The mechanical moulding of tiles comprises three operations : (1) the preparation of the clay; (2) the formation of slabs or regular plates of clay, the dimensions of which depend upon those of the tiles; and (3) the transformation of these slabs into tiles by stamping with plaster or cast-iron moulds of the required shape.
This is carried out in the same way as for bricks, but requires more attention. The choice of clay is very important, and thin clays no longer being suitable alone, there must be added to them a certain quantity of rich clay which gives to the paste the degree of cohesiveness and tenacity necessary to ensure for the tiles their essential qualities, especially itnperviousness and resistance. The paste, thus rendered as homogeneous and ductile as possible, should be free from all impurities.
Here an important question presents itself: What quantity of water should a paste contain to make the best tiles?
Pastes are placed in three classes according to the quantity of water contained in them and evaporated at 100° C.
Soft pastes containing 20 to 25 per cent. Firm ,, ,, 15 to 20 ,,
Hard „ ,, 10 to 15 „ that is to say, not much more than the amount naturally contained in clays.
Each kind of paste requires a different method of manufacture, possessing advantages and disadvantages of its own.
The clay blended in this manner acquires the maximum of ductility and homogeneousness which can be expected from its plasticity. The separation of the paste into thin laminae, which slide over one another, as is always observed when it is expressed from the die, will be reduced to a minimum. Besides, the passing of the soft slab under the press will perfectly weld together the molecules which might become separated, and a good firing will give as a final result a product with a good ring, a clean fracture, and fine grain, signs of excellent quality.
The drawbacks of soft paste for manufacturing purposes are: difficulty of handling slabs and tiles of no consistency; the adhesiveness of the paste renders the use of cast-iron for moulding impracticable, and recourse must be had to plaster moulds, the quick wearing out of which necessitates frequent renewal; as we work with soft paste a strong pressure cannot be used, and this diminishes the clearness of reproduction of the details of the mould, for tiles are not stamped like bricks; finally, the large quantity of water contained in the paste lengthens the time of drying, - hence an increase in the dimensions of the drying-sheds, - and its removal causes hollows - hence the products are more porous.
All being considered, the production with soft paste is small, and the cost somewhat high.
The quantity of water being reduced, the paste is more easily handled than the preceding kind; it can support a greater pressure, and can be moulded in cast-iron, which gives the tiles very clean-cut faces, a great delicacy of shape, and sharp edges. The drying is quicker and the porosity not so great.
The inconvenience of this method, which largely counterbalances its advantages, is that it requires more powerful machinery and in consequence absorbs more motive force.