The very greatest importance is attached in some industries to not having a mixture made by a machine. In many places even to this day the inhuman method of heel-treading is used, because more care is then exercised, or because smaller quan tities being mixed at once, better results are obtained. The more the operations of mixing are repeated the better the material, and it is undoubtedly true that with mechanical means such a homogeneous paste is not produced as can be made by human labor, because the whole object of the machine is to operate on large quantities at a time.
Every mixture has its own peculiar rate of expansion and contraction. This expansion not only takes place when the brick are made, but if when used they are submitted to a higher degree of heat they expand still further, and contract again on cooling to such an extent that at Dowlais, Wales, the tie-rods of the steel furnace are slackened when the furnace is getting into heat, and are tightened again as it cools. At Crewe, England, this is made self-acting by means of springs. At Creusot the furnace-casing is made so strong as to resist the pressure, so that the roof-arch must rise and fall, to allow for the expansion and contraction. When neutral brick must be had for any reason, it is mixed with just enough clay and burned brick to make it keep its form, and such a brick is generally less fusible the less silica it contains.
The proportions of flint, " lean," calcined and plastic clays which shall compose the brick or other fire-clay product having been determined, the materials are selected from the pile ; the mixture of so much of each factor is made by counting the shovelfuls with which the charging barrow is loaded. As a rule no closer proportion is kept anywhere than careful shoveling will make. In the manufacture of silica brick, however, the most scrupulous care is necessary, and it is the practice to carefully weigh the constituents, platform-scales being used upon which the loaded barrows are run.
Washing. The barrow being loaded, the clay, be it plastic or non-plastic, should be washed. The washing is accom plished by running the barrow over a sink or drain and drench ing from a hose above. The barrow being perforated on the bottom speedily drains dry again. This treatment, though not thorough, tends to free the clay from dust, mud and dirt, which stick to it from the diggings. The best method of washing is at the excellent works of the Harbison & Walker Co., Pitts
burg, Pa. Their machine consists of a cylinder revolving in a slightly inclined position in a trough of water. The wall of the cylinder is made of coarse iron gauze or netting, and on the inside is bolted a spiral flange beginning on the upper end and running to the other extreme. A charge of clay is introduced into a hopper at the upper end, and by the flange is slowly carried down the length of the cylinder, being agitated in water, which is about six inches deep in the lower part of the cylinder. This machine is only used to wash hard, uncalcined clays, for the plastic grades would not stand so severe a treat ment, and calcined clays do not need it. Washing is only useful or advisable where the hard clays in use are mined by benching or stripping and come to the works covered with mud or dirt.
Grinding and Tempering. We may now follow the clay through the successive steps of grinding, tempering, molding, pressing, drying and burning.
The correct mixtures of the constituents of fire-brick having been accomplished, it is necessary in the further working of the mass, when moistened in the proper proportion with water, to be especially careful to obtain complete homogeneity in the interior of the brick, and the utmost mechanical strength. This is attained by a thorough and intimate kneading of the mass, by firmly pressing it into the moulds and by slowly dry ing the moulded brick. By over-hastening the drying the mass is again loosened, which injures the quality of the brick as regards mechanical strength.
For most uses in practice a high degree of mechanical strength is demanded from fire-brick, which depend for their refractoriness upon the nature of the clay used in their manu facture. With comparatively less refractory clays showing an inclination towards slagging at a not very high degree of heat, the ordinary burning, which is the final operation of manufac ture, frequently corrects any carelessness committed in working the mass, the brick after burning appearing sufficiently strong and hard. With very refractory clays this is, however, not the case, since they do not readily slag in the heat of an ordinary kiln. For these, sharp burning, which is absolutely necessary, must be preceded by the most careful and thorough working of the mass, and, if possible, a strong pressing of the brick moulded by hand.