In later years a demand arose for sheets paler in colour than the old type, and in order to meet that demand, a change had to be made in methods. This led to a system of working whereby it was possible to smoke-dry sheets thoroughly in from twelve to fourteen days. This interval was further reduced on many estates, until some were producing rubber which appeared to satisfy all requirements after only five or six days’ curing. This does not refer to the case of estates having smoke-houses of “continuous-working” type, but to those on which smoking was confined practically to the hours of night. Under former conditions of rate of production and consumption, this short period of smoke-curing would possibly have been ample; but even this is very doubtful, as often the rubber would not stand the relatively short journey from the estate to Singapore without mildew-growth being incipient. We have often received specimens of rubber sent from estates for criticism, and have noted that within a comparatively brief period mildew was to be seen.
The whole matter resolves itself into a question of thorough efficiency of smoking. This is not dependent on duration of smoking alone, but involves other factors, such as the kind of fuel employed, the rate of combustion of fuel, the average temperature sustained, the ventilation of the smoke-house, and the situation of the building. Other occasional contributory factors are contemporary adverse climatic conditions and the possible abuse of an anti-coagulant such as sodium sulphite.
It has been shown that after a time, given suitable conditions involving the presence of moisture, moulds may appear on sheets which were apparently fully smoke-cured, and that under the same conditions other and older samples were unaffected. It is argued that the latter sheets had evidently been smoked more efficiently than the others. Hence it is fair to assume that, except under very special conditions, which do not apply to the ordinary procedure in the shipping, storage, and sale of rubber, moulds will not develop upon sheets which have been properly smoked. The term “properly smoked” signifies efficient smoking for all practical purposes under ordinary procedure, and implies or includes all the advantageous factors which have been discussed or alluded to in preceding paragraphs.
Without discussing in wearisome detail conditions which may give rise to the incidence of mildew on properly smoked rubber, it may be pointed out that the following are favourable to the growth of moulds:
(a) Storing sheets in a damp place before packing.
(b) Packing sheets in wooden cases which are not thoroughly dry.
(c) Piling up cases of rubber in a badly ventilated store-room.
(d) Placing the cases on a cement floor.
(e) Wetting of cases by sea-water or by rain during transport, etc.
Black Streaks, Spots or Patches.—The origin of these is not difficult to trace. They are caused by drippings from the roof, and contain condensation products from smoke plus moisture. The ventilation of the roof-ridge should receive attention, and if the trouble persists it will be necessary to place some absorbent screen below the sloping roof. Sackcloth is sometimes used, but leads to a worse state of affairs unless changed frequently. In most modern smoke-houses having an iron roof there is an inner lining of soft timber.