considerable pressure. This is not the true type of block, and the layers are quite distinct--_i.e._, they do not amalgamate. Usually this pressed rubber consists of lower grades of crepe, and it should not be popular, inasmuch as it leaves too wide an opportunity for the inclusion of dirt, bark particles, and other impurities, which cannot be seen generally on account of the protective colour of the rubber.
In the true type of block, the layers are in a plastic condition, due to heat, when they leave the drying-chamber; and being immediately submitted to great pressure the result is a homogeneous mass in which the layers disappear by amalgamation. Only the best grade of crepe is employed, and given the absence of defects in the layers there should be no complaint regarding the final block.
Prepared in slabs which are three or four inches in thickness, the product is easily handled, and should be sufficiently translucent to make it possible to distinguish the shape of the hand when held between the block and the light. This is not possible when blocks are made of greater thickness.
The only complaints which it should be possible to lodge against block rubbers are:
(_a_) The inclusion of dirt and other matter.
(_b_) The use of layers of crepe which have some defect.
The remedy for (_a_) and (_b_) lies in the hands of the factory superintendent. The last ground of complaint is dependent upon the style of preparation of the original layers of crepe.
When layers of crepe are placed one upon the other, and submitted to great pressure, it is natural to suppose that air would be contained in spaces, and would be unable to escape. To guard against this, it would seem necessary to prepare the crepe thin and with a fairly good surface finish.
It must be obvious to all acquainted with the processes involved in the preparation of block rubber, that no possibility exists for the presence of air-bells actually enclosed _in_ thin crepe. When the vacuum-dried crepe is folded preparatory to the blocking process it is apparent that between the layers there must always be a considerable volume of air, a small proportion of which is bound to be retained owing to the nature of the surface of crepe rubber.