India rubber presents some difficulties in its solution. If a piece of pure gum just as received by the factory is placed in hot water it will swell and whiten after a while, but will not dissolve. If a similar piece is placed in benzole a similar but greatly exaggerated action takes place. The piece if left to soak for a day or more swells enormously, but very little solution is effected.

The swollen india rubber can be removed from the benzole in a single piece. It will display all the layers and marks of the original piece which was perhaps of not one hundredth part of its volume. Some parts will be a perfect transparent jelly.

It has been found that masticated india rubber dissolves with comparatively little difficulty. If the experimenter will place in a porcelain mortar, the jelly-like mass obtained as above detailed, and will rub it up thoroughly, it will be effectually masticated. This requires a little patience, as the slippery material seems to elude the pestle. Yet eventually it will all be reduced to a perfectly homogeneous mass. Its action while being rubbed up is very peculiar. At first no progress seems to be made. After a little the lumps yield to the friction. The rubber then begins to attach itself to the pestle and mortar, and begins to be drawn out into ever changing webs and threads. As the operation approaches completion the material makes a snapping, crackling noise familiar to all rubber workers. When complete there will be no lump left, and the whole will be a uniform pulp.

If benzole or a volatile solvent has been used, the rubber will easily be removed from the mortar with a spatula or palette knife. If turpentine was the solvent it will be impossible to remove the last traces except after long standing or by solution.

If replaced in the original solvent it will now come into nearly or quite perfect solution. This is the best way of masticating on the small scale. It is almost impossible to masticate untreated gum in an ordinary mortar.

The dealers sell a special india rubber for the manufacture of cement and solutions. This is so treated by mastication that it dissolves with great readiness. It is also said that heating under pressure is used to dissolve it in some factories.

Many solvents have been used and none work without some difficulty. Benzole, coal tar naptha, petroleum naptha, carbon disulphide, ether and chloroform, oil of turpentine and caoutchoucin are the best known. The naptha best suited for its solution is termed solvent naptha. It has a specific gravity of .850 at 60° F. (15½° C.); it boils at from 240° F. (115½° C.) to 250° F. (121° C.) and on evaporation should leave no more than ten per cent. of residue at 320° F. (160° C.)

Payen recommends a mixture of 95 parts bisulphide of carbon with 5 parts of absolute alcohol.

Commercial chloroform is apt to be too impure to act as a good solvent. It is apt to contain alcohol mixed with it as a preservative, which impairs its effectiveness.

Some of these solutions are better suited than others for the deposition of thin layers by evaporation. Turpentine gives a very sticky and unmanageable solution, which dries very slowly. Payen’s solution and the chloroform and the benzole solutions may be cited as especially adapted for this purpose. Careful vulcanization by the cold curing method can be applied to articles made by such deposition from evaporation.

In the case of all of them some form of mastication for the india rubber is needed. The simple mortar grinding of the gum swelled by the solvent is the only practical treatment without special apparatus.

When it is remembered that fixed oils are destroyers of vulcanized or unvulcanized india rubber it will be obvious how important it is to use pure solvents. Too great care cannot be taken to preserve the liquids pure and free from such matter.

A solid hydrocarbon may be used. Thus paraffin wax, such as candles are made of, when melted acts as a solvent. The resulting liquid solidifies when it cools, retaining an almost greasy feel.

Boiling oil of turpentine is recommended by some for the solution of vulcanized india rubber. Phenyle sulphide, it is stated, will soften it so as to render it workable. The latter discovery is credited to Dr. Stenhouse.

It is stated that a solution or pasty mixture of one part of caoutchouc in eleven parts of turpentine with one half part of a hot concentrated solution of sulphur (potassium sulphide) gives on evaporation a film neither tacky nor soft, a species of vulcanization taking place.

It is of much interest to note that an aqueous solution of india rubber has been proposed in which the vehicle is a solution of borax in water. This is well known to be a solvent for shellac and other resins. It has been recommended often as a vehicle for rubbing up india ink. The ink made by mixing lampblack with the shellac solution is nearly waterproof. A shellac varnish is given by the plain solution.

The experiments upon india rubber were published in a recent trade paper. One method of making the solution is as follows.

A solution of borax two fifths saturated is made by adding to two volumes of saturated solution three volumes of water. To this is added a solution of india rubber in benzole or other hydrocarbon of such strength and in such quantity as to contain from three and one-half to four and one-half per cent. of india rubber referred to the borax solution. It is now vigorously shaken and heated to 120°-140° F. (49°-60° C.) and the agitation, not too violent, is continued until it cools. Ceara or Madagascar rubber answers best; Para is not so good for this formula. This may be termed the indirect or emulsion method.

For direct solution from two to three volumes of water may be added to three volumes of saturated borax solution. The india rubber is added in extremely thin shavings and the solution is heated. For weak solutions the boiling point need not be reached. For strong solutions the heating should be done under pressure so as to bring up the pressure to one to three atmospheres.

Such solutions may contain as much as eight per cent. of the gum. The mixture is liable to coagulate or gelatinize just at the wrong time, but it may be of value as a vehicle or as a waterproofing agent. It deserves further investigation, which it is to be hoped it will duly receive.

Great care is necessary in working with naptha, benzole, carbon disulphide and similar liquids. Their vapor is given off at ordinary temperatures and may travel some distance to a lamp or fire and become ignited and carry the flame back to the vessel. Their vapors are also anæsthetic and should be avoided as regards inhalation.