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The Clay

brick, paving, fire, strength, shale, presence, composition and clays

THE CLAY. Three distinct classes of clay are employed in the manufacture of paving brick: surface clays, impure fire clays, and shales. Surface clays are almost exclusively used for the manufacture of building bricks; but are not ordinarily suitable for making paving bricks, since it is practically impossible to burn them hard enough without their losing their shape. On account of its infusibility, pure fire clay is unsuitable for making paving brick, the brick being expensive to burn and lacking density, hard ness, and strength; but quite impure fire clay makes a fair quality of paving brick, although the process of manufacture is compara tively expensive. Bricks made from impure fire clay are usually light in color, varying from cream to buff, and ordinarily are quite porous, absorbing from 2.5 to 7.0 per cent of water. Most paving bricks are made from shale,—an impure, hard, laminated clay which has been subjected to great pressure by the superincumbent earth strata. Shale is the most widely distributed of the laminated rocks, and makes a much better and cheaper paving brick than either surface or fire clay.

The different classes of clay so shade by imperceptible degrees one into the other that it is impossible sharply to discriminate them. Surface clays are soft and unconsolidated, and arc found at or near the natural surface. Shales are dense and but are easily reduced to powder and are readily worked into a plastic mass when mixed with water. Shale is often incorrectly called soapstone, from which it differs in nearly every respect. Shale is also frequently, but erroneously, called slate, from which it differs only slightly in origin and composition; but slate, unlike shale, can not be rendered plastic by mixing it with water. The only method of distinguishing between shale and impure fire clay, except by a kiln test, is the fact that shale gives a conchoidal frac ture while fire clay does not.

Chemical Composition.

It is not wise to enter into any extended consideration of the chemical composition of brick clays, since the subject is very complicated, and since the engineer is interested only in the physical properties of the finished product and should not attempt to prescribe the materials or to limit the methods employed by the manufacturer. However, a brief diti cussion of the subject may be of value as showing some of the linii tations upon the manufacture. Clay varies widely in its chem ical composition, the essential ingredient being kaolin, a hydrous silicate of alumina having the composition : Kaolin may be considered pure clay, but is rarely if ever Auld pure in nature. It is commonly mixed with varying quantities of silica,

lime, magnesia, ferric oxide, potash, and soda. The presence of these substances, which may be regarded as the impurities of clay, and the physical condition under which they exist, cause the wide variation in the clays themselves and to a great extent in the product made from them.

The chemical composition of shale suitable for making paving brick usually ranges between the limits given in Table 49.* Silica is practically infusible, and its presence prevents, or at least reduces, the tendency of the clay to crack, distort and shrink; but the more silica the greater plasticity and adhesiveness of the clay, and the less the strength and the toughness of the brick. Alumina will resist the highest temperature, and gives plasticity and adhesiveness to the clay and strength to the brick; but it causes the clay to shrink, warp, and crack in drying. Iron in con siderable quantities has a fluxing effect with silica, cementing it to gether and giving it strength. Iron is not the most valuable of constituents in this regard, and its presence is not essential to a first-class paving brick. The red color of brick is due to the presence of iron, but more to its form than to its amount. Many erroneously believe that only red brick have sufficient strength and durability for paving purposes. Lime and magnesia are infusible by themselves or with alumina, but fuse in the presence of an excess of silica and give strength to the brick. If the lime is in the form of feldspar or a silicate, the more the better. But if it is present in the form of carbonate, it weakens the brick; and if present as concretions or pebbles, and not finely ground, the quick lime resulting from the burning is likely to cause swelling or crack ing when the brick is wet. Potash and soda (Na()) fuse at lower temperatures than the other constituents of clay, and their presence in suitable quantities is very desirable.

Physical Properties.

A chemical analysis of a clay may furnish sufficient evidence upon which to condemn it for brick making purposes, but never enough for its indorsement. The following physical properties are important factors in determining the value of a brick clay: 1, its plasticity; 2, the amount of water required to make a plastic mass; 3, the amount of shrinkage, both in burning and in drying; 4, the rapidity of drying and also of burning; 5, the temperature of incipient and complete vitrifica tion; 6, the density before and after burning; and 7, the strength of the burned brick.