289. Various materials have been introduced at different times for the purpose of making buildings fireproof. Experience has shown, however, that the only practical method of producing a really fireproof building is by using only incombustible materials for its struc-tural parts and protecting all structural metal work with some fire, water and heat-resisting material. The ideal fireproof building would undoubtedly be one that was constructed entirely of brickwork and terra cotta, with brick, concrete or tile floors or roofs, built in the form of vaults sprung from brick piers and without the employment of structural metal work. Such a building, if properly designed and built, would withstand the combined action of all the elements for centuries. Modern commercial requirements, however, demand that the vertical supports shall be as small and as far apart as possible, and that the floors shall be thin and have level ceilings, and these can only be obtained by the use of metal work.

The materials that have been found to successfully answer the purposes of modern fireproofing are confined to the products of clay, some concretes and lime and cement mortars under certain conditions.

290. Clay Products

Of all fire-resisting materials burnt clay has the most numerous applications in incombustible building. For the construction of floors and partitions, and for the casing of posts and girders, the clay is moulded into hollow tiles or blocks of two general kinds.

These are known by several different names : The one by such as porous terra cotta, terra cotta lumber, cellular pottery, porous tiling, soft tiling, etc.; the other by fire clay tile, hollow pottery, hard tile, terra cotta, dense tiling, etc.* For convenience the first will be hereinafter referred to as porous tiling and the second as dense tiling. The terms "hollow tiling" and "fireproof tiling" will be used when both are referred to in a general way.

291. Porous tiling is formed by mixing sawdust and finely cut straw with pure clay and submitting it to an intense heat, by the action of which the sawdust is destroyed, leaving the material light and porous like pumice stone. When properly made it will not crack or break from unequal heating or from being suddenly cooled by water when in a heated condition. It can also be cut with a saw or edge tools, and nails or screws may be easily driven into it for securing interior finish, slates, tiles, etc.

* The Pioneer Fireproof Construction Company have also recently introduced a new material which they call " semi-porous hollow tile." This material is considerably lighter than the dense tiles formerly made by them, and is claimed to stand the fire and water tests equally as well as porous tiling.

For the successful resistance of heat, and as a non-conductor, the author believes there is no building material equal to it, especially when used in thin sections. To obtain the above qualities in their fullest extent the blocks should be manufactured from tough plastic clays, with which a small percentage of fire clay should be mixed.

Porous tiles, when properly made and burned, should be compact, tough and hard, ringing when struck with metal. Poorly mixed pressed or burned tiles, or tiles from short or sandy clays, present a ragged, soft and crumbly appearance, and are not desirable.

Porous tiles for floor construction, or wherever they may have to carry considerable weight, should be made with not less than 1-inch shells, and the webs or partitions dividing the spaces should be from to 7/8 inch thick, according to the size of the hollows.

Porous tiling possesses the advantages over hard tiling of being light, tough and elastic, while dense tiles are hard and brittle.

292. Dense tiling is made generally of fire clay, combined with potters' clay, plastic clays or tough brick clays, moulded by dies into the various hollow forms required for commercial use. The clay is subjected during its manufacture to a high pressure while in a moist or damp state, which gives the finished material great crushing strength. After drying the tiles are burned like terra cotta in a kiln.

Previous to the year 1890 dense tiling was almost exclusively used for the construction of floor arches, and even at the present day it appears to be more extensively used for this purpose than the porous tiling, the latter being confined principally to the end-method system of floor arches.

Dense tiling in solid blocks is unquestionably stronger than porous tiling, although more brittle. When made from fire clay it is undoubtedly a thoroughly fireproof and non-conducting material, but it will not stand the combined effects of fire and cold water as well as the porous tiling. In outer walls, exposed to the weather and required to be light, dense tiling is very desirable. Some manufacturers furnish it with a semi glazed surface for outer walls of buildings. For such use it has great durability and effectually stops moisture.

In using dense tiling for fireproof filling care should be taken that the tiles are free from cracks and sound and hard burnt.