In the manufacture of fire-brick the correct choice of the raw materials is the basis of success. The methods of working vary in different countries. But as the refractory clays present very dissimilar properties, and these dissimilarities show themselves in various grades and differences in quality, a factory where a great variety of products is to be turned out should not be re stricted to the working of but one bed of clay. A free and unrestricted choice of material, on which the value of the pro duct is necessarily dependent, must always be considered as a special advantage in locating a factory. In working crude clays it is, however, almost always necessary to add chamotte, or, as a substitute for it, old fire-brick and fragments of worn out clay vessels (glass pots or seggers, crucibles, muffles, etc.), whereby the mass gains in density, resisting capacity, solidity, inalterability.
Storing, Weathering and Elutriating The piles of clay from which the selection of clays for mixing is made, usually adjoin the works as closely as possible on the side next to the grinding machinery. In many places the amount kept on hand is large, amounting to 7,000 or 10,000 tons. There is no object in thus storing fire-clay, unless it be either to insure a supply for some time in advance and guard against transient interruptions, or to allow the clay to slack and break up fine, thus curtailing part of the mechanical preparations otherwise needed. There is a belief largely current that allowing a clay to be exposed to the influences of the weather acts advanta geously in ridding it of impurities. Though it cannot be denied that under certain conditions this would be so, yet it is equally certain that these influences are much overrated. The impurities which would thus escape are potash and soda, from such compouds as feldspar and mica ; yet the decomposition of these minerals having been effected by weathering, the mechanical conditions which would aid in the escape of the impurities are seldom found. A strong slant to the floor of the clay pile, so that water would drain away quickly and well after raining, and only a thin layer of clay on the floor so that impurities from the top layers might not lodge in the bottom, would favor the escape of impurities ; but the exact opposite of both these conditions as a rule prevails. Iron sulphide and carbonates of lime and magnesia would also tend to decompose and leave the clay, but their action would be very gradual. But the mechanical subdivision of the clay which takes place is undoubtedly advantageous ; alternate frost and heat has long been reckoned as a valuable agent in increasing the plasticity of hard clays.
Some clays, however, are naturally highly plastic, a condi tion which detracts from their refractoriness, and as weathering promotes plasticity it does not improve this class of clays. Clays of the non-plastic type and those that are very hard when freshly mined should all be weathered before using. It takes off that rawness noticeable in freshly-mined clays of this class, conduces largely to an increase of purity, renders the clay much better to temper and easier to work, and makes sounder, better brick. It also facilitates the sorting out of im pure matters that have escaped the miners, and also makes it easier to grind, thus reducing the wear and tear and increasing the capacity of the plant. To obtain the full benefit from weathering, the clay should be kept exposed some months ahead, and not stored in large heaps, except it be turned over periodically, or much of the benefit will be lost. In storing the clay it should be arranged so that it can be used systematically and in order of its age.
A more effective but also more artificial means of improving fire-clays is by elutriation, the separation of heavier and coarser particles of iron combinations, sand, quartz, etc., being best effected thereby. Besides, the elutriating waste, especially when elutriation is preceded by an organic disintegrating pro cess, carries away the alkalies as well as a portion of the silica in soluble form. The treatment of the clays with acids should be entirely rejected ; therefore the only practical means for im proving the clays which can be recommended are weathering and elutriating, and only these means have come into use in manufacturing on a large scale.
A certain portion of the clay used in the manu facture of refractory materials is calcined and converted into " chamotte" or cement. With some clays calcining is a prime necessity. These clays are such as are highly plastic, and if made into brick entirely from the raw clay, form a dense, close body, predisposed on that account to incipient vitrification, and will not stand sudden and extreme changes of temperature. By using from one-third to one-half of calcined clay a granular body is obtained, and the defects just mentioned are overcome. From this necessary method with plastic clays, the practice of calcining has also become quite common in treating the flint or non-plastic clays.