FIRECLAYS

are clays which will withstand a high degree of heat without exce ive shrinkage or warping. The varieties of F. differ in their degrees of fusibility, owing to the variations in the proportion of free and combined silicon; they are e entially hydrated aluminous silicates with lime and magnesia in the form of carbonates, iron pyrites, free silica, potash, and soda, with a percentage of water. No fixed standard of refractorine can be given, but in all good F’. the fusion point is over 1600° GC. Such materials as ganister, sand, saw-- dust, etc., are mixed up with F. before burning to ensue the ‘body’ of the brick being sufficiently open in character. Ordinary F, is extensively used for making bricks, crucibles, chimney-pipes, etc., but when special properties are required in the bricks, such materials as lime, bauxite, etc., are added. See Cuay Propvucts.