“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.