The chambers used for vulcanising by the dry process are often of very large size. They may take the form of great iron tunnels, into which the articles to be vulcanised are run on rails. Hose pipes are often vulcanised in lengths of 60 feet or more. The rubber becomes soft and plastic at the temperature employed, and must therefore be supported in order to preserve its shape. Powdered French chalk is largely employed as a bed for the rubber articles to lie on. Closed objects such as balls, india-rubber dolls and other toys, are vulcanised in moulds, and before the cavity of the rubber is closed up some substance is introduced which will volatilise at the temperature of vulcanisation, and so press the object firmly against the mould. The outer covers of motor tyres are vulcanised in heavy presses, in order that their substance may be firmly compacted during the process. The most complicated of all vulcanising machines is the autoclave press, which consists of a hydraulic press completely enclosed in a steam pressure chamber. In this way the difficulties of obtaining an even temperature in a press heated by-steam pipes are overcome. Vulcanisation by steam is generally carried out at a pressure of three to four atmospheres (45 to 60 lbs. per square inch) corresponding to a temperature of 134°C. to 144°C. The temperature and pressure are first raised gradually, and afterwards kept high for three or four hours. Self-recording thermometers and pressure gauges are fitted to the apparatus in order that the process may be kept under complete control. The time required for vulcanisation varies according to the source of the rubber, being shortest in the case of Hevea rubber. The previous treatment of the rubber also affects the process. Thick articles naturally require to be heated for a longer period than thin ones. The proportion of sulphur added to the rubber varies according to the nature of the articles to be manufactured. For ordinary goods the quantity is about 7 to 10 per cent, of the amount of rubber.
In Hancock's bath process, which is comparatively little used, small objects are immersed in molten sulphur for a period of two or three hours at a temperature of 130°C. to 135°C. Test pieces of rubber, of similar thickness and composition to the articles to be vulcanised, are placed in the same bath and removed at intervals. From the appearance of these the operator is able to judge when the process of vulcanisation is complete.