When the surface is of one clay and the body of another, each clay is to be separately compressed, unless sgraffito effects are desired, in which case the surface-layer must be carefully applied, so as not to cover the pattern desired to be of the color of the body- clay.
The sharpness and definition of texture of reliefs made from dust-clay intaglios are very remarkable, and tile compressed from dust from its homogeneous quality is much less likely to warp or shrink unevenly in firing than any other, particularly if packed in a less fusible powder, like quartz grains or canister in firing, as is not unusual with terra-cotta relief work. By these means is obtained what has long been a desideratum in relief tile-work—a compact homogeneous embossed tile of uni form quality and slight shrinkage—more surely than has ever been done before.
It may often be desirable to obtain in tile both the relief and intaglio of the impression in the clay-dust. In this case the relief can be used upon the platen in the same way as the in taglio. Two of these tiles, an intaglio and a relief, may be placed face to face in the sagger for firing, and usually will separate on removal ; but it is best to insure this by leaving the paper diaphragm between them.
In case the design is to be reproduced smaller, the shrunk fired tile may be black-leaded and electrotyped.
Of course these tiles may have their intaglios filled with, or their reliefs covered with, kiln colors, slip, or enamel, either while simply clay or after firing, in any way and at any time proper in tile-making for such application.
No good method of fixing wall-tiles has yet been contrived, except those used by the ancients of flanging or beveling the edges backward and forward on alternate sides or in alternating section on the same side, or in constructing them with holes partly parallel to their surfaces for cramps or wires extending into the plastic cement, all of which are costly, and none of which are adapted for compressed clay-dust work.
Lately on occasions, in wet-clay work, undercut cramp grooves have been made by hand ; but these are costly and inapplicable to dust-work.
Mr. Low employs the following means for forming dovetailed grooves on the backs of tile : He cuts one or more pieces of wood of dovetailed cross-section to such length as may be de sirable, usually long enough to extend clear across one way, and lays them on the platen of the tile-machine narrow side down, and fills in the clay-dust upon them, or he places them on top of the filled-in dust, narrow side up, according as the face of the tile is to be up or down. In compression the nar row face of the wood will be level with the back of the tile, and the clay-dust will mould round it. In firing, these formers will burn out, leaving their grooves, and this, if the wood be soft, light, and dry, without much, if any, chance of injury to the tile.
Many things may be used as substitutes for paraffines, such as waxes, and compounds of waxes, resins, gums, etc.; but we have not considered it requisite to enumerate them, as they would clearly be equivalents if their qualities of toughness, flexibility, and plasticity resembled those of paraffine.
The clay-dust used should be fine enough to pass a one hundred-mesh sieve at largest.