Having incorporated sulphur and other ingredients, the plastic mass is sheeted and run between layers of calico to prevent the superimposed sheets from adhering. From this "calendered sheet" the article, whatever it may be, is built up. The calender rollers are heated so as to keep the rubber compound plastic. There is a limit to the thickness of the sheet which can be produced. It is a difficult operation to perform satisfactorily so as to yield a smooth surface and a sheet free from enclosed air. When cool the rubber hardens and is readily handled. The object to be manufactured is then built up from the calendered sheet. Thus in the manufacture of a motor tyre the tread is built up on the casing or carcase by laying the sheets on the canvas and rolling these with a hand or power operated roller, so that they adhere firmly, the first layer to the canvas of the casing and subsequent layers to one another. This rough description will suffice to illustrate how important it is that the rubber when mixed should be plastic enough to give a smooth sheet, and to allow the sheet to be manipulated in building up the article in process of manufacture. The testing of rubber in regard to its plasticity and power to absorb finely divided mineral matter will be discussed in a later chapter. We may, however, point out here, that the mineral matter is not generally added as an adulterant, but because of certain specific properties it confers on the product. To proceed with our outline of vulcanisation, we have now arrived at the stage at which the goods are built up and ready for vulcanising. For this purpose they are generally enclosed in some manner, either in metal moulds bolted together, or tightly wrapped in cloth, as, _e.g._, in the manufacture of inner tubes, hose, etc. In the latter case, you can detect the cloth mark on the finished product. Sometimes the rubber is spewed--that is, driven out of a barrel by means of an endless screw revolving in it. In this way rubber tubing, perambulator tyres, and such articles, may be made. More recently even tyre treads and the shaped rubber for band tyres (heavy solid tyres) have been extruded in this manner, for the process is much cheaper than building up a tyre from calendered sheet, and then cutting the mass to shape by hand. But for spewing the rubber mass must be very soft and plastic; this condition is not obtainable unless the raw rubber originally used can be made thoroughly plastic without damage. Nor can it be effected with a rubber mass containing much finely divided mineral matter, as this hardens the mixture.