The influence of the effect of the hypsical condition of the wet rubber upon the final colour must be thoroughly grasped. One may take two sheets of apparently the same thickness, and smoke-cure them in juxtaposition within the same house, only to find that one dries much more rapidly than the other. As a consequence, the first, when fully cured, will be of a medium golden brown colour; while the other, owing to protracted smoking, will be dark. Evidently there must be some distinct difference between the two in physical condition prior to the smoking. Here the factor involved is the rubber-content of the latex. Given two pieces of coagulum of identical thickness, but prepared from latices of different dry rubber content, it will be obvious that to reduce them to similar thickness, more pressure will be necessary in one case--_i.e._, that piece of coagulum will be much more dense (more consolidated)--while the other will be comparatively soft and porous. Into the latter warm smoke can penetrate much more easily, and the internal moisture can escape more rapidly. The full cure may be made, say, within twelve days, while the tougher sheet may demand up to twenty days.
To attain uniformity in colour, therefore, the following points must be studied and controlled as far as is possible:
(_b_) Uniform dimensions of coagulating receptacles.
(_c_) Uniform volumes of standardised latex.
(_d_) Uniform quality and quantities of chemicals.
(_e_) Uniform methods of manipulating the coagulum.
(_f_) Uniform conditions of fuel and accommodation during smoke-curing.
SURFACE GLOSS.--In the choice of fuel there is room for control if one has good timber available. This point opens up a discussion on the vexed question of "over-smoking," as the term is sometimes applied to a pronounced dry glossy appearance of the surface of sheets.
(2) The ratio between furnace capacity and the capacity of the smoke-house.
Obviously any fuel which yields an excessive quantity of tarry matter or creosotic substance would conduce to the formation of a heavy glaze on the rubber. Such fuel, therefore, should at most only be employed as the smaller portion in a mixture with "dead" timber.
It is impossible to lay down any general rules for the guidance of estates, as the timber available varies so widely in nature. Experience must be the only guide, and it should not be difficult to obviate the defect. Even so, there must be minor differences between the results obtained on estates, so that it is not possible to make strict estimations of the smoke-curing period by an examination of the surface appearance of rubber, even under the best of conditions. Some estates find that the rubber has a distinct gloss in ten days, while others may smoke-cure for twice that period without difficulty. Evidently, therefore, the question of available fuel is of prime importance. It may be remarked that very satisfactory results are always obtained from the use of fairly dry timber obtained from thinned rubber trees, mixed with the "dead" timber of old logs and stumps found on the estates.