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.
(c) The inclusion of air-bubbles.
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.
That this has always been true of the preparation of block rubber cannot be denied. It is possible, of course, for one type of block to show the presence of air-bells more than another type, the proportion of air enclosed in blocking depending upon the nature of the crepe of which the block is composed. A block built up of layers of smooth, fine crepe would be expected to contain less air-bells than a block composed of layers of a rough crepe.
Block rubber has been seen which was free from air-bells, but this was the thin variety of block prepared for show purposes with far greater care, probably, than would be expended in commercial preparations.