When chamotte is used (i. e., fire-clay previously burnt and comminuted) the material for the chamotte is comminuted by grinding, and consists partially of clay as refractory as possible, especially burnt for this purpose, and of worn-out glass-pots or seggars, crucibles, muffles, and other waste of refractory clay articles. The more refractory the clay, as well as the chamotte used, the more refractory the brick will be.
The finer the grain of the chamotte the more of it may be added to a clay, whereby the mass gains in homogeneity and strength, but, on other hand, loses in capability of bearing changes in temperature, and inversely, the coarser the grain of the chamotte, the better the composition will bear rapid changes in temperature. It is best to endeavor to unite both properties by adding to the quite finely ground crude clay a mixture of finely and coarsely ground chamotte, taking a somewhat larger proportion of the latter. As a rule the coarse grain should not exceed one-quarter inch in size.
The proportion in which the burnt clay or the substitute for it is mixed with the crude clay varies according to the degree of fatness or cementing capacity of the latter. If the cement ing capacity is considerable, for instance, about to to 12, as in Belgian clay, and the chamotte is sharply burnt, which is neces sary for the production of good brick, I part of crude to 2 parts of burnt clay is used. If, however, the crude clay is quite mea gre, and possesses but little cementing capacity, for instance only 3 to 5, the addition of chamotte must be considerably smaller in to obtain a mass of sufficient plasticity and suitable for transport, and the proportion may be reduced even to I : I i. e., I part of burnt clay .to I part of crude clay. If the rule to use the most refractory clay at the disposal of the manufac turer, as chamotte, is to be considered as valid, it follows that nearly everywhere the fat clays should be used for the crude portion of the composition, and the meagre clays, which are frequently more refractory and generally burn very hard, for the burnt portion, since the refractoriness of the entire mass is thus increased by the greater addition of burnt clay required. Variations from this rule will, however, be sometimes neces sary, according to the different pyrometrical value of the clays at disposal.
Mixing is in the mixing of the clays used in the manufacture of refractory materials that the skill of one operator over another is manifested.
Clays which are to be used in the manufacture of fire-brick and other refractory materials, after being stored in the sheds, cleaned, and carefully dried, and in all other ways properly prepared, are afterward mixed with the substances with which they are to be incorporated, which are classified by numbers, varying according to the size of the sieve-holes through which they will pass. The quantity and quality of the mixture will determine the refractory nature of the material to be produced. A friable paste with large grains, and quite porous, resists a great heat. One with fine grains, close and compact, splits at a high heat, especially if it is not homogeneous. The manner in which the mixture is made also influences the quality of the brick quite as much as the material. In some works in Bel gium, after taking all the ordinary precautions to make the mixture perfect, it is submitted to a succession of shocks con tinued for some time, until it is found by experiment that the materials are perfectly mixed. It has been found by long ex perience that the brick so made keep their form perfectly, while others made of exactly the same mixture in the ordinary way contract. The quantity and size of the mixture depend upon the size of the article to be manufactured. When coarse grains are used, greater thickness must be given to the sides of the articles if they are hollow, and they must be made larger if they are solid, thus giving a mechanicnl cohesion where a chemical one is wanting. The usual quantities of the mixture for brick are three-fifths to two-thirds of the substances added to two-fifths to one-third of the clay, these quantities being determined by volume and not by weight. When coke-dust is used it does not seem to have any decided effect beyond one tenth. The action of coke or graphite is to decompose the metallic oxides as they form, and thus prevent their union with the material of the crucible. Coke may be profitably used in the place of graphite when the ash is in small quantity, free from iron and highly aluminous. Beyond 2 to 3 per cent. of graphite cannot be profitably used, as it weakens the article and renders it liable to break. The mixture which gives the very best results for small articles is, however, worthless for large. It will generally be found that the pieces which crack up and down in drying have had too much material mixed with the clay, and those which crack laterally have had too much clay.