Refractory is used here to denote the high heat-resisting properties of a material. The subject is very wide and involved, and is not yet thoroughly understood in all re- * spects. It frequently happens that clays from different sources, but having the same chemical analysis, differ very much in their behaviour, and this is assumed to be due to a different atomic construction or arrangement. Various theories have been put forward to account for this, but none seem to fully satisfy all the conditions.

There are many materials which have a fusion point high enough to justify their being termed refractory ; among the best known of such are asbestos, mica, fireclay, alumina, magnesia, silica, carborundum, chrome iron ore, carbon, etc.

While any of these may have a known fusion point in the pure state, yet the impurities always present in a commercial article so affect the fusion point that the same is decreased. The fusion point is also very much affected by the conditions under which the material is being used, or is subjected to, such as rate of heating, or rate of cooling, shocks or jars, etc.. that no exact figures can be given that will cover all possible conditions. Refractory materials may be classed as either acid, basic, or neutral in character. Acid refractory materials are those such as silica and fireclay, with an excess proportion of a material such as silica, which combines freely with the basic oxides. Basic refractory materials are those such as mag-nesite in which silica is only present in quite small proportions and therefore the basic material is unsatisfied, and is open to attack by silica at a high temperature. Neutral refractory materials are those in which the acid and basic components are so satisfied that no excess exists to attack, or to be attacked by either acid or basic substances brought into contact with them chromite is neutral in character. It should be remembered that the constituents of a heated slag are always ready to attack anything with which they may be in contact provided that by so doing they can form a neutral salt, hence the importance of using such refractory materials that no such chemical action can be started between the slag and the firebricks

By chemical definition an acid is a body containing hydrogen, which hydrogen may be replaced by a metal, or the equivalent of a metal, when presented to it in the form of an oxide. Bv similar definition a base is a body, generally an oxide of a metal, or its equivalent, which metal is capable of replacing the hydrogen of an acid, given the opportunity. An Alkali is a base of an especially active character, generally soluble in water.

For the ready determination of the high temperatures to be met with in refractory work, Dr. Seger has developed a series of small tetrahedral cones, generally known as Seger Cones. These are composed of carefully determined different proportions of mainly such materials as marble, felspar, quartz, and kaolin. They are numbered to correspond to the temperatures to which they relate, such temperature being determined by the top of the cone bending over towards the base, due to softening by the heat applied. It should be realised that the temperatures noted for each cone are only applicable for the cone when heated at a certain rate of heat increase, and in an oxidising atmosphere, otherwise the results differ, and the figures of numbers and temperatures do not agree. For the purposes of refractory materials, cones number 20 to number 42 may be required (21 to 25 are not made now) and the temperatures at which the above mentioned bending takes place are :-

The chief use for refract ory materials are for the making of firebricks, and among various kinds or such are the following :

The best known is fireclay; this is generally found in the carboniferous districts, is crushed, mixed with water, moulded either by hand or machinery, and burnt to a temperature of about 1350 C. Fireclay, when fresh, may be liable to extreme shrinkage, and grog is frequently added to it before the burning. The grog consists of selected fireclay, burnt, crushed to a size that will not pass through a specified sieve; the quantity added depends upon the conditions, etc., for firebrick it may be up to 20 per cent Fireclay exists in three main varieties, i.e.. Aluminious, Siliceous, or Silica, these containing about 65 per cent., 80 per cent, to 92 per cent, and above 92 per cent, of silica respectively. All fireclay bricks are acid in character.

Chromite firebrick is made from chrome iron ore, which is a double oxide of iron and chromium, with generally less than 6 per cent, of silica; it has a fusion point of about 1800 C, and is inert to the extent that it acts as a neutral material scarcely affected by either basic or acid conditions. Canister is composed largely of silica, up to perhaps 97 per cent.; it occurs in a natural state in the English Midlands and elsewhere, is acid in character, and is largely used in lining the convertors for the ordinary or acid Bessemer process.

Magnesite or Magesia firebricks are composed largely of magnesia heated in a suitable kiln at a temperature of about 1600 C, is crushed, the bond added, mixed well, moulded at a high pressure, dried and burnt; it is highly basic in character, and under certain conditions will stand up to 2000 C. before fusion. Dolomite firebricks are composed of magnesian limestone, which is a definite mineral containing the carbonates of lime and magnesia in nearly equal proportions, is highly basic, and is sometimes used to line the convertors for the basic Bessemer process, although now magnesia firebricks are more frequently used. The raw material after crushing to a coarse powder is mixed with the selected bonding material, frequently tar, is then moulded, dried out, and burnt in a suitable kiln. Dinas firebrick are so called from having been first made from the Dinas sand near the Yale of Neath, are very refractory, and acid in character. It may contain up to 95 per cent, of silica. Bauxite is a hydrated aluminous ferric oxide, containing about 3 per cent, of silica, about 20 per cent, of ferric oxide, and about 60 per cent, of alumina, is basic in character, and with about 5 per cent, of a bonding material added to it makes up into an excellent firebrick, with a fusion point of about 1780 c.