One of the most frequent sources of error is the control of steam pressure which is responsible for the temperature of the drier. It is quite unnecessary and unwise to maintain any steam pressure once the drying is well under way. All that is necessary is to heat the chamber well, with a steam pressure of 5 lbs., before inserting the rubber. As soon as the maximum vacuum pressure has been obtained, steam should be shut off from the heating pipes, and it will be found that the temperature is well maintained throughout the operation with a rise of ten to twenty degrees at the end. If the drier is working at a vacuum pressure of 28 inches, and if the crepe has been prepared thin enough, the rubber should be quite dry within two hours. Should the operation have to be extended to two and a half hours at 28 inches vacuum pressure, it is a sign that the crepe is too thick. On such occasions it is often noticed that these thicker crepes are not thoroughly dry, having moist spots enclosed in them. On re-rolling, these moist patches become easily visible, and are a source of great annoyance, inasmuch as they take quite a long time to dry out. As mentioned before, the crepe for vacuum drying should be thin. There is no necessity to give it a superfine finish, and the presence of small holes is quite permissible, as they disappear on subsequent re-rolling. The thin crepe may be folded loosely to the length (or breadth) of the tray several times, but in no other way can the drier be expected to perform its work satisfactorily. A case was noted in which thin crepe was excellently prepared, and four or five layers were rolled together for vacuum drying. Naturally this mode of procedure does not give the drier a fair chance, and it would be ridiculous to judge vacuum drying on the results. After two and a half hours at a temperature of 145° F. the rubber appeared to be only about three parts dry, and the subsequent air-drying extended well into a fortnight.