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.
Sun-Drying Sheet Rubber.—Among the first curious sights which impress the visitor or newcomer to this country is the spectacle of sheet rubber hanging in the sun on native holdings. From what one has learned of the extraordinary care which must be exercised in all the processes of rubber preparation, one fails to understand how such rubber reaches the market without becoming tacky. That some of it does become slightly tacky is certain, but on the whole native rubber, though crudely prepared, is usually sound. The native idea of giving sheet rubber a preliminary drying in the sun is to hasten the total period of drying. That the period is curtailed would seem to be the case, but it is open to doubt, as the effect of sun-drying, if unduly prolonged, is to create a thin surface film of dry rubber which retards the drying of the rubber below the surface. Working with wet crepe rubber, the writer found that, to all external appearances, there was no effect upon the rubber when it was allowed to sun-dry for four or five hours. With periods of from six to ten hours the crepe becomes slightly sticky, chiefly on that portion across the support. When removed to the air-drying house this tackiness developed further, and the rubber, on the line of support, became so weak that it stretched and broke.
Reasoning by analogy, it would appear that no apparent harm would result to sheet rubber from sun-drying for periods up to four or five hours. From experience (see [Chapter IX.]), not the slightest ill-effect is found to result from the short interval of preliminary drying or dripping practised on many estates preparatory to smoke-curing.
Artificial Driers for Sheet Rubber.—It is understood that when vacuum driers were first applied to the drying of rubber it was thought possible to dry sheet rubber in this way. The practice was found to be impossible, as the length of time required and the temperature were responsible for the destruction of the form of the rubber; it became tacky and semi-liquid.
The “Chula” Drier.—Although several suggestions of devices for artificially drying sheet have been made, only one is known to be in use at the present time. In the original form this was used for drying other tropical products. It consists of a large iron chamber, in which are several compartments divided by means of baffle-plates. At one end there is a small furnace and, by means of a fan, smoke and hot air are drawn through the compartments. Owing to the temperature attained (140° to 160° F.) sheet rubber cannot be completely dried in the chambers, and is, as a rule, only treated in this manner for one or two days. Drying is then completed in an ordinary air-drying house. It is claimed that drying is expedited, and that the rubber can be packed in ten days.
In the more recent modification, the smoke and hot air which leave the Chula drier pass through a large room in which may be hung either sheet or crepe rubber. It would seem that all sources of danger have not been eliminated from the process, as on one estate a wooden room containing rubber was ignited by a spark which passed through the drier.
Yet another form exists in which the furnace is outside the main building, and in the ordinary course of working only heats a series of open pipes through which air is drawn by a powerful fan. By means of a valve it is possible to allow smoke from the furnace to pass into the room with the hot air for the preparation of smoked rubber. The hot air or smoke is distributed in the lower room by means of main and branch pipes, and passes through an open floor to the room above. With such an arrangement it is possible, therefore, to prepare either air-dried or smoke-cured rubber. If the method could be successfully applied to the drying of crepe it would be of great assistance on some estates. There would seem to be a difficulty in working it for the drying of sheer rubber and crepe together, as the temperature suitable for the one is excessive for the other. Given an efficient control over the temperature of the hot air, the house should be successful in the drying of crepe, provided the rubber is not hung in folds of too great length. For smoke-curing sheet rubber the period is said to be reduced by several days in comparison with the time occupied in an ordinary smoke-house, but it is not clear that such a system would have any advantage over a modern smoke-house, in types of which rubber can be fully cured in periods ranging from five to ten days.