Freshly drawn latex is alkaline in reaction. The addition of a suitable amount of any kind of acid leads to coagulation and to the separation of the rubber, with most of the resins and proteids, from the watery and soluble constituents of the latex.
According to the older view of coagulation, this was supposed to be due to the bursting of the skins surrounding the globules, followed by the coalescence of the latter. Recent observations have shown that the globules may persist in the coagulated latex. This fact seems to support the view that the coagulation of Hevea latex by acid consists in the formation of a network of coagulated proteid, which entangles the rubber globules in its meshes and contracts upon itself. The coalescence of the globules is only effected during the subsequent pressing and working of the wet rubber. The question of the extent to which polymerisation occurs during coagulation appears to be still a disputed one.
Fresh latex also contains an oxydising enzyme which leads to a darkening in colour on prolonged exposure to the air. The enzyme may be destroyed by immersing the coagulated rubber for a few minutes in water at 80°C. and this treatment has been recommended as a commercial process in order to obtain light coloured rubber. Such treatment can be replaced to some extent by thorough washing, and completely by the use of vacuum driers, in which the rubber is rapidly dried at a fairly high temperature.