THE MANUFACTURE OF FIRE-BRICK ; SILICA FIRE-BRICK ; BON FIRE-BRICK FOR FURNACES; GLASS POTS AND GAS RETORTS.
The essential qualities of a good fire-brick may be stated as follows: First, infusibility; second, regularity of shape and the power to retain it under all circumstances, which involves per fect uniformity of composition ; third, strength to resist the pressures required under different circumstances; and fourth, its cheap price. No material yet manufactured fulfills all these conditions ; but there seems to be no reason why, with the proper investigation, a material should not be made which will fulfill most of the requirements above stated. No brick can come up to the modern standard of infusibility which contains 5 per cent, of iron, or 3 per cent. of combined alkalies or alkaline earths ; and yet the most infusible brick that is known, which in the roof of a Siemens-Martin furnace will resist 25o charges, and then wear out by abrasion, when required to come in contact with metals, oxides and alkalies in a Spiegel cupola, will hardly stand 25 heats, although an iron pipe coil, which is easily destroyed by heat, will last almost indefinitely in the same cupola, provided only a sufficient stream of water is run through it. Different furnaces, and different parts of the same furnace, should, therefore, be treated differently, instead of being treated by the same procrustean methods, as is frequently the case. If silica makes the best roof, it makes the worst hearth. Alumina, when present in very large quantities, even in the presence of a small amount of silica, makes compounds which are almost infusible, so that it should be used for the fire-bridges and hearths, and not be put- into the roof, where its tendency to contract would endan ger the structure of the furnace.
Far too little attention has been given to the abrasive and corrosive power of coal-dust and ashes carried by the draft in gradually cutting and fluxing away the parts of the furnace ex posed to its action, and many qualities of brick which are in fusible in the assay, owe their small power of resistance to its effect. A brick to be used where it is to be exposed to such action should always be tested by placing it for a considerable time on the bridge of the furnace where it is to be used, for the destructive effects of this almost unobserved agency seem to be greater than those of long-continued heat.
A good brick should not only resist high temperatures, but sudden changes of temperature, without alteration of any kind, such as crushing, splitting, etc., and at a high temperature,
should undergo the least possible change of form. In general, it may be said that brick which have undergone a very high temperature in the manufacture are less liable to contract after ward. Shrinkage is generally due to insufficient burning, or to a small proportion of old material in the mixture, and gener ally occurs in aluminous brick. Its chief evil is in allowing the flame to penetrate the open joints and give the dust an oppor tunity to cut between the brick, for any cause which produces eddies in the flames, such as hollows or projecting surfaces, is certain to effect the destruction of that part of the furnace.
Refractory materials may be classified as fire-stones and fire clays. The former are usually silicious rocks, but sometimes talcose slates or soapstones are used,,which stand heat well in the presence of basic slags. All fire-stones are used in the native state, with no other preparation than the necessary shaping. Fire-clay is the main refractory material, and is used only in the manufactured state.
Many managers of rolling-mills have condemned a first-class fire-brick because the crown, or roof, of the puddling furnace had worn in an irregular manner, holes or eddies being- warn through the whole depth of the brick,- While' the brick- iii the same roof and immediately around are nearly of original length. The fire-brick manufacturer receives the complaint and is sorry to lose his customer without any personal investigation, or probably he may run over and see for himself. He sees an irregular roof with holes fused through, or nearly so, and the bulk in good condition, and is at a loss to explain the reason. He may be satisfied in his own mind that the brick he sent were all of the same quality, and so tell the manager. This failure may, perhaps, have been caused by some No. 2 quality having become mixed with the others in loading, or very prob ably, as I have found myself under such circumstances, that the bricklayers wanted some keys or wedges to get the proper radius, and have been obliged to take a make of brick of an other manufacture and of inferior quality from the stock-house. These have fused more readily, as they have worn past the other brick, forming holes or eddies in which the flames and dust have played, thus destroying the inferior brick much more rapidly than if the whole roof had been built with them.