CHAPTER VI.

FIRE-CLAYS, FIRE-BRICKS, AND OTHER PRODUCTS, AND THE NECESSARY MACHINES, ETC.

FIRE-CLAY is a substance which differs very greatly from the brick-clays, and also in many particulars from the terracotta clays; and the best fire-clay for any special purpose is that which will, for the longest period of time, effectively perform all that can be reasonably required of it, at the least cost or for repairs. Like the clays which have been named, fire-clays cannot be properly used as found; they must, as it were, be suspended in some infusible material, which will be a check upon the great mechanical effects of the heat, and at the same time concede a certain amount of expansion as well as of contraction, and exert a positive influence over both tendencies, should there be a disposition in either to go to too great an extreme.

The materials employed for this purpose are generally pulverized quartz or quartz-sand, finely ground old bricks, burnt clay, serpentine, which is a hard rock, veined or spotted, in which the colors are variously disposed, but the most prominent are greens and reds; talc, which is a mineral, occurring in granular and flaky conditions; graphite in powder, and when there is no fear from the ash, small coke

is used; but the last should be very cautiously employed, and only when graphite cannot be had, or when it is not desired to use it on account of its high price.

When a cheap mixture is to be made at the place where it is to be used, without previous burning, it is usually composed of one-fifth plastic clay, and the other four-fifths of burned clay or quartz, or it can be varied to one-fourth meagre clay to three-fourths burned clay or quartz.

The mixture avoids contraction, and is the most economical compound that can be constructed for many purposes, and it is coming rapidly into use, even for small blast furnaces that are not in constant or hard use.

The composition of fire-clays differs very materially; but those which contain the largest per cent. of alumina are the best; in these there should be not over three-fourths of one per cent. of lime.

In a large number of industries, an increase may be made in the proportion of combined silica, and thereby increase the resistance to the fire; but this will answer and perform the requirements only at a low temperature.

But the greater the per cent. of combined silica in intense heat, the greater is the fusibility, and for all ware to be so employed, the longer should be the exposure to the heat in the kiln.

The refractory power of all fire-clay wares is greatly enhanced at very high temperatures by the presence of a large per cent. of alumina.

There should be a perfect understanding of the difference between combined silica and free silica. In speaking of the

composition of fire-clays, the first, as has been stated, will melt in very high heats, in proportion to the per cent. of combination; but clean, free silica, i. e., in crystals, mechanically combined, will not melt in our melting heats, unless fluxed.

Consequently, a high proportion of free silica, in the absence of a high per cent. of the fluxes, lime, magnesia, alkali, and oxide of iron, is not near so injurious as when the silica is combined. Any additions in the fluxes, above three and one-half per cent., very quickly increase the fusibility of the clay, in the order in which they have been named in the preceding paragraph. They are thought to be damaging in the following order, viz: magnesia most, then lime, next oxide of iron, and least alkali.

The average amount of alumina, silica, etc., contained in the best fire-clays may be expressed in the following table:

There should be no organic substances.

The table is prepared for comparing the relative values of fire-clays.

All the above substances do not usually appear in good clays; in some there are none or only traces of lime and the different fluxes, and in other clays there may be a

larger or smaller proportion; but the ingredients in all good fire-clays should be somewhere in the neighborhood of the averages given above.

But, as has before been stated, any increase of alumina intensifies the resistance to heat at a high temperature; but in a combination when the alumina reaches as high as 40.5 and the silica decreases to 45, and the hygroscopic water appears in about the same proportion as the average of combined water, then the compound is probably china-clay or kaolin.

According to M. Brogniard, the clay which is most refractory when deprived of its hygrometric water has the composition: silica, 57.42; alumina, 42.58. Silica alone cannot be used unless it be ground, and, having no binding or plastic qualities like alumina, a small proportion of binding material is added to it.

For the Dinas fire-brick, which is the best substance to resist heat alone-the binding material is lime. The Dinas bricks are sometimes called "stone bricks," and are largely produced at Neath, in Glamorganshire, and are much used in the construction of copper furnaces at Swansea. materials of which the bricks are made are from quarries in the neighborhood; there are three kinds of material, which are faithfully mixed together; the composition of the three substances is as follows:

The

The bricks are made from these quartzose sandstones, which are first heated in quantities of twelve or fifteen tons in a Rumford oven, and then thrown into water to break them up, and then pulverized to a coarse powder between iron rollers. The amount of lime required to bind the bricks is one and one-half per cent., and sufficient water is added to make the mass slightly cohere under pressure; the joints between the bricks when laid are filled with the same material.

These bricks expand under the action of heat, but care must be taken not to expose them to the action of slags that are rich in the metallic oxides. These fire-bricks resist a much greater heat than those made from the Stourbridge clays. The Dinas bricks will last six weeks in the roof of an ordinary reverberatory furnace at a temperature which, if it could be measured by mercury, would be about 2200° C., equal to 3992° Fahr., and in that period will be reduced from nine to two inches by abrasion of the flame and dust, and partly from chipping.

The bricks conduct the heat so badly that at this great temperature, which is a bright white heat in the interior of the furnace, the exterior is not more than pleasantly warm to the touch of the hand. These bricks are carefully dried on floors warmed by flues after being pressed; seven days' hard firing are required for the bricks, and a little longer for cooling the kiln.

The ash-dust which circulates with the gases is ordinarily a chief cause of the wasting away of fire-bricks, and in furnaces where there is no dust, such as the Siemens, they give out from weakness.