Mastication is sometimes carried out in a machine resembling a powerful churn or sausage machine. For most purposes rollers are employed similar to or identical with the mixing rollers. Mixing is always preceded by a masticating process, introduced in order to soften the rubber. The dried rubber is passed repeatedly between a pair of hollow rollers, which can either be heated internally by steam, or cooled by passing cold water through the cavity. The rollers revolve at different rates, and the distance between them is adjustable. The rollers are gradually brought closer together until the soft rubber adheres to the slower moving roller and passes round with it, being subjected to a kneading process as it passes the second roller, which revolves more rapidly. At the beginning of the process the rollers are warmed, but later on friction may give rise to so much heat that it is necessary to pass cold water into the interior of the rollers. The whole process thus calls for considerable skill and judgment on the part of the operator in regulating both the temperature and the action of the machine.
Vacuum driers are also under trial in certain factories, but it is said that rubber dried in this way does not recover its "nerve" so completely as it would if the slower process of air drying were adopted. Drying machines of this type are, however, frequently employed for desiccating the materials used for mixing with the rubber, thorough dryness being essential in the latter process.
After undergoing a certain amount of mastication the rubber is judged to be ready for mixing. In the mixing process, the sulphur required for vulcanisation is incorporated with the rubber, together with other substances known as fillers. The rubber is passed repeatedly between the rollers and the sulphur, and filling materials are gradually sprinkled over it in the form of a fine powder. The process of rolling is then continued until the fillers are evenly distributed through the rubber, and the whole substance has become nearly homogeneous.
Sulphur is introduced either in the form of flowers of sulphur or as precipitated sulphur. Other substances are used in the mixing for several definite purposes. Thus various sulphides aid in the process of vulcanisation. These and other chemicals are also introduced in order to impart definite qualities of toughness and durability to the finished product. Among the most important of these substances are zinc oxide, magnesia, antimony sulphide and litharge (lead sulphide). Some of these chemicals impart characteristic colours to the rubber, whilst a further series of materials is used simply for colouring. Thus red rubber may contain vermilion, red lead or antimony sulphide; whilst white or grey rubbers usually contain zinc oxide, and black rubber contains either litharge or some form of carbon black. Finally, there are the filling materials which are introduced in order to reduce the specific gravity of the rubber or to lower the cost. Rubber is sold by weight, and the materials used for toughening have mostly high specific gravities. Substances of low specific weight are therefore introduced in order to counteract the effect of the heavy bodies. For this purpose whiting and French chalk are largely employed. For cheapening, a large variety of substances is used, foremost among them being old vulcanised rubber in a more or less reclaimed condition. Fatty oils undergo a kind of vulcanisation with sulphur, and are used as cheapeners in combination with reclaimed rubber. With low grades of rubber such substances as leather waste and sawdust may even be incorporated.
The further treatment of the mixed rubber varies according to the kind of article which is to be manufactured. Very commonly it is treated in one of three ways. It is either pressed into blocks, or rolled into sheets, or moulded in what is known as a forcing machine.