THE MAKING OF STAMPS AND TYPE FROM VULCANIZED INDIA RUBBER.
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.