A fast curing raw rubber is not necessarily a desirable type for all manufacturing purposes. In the vulcanising of large masses of rubber, a slower rather than a faster vulcanising rubber may be desirable, so as to give ample time for the heat to penetrate and spread evenly throughout the mass. But for many purposes a fast curing rubber enables a larger output to be obtained, so that artificial organic accelerators are coming more and more into use. The addition of such accelerators might be obviated, if a suitable fast curing rubber were available, but it is essential that such rubber should be uniform. It is just in this respect that slab rubber or crepe made therefrom is found to be deficient.[56] The rate of cure depends on the functions of wild bacteria, which are naturally sensitive to changes of conditions, such as temperature, etc. The coagulated rubber depends on chance circumstances for infection, and, as a natural result, the activity of the bacteria and the nature and amounts of active vulcanising agent produced will vary and be difficult to control. Consequently, the rate of cure of slab rubber shows considerably greater variation than ordinary crepe or sheet.[57] This, in our opinion, is the main difficulty of utilising "slab," or crepe prepared from it. Experience in other industries, using micro-organisms, has shown that the only method of control has been to replace the wild growths by cultures of some particular strain, as, for instance, in yeasts for brewing. To control the rate of cure of slab, it might be possible to use a special culture for the purpose. [56] Bulletin R.G.A., January, 1920, p. 6; January, 1921, p. 47. [57] _Ibid._, January, 1920, p. 68. Other less usual methods of preparation, referred to in the earlier part of this book, do not call for particular mention, as the properties of the rubber do not differ much from ordinary sheet or crepe. It is mainly a matter of variation in rate of cure.