This is done to reduce to shavings the vegetable mould (lehm or limon) used for the manufacture of so-called native bricks. The earth is scraped vertically with the round knife of which we have spoken, and which the workman holds by two handles, one in each hand. Layers of about 60 to 80 centimetres high are taken in turn; the earth falls to the foot of the bed, splitting up and becoming sufficiently granulated to be put as it is into the press-moulds.
This process is applied to non-calculous loams which are treated as they come fresh from the pit. As they are compact and in large lumps which cannot be crushed between cylinders, they are cut up into small pieces which absorb water more easily. For this purpose special machines called mixing mills are used, the principle of which is the same as that of chaff-cutters; rotating steel knives meet the motionless mass of clay and cut it into regular shavings. Cylindrical and conical mixing mills are manufactured.
This consists of a cast-iron framework in the middle of which is a cylindrical pan with movable bottom. The base is a cast-iron plate pierced with oblong holes in the direction of the radii. Each of these openings is furnished with a steel blade which acts as a knife and is fixed with bolts to the plate and projects from it. The plate is fixed to the end of a strong shaft which is moved by bevelled gear. The pan is divided into two parts by bars or by a plate fixed against its sides (Fig. 19).
Fig. 19. Mixing Mill with Horizontal Tray (Pinette).
When the machine is set in motion, the plate with the knives begins to turn with a speed of 60 to 70 revolutions a minute. The clay is introduced into the pan, and, the fixed bars preventing it from following the rotation of the plate, the steel blades cut it below. The slices so formed pass through the openings between the blades and the plate and fall to the ground, their size being greater or less according to the dimensions of these holes.
The machine is strong enough to stand shocks caused by little stones being broken by the knives. If the stone is too large the driving-belt slips, and the machine must be thrown out of gear in order that the stone may be taken out by the hand.
The yield of a mixing mill depends upon the amount of moisture in the clay and the size of the machine.
The principle is the same in this, but the knives are fixed on the surface of the frustum of a cone. The clay, being thrown into the frustum, is kept motionless by a fixed partition as in the flat-bottomed machine.
Fig. 20. ConLca1 Mixing Mill (Lacroix).
Usually the mills are placed immediately above the soaking-trenches (Fig. 21). It is easy to so arrange the machine that it shall supply several trenches by placing it at a certain height, and guiding the clay into the more distant ditches by means of inclined planes kept damp by a slight stream of water.
Sometimes an endless band is placed under the mill, as in Fig. 19.