Dry crushing is done with the crushing mills described under the head of the preparatory treatment of clays. In the case of white pastes, the use of iron must be avoided, and flint grindstones substituted. It is better to pulverise the felspar in presence of water in special machines like those shown in Figs. 748, 788, 789.
The Villeroy crusher (Fig. 788) consists of a ball of granite weighing from 1000 to 2000 kilog. and rolling on a tray which is also of granite. Scrapers keep the sieve clean and bring the substances under the ball. This machine has a large output; with five horse-power, and the holes in the sieve being .004 m. (about |th inch), it crusher: 1500 kilog. of coarse felspar per hour.
Fig. 788. Ball Crushing Machine with Sieve.
The Alsing cylinders (Fig. 789), which may be of cast-iron or steel, are coated inside with a casing of millstone grit in which fragments of flint are incorporated. Flint pebbles or balls of granite or millstone grit are introduced into this casing. The cylinder, which moves on an axle, draws the balls along in its rotation, and the resulting friction wears and pulverises the mass to be crushed. The crushing may be effected on the substances when either dry or moist. The opening through which the substances to be crushed are introduced is closed by a stopper (to the right of the figure), and when the operation is finished this is replaced by a grating (to the left of the figure) which allows the crushed substance to pass but retains the balls.
Fig. 789. Alsing Crushing Cylinder.
When the clay is crushed in presence of water, it is afterwards passed through the filter press, and thence to the oven; when it is sufficiently dried, it is mixed with the other substances required to form the paste.
If the crushing takes place on dry clay, the powder is damped in the proper machines (p. 64) and is then mixed with the other materials. The required quantity of metallic oxides is afterwards added to the pastes, if they are to be coloured, and the mixtures, closely blended together, are placed in pigeon-holes within reach of the workwomen who are to perform the moulding.