India rubber can be so readily shaped in moulds and the latter are so readily made of plaster of paris that any one who is interested in such things will find endless amusement in working out different designs. Before suggesting any specific articles the following are the general points to be kept in mind.

The material may be uncured mixed sheet of any thickness. As we have seen this material when heated and pressed runs together. It can be forced into any shape by comparatively slight pressure. So exactly does it reproduce the smallest line or mark, that care must be taken to have the moulds very smooth and free from defect. Powdered soapstone is used to prevent adherence to the mould, but great care must be taken not to mix it among the pieces of the india rubber, where several are used in one article, as it will prevent their coalescing or running together.

Another point is to contrive to introduce the proper quantity of rubber. The aim must be to have a slight excess, but to avoid waste this should be as little as possible. Unless some rubber is squeezed out there is no certainty that the mould has been filled. Any projecting “fins” from the overflow are cut off with a knife or scissors after the article is removed from the mould.

Plaster of paris or dental plaster mixed with dextrine or gum arabic water or the zinc oxychloride cement, already described, is to be recommended for the moulds. They should be made, if deep, in frames or “flasks” of tin, as plaster if unsupported is liable to split open when the rubber is forced home.

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.

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.

Sometimes the tips are cup shaped. For these the mould is made in two sections fastened by catches or by pins set in the plaster as shown in the cut. The hole is made larger at bottom than at top, and at the top is a little smaller than the shaft of the pencil. A plunger that nearly fits the small end is provided. The india rubber is placed in the mould and heated. When soft, the plunger is forced down to the proper distance in the press and the article is cured. Care must be taken to give the plunger a good coating of talc, and it must be made to sit vertically. The arrangement of a cylindrical hole shown in the cut secures this result perfectly. As distance piece a pin is passed through the plunger.

Cane and Chair Leg Tips, etc.—By carrying out the process just described with larger moulds and of slightly different section very convenient tips for chair legs and walking canes can be made. Such tips can be modified in size and thickness to answer as covers for the mouths of bottles, test-tubes, etc.

Corks.—These may be made in moulds tapering from top to bottom. The india rubber must be packed in with great care to secure as solid filling as possible. A plunger is used that enters the larger end and is a very little smaller in diameter, so as to descend a little way into the mould. This distance determines the length of the cork. As the perimeter of the plunger strikes the walls of the mould it cuts off almost completely the excess of rubber that has squeezed up past it. An excellent modification of the mould is shown in the cut. The upper part with parallel sides serves as a guide for the plunger. It is a similar extension as the one recommended to be used for the plunger in the hollow pencil and chair leg tip moulds just spoken of.

Mats.—These may generally be made in the hot press. Designs for them in great variety may be found in cut glass and pressed glass dishes. Many of these have patterns on their bottoms that can be moulded in plaster to serve as matrices.

Cord, Thread and Seamless Tube.—By placing the mixed india rubber in a cylindrical mould fitted with piston and with one or more round holes in the bottom, the material may be softened by heat and forced out of the holes by depressing the piston. This will form cylindrical thread or cord. As it descends it may be received in a box of powdered talc and be afterwards cured. By providing the hole with a mandrel seamless tubing may be thus made. In making such the mandrel usually remains in place during the curing. Plenty of powdered talc must be used.

Skeletonized Leaves as Models.—These would form interesting models from which matrices could be made in plaster. It would be possible to produce some very pretty stamps or mats from these and similar models.

After some experience inspection of any article will show how it was moulded. The fin will indicate the joint in the mould, and with this as a clew the mould can be almost certainly constructed like the original.

India Rubber Bulbs.—Bulbs and hollow articles generally, such as dolls, toys and the like, cannot be made without special high pressure hollow moulds. The general process consists in cutting out gores from mixed sheet as for a balloon. The edges are coated with cement (thick benzole or carbon disulphide india rubber solution) and while the rubber is warm the seams are pressed and knitted together with the fingers. A hole is left in one place through which some pure water or water of ammonia is introduced. The bulb is now blown up with the mouth or otherwise, and while inflated the hole is pressed shut. This is often done with the teeth. Any projections around the seams are cut off with curved scissors. The mould is of iron and in two halves. Powdered talc is applied, and the bulb is placed in and shut up in the mould which it should exactly fill. The mould is clamped together and the whole is put into a vulcanizer, and the rubber is cured. The steam and vapor formed by its liquid contents expand it and press it with great force against the sides of the mould. After curing the mould and bulb are removed from the vulcanizer, cooled by a shower bath of cold water, the mould is opened and the bulb is removed. Often an iron pin is left projecting through the side during the vulcanizing, which pin, when withdrawn, leaves the necessary aperture, or it is perforated. The bulbs are polished by tumbling in a revolving cylinder. Considerable skill and practice are needed to succeed in making hollow bulbs. Great accuracy is needed in cutting out the gores and in joining the seams.