[37] The terms "curing" and "vulcanising" are generally employed as if synonymous. Twiss has suggested that the former be applied in regard to a change in physical properties, and the latter to the chemical change whereby sulphur is combined with the rubber. The term "curing" is also applied to the process of preparation of raw rubber. This must be kept in mind so as to avoid confusion. The degree of vulcanising will vary with the type of article to be produced, and where a long life is desired, the tendency will be to "undervulcanise"; but if the best mechanical properties are desired, the tendency will be towards "overvulcanising," or, more correctly, "fully" vulcanising. These considerations are aptly illustrated by reference to pneumatic tyres. The inner tube need not possess high tensile strength, provided that it is easily distensible, for the reason that, during use, it is protected by the casing of the tyre proper, which confines and supports it against the air-pressure applied. Inner tubes are therefore cured to give a long life without developing the maximal physical properties. On the other hand, the casing and tread of the tyre are required to withstand severe mechanical conditions--particularly the constant flexing of the cover, and the abrasion of the road surface. Tyres are not stored for any long period, and, when put into service, have a limited period of useful life. Consequently it is needful to develop maximal mechanical properties, and vulcanisation is therefore carried further than in the manufacture of inner tubes. The rate of cure is controlled by a number of factors in addition to the period and temperature of vulcanisation, in particular by the proportion and nature of the other ingredients, especially sulphur and accelerators, and also by the rubber itself. The main complaint as regards plantation rubber is that it varies excessively in this respect. This matter will not be discussed here, but is only introduced in order to explain the importance of a constant rate of vulcanising to the manufacturer. Plantation rubber should, therefore, be prepared so as to be as uniform as possible in this respect, and the earlier part of this book gives full details of the precautions advised, and in many cases adopted on the plantations. Unfortunately, it is impossible to secure uniformity of methods among all producers, even when they are Europeans, to say nothing of the native producers, who account for perhaps one-third of the output. Hence the importance of branding the rubber whenever possible, so that the manufacturer may identify the rubber he purchases. If found satisfactory, he can then secure further supplies from the same estate. CHAPTER XXII _TESTING OF PLANTATION RUBBER_ This subject may be subdivided into (_a_) Tests on the raw rubber; (_b_) tests on the vulcanised rubber.