Rubber Hand Stamps and the Manipulation of Rubber / A practical treatise on the manufacture of India rubber hand stamps, small articles of India rubber, the hektograph, special inks, cements, and allied subjects

by T. O'Conor Sloane

Next the plate with the plaster is inverted and is pressed steadily down upon the model until it strikes the distance pieces. It is left to set. In about ten minutes it can be raised, when it will be found to give a beautiful impression true to the smallest detail of each letter.

It has been said that water may be used as the mixing fluid. If this is done it is well to strengthen the mould by saturating it with an alcoholic solution of shellac, after it has dried thoroughly, best for a few hours in an oven. This operates to strengthen the small projections that are liable to crumble or to break off in use.

The dealers in rubber stamp supplies sell a lever press for conducting the operation of producing the matrix. The type is locked in a special chase, which is carried on a bed that travels under and out from under the platen of the press upon rollers. From each corner of the chase in which the type model is locked, a pin rises which is encircled by a spiral spring. A square frame of flat iron with holes at the corners for the pins to pass through, rests upon these springs well above the type. The pins pass through holes in its corners. The matrix plate with its coating of plaster is placed upon this frame, which supports it above and not touching the type. The whole is now rolled under the press and the lever pulled to produce the impression. As the pressure is released the frame with the matrix is raised from the type by the action of the springs. This can be done immediately, and before the plaster has set. It is almost impossible to raise it by hand with the requisite steadiness. The same chase with corner pins and springs can be used in a screw press, the one press answering for making the matrix and for moulding and curing the stamps. The plaster matrix can also be made by casting from a thinner mixture of plaster and water. After the type has been set up, or the model has been selected and placed face up and horizontal, a little ridge or projection must be made all around it. Paper can be pasted around it, and wound with thread for this purpose. It is oiled and wiped off as before. The plaster is now mixed with water to the consistency of cream, and is poured upon the model until it lies even with the projecting ledges or paper border. In an hour or less it can be removed. If water is used the mould should before use be treated with shellac solution as already described. The plaster may also be mixed with gum arabic solution, or with three to ten per cent. of powdered marshmallow root. This increases its toughness.

What is known as the oxychloride of zinc cement appears to the author to be far preferable to common plaster of paris. It is a trifle more expensive, but it costs so little that it is well worth trying. It is made by mixing oxide of zinc with a solution of zinc chloride. No particular strength of solution or proportions are prescribed; the zinc chloride solution should be a strong one, and the mixture should be of about the consistency of soft putty.

Zinc chloride may be bought as a solid substance or in strong solution. The latter answers for the mixing directly. It may also be simply made by dissolving metallic zinc in strong hydrochloric acid. The manipulation is exactly the same as with plaster of paris.

The manufacture of papier maché and of other matrices is given in a special chapter. For all ordinary purposes the plaster or cement matrices are ample.

The stamp is made from the mixed uncured sheet rubber, whose preparation in the factory, including the operation of calendering it into sheets, has already been described. The best advice the reader can be given is not to attempt to make it except as a matter of interest and experiment. It can be purchased especially prepared for stamps from the dealers in india rubber.

A piece is cut from the sheet large enough to cover the face of the matrix. It should have a perfectly smooth surface, without cloth wrapper marks sometimes found impressed on it. The sheet as received from the maker is about one-eighth of an inch thick. It is thrown into a box of powdered soapstone or talc to secure a coating of the same on both sides. A little is dusted over the matrix and the excess is blown off. The matrix is now placed upon the base of the press, and heat is applied.

To carry out the process most simply the press if of metal may be placed upon a support over a gas burner or kerosene lamp, or even on a kitchen range or stove. It will in a few minutes become warm. The sheet of india rubber is now dusted off and is placed in the press upon the matrix. The platen of the press is screwed down upon it.

As the india rubber becomes hot it begins to soften and flow. By the action of the screw of the press it must be forced down from time to time as it softens. This drives the putty-like material into all the interstices of the mould. The excess escapes from the sides of the tympan in cases where the latter is of restricted area. The press theoretically should be heated to the vulcanizing temperature, which is 284° F. (140° C.). In practice the heat is not determined with a thermometer. The operator learns by experience how much heat to apply. The regulation type of gas heated press or stamp vulcanizer is shown in the illustration on [page 53].