It is preferable, for many reasons, not to use clays which contain too much foreign matter, - stones, sands, pyrites, etc., - but there are cases in which one cannot do otherwise, and then the foreign substances have to be removed.

Cleaning by hand is only suitable for expensive clays like the kaolins; generally washing is used to remove the sand and gravel, and stone-removing machines for the larger stones.

Fig. 21. Conical Mixing Mill (Boulet).

Washing Of Clays

This is an excellent but a costly process for freeing clays from the heavy products contained in them. It is used with kaolins, and for preparing the fine pastes used in decorative terra-cottas and certain kinds of pottery.

The substances which clays deposit, when washed with water, are pyrites or quartz sands (silicious), with felspar or mica,

The apparatus used depends upon situation and the quantity of water available. In works where both are satisfactory, the process is carried out in large walled ditches, through which a current of water is passed over, the clay. When there is a sufficient quantity of water, the clay is stirred up so as to make a kind of clear pulp, while the dense substances remain at the bottom; when all the clay is in a state of pulp, it is passed into reservoirs, where it falls to the bottom. The remaining water is removed by decantation, and for this purpose the reservoirs are arranged in cascades.

Fig. 22. Filler Press for Clay (Wegelin and Hubner.

In order to prevent the current of water from carrying with it part of the foreign substances, the pulp may be passed over a kind of filter which retains them. When the deposit, which has been exposed to the air, is firm enough to be handled, it is taken out to be compressed by different methods, the quickest and least cumbrous of which is the filter press (Fig. 22).

The use of deposit-reservoirs necessitates a very large space and considerable cost of installation, when the output is large; therefore it is desirable in certain cases to substitute for them a system of diluting the clay in sheet - iron tanks with sieve bottoms which retain the foreign bodies and let the pulp pass. The latter is forwarded by means of a pump to the filter press. The apparatus is easily moved complete, and sometimes is combined with a motor which works the pump for transferring the pulp to the filter press. This latter contains a certain number of compartments which communicate with the pipe bringing the pulp under pressure. The water runs off, and when the filter press is full of clay, the entrance tap is turned off and the pulp conducted into another filter. The clay is thus obtained in the form of a paste still containing much water, which is removed by compressing the plates strongly by means of a fly-wheel moved by a central screw. The pump (Fig. 23) has an elastic membrane which separates the piston from the muddy liquid to be raised. There is therefore no damage from the hard bodies to be feared.

Fig. 23. Membrane Pump for Filter Press (Wegelin and Hubner).

To make the production continuous, it is sufficient to constantly remove the stones. This is done by using a rotating sifting cylinder slightly inclined.

The pulp is prepared in a tub, A, and flows with the foreign substances into a perforated rotating cylinder, C. The pulp falls into a reservoir, D, while the impurities, drawn by the rotatory motion of the cylinder, are poured into the waggon, W, which is changed when full. The pulp is drawn up by the pump and sent on to the filter press F.

We must not forget that the washing of clays only removes certain impurities such as sands and stones, foreign bodies like limestone, pyrites in an impalpable condition, oxide of iron, etc., becoming shaken up with the clay and remaining in suspension with it.

Fig. 24. Arrangement of an Installation for continuous Washing of the Clay.

Mechanical Stone-Removing

This process is not as perfect as the foregoing, but it allows of the expulsion of stones or other hard bodies of a certain size. The apparatus used is composed of two conical rollers placed according to their generating lines (Fig. 25), and moved by a conical gear, The machine is fed without interruption like the other rolling-machines. The foreign substances advance automatically and are expelled by a gutter which may be seen on 4 the left of the figure. The clay afterward passes between the rollers and undergoes a first flattening. The machines made by Messrs. J*×ger (Fig. 25) treat in 10 hours a quantity varying according to their size, from 10,000 to 70,000 bricks, and require from 3 to 7 horse-power. Their weight varies from 1500 to 4000 kilos (1.5 to 4 tons).

Fig. 25. Stone-removing and Rolling Machines (J*×ger).

Another machine of the same kind is shown in Fig. 26.

Fig. 26. Stone-removing Machine [Penfeld).

Crushing And Pulverising

Crushing

This is done, in fresh clays, in order to destroy hard lumps and to crush the foreign substances contained in them: limestone, schists, quartz, flint, etc. Dividing and crushing cylinders are used.

Dry crushing is used with dry clays which are reduced to powder, either for use as they are or for mixture with others. Finally, substances are crushed for use as antiplastics, such as sands, slag, fragments of pottery, etc.

Damp Crushing. Dividing Cylinders

When the clays are too hard or in pieces difficult to treat with the mixing mills, special cylinders are used to split them up. Some carry tempered steel points (Fig. 27) to catch the large pieces of clay which might slip over smooth cylinders. The fragments of clay thus obtained are sent on for rolling or soaking.

Fig. 27. Crusher and Divider with Interchangeable Points (J*×ger).

In the machine shown in Fig. 28, the diameter of the cylinders plays the same part as the steel points of the foregoing machine. This machine, which is very strong, is supplied direct from waggons, and the large pieces or unseparated parts of the clay are reduced to the size required for a satisfactory soaking.