It is difficult to say under what circumstances machines of this kind are preferable to screw machines; but we can state in what cases they can not be used. If it is true that clay can be used for these machines just as it comes from the pit, after having undergone a suitable moistening, the work will nevertheless be facilitated and the production increased by a previous pugging. This pugging is even indispensable for rich clays which have to be thinned; the mixture of the rich and the shortening matter is thus rendered a close one.

It is equally evident that if the clays contain stones or foreign matter they must undergo crushing before the pugging.

Personal experience has shown us that vegetable moulds (lehm or loess), which are always thin, cannot be worked alone by the powerful screw machines, as a too strong pugging makes the products brittle and unfit to be cut, while the same clay treated with a cylinder machine, even without previous pugging, gives fairly good results.

We may say then that, in questions like this, experience alone can guide us, according to the nature of the clays, and we must not hesitate, when circumstances require it, to abandon the screw machines, and replace them by cylinder machines with pug-mills attached. The Vaugirard brickworks in Paris, which use plastic clays, employ cylinder machines.

The principal parts of a machine with propelling cylinders (Figs. 100 and 101) are a solid cast-iron frame, X, supporting two hard cast-iron cylinders, G and H, of different diameters, which are moved by the gear-wheels B and E, themselves moved by the pulleys C D. The upper cylinder, H, turns between movable iron wedges in slide-bars the height of which is regulated by the screws F F. Behind the cylinders is a movable hopper to receive the clay. In front is placed an open cast-iron box, O, which has one orifice turned towards the two cylinders, and the other closed by a cast-iron plate held in place by cotter-hooks, and pierced in the centre with a square hole, S, through which the clay passes.

Fig. 100. Section of a Cylinder Expression Machine (Joly).

This plate is called a die. At the bottom of the box A are two blades, I and J, furnished with two oval openings for the bolts which fix them to the sides of the box; these blades, called scrapers, can then be moved nearer to the rollers as they wear away.

The prism of clay when it issues is received on a table furnished with rollers and called the cutting - table. We shall give a description of this in a special chapter.

The machine works as follows: the clay introduced into the hopper, K, is drawn along by the cylinders, which, in consequence of their difference of diameter, crush and blend it. It accumulates in the space O, and is gradually compressed there. When the pressure is sufficient, it issues by the opening, S, in the form of a regular prism, which is cut up on the table.

Fig. 101. Machine with Expression Cylinders (Joly).

All cylinder machines are based on the same principle, the dimensions and shape of the parts only being different. Thus the Joly machine is so constructed that it can also be used for pottery (Fig. 575). If the size of the machine be reduced, it can be worked by hand (Fig. 102), but we cannot recommend this unless unavoidable.

The model in Fig. 103 is the preceding machine made stationary; this simplifies the construction and consequently reduces the price (see table, p. 146).

When the propelling cylinders have the same diameter, as in the Sachsenberg (Fig. 104) and Groke (Fig. 105) machines, they have a different speed, so that friction may be produced for pugging the clay and driving it towards the die.

The Jager machine (Fig. 106) possesses the same parts as the foregoing machines, but the space in which the clay is com-pressed is reduced to a minimum: the clay passes straight to the due from the cylinders. These latter are sometimes fluted instead of smooth, the (lutings being angular or straight like these of the Johnson machine Fig. 107. This machine has three rollers: two to compress the clay and placed one over the two first with Pug-mills. - Except hit the clay to a previous cylinder machine. Thus the two machines are so the pug-mill is at once the foregoing machines, but the space in which the clay is compressed is reduced to a minimum; the clay passes straight to the die from the cylinders. These latter are sometimes fluted instead of smooth, the flu tings being angular or straight like those of the Johnson machine (Fig. 107). This machine has three rollers: two to compress the clay and placed one over the other; the third passes the clay to the two first.

Expression Machine with Fluted Cylinders (Johnson).

Fig. 107. Expression Machine with Fluted Cylinders (Johnson).