The uniform moistening of the clay is effected by means of special machines, the object of which is to cause a complete mixture of the clay with water. These machines are especially useful in the case of powdered earths, but they can also be used for clay from the pit. They consist principally of a cast-iron or sheet-iron trough, in which is a shaft furnished with curved pallets or mixing blades according to the clay to be worked, and moved by a belt-wheel.
Above the trough is placed a pipe pierced with holes through which the water falls on to the clay while it is being stirred by the knives (Fig. 44).
These knives, something like screw - propellers, cause a motion of translation which carries the clay from the front of the machine, where it is introduced, to the back while mixing it closely with the water.
The discharge is effected either direct on to the ground (Fig. 45) or through an aperture in the lower part of the trough(Fig. 44) or in the vertical wall. The clay, thus uniformly moistened, is afterwards transferred to other machines for working.
Fig. 44. Moistening Machine (Whitaker).
The belt-wheel of the machine is either placed in a separate frame, as in Figs. 45 and 46, or fixed directly on to that of the machine, as in Fig. 47.
Fig. 45. Moistening Machine (Groke).
The transmission of motion is effected by conical and cylindrical gear-wheels.
Instead of a single shaft there may be two turning in opposite directions. A machine of this kind is shown in Fig. 48.
When the machines are constructed to moisten powdered substances, the useless knives are replaced by curved pallets which stir the powder better and facilitate the incorporation of the water.
Then the clays have to be crushed before damping, instead of having two machines they are combined in one. All that is required is to arrange the framework of the moistening machine to receive crushing cylinders as we see in Fig. 49.
Fig. 49. Moistening Machine with Supports to recieve Cylinders (Jacobi).
The cylinders may be conical or cylindrical, and two or four in number, as required. The machine represented in Fig. 50 has a pair of spiked crushers and another pair of conical rollers.
The machine in Fig. 51, called damping trough, is also provided with flatting friction cylinders to which is attached an automatic distributing tray.
In the trough are arranged two horizontal shafts furnished with screws crossing one another in their motion. The water is thrown on the clay by means of percolators.
Rich clays are damped with difficulty. Simple moistening as used for thin clays is not sufficient to damp them enough for working; a long stay in water is required in ditches as water-tight as possible.
This process, called soaking, is carried out after cutting or crushing, but before blending with shortening substances. Generally the ditches are situated below the cutting or crushing machines; when they are at some distance the clay must be carried to them by means of inclined planes or endless bands.
Fig. 50. Moistening Machine fitted with Crushing Cylinders (Jager).
The size of the ditches depends upon the importance of the factory and the manner in which it is arranged.
In some cases, their capacity is as much as 20 or 30 cubic metres; the depth of the clay in the ditches is as much as 1 to 2 l/2 metres, not more. As twenty-four hours are required for uniform and complete soaking, it is arranged that the total volume of the ditches should represent double the daily consumption of clay. To avoid waste of time the ditches are sometimes made with removable ends, so that, if one end is removed, it is only necessary to cut the mass, and put the clay into the pug-mill close by, mixing it with anaplastics which arc also placed near at hand. This being so, only just enough water should be added to give the paste a sufficient consistency without making it too soft. When the local conditions and the nature of the clay are favourable, this is undoubtedly an economical arrangement.
Fig. 51. Moistening Machine (Damp Trough) fitted with Rolling Cylinders (Lobin).
In other manufactories the clay is submerged for twelve or twenty-four hours, and then the water is drawn off; the clay is then taken from the ditches and placed on slightly sloping ground in thin layers between which is put the necessary quantity of thinning matter. When the heap reaches a certain height, it is drained until it reaches the required degree of dampness, then it is cut in vertical slices and put either into the pug-mill or straight into the hopper of a machine. Finally, in other factories the clay, or clays if there are mixtures, and the thinning substances are put together for damping.
Soaking is equally important for rich clays as for thin one therefore it must be watched with great care; on it depett. successful manufacture.