Page:The American Cyclopædia (1879) Volume III.djvu/272

 266 BRICK sition, and much of it requires some admix- ture of other substances. The purer aluminous earths consist of about two parts of silica to one of alumina, together with a larger or smaller proportion of water. They are remarkable for their plasticity, and mix freely with any quantity of water. But such materials, if moulded and baked, would shrink greatly and bend and warp ; cracks would be produced from the outside hardening before the moisture of the interior could escape through the viscid mass. Such rich or fat clays require to be tempered with sand, or cinders and ashes, which render their texture more open, so that they retain their form ; but they may without this tempering serve for baking into thin sheets as tiles. The quantity of sand or other sub- stance required for any clay can only be de- termined by actual experiment. Some clays contain a proper proportion of sand naturally mixed with them. Others contain too much, and the bricks from these will fall to pieces. Admixture of fatter clays is the only method of making such useful, unless an expensive pro- cess be adopted of suspending the earth in water, and drawing off and collecting that which is held longest in suspension. Besides sandy clay or loam, calcareous clay or marl is sometimes used for the manufacture of brick ; but if much lime he present, the compound may be too fusible to answer the purpose of making good brick. Oxide of iron is rarely absent. In the process of burning it is con- verted into the peroxide, and imparts to the whole brick its red color, more or less deep according to the degree of oxidation. The first of the following analyses is of a clay highly charged with oxide of iron, given in Rnapp's " Chemical Technology ; " it is largely used in the neighborhood of Glasgow, Scotland, for making brick ; the quantity of water is less than is commonly given in the analyses of clay ; the second is of a clay suitable for potters' use or the manufacture of brick : (1) (2) Silica 4<J-44 485 Alumina 84-26 83-2 Protoxide of iron 7-74 1-0 Lime 148 8-S Water 1-94 18 Magnesia 514 Loss, 0-8 100-00 100-00 The more free the clay is from other ingredients than silica and alumina, the bettor the bricks made from it withstand high temperatures. Clay taken freshly from its bed, even if of suitable composition, is not in a condition to be at once moulded into brick. It must first be exposed to the weather until its particles are disintegrated, when it can be kneaded into a mass of uniform consistency. This is best ef- fected by the action of frost, the water diffused through the substance expanding by freezing and breaking it in every direction. The longer the exposure is continued, the more effectually is the clay reduced. This is followed by cov- ering the clay with water and leaving it for a short time in a pit or tank. The kneading was formerly conducted by treading of horses, oxen, or men, and the work was no doubt more efficiently done by the naked feet of men than by the machinery afterward introduced for this purpose; for the lumps, stones, sticks, &c., mixed with the clay were thus readily de- tected and removed. The pug-mill is the first form of machinery introduced for grinding the clay. It is a conical or cylindrical tub, stand- ing on end, with a shaft passing vertically through it, armed with blades, which cut and knead the clay delivered in the top, forcing it down by their oblique position to the line of the shaft, as this is carried round by a horse attached to a horizontal arm. The clay thus ground and kneaded continually passes through an opening in the bottom of the mill, and is then cut into convenient pieces and stacked away for use. It must then be handled again for moulding it, and the practice was formerly to forcibly throw a quantity into the mould, which was a box of wood or of brass without top or bottom, and then strike off what was superfluous. The mould is always sanded to prevent the clay adhering to it. A box con- taining a row of five or seven moulds open at bottom was afterward contrived to run in un- der the lower part of the pug-mill and receive the clay, the further exit of which was at the same time arrested until another box of moulds replaced the one just removed. The work was th us rendered more expeditious w ith less expen- diture of labor. In whatever way the kneading is conducted, especial care is taken to separate from the clay roots, sticks, and pebbles, the presence of which in the bricks would disfigure and weaken them. Even if the stones were buried in the interior of the bricks, a cavity would be left around them, for the reason that the stones first expand while the clay contracts by heating, and afterward contract by cooling in a much greater degree than the clay. In tempering the clay, it was found highly advan- tageous at the great brick yards near London to introduce a portion of coal ashes, which always contain more or less fine coal. The use of fine anthracite was introduced for the same purpose at the kilns on the Hudson river in 1838, and has been found so serviceable that it has been ever since continued. The quantity employed is about 75 bushels to 100,000 bricks. It is thoroughly intermixed in the kneading, and has the effect of saving a portion of the fuel, while it diminishes the time of burning ; the quality of the bricks, however, is not so good as of those made in the old way. For drying the bricks previous to baking, the first requisite is a smooth level yard fully exposed to the rays of the sun, or, if covered by a roof, open to a free circulation of air all around. To this the moulds containing the bricks are brought, and, being placed upon the ground, are cautiously lifted off, leaving the bricks behind. They are arranged in rows, and in case of rain, if not under a roof, must be