Fire Bricks.
Mr. W. Y. Dent, in a Cantor lecture at the Society of Arts, London, on building materials, gave an account of some of the chemical problems involved in the constituency of fire clay and fire bricks.
The plastic clays consist of silica and alumina chemically combined with water. They are hydrated silicates of alumina, the plasticity depending upon the water that enters into their composition. The water with which the clay is chemically combined can be expelled at a temperature a little above that of boiling, without detriment to its plasticity, but the whole of the water contained cannot be driven off without raising the temperature to dull redness. Silica, alumina, and lime are separately very infusible substances, and are capable of resisting exposure to very high temperatures without softening. It is on account of its extreme infusibility that lime is found to be the most suitable material for the cylinders upon which the oxyhydrogen flame is made to impinge to produce a brilliant light, the intensity of the light being due to the extremely high temperature to which the lime is raised. Lime, however, from its want of cohesion, could never be brought into general use for such purposes as fire clay is employed, and this is also the case as regards silica, which requires the addition of some substance of a basic character, with which it will unite, and so cause the particles to bind together. The nearest approach to the use of silica alone as a fire brick is in the case of the Welsh brick, made from the Dinas rock in the Vale of Neath.
This material, before being made into fire bricks, had long been used for repairing the furnaces at the copper works of South Wales, for which purpose its peculiar property of expanding when subjected to the influence of a high temperature, instead of contracting, as in the case of some other fire clays, renders it particularly suitable, the cementation of the bricks being facilitated by the increase of temperature. This Dinas rock occurs in various conditions, from that of a firm rock to that of disintegrated sand, and a mixture of about 1 per cent. of lime is, therefore, necessary in order to make it into bricks. Dinas bricks will stand very high temperatures, but are more friable than ordinary fire bricks, and will not resist to the same extent the action of basic substances, such as furnace slags, containing much oxide of iron. They are, besides, porous and readily absorb moisture, rendering it necessary for furnaces built of them to be gradually heated, as they are liable to crack if sufficient time is not allowed for driving off the moisture. The composition of the clay used for fire bricks is a question of great importance, inasmuch as its quality depends greatly upon its chemical constituents, although its power of resisting fusion, when exposed to intense heat, is effected by its mechanical condition.
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.