"The clay is brought from the pug-mill so dry that, when squeezed through the machine, not a drop of water exudes,—moisture is, indeed, scarcely apparent on the surface of the raw pipe. Hence, the tiles undergo little or no change of figure while drying, which takes place very rapidly, because of their firm and slight substance."
Fig. 42 - PUG-MILL.
Tempering.—After the fine clay is relieved of the water with which it was washed, and has become tolerably dry, it should be mixed with the sand, or other tempering material, and passed through the Pug-Mill, (Fig. 42,) which will thoroughly mix its various ingredients, and work the whole into a homogeneous mass, ready for the tile machine. The pug-mill is similar to that used in brick-yards, only, as the clay is worked much stiffer for tiles than for bricks, iron knives must be substituted for the wooden pins. These knives are so arranged as to cut the clay in every part, and, by being set at an angle, they force it downward toward the outlet gate at the bottom. The clay should be kept at the proper degree of moisture from the time of tempering, and after passing through the pug-mill it should be thoroughly beaten to drive out the air, and the beaten mass should be kept covered with wet cloths to prevent drying.
Moulding the Tiles.—Machines for moulding tiles are[pg 180] of various styles, with much variation in the details of their construction, but they all act on the same general principle;—that of forcing the clay through a ring-shaped aperture in an iron plate, forming a continuous pipe, which is carried off on an endless apron, or on rollers, and cut by wires into the desired lengths. The plates with the ring-shaped apertures are called dies; the openings are of any desired form, corresponding to the external shape of the tiles; and the size and shape of the bore, is determined by the core or plug, which is held in the centers of the apertures. The construction of the die plates, and the manner of fastening the plugs, which determine the bore of the tiles, is shown in Fig. 43. The view taken is of the inside of the plate.
Fig. 43 - PLATE OF DIES.
The machine consists usually of a strong iron chest, with a hinged cover, into which the clay is placed, having a piston moving in it, connected by a rod or bar, having cog-teeth, with a cog-wheel, which is moved by horse or hand power, and drives the piston forward with steadiness, forcing the clay through the openings in the die-plate. The clay issues in continuous lines of pipe. The machines most in use in this country are connected directly with the pug-mill, and as the clay is pugged, it at once passes into the box, and is pressed out as tiles. These machines are usually run by horse-power.
Mr. Barral, in his voluminous work on drainage,[25] describes, as follows, a cheap hand machine which can be made by any country wheelwright, and which has a capacity of 3,000 tiles per day (Fig. 44):
"Imagine a simple, wooden box, divided into two compartments. In the rear compartment there stands a vertical post, fastened with two iron bolts, having heads[pg 181] at one end, and nuts and screws at the other. The box is thus fixed to its support. We simply place this support on the ground and bind its upper part with a rope to a tree, a stake, or a post. The front compartment is the reservoir for the clay, presenting at its front an orifice, in which we fix the desired die with a simple bolt. A wooden piston, of which the rod is jointed with a lever, which works in a bolt at the top of the supporting post, gives the necessary pressure. When the chest is full of clay, we bear down on the end of the lever, and the moulded tiles run out on a table supplied with rollers. Raising the piston, it comes out of the box, which is again packed with clay. The piston is replaced in the box; pressure is again applied to the lever, and so on. When the line of tiles reaches the end of the table, we lower a frame on which brass wires are stretched, and cut it into the usual lengths."