India rubber type are often used to set up different inscriptions in wooden handles, or different date figures in rubber stamps. The latter are in such cases made with slots or recesses to receive them. Rubber type are much shorter than regular type, and as a rule are larger in the body in proportion to the face of the letter. Where only a few are required the following process is the simplest way of making them from mixed rubber sheet.
The type which are to be copied are set up on a level base or imposing stone, and quads or spaces are put between them. High quads and spaces should be used; otherwise they should be pushed up until even with the shoulders of the type. After oiling the faces a matrix is produced exactly as described for stamps. Before it has set quite hard the plaster or cement is cut off so that it will just fit within a little “flask” or frame.
The latter may be made of tin or wood and may be rectangular or circular, provided it is large enough to include within its area the full working face of the matrix. It should be about half an inch or five-eighths of an inch deep. Its object is to prevent the softened india rubber from spreading, so as to secure the requisite height of the type produced.
India Rubber Type Mould.
A piece of wood or metal is cut so as to fit closely within this frame like a plunger. It is provided with shoulders or cross pieces, so as to limit the depth to which it can be inserted. It will be seen that when matrix, flask, and plunger are all put together a complete mould for a block of type is produced, as shown in the [illustration], the matrix with its plate forming the bottom of the box. After the flask is placed upon the matrix it is filled with the mixed uncured india rubber sheet. As a matter of preference thick sheet is used, but scraps of all shapes can be employed as it all fuses together. The mould and matrix are of course first well dusted with talc powder. The plunger is put on and the whole is pressed. Heat is next applied in a vulcanizer or hot air chamber, such as the flower pot arrangement, or in boiling water. As the sheet reaches the boiling point 212° F. (100° C.) the flask is removed and the plunger examined. If it goes down to its seat without expelling any india rubber more of the latter is required and is accordingly inserted, the plunger being taken off for the purpose. The softened gum should ooze out around the sides of the plunger. The whole is again put under pressure, and the platen is screwed down, and if all is right an excess of rubber showing itself, the whole is put in the hot chamber, the heat is raised to 284° F. (140° C.), and is maintained there for half an hour.
It is almost a necessity to secure the matrix plate to the bottom of the flask. This for a single operation may be done by screws, or for several operations by hooks or catches.
When the curing is complete the mould is removed from the vulcanizer, is allowed to cool and is opened. The block of type will come out with perfect reproduction of the letters upon one side. If all the directions have been followed as regards distance pieces, level imposing surface, etc., both faces will be exactly parallel, and any number of other blocks can be reproduced of exactly the same height, not necessarily from the same matrix, although one good matrix can be used many times.
The type have now to be cut apart. This is done with a sharp knife which is kept wet. It is worked with a sawing motion, and if sharp and properly used will cut with regularity, and smoothly. Type with knife marks on the sides are always unmechanical in appearance and seem to be “home made.”
The object of using high quads and spaces or of pushing them up, will now be evident. It secures the evenness of the general face of the block of letters, which otherwise would have a deep depression between each pair of letters. If the quads and leads are properly arranged, the letters will project upwards from a smooth, plane surface.