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. CHAPTER XII _SORTING, GRADING, AND PACKING_ The question of standardising the output of our plantations is one which has occupied attention for some years, with a not inconsiderable degree of success. Meanwhile opinion is growing in favour of proceeding along the line of reducing the number of plantation grades to a minimum. At present some confusion exists. Some estates make up tree-scrap and bark-shavings together; one estate puts tree-scrap, earth-scrap, and bark-shavings into one uniform crepe; other estates have three or more separate scrap grades--_e.g._, lump-rubber and "washings," tree-scrap, earth-scrap, and bark-shavings scrap. There is a movement on foot at present to try to restrict plantation rubber to three grades: