INDIARUBBER (Fr., Caoutchouc ; Ger., Kaut schuk) Synonyms, caoutchouc, gum elastic, rubber. Insoluble in water and alcohol, soluble in carbon disulphide, chloroform, benzene, etc. A natural product, the coagulated juice of various plants belonging to the natural order Euphorbiacece. It occurs in brownish black cakes, balls or hollow shaped pieces. It is very elastic and of charac teristic odour. When heated with ro per cent. of sulphur it becomes vulcanised. For photo graphic purposes, the pure washed and masti cated rubber, not the vulcanised rubber, should be used. It is employed as an edging for collodion plates and as a mountant. The most satisfactory solvent is carbon disulphide with 5 per cent. of absolute alcohol, in which the rubber, cut in small pieces, should be allowed to stand with agitation till dissolved. In order to save the trouble of making, the ordinary cycle tyre cement may be used and thinned down with disulphide or chloroform. The great draw back to its use as a mountant is that the rubber perishes in time, and the print leaves the mount.
In process work, a solution of indiarubber in benzole of 2 per cent. strength is used in stripping wet collodion films. The collodion negative is flowed with the rubber solution, and when this is dry it is again flowed with stripping collodion. The rubber solution prevents the solvents of the stripping collodion reaching the original collodion film, and at the same time increases the flexibility and toughness of the film.
Indiarubber solution is used in the collotype process for attaching to the negative the tinfoil used for masking.
In aerograph retouching, some workers use indiarubber solution for stopping-out certain portions of the print on which the aerograph spray is not wanted. After the spraying is done the rubber film easily peels off by rubbing with the finger-tip, and carries with it any spray that has overstepped the boundaries of the stopping out medium.