FIREPROOFING, STRUCTURAL, OR HOLLOW TILES are rapidly growing in use (see FIREPROOF CONSTRUCTION) particularly for fitting around steel columns and girders forming partitions, floor arches, and ceilings. Their lightness is greatly in their favor. They are divided into three classes: Dense, porous, or terra cotta lumber, and semi-porous. Dense tiles are pre pared for molding much like terra cotta. They are given heavy pressure and a long burning, and are sometimes made from fire clay. Por ous tiles are made by mixing one part by bulk of soft wood-sawdust or finely cut straw with two parts of clay, passing, the mixture through a disintegrator or cutter, and then between two sets of corrugated rolls, one below the other. From the rolls conveyors take the mixture to the tile machines described below. The subsequent burn ing destroys the sawdust or straw, leaving a very porous tile into which nails may be driven, or which may be cut with a saw; hence the name, terra cotta lumber. Semi-porous tiles are
made of good fire clay containing GO per cent. of silica, calcined fire clay, and coarsely grolind bituminous coal. These materials are mixed, molded, and burned much like porous tiles.
lloLtow TILE :MACHINES are of various forms according to the character of the clay. The tempered clay is forced through forms and around plugs. Tile forms give the outer and the plugs the inner shape to the tiles. The plugs ale of metal, with their front ends just inside the form. Plungers force the clay over or around the plugs, then through the forms. The con tinuous shapes are separated by wires or knives. Some of the special forms require partial hand shaping.