The cold process, discovered by Parkes, can only be used for vulcanising very thin sheet. This is due to the extreme rapidity of the process, which is such that if it were applied to thick rubber, the surface of the object would be converted into vulcanite before the interior was properly vulcanised. The reagent employed is sulphur monochloride, prepared by passing dry chlorine over heated sulphur. This substance is very active chemically, and is decomposed by water. As a solvent, carbon bisulphide is almost universally adopted. Carbon bisulphide is exceedingly poisonous and inflammable, but no other solvent has been found to give equally good results. The strength of the solution used is generally about 2½ per cent The sheet rubber, which must first be very thoroughly dried, is immersed in the solution for three minutes or less, according to its thickness.
Whatever the method of vulcanisation employed, there is always a certain amount of after effect, owing to the prolonged slow action of the residual rubber. Articles are therefore almost always rather less perfectly vulcanised than their final condition requires. The goods afterwards improve to some extent by keeping. In the case of articles in which a high degree of permanent elasticity is required, the excess of sulphur must be removed by boiling in caustic soda, followed by a thorough washing in water in order to remove the alkali. This process is necessary, for example, in the manufacture of elastic thread, which is cut from vulcanised spread sheet and afterwards freed from sulphur in the manner described.