An important feature demanding particular attention in all furnaces is the interior lining, because of the intense heat and the chemical action to which furnace linings are subjected. A lining must (1) resist oxidation and reduction,

(2) must not melt nor disintegrate from heat, (3) must reasonably stand the mechanical wear of the furnace charge, and (4) must not, unless so intended, enter into chemical combination with the furnace charge. Crucibles, in which metal is melted, the linings of ladles for receiving and pouring molten metals, and the linings of all classes of heating furnaces must fulfill substantially the same requirements.

The refractory materials commonly used in all branches of smelting, refining and melting of metals, including the processes of steel making, and in heating furnaces, are

(1) Silica (common sand), SiO2.

(2) Silicate of Aluminum (clay), Al2O32SiO22H2O.

(3) Magnesite (magnesia), MgO.

(4) Chromite (chromium oxide), Cr2O3FeO.

(5) Dolomite (magnesian limestone).

(6) Bauxite (alumina), A12O3.

All of these substances are more or less impure as found in nature, containing iron oxide, sand and clay, and in some cases, soda, potash, and organic matter. Practice has established the limit of impurities not detrimental to the uses of these materials for specific purposes.

Silica and clay are closely associated in use as refractory materials. They are used far more than the other substances named, and are adapted for use in many kinds of furnaces. A mixture of the two materials is known as fire clay, and different proportions of the mixtures are employed in different furnaces, according to results obtained by experience. Silica resists the action of heat and alumina resists the action of metallic oxides. Silica is not plastic when wet and must be mixed with enough clay to make the grains stick together. Ganister is a natural rock composed of silica and clay in the proportion for making high-grade fire brick.

Silica is an "acid" material and cannot be used for bottoms of furnaces for basic steel making, and for linings of other furnace bottoms requiring basic material. For these uses, magnesite and chromite are used in brick form, and dolomite is used for patching, but it is more or less objectionable because of its lime content, an excessive amount of which causes disintegration at high temperatures. Chromite is a neutral material, that is, it is neither acid, basic, oxidizing nor reducing in its chemical action, and is unexcelled for use where a material is needed to resist high heat and chemical action.

Bauxite is exceedingly refractory and is neutral, but has very limited use because it is subject to excessive shrinkage and loses plasticity when highly heated.

Refractory materials are much used in the form of fire bricks. There are many standard commercial shapes of fire bricks, and many grades also, according to the purity of materials, skill in shaping and care in burning. The selected materials are thoroughly calcined, ground, well mixed in correct proportions with water, moulded to shape, dried in the open air, and slowly burned in a kiln which must be heated to a white heat and gradually allowed to cool. About 1 per cent of lime in silica bricks is necessary to fuse the particles together when burned. All bricks should be true to shape, and their surfaces must allow them to lie in close contact when placed for use. Each grade of bricks must be laid in a cement or mortar of like material ground fine. This cement fills spaces between the bricks, uniting the whole in a compact mass.

A good fire brick when broken should not show a crumbly mass of ingredients, with large grains of material loose and ready to fall out, but the mass should be dense, strong and thoroughly fused together.