Although all reference hitherto in the matters of stamps and type has been to their manufacture from uncured india rubber, a good deal can be done with vulcanized and cured gum. The stock that is known in the trade as pure gum, such as is used for bicycle tyres, for steam packing and the like, can be made to yield to moulding to a certain degree. It will not flow and unite as will the uncured gum, but it is evident that in certain cases its stiffness is even an advantage. Thus with it, rubber type can be made without any flask or frame. The material has stiffness enough to support itself.

The manipulation is of the simplest. A piece is cut out with a knife so as to be of proper thickness and size. It should be a little thicker than will ultimately be required. The two opposite surfaces should be smooth and parallel. It is talced, and placed in the press with the matrix beneath it, and subjected to pressure by the screws being turned down. It is then placed in the vulcanizing chamber and heated to about 284° F. (140° C.). After it has become hot it softens a little. The press is removed from the hot chamber and is again screwed down as hard as the matrix can stand. This point is largely a matter of judgment. The heat is largely indifferent as long as it is anywhere near the above temperature.

By one or two repetitions of the pressing and heating the softened india rubber can be made to take quite a deep impression from a suitable matrix. It is allowed to cool under full pressure. When removed from the press, it will retain the characters.

It is evident that impressions in as high relief or as deep and clear as those yielded by uncured india rubber need not be expected. But where the other cannot be had, or where some experimental or temporary work only is on hand, this process will be very convenient.

The material may be half an inch thick. From such india rubber type can be cut with advantage.

Old rubber can be thus used. The writer has obtained excellent results from pieces of an old discarded bicycle tyre.

The great point is to apply a heavy pressure to the hot material. Many other articles can be thus produced extemporaneously. At the same time it must be considered only a makeshift. One who has used the soft, easy flowing uncured gum would never be reconciled to the use of so rigid and difficultly moulded a material, one too that can never be trusted to reproduce intricate moulds of considerable depth. In the slow yielding of the half melted uncured gum, so amenable to slight pressure, a quality of availability is found that is missed in the other. One is worked by main force where the other readily yields and takes the most complicated shapes.

By the above process stamps of such thickness may be made that they can be used without handles. It is also useful for impressing a designation of any kind upon ready cured articles. It suggests a very useful department of manipulation of india rubber.

The heating and moulding can be done also in a hot liquid bath such as described in chapter XI.