A rubber stamp is type matter molded in rubber which is then mounted on a block with a handle as shown in Fig. 77. When the stamp is inked and then pressed on a smooth surface it leaves an impression on it.
Rubber stamps are useful for marking tags, books, boxes, bundles, etc., but it is very bad taste to use them for letter-heads and envelopes.
To make rubber stamps you will need (1) one or more fonts of type; (2) a composing stick if you intend setting up more than one line; (3) a frame called a chase, 4 x 5 inches on the sides made of a strip of wood 1 inch wide and 13/16 inch high, which is the height of type from its feet to its shoulder; (4) another frame, called the matrix frame, made of a strip of wood 1 inch wide, 3/8 inch thick and 4 x 5 inches on the sides. These two frames are shown at A and B in Fig. 74. (5) Two smooth boards 1 inch thick, 6 inches wide and 7 inches long; (6) a toothbrush; (7) a couple of pounds of dental plaster of Paris, and (8) a 1/4 pound of pure unvulcanized rubber. Dental plaster is finer than the ordinary kind and you can buy all you will need from your family dentist for 5 or 10 cents a pound. Unvulcanized rubber for rubber stamps comes in sheets about 3/16 inch thick and is sold under the name of signature stamp gum and you can buy it for about $1.00 a pound.68
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.
68 You can buy signature stamp gum of The Goodyear Tire and Rubber Company, 10 Central Park West, New York City. In ordering of this firm ask for No. 4093, 3/16 inch thick.
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 lead69 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.
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,70 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.
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.
69 See the preceding chapter on printing.
70 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.
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
Cut a piece of the un-vulcanized 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 vulcanizes71 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,72 which is simply a little boiler with a thermometer on top, is used by rubber stamp makers.
71 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.
72The S. S. White Dental Mfg. Co., 5 Union Square, New York, sells them, and unvulcanized rubber as well.
All that you need to do now is to trim off the edges of the rubber stamp with a pair of shears and mount it on a smooth block of wood having a handle as shown in Fig. 77.
Fig. 77. The Rubber Stamp Ready To Use
A special kind of ink is used for rubber stamps, as writing ink is too thin and printing ink spoils the rubber.