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.
CHAPTER XIII.
EBONITE, VULCANITE AND GUTTA-PERCHA.
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.