153. Tiling consists of clay molded in either solid or hollow shapes, and thoroughly burned. It may be divided into two general classes. The first is variously termed porous terra cotta, cellular pottery, porous tiling, soft tiling, etc.; the second is called fireclay tile, hollow pottery, hard tile, terra cotta, dense tiling, etc. In this section, the first kind will be hereafter referred to as porous tiling, and the second as dense tiling.

154. Porous tiling has the advantage over hard tiling in that it is elastic, tough, and light. It is composed of sawdust, finely cut straw, and tough plastic clay, to which a small proportion of fireclay has been added. The mixture is exposed to an intense heat, which burns out the sawdust and cut straw, leaving a light, porous material resembling pumice stone. When properly made and burned, it will endure unequal heating or sudden cooling without cracking. It may be cut with a saw or chisel, and nails or screws may be easily driven into it. Properly made porous tile has a compact, hard, and tough texture, and gives out a ringing sound when tapped with a hammer. Tile made from sandy clay, or from material poorly mixed and burned, shows an uneven, soft, and crumbly fracture, and should be tested before using. Whenever porous tile has considerable weight to carry, as in floor construction, the shell should be at least 1 inch in thickness, and the webs, or partitions, between the cells should be about 3/4 inch thick.

155. Dense tiling is stronger than porous tile, but more brittle, and is made of fireclay, combined with either potter's, plastic, or tough brick clays, molded into shapes to suit various constructional purposes. While the clay is still in a moist condition, it is subjected to heavy pressure, which makes it very dense, and gives the finished material great crushing strength. The tiles are then dried and burned in a kiln like other terra cotta.

Dense terra cotta is used principally in the construction of floor arches. For use in exposed situations, it is more suitable than the other kind, and is often made with glazed outer surfaces, to prevent penetration of rain. When exposed to heat it does not endure as well as porous tiling.

156. Concrete

Concrete. Portland cement concrete stands fire and water tests well, and may be considered a thoroughly fireproof material. Concrete columns have been repeatedly heated red hot, and then drenched with water, without injury.

A concrete consisting of plaster of Paris, broken brick, wood shavings, etc. is much used in Paris for fireproofing purposes. A composition made of 5 parts of plaster of Paris and 1 part of fine wood shavings, with enough water added to make a thin paste, is much lighter than ordinary concrete, and is used in one of the fireproof floor constructions to be described hereafter. Instances are given of this material being exposed to intense heat for a considerable time, the results showing that it is affected only to a depth of from 1/4 to 5/8- inch, and does not check or crack when water is thrown on it.

Mortars, both lime and cement, applied on metal lathing, will successfully endure a great degree of heat, and will also withstand well the action of water.