Firebrick

A brick capable of sustaining, without fusion, the extreme action of fire. They are used for lining furnaces, and for all kinds of brick-work exposed to intense heat which would melt common bricks. They are made from a natural compound of silica and alumina, which, when free from lime and other fluxes, is infusible under the greatest heat to which it can be subjected. Oxide of iron, however, which is present in most clays, renders the clay fusible when the silica and alumina are nearly in equal proportions, and those fireclays are the best in which the silica is greatly in exce over the alumina. When the alumina is in exce , broken crucibles, gla -house pots, and old F. B., ground to powder, are substituted for the common silicious sand used in the ordinary proce es of brick-making, but which, in this case, would be injurious, as having a tendency to render the clay fusible. Fire-clay being an expensive article, it is usual, when making F. B. at a distance from mines, to mix with it burnt clay, for the sake of economizing the clay and diminishing its con traction.