Temperature of Smoke-Curing.—In the question of temperature of drying, it is well to be as strict as possible; not that any great harm will result from a rise of 10° above that recommended, but because the higher the temperature recorded the larger the fires must have been, and consequently the more real danger there was of the store becoming ignited. It has been shown[14] that the temperature giving the maximum benefit of drying and quality was found experimentally to be rather above the temperature usually prescribed for smoke-houses, but in the experimental work there was no danger from fire.
[14] “Preparation of Plantation Rubber,” Morgan, 1913, chapter x.
The figure given in previous publications as a maximum working temperature for smoke-houses was 110° F., but certainly the temperature may be as high as 130° if it is considered safe to allow fires to be so arranged. One or two estates are known to work at temperatures of 130° F. and over, in spite of the recommendations of the writers. If those estates care to risk it they may do so, with increased rapidity of drying; but no responsibility can be taken for whatever may happen in smoke-houses where the temperature is allowed to remain, as in one case, at 160° F. Naturally the range of temperature is strictly limited by the properties of the substance to be treated, and with a substance such as rubber it would be far better to err on the side of caution than to risk damage to such a commodity, apart from the consideration of the possible destruction of the building.
Period of Drying.—Considerable differences are noted in the periods of drying on various estates; but, as there is more than one factor influencing the results, it is not easy at first to find why these differences should exist. Really there are three factors:
(1) Relative thickness of rubber.
(2) Extent and quality of rolling.
(3) Temperature of drying.
It is presumed that the smoke-houses are identical in type and efficiency, and that smoking is in force for the same length of time each day. There need be no discussion of these points; the effect of each is so obvious. The thinner the sheet, the quicker the rate of drying; the better the sheet has been rolled, the shorter the period of drying; the higher the temperature, the more rapid the drying.
It has been shown in [Chapter IX.] that the condition of the sheet after rolling depends primarily upon the standard of dilution of the latex and the original thickness of the coagulum. If these factors are correctly controlled, the rolling should give a sheet which is fairly soft and porous—i.e., it should not have been subjected to such pressure as to make it both thin and hard. An average sheet of rubber which has been well rolled should be smoke-dried at a temperature of 120° F. in about ten days. If sheets take appreciably longer to dry, then the three foregoing factors must be examined.
On the other hand, it is often found that thin sheets made from very dilute latex dry so quickly that they are considered to be fully smoke-cured in from five to seven days. It frequently happens in such cases, however, that the smoking is insufficient, and by the time the rubber reaches home it has begun to show signs of surface moulds. It is evident, therefore, from this discussion that: