The same materials, when mixed together in the form of a coarse powder, will require a higher temperature to fuse them than would be the case if they were reduced to a fine state of division. The qualities required in fire bricks are that they should bear exposure to intense heat for a long time without fusion, that they should be capable of being subjected to sudden changes of temperature without injury, and that they should be able to resist the action of melted copper or iron slag. The Dinas brick, which contains 98 per cent. of silica, will bear exposure to a higher temperature than most others, but it will run down sooner when in contact with melted iron slag. Ganister is the name given to a fine grit which occurs under certain coal beds in Yorkshire, Derby, and South Wales, and the black ganister from the neighborhood of Sheffield is especially adapted for lining cupola furnaces, owing to its capacity to stand high temperatures without shrinking, in consequence of the large quantity of silica it contains.
Fire bricks made of silicious clays from granitic deposits in various parts of Devonshire also contain a large proportion of silica, but their powers of supporting exposure to high temperatures are materially increased by the coarseness of the particles of disintegrated granite of which they are composed. The material employed for the Dinas bricks, as well as the others mentioned, differs considerably in its character from what is ordinarily understood by the term fire clay, as used in the manufacture of the celebrated fire bricks of Blaydon Burn, Stourbridge, or Glenboig; the quality of which, as regards their chemical composition, depends upon the relative portions of silica and alumina, and their freedom from iron oxide and alkaline salts, the presence of which tends to render the clay more fusible.
Clay is taken as dug from the pit, without being dried, and mixed with the usual proportion of lime, a portion of which is used as limestone, the remainder as freshly burnt lime. The burnt lime is first added in such proportion that the water in the clay exactly suffices to slake it, and the heat given out effects the necessary drying. The limestone is then added and the mixture ground in a mill to the usual degree of fineness, made into bricks, calcined, and the “clinker” reduced to a fine state of division as usual.