About one-half, by volume, of the charge of the furnace in almost all cases is coke, one of the most highly abrasive substances known. In practically all cases also limestone is added to the charge in the condition of lumps sometimes of the size of a man's head or larger, but more generally crushed down at least to go through a 4-in. ring. The balance of the charge is made up of ore which varies from the soft condition of a plastic clay to the condition of one of the hardest of the common rocks, according to the variety. These materials are put through a modern furnace 90 ft. high, at the rate of something like 1700 or 1800 tons in twenty-four hours. The limestone and the ore as they approach the hotter zone are softened if not reduced to powder by the action of the gas upon them, but the coke remains firm and hard until it is burnt up at the tuyeres.

The abrasive action of such a mass of material even at atmospheric temperature would be considerable, but when it is remembered that in the lower portion of the furnace temperatures prevail which are above the softening point of any material available, it is obvious that the abrasion by the descent of the charge alone must present a problem to be reckoned with.

Resistance To Abrasion By The Gas Current

The velocity of the gas through the shaft is exceedingly high, probably higher than is realized by those who have not actually estimated it. I believe in ordinary modern practice it is never less than 1000 ft. per minute, and more probably double that. This gas is necessarily filled with fine particles of material from the stock, which is in a constant state of agitation due to its descent, so that the fines are swept out of it almost as chaff is separated from wheat. These fines are picked up and carried by this great volume of gas moving at this high velocity. Most of the particles impinge on the solid stock, but many of them strike the furnace walls and exert thereon the action of the familiar sand blast. In this case also we must remember that the resisting material, at least in the lower regions of the furnace, is softened by heat so that much of its resistance is gone, and it becomes evident that we have here a form of destructive energy whose effects cannot be ignored.

Resistance To Pressure

It is absolutely necessary that the furnace be gas-tight in the same way, and as nearly as possible to the same extent, that a good boiler is steam-tight. The pressures, it is true, are not as high as modern boiler pressures, but they range from 10 to 30 lbs. per square inch in modern practice, probably averaging in Lake ore practice around 15 lbs.

The effect of pressure in promoting reaction, especially when gases are concerned, and its effect in giving density and carrying power to the gas currents, are probably factors in the situation, but the principal point is that if this pressure be not resisted at every point gas will find the weak spot, and, traveling through it will carry heat and chemical action to assist the destructive effect of the pressure. The pressure-resisting quality of the structure comes from the steel shell outside the lining, but this is quickly destroyed if the conditions prevailing within the furnace proper strike it, and with its destruction the whole structure fails. For this reason resistance to pressure is one of the prime conditions to be met.