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

For many articles the hot press can be used. Such articles are mats and other thin flat pieces. The rubber stamp sheet is a good material for them. For thicker articles a thicker sheet can be used, and sheet of any gauge can be procured from the maker. Much of what has been said about india rubber type applies to the making of miscellaneous shapes. It will also be understood where wooden moulds are spoken of that plaster, or, still better, metal can be substituted, and is to be recommended for nice work as the grain of the wood is very apt to show where the india rubber comes in contact with it.

Suction discs and similar small articles into which an extra thickness of india rubber enters are best cured in a vulcanizer. The flower pot arrangement is excellent for such. The time for curing may be somewhat extended on account of the greater thickness of material to be acted on.

Suction Discs.—For suction discs a mould is required which will produce a shallow cup with the edge feathered or reduced to a very slight thickness. Its outer surface should be raised in the centre so as to give a projection for attachment of the hook. The discs are generally made small, not over an inch in diameter, as they are not reliable for any heavy service. Their principal use is to suspend advertising cards and light articles to the glass of show windows. The following is a method of making a simple mould.

A hole to give the outside contour should be bored in a small piece of wood. A marble which will exactly fit the hole is next required. Some plaster of paris is mixed with water and put into the bottom of the hole, and the oiled marble is pressed down until the plaster rises and fills the entire space under the marble. After it has set the marble is removed. The proportions should be so arranged that the plaster will have risen at the sides within an eighth of an inch of the surface of the wood. This gives the exterior mould. For the cup or hollow a marble a shade too large to enter the hole may be used.

One or if necessary two thicknesses of mixed sheet rubber cut into disc shape so as to fit the hole are inserted in the block, and the larger marble is placed on top and screwed down by the press. Heat is now applied in the vulcanizer. When the thermometer indicates 212° F. (100° C.), or better a little more, the mould is withdrawn and the screws turned until the rubber is forced down and the excess begins to squeeze out between the marble and the wood, which two should now nearly touch. It is replaced and the heat is brought up to the curing temperature 284° F. (140° C.). It is possible that a second screwing up may be needed. The spring press is in such cases particularly convenient as it avoids the necessity for removing the press from the vulcanizing chamber. After half an hour it will be thoroughly cured. A hole is made through its centre from side to side thereof, but not penetrating the disc, and through this hole a brass nail is thrust and bent into hook form.

Mould for Suction Discs.

In the cut the correct shape for the mould and consequently for a suction disc is shown. This can be easily secured where a disc already made is procurable by casting in plaster, or, with a little ingenuity the template for the mould and the plunger to be used instead of the marble can be whittled out of wood. The lower body of the mould in such a case can be made of plaster of paris. To secure the alignment of the two parts of the mould, dowel pins, indicated in dotted lines, should be placed near the periphery. The gum should be introduced in a lump near the centre, in order that it may sink well downwards to the bottom of the mould before spreading laterally. Sometimes the tips have a recessed end. This is secured by the use of a mandrel, shown in dotted lines in the axis of the mould. Such discs are sometimes made to be cemented to arrows to be discharged against smooth surfaced targets, to which they adhere on impact by atmospheric pressure, giving rise to a very interesting game.

Another use of suction discs is as photographic negative holders. They can be fastened to a wooden handle and be attached by suction to the back of a negative under treatment. For this purpose they should be at least two inches in diameter.

Pencil Tips.—These are generally little cylinders of india rubber, which fit into a tube that slides over the end of the pencil. They can be thus simply made. A hole is bored in a piece of wood the diameter of and a little more than the depth of the pencil tip. A short cylinder that exactly fits the hole is required for plunger. The gum is put into the hole in little discs, or rolled up into a cylinder, the plunger is placed on top, and the mould put in the press. It is shaped by pressure and cured as described.