The most important constituent of all refractory wares is the calcined clay or " chamotte." This will not shrink, and pos sesses the power of union in the greatest possible degree. These two important qualities have more to do with the pro duction of fire-clay goods, regular both in size and in quality, than any other features in the material or process employed.
Another advantage in calcining clay is that it enables one to throw aside any material in which there are impurities that may have been previously overlooked, since these are much more easily seen when the clay has been burned. The proportion of calcined clay or " chamotte" used must, of course, vary with the size of the brick or tile, and the particular use for which it is intended.
In calcining fire-clay, the raw clay as it comes from the mine or pit is used, and of this the lumps are preferable for this purpose. The reason for this is that the small or fine clay is more trouble to handle and to burn. The lumps can be more easily placed in • the kiln and the fire can get a draft through between them. Of course a certain quantity of small or fire clay can be used along with the lumps. Two points here should be observed. First, it should be dry when set, or it will be liable to fall in the kiln and obstruct the draft. Second, it should also be freshly mined, as, if exposed to the weather too long it falls to pieces and cannot well be handled. These remarks are intended to apply more particularly to the clays of the coal measures, which are either non-plastic or plastic, and which come out of the mine in hard blocks. The soft plastic clays, either fire-clays or terra- cotta clays, that can be dug out of the bank in large pieces or blocks, must be dried before being burnt, and can then be treated in the same way as just described.
Some manufacturers of refractory materials have the clay in tended for calcining cut in twelve-inch lengths as it emerges from a 6x6 inch hole at the bottom of an ordinary pug-mill, and then set these crude blocks of fire-clay next to the walls and in the same kilns in which the wares are fired. These rough blocks thus calcined are, on removal from the kiln, broken by a six-stamp mill, and afterwards ground to three or more degrees of fineness in ordinary grinding mills.
The more advanced fire-brick makers, however, have separate calcining kilns, which are built of brick, with a boiler-iron shell, as will be described. These kilns in large works are com
monly 15 feet high and 8 feet in diameter, with fire holes a few feet from the bottom. The top is dome-shaped, with a chimney from the centre having a damper on top. The clay is charged in through a hole near the top of the dome, and is drawn out at the bottom of the kiln on iron plates, through two drawing doors, one on each side of the kiln, 20 tons being the daily product of one kiln.
" Lean" few clays can be used as mined. They must be, as it were, suspended in some infusible material which will prevent, as far as possible, the mechanical effects of the heat, and allow, at the same time, of a certain amount of expansion and contraction, while preventing both in too great a degree. These materials are generally called " lean," that is, they do not make a paste with water, and require some binding material to keep them together. They are usually quartz-sand or pulverized quartz, burnt clay, old brick, serpentine, talc, graphite in powder, and not infrequently small coke, when the ash is not to be feared, and when graphite either cannot be had or cannot be used on account of its high price. Some fire-clays from Spain contain this " lean " material, which comes from the decomposition of talc-shale in which they have been suspended by nature, but this is a rare exception. The mixture must gen erally be made artificially. Of all these substances quartz-sand is the cheapest, but it has been found by experience that round grains of sand are less liable to become thoroughly incorpo rated with the binding material than the angular pieces of crushed quartz, so that when a very refractory material is re quired crushed quartz is always used. As the clay contracts and quartz expands, a mixture may be made which will not change its form ; but in a given case this may not be the best mixture for a special use. If the material has only to resist a great heat, an excess of quartz is preferable ; but if it must also resist the corrosive action of basic slags, clays burnt at a high heat, graphite or coke, can be used. When the mixture is made in the place where it is to be used, without previous burning, it is generally made of one-fifth plastic clay and four-fifths burned clay or quartz, or one-fourth lean clay and three-fourths burned clay or quartz. This is done to avoid contraction.