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.
CHAPTER VIII.
VARIOUS TYPE MATRICES FOR RUBBER STAMPS AND TYPES.
Matrices for stamp moulds can be made by several of the methods used by stereotypers. Thus an electrotype could be taken directly from the face of the type. There would be little or no utility in doing this where the simpler processes are available.
PAPIER MACHÉ MATRICES.
The stereotyper for daily newspaper work uses very generally the papier maché or “flong” process of reproducing the page. This is also available for rubber stamp making.
The first requirement is paste. This is made by softening twelve parts of whiting in forty parts of water, letting it soak for an hour or more. Nine parts of wheat flour are added. This is best mixed with a little water before adding to the main mixture. It is then brought to the boil and seven parts of glue softened by soaking in twenty-one parts of water, are added. For each gallon of such mixture, one ounce of white crystallized carbolic acid is added if it is to be kept for a long time.
The “flong” is made by pasting together, one on top of the other, a sheet of fine hard tissue paper, three sheets of blotting paper (about 23 pounds to the ream), and a heavy sheet of manilla paper. The pasting must be smooth and each layer must be pressed and rubbed down, but not too hard. It is very important to secure perfect smoothness and regularity, and entire absence of air bubbles.