Fig. 74. the matrix frame, chase and boards for making rubber stamps
Unvulcanized rubber is crude india-rubber mixed with sulphur and when this is heated it gets very soft and can be molded by putting it under pressure; when it gets cold it is not only much stronger than before but it is very elastic as well.
Making the Mold.
—Since you know how to make pewter castings and how to set type, making a rubber stamp will be as easy as rolling off a log.
When you have the type set up that you want to make the rubber stamp of, put a lead[73] on each side of it, oil it all over with sweet oil and tie a string around it tight. Lay the thick, shoulder high wood frame over the type matter you have set and see that it is in the middle of it.
[73] See the preceding chapter on [printing].
Next mix up a little more than enough plaster with cold water in a bowl to fill the frame; stir it with a tablespoon and make it about as thick as sorghum molasses. Pour the plaster all around the type in the frame and fill up the space between them as high as the face of the type. Now let the plaster set,[74] that is, get hard, which it will do in a very few minutes. When it is hard enough to hold the type in place and yet before it gets solid take the tooth-brush, dip it in water and brush away the plaster until it is exactly even with the frame, and hence, even with the shoulders of the type as shown in [Fig. 75].
[74] When calcium sulphate is heated it loses its water of crystallization and forms a powder, which we call plaster of Paris; the plaster has the power of taking up water and forming a solid substance, and this process is called setting.
When the plaster has set hard oil the face of the type and the plaster with sweet oil; now lay the thin frame over the thick frame; mix up some more plaster with water and make it thin enough so that it will flow easily into every little space of the type and fill the frame up with it.