These are used to crush hard bodies, such as limestone or flint, which may be contained in the clays. They should be very hard, and the whole of the machine extremely strong in order to resist the violent shocks. Usually two or three pairs of cylinders are placed one over the other, the distance between them decreasing, and the clay falling from one pair to another.

Fig. 28. Large Dividing Cylinders (Boulet).

The cylinders are smooth or fluted, and their diameter varies from 35 centimetres to 1 metre, the length being always 60 centimetres. The clay is brought to the cylinders by endless bands, or poured in direct from waggons, or thrown in with the shovel.

The cylinders represented in Fig. 29 arc also used to crush the clays, but have also this peculiarity, that by the addition of one or two extra pieces of machinery, they can be easily employed for the crushing of dry substances such as fragments of tiles and pottery; they can thus in a certain measure take the place of the grindstone apparatus generally used for that work.

The extra pieces comprise a lever with counterpoise and an oscillating grating to separate-the powder from the large pieces. To increase the effect of this kind of machine, a differential speed is sometimes given to the rollers, so as to add to the crushing action a powerful rubbing. The crushing cylinders of the machine shown in Fig. 30 are arranged in this manner, and so are those of the Whittaker machine (Fig. 31).

Fig. 29. Crushing Cylinders (Joly).

As shocks are numerous in these machines, automatic belting is used to avoid breaking the gear-wheels; the movement is transmitted by means of a friction pinion, and a catch which can be put in or out of gear by a lever.

In the foregoing machines the rollers are furnished with scrapers as in flatting-machines, but instead of leaving a layer of clay on the cylinders, these are always kept clean. As the rollers wear out rather quickly, they have to be brought closer together in order to keep them at the required short distance apart.

Fig. 30. Differential Speed Crushing Cylinders (Johnson).

A time comes when the gear-wheels transmitting the motion work deep into one another, in spite of their long cogs, and this will infallibly cause their fracture. To avoid this accident, then, we must have an extra wheel of the same speed but less diameter and substitute it in good time. The one removed is not useless, for it will serve again when the worn-out rollers are replaced by new ones.

Fig. 31. Differential Speed Crushing Cylinders (Whittaker).

Fig. 32. Crushing Cylinders with Arrangement for Throwing Out of Gear (Whitehead).

Dry Crushing

This process is used in certain cases when the clays are dried, and also to pulverise bodies for use as anti-plastics.

Drying Of Clays

The manufacture of pottery with dry clay only flourishes in warm countries where the extreme dryness permits of easy desiccation of the earths. In other climates the cost of drying prevents a general use of this method. Nevertheless certain clays are worked in a dry state by leaving them for a long time in the air under covered sheds.

For artificial drying, large hot-air ovens are used; the clay is placed in layers of 15 or 20 centimetres thick, and moved from time to time. For large works, continuous kilns are used. The clay is thrown into an upper hopper, and thence falls into the oven itself, which is heated by coke furnaces giving a moderate heat to a large extent of air. This air, loaded with water vapour, issues by means of openings made in the arch supporting the hopper, and passes into two longitudinal conduits, whence it goes into a chimney. A kiln 7 metres high, 2 broad, and 4 long dries in 24 hours about 15 cubic metres of clay containing 13 per cent, of water, and uses 400 to 500 kilos of coke.

Pulverisation

This is done by grindstone mills in the case of clays and in general for substances of medium hardness. In the case of very hard and bulky substances, it is advisable to use pounding mills and special crushers.

Crushing Mills

These consist of vertical grindstones of varying weight, and circular cast-iron pans into which the substances to be crushed are placed. Sometimes these pans are fixed, sometimes they are movable. The separation of the pulverised portions from the coarser ones is effected in several ways: either by a central inclined sieve, on to which an automatic feeder throws the substance; or by the bottom of the pan itself being perforated with a number of holes, whose size depends upon the coarseness of the powder to be produced.