Ebonite and Vulcanite.—These two well known substances are india rubber, in which the vulcanization process has been intensified. From twenty-five to fifty per cent. of sulphur is added in the mixing, and the curing is prolonged to several hours. A temperature of 275° F. (135° C.) for six to ten hours is sometimes recommended, but generally a shorter period at the regular temperature, 284° F. (140° C.), may be employed.
The mixed sheet is made and sold extensively for dentists’ use. It is soft and flexible and very easily moulded. It is treated like the regular mixed sheet in every respect, except that plumbago brushed on the slightly oiled surface of the mould is recommended instead of the light colored talc, to prevent adherence. Wax where available is better than oil.
Sometimes specimens are built up in sections. About an hour before full vulcanization in the fourth stage, new material can be added and will attach itself to the old. The stages of vulcanization are thus given by Bolas.
“Several distinct stages or steps may be traced during the curing of ebonite; and I wish to call your attention to some specimens illustrating these various stages.
“Here, in the first place, is the plain mixture of sulphur and rubber, this being nearly white, and capable of becoming quite plastic or soft by the application of a gentle heat.
“The second specimen illustrates the action of a very moderate degree of heat on the mixed material, this particular sample having been heated to 128° Centigrade for twenty minutes. It is, as you see, somewhat darkened, and has lost a little of its original softness; while a degree of heat which would have rendered the original mixture plastic, like putty, fails to make much impression upon it.
“The third specimen illustrates the effect of a more prolonged heating, this sample having been heated for an hour to 135° Centigrade. It is olive green in color, and has acquired a certain amount of elasticity, resembling that of a rather inferior quality of vulcanized caoutchouc.
“The fourth stage of curing is illustrated by this specimen, which you see is brown, and tolerably hard. Ebonite in this state refuses altogether to become plastic by heat, and a temperature of 150° maintained for half an hour or less would suffice to bring it to the fifth stage, or that of finished ebonite.
“The fifth stage, or that of properly cured ebonite, is the goal to be arrived at in manufacturing the material. There should be no places where the curing is imperfect, a kind of defect which is likely to happen when articles of unusual thickness are vulcanized, and no portion of the ebonite should be spongy or honeycombed by air bubbles.
“The sixth, or spongy state, is generally the result of over-heating, bubbles of gas forming in the material, and converting it into a kind of porous, cinder-like mass.
“A specimen will now be handed round, which illustrates the third, fourth, fifth and sixth stages, as already described. The specimen in question was cured on a hot plate, this having probably been heated to 160° or 170° Centigrade; and you will be able to trace all gradations in the curing operation, from the first setting of the plastic material to the destruction of the ebonite by overheating.”
Cement for uniting pieces of the partially cured material may be made by rubbing up some of the untreated scrap with benzole.
At the heat of boiling water, ebonite can be bent to a certain extent, which bend it retains on cooling. When warm an impression of a coin or relief die may be made on it by heavy pressure which it will retain. On heating the image disappears. If before heating the surface is planed off and the piece is heated the image formerly in intaglio will expand into relief.
By the exact process of rubber stamp making excellent stereotype plates may be made of ebonite.
It can be turned at high speed in a lathe and polished with fine 000 emery paper followed by a cloth bob with rotten stone, etc., and water or oil. Blotting paper, charged with the above or with tripoli, is excellent for polishing small surfaces by hand.
Ebonite is a good connecting material between softer rubber and iron, the whole being vulcanized together; the iron should be well roughened or cut into rasp-like or file-like projections.
Ebonite is properly the name for black hard rubber, and vulcanite for the colored products such as used by dentists and others.
Gutta-percha is prepared by coagulation from the juice or sap of several trees, among others the Isonandra gutta, of Borneo and the East Indian Archipelago. The product gutta-percha is identical in composition with india rubber. It is hard at all ordinary temperatures.
Its manufacture includes purification and mastication. It is far more amenable to treatment than is india rubber. Many materials are mixed with it as adulterants or otherwise in the factories.
It is more useful in the form of sheets. These when heated to 122° F. (50° C.) become pliable and can be moulded by pressure to any degree. At the temperature of boiling water it becomes pasty and adhesive, and at 266° F. (130° C.) it is so soft that it may be considered as melted.
It is an admirable moulding material. Stereotypes and other relief or intaglio images can be made by pressing it while heated. These are often absolutely perfect reproductions of the original.
Dishes for photographic purposes, etc., are easily made out of the sheet. By gentle warming they become pliable, and a greater heat makes surfaces capable of adhering by pressure.
Tubes can be made by the squirting process, as used for india rubber. Wires are coated with it in a similar manner.
It has several defects. It is not durable if exposed to the air with consequent changes of temperature. It is also too easily softened by heat, as of course no hot liquid can be introduced into a gutta-percha vessel. The Parkes cold curing process can be applied to it, which makes it more indifferent to heat. This is applied by dipping an instant and drying. After several repetitions the period of dipping is prolonged and ultimately it is left immersed some time. If left immersed at first it would dissolve.
It is soluble in most caoutchouc solvents, particularly in carbon disulphide.