When the tiles are made of soft clay, the moulds of the presses are of plaster. The pressure and damp soon destroy them, especially the upper ones which transmit the pressure, therefore spare ones should always be at hand.
The plant necessary for making plaster moulds is as follows: (1) two matrices of chased and polished cast-iron, one representing the lower part (Fig. 403), the other the upper part (Fig. 404) of the tile; (2) two cast - iron frames (Figs. 400, 402) which are to receive the plaster moulds and are fixed by bolts to the mould-carriers of the presses; (3) an oak table (Fig. 401) provided with iron ribs, and a screw which passes through a bronze nut fixed in an iron arch.
Each matrix, having been well greased with black soap or resin oil, receives the plaster (of Paris), which is poured in in a fairly liquid condition, without, however, being too soft; it is then covered with one of the cast-iron frames, and carried to the table, where it is held tightly pressed down by the screw until the plaster has become hard. The matrix is now removed, and the plaster mould remains fixed in the cast - iron frames. The frames thus prepared arc then attached, those containing the upper part of the tile to the upper mould-carrier, and those containing the lower part to the press supports (Figs. 383 to 389) or the revolving mould-carrier (Figs. 390 to 399).
Fig. 400. Frame to receive the Upper Mould. Fig. 401. - Table on which the Moulds are pressed. Fig. 402. - Frame to receive the Lower Mould. Fig. 403. - Matrix of the Bottom of the Tile. Fig. 404. - Matrix of the Top of the Tile.
To make sure of a continuous supply, there should be a pair of matrices (Figs. 403, 404) for every five presses, and for each press, seven upper frames (Fig. 400) and four lower frames (Fig. 402), which makes five bottoms and one top in use; two bottoms and three tops being used in remaking the plaster moulds. For carrier - presses, we must have, besides the two matrices, two or three pairs of moulds according as the carrier is single or double.
When we are working on hard clay, the plaster moulds of small resistance are replaced by cast-iron moulds, which are lubricated for each tile to prevent the clay from adhering to the metal.
In either method each kind of tile requires special matrices as well as, in the case of plaster moulds, cast-iron frames.