Let this frame stay on the lower frame over night so that it will get very hard and you can then lift it off, when a very sharp impression of the type faces will be formed in it shoulder deep, that is as deep in the plaster as the face of the type is high. This frame with its plaster impression is called the matrix.
Fig. 75d. the type in the chase. e. plaster of paris impression in the matrix frame
Vulcanizing the Rubber.
—Cut a piece of the unvulcanized gum rubber ¹⁄₄ inch wider and longer all round than the impression of the type; peel the strip of muslin from the strip of rubber gum and lay it on the matrix. Put one of the boards on top of the rubber and the other on the bottom of the matrix and screw them together tight with the iron clamps as shown in [Fig. 76].
Half fill a kettle with water; lay the mold on top of the kettle—but not in the water—and put both of them in a hot oven for 30 minutes. When the rubber gum gets hot it softens and the pressure of the screws forces it into the letters of the matrix and so makes rubber type of them. The steam from the kettle will keep the wood from charring and the rubber from burning but has no other action on it.
Fig. 76. the matrix with the rubber gum in place ready to vulcanize
The heat vulcanizes[75] the rubber gum and makes it springy and stretchy, but if it gets too hot it will become hard and you will have hard rubber instead. To get just the right degree of heat a vulcanizer,[76] which is simply a little boiler with a thermometer on top, is used by rubber stamp makers.
[75] Vulcanizing is the process of heating raw India rubber with sulphur; the sulphur combines with the rubber to form a new compound. If a large amount of sulphur is used and great heat is used hard rubber, or vulcanite, or ebonite is formed. If a small amount of sulphur and a low heat are used the elastic rubber that is so common is formed.