44 METALLURGY OF CAST IRON.
gases which, though probably important, produce an effect the amount of which is at present not accurately determined. 3. The action of sodium chloride or other alkaline substances contained in coke; this is probably one of the most important causes of wear, as at a high temperature salt is decomposed by silica, and a fusible silicate is obtained. 4. The flaking of the bricks due to decomposition of carbon from carbon monoxide around any iron particles reduced from impurities in the original bricks.''
The best grades of fire-brick are necessary in lining
furnaces. Absolute fire-proof bricks, it may be said, are not obtainable. Several kinds of material have been tried in an effort to secure a lining for furnaces that would exceed the life of the general character of fire-bricks used. We have what are called silica, carbon, ganister, coke, magnesia, and asbestos bricks, all of which have been experimented with, and, to some degree, all have advocates of their utility in certain lines of work. Carbon bricks, it is claimed, have worn well, made of fine coke (poor in ash), or charcoal mixed with clay with tar as a binder. If such bricks contain more than 70 per cent of silica, as used for high temperatures, they are generally very friable and disintegrate with the least friction, so that bricks of this character would be suitable only for the lower body of a furnace. As clay is chiefly silicate of alumina, which is also a good substance to resist high temperatures, it works well as a binder with silica in making fire-bricks. The other substances in clay are iron oxide, lime, magnesia, potash and soda, which, to some degree, decrease the durability of fire-bricks. As fire-bricks come to the furnace or foundry they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are if j