Finally, if there is a difficult mixture to be made, or a perfectly homogeneous paste is desired, a pug-mill is added to the screw machine.

Fig. 129 shows an installation of this kind.

Fig. 127. Screw Machine with Rollers, Distributers, and Cylinders with Points (Whitehead).

As can be seen from the numerous figures above, which do not by any means represent all the types of machines constructed, the manufacturer of pottery has a large choice, which cannot fail to embarrass him. The composition of the clay to be treated, and the way in which it is to be treated according to the articles to be made from it, are the only things which should influence him; in the last resort experience is the only guide.

Fig. 1l8. Screw Machine with Rollers and Distributing Cylinders (Groke).

Screw machines are intended for treating clay in the state of a semi-firm paste, the most suitable state for getting the maximum of output with the minimum of motive power.

Fig. 129. Screw Machine with Distributing Cones and Vertical Pug-mill (Jager).

Piston Machines

These machines are intended for treating clay when in a state of firm paste. It will be understood that the firmer the clay to be worked upon is, the greater is the effort required to force it through the die. Hence machines with propelling cylinders or pug-mills with expression screws are unsuitable for this kind of work. They are replaced by machines which compress the previously pugged clay into boxes with a piston.

One of the ends of these boxes forms a die. The piston has an alternating motion, therefore the production is not continuous, but in order not to lose the return motion the machine is double and furnished with two cutters. In machines with large output like that of Fig. 37 the feeding is done by hoppers into which the clay is thrown during the return journey of the piston.

In small machines (Fig. 564) the clay is placed in two boxes, the upper part of which form lids: while one is emptied under the pressure of the piston the other is being filled with clay.

Large piston machines are called "galetticres"; they are used for the preparation of slabs for tiles; the others are specially employed for the preparation of hollow products and especially pipes. We shall describe them when we speak of the manu-facture of these.