(_a_) The relative thickness of the rubber. (_b_) The preliminary treatment of rolling. (_c_) The temperature of the smoke-house, and (_d_) The type of smoke-house used. This point will be treated in a subsequent chapter. FUELS FOR SMOKING.--The general idea formerly held was that the beneficial effects of smoking were to be attributed to the constituents of the smoke, and chiefly the creosotic substances. This is not now the opinion of the writers, who attribute the effect largely to the temperature of drying and constituents of the smoke other than creosotic substances. There can be no doubt that the presence of creosotic bodies is responsible largely for the absence of moulds and the existence of the typical odour, but it is becoming increasingly known that the employment of substances rich in creosote is not required or desirable. Estates used to be put to considerable expense in the purchase of "bakau" (a mangrove timber rich in creosote and creating much heat), under the idea that it was the best material and almost indispensable. Most estates now restrict themselves to the consumption of timber obtained from their own areas. Thinning-out programmes are largely responsible for the supply, but the local authorities are much concerned regarding future supplies; and consideration has been given in some quarters to the question of the development of quick-growing trees on estates with a view to safeguarding the future. This seems to be desirable, as it is difficult to imagine that the place of timber can be taken by any other material in the smoke-curing of rubber. Unless some such precautions are taken it is not difficult to predict that, in course of time, some estates will be able to continue the preparation of smoked sheets only at considerable expense in obtaining suitable fuel from a distance. It is not true that _any kind of timber_ is suitable as a fuel to be used in a smoke-house. All timbers are suitable, either alone or in mixture with others, provided that the wood is not too green. Naturally an absolutely dead and crumbling wood will smoulder, but does not develop sufficient smoke. A green timber will give an acrid and moist smoke, but demands the consumption of a certain amount of dry timber in addition if it is to be used. Rubber-tree prunings and sawn rubber trees obtained by thinning-out may be used in mixture with dead wood, provided the logs are stacked to dry in the sun for some weeks before use. If the timber is too green, steam is formed as well as smoke, and the sheets of rubber may have a moist surface glaze.