IMPORTANT PROPERTIES OF FIRE-CLAYS.
The compositions of fire-clays differ very materially, as is shown by the results of the various analyses which have been previously given.
The refractory power of all fire-clay wares is greatly en hanced at very high temperatures by the presence of a large per cent. of alumina.
Chemists tell us that fire-clays will melt the more readily in very high heats in proportion to the per cent. of combined silica which they contain ; but that clean, free silica, i. e., in crystals, mechanically combined, will not melt in our melting heats, un less fluxed. Consequently, they say, a high proportion of free silica, in the absence of a high per cent. of the fluxes, lime, magnesia, alkali and iron, is not nearly so injurious as when the silica is chemically combined.
Every fire-brick manufacturer, who has ever given the sub ject of combined and free silica attention, and who has made furnace tests of brick containing an excess of free silica, in com petition with brick carrying an excess of combined silica, by placing such brick side by side in a furnace, knows that fire brick are more refractory in proportion as the free silica is re placed by combined silica.
Free silica alone is, of course, infusible, but when it enters into combination with the other bodies commonly present in fire-clays, such clays become the more easily fused the larger the proportion of free silica present.
This raises a very important question, viz. : Whether it is not really essential that there should be made two analyses of the silica contained in any fire-clay; one analysis to determine the per cent. of free silica, and the other to determine the per cent. of combined silica which the clay carries.
The hydrated silicates of alumina used in fire-brick manufac ture contain as a rule from so to 65 per cent. of silica, so to 75 per cent. of alumina, and I I to 15 per cent. of water. The re lation between the silica and the alumina is exceedingly vari able, owing to the fact that a part of the silica, which is not always the same, is combined, and a part uncombined ; hence the necessity of the dual analysis of the silica just mentioned. The quantity of water is also variable, as part of it is hygro scopic and can be driven off without injury to the clay. The plasticity generally depends on the water of combination, which, when driven off at a red heat, cannot be made to combine again, so that this property is then entirely lost. It contains, beside, a small quantity of other elements, such as potash, soda, lime, magnesia and iron, and is generally less refractory the more it contains of them. When it contains from 6 to to per
cent. it will generally melt. When the clay is silicious, 3 to 4 per cent. of other substances make it fusible. When it is alu minous, 6 to 7 per cent. of oxide of iron does not make it lose its refractory qualities, owing to the very refractory nature of most aluminates. When, therefore, the corrosive effects of basic slags are to be feared, aluminous clays must be used.
Almost all clays contain organic matter. The presence of Organic matter in fire-clays is, however, unimportant, as it is consumed or removed when the brick are passed through the kiln, as would be the case with an admixture of coal free from ash.
Pure material, composed exclusively of silica and alumina, would be completely infusible. Such material is, however, ex ceedingly rare. The property of infusibility is always more or less compromised by the presence of foreign substances, which tend to reduce it or take it away altogether. The clay which, according to Brogniard, is the most refractory when deprived of its hygrometric water, has the following composition : Silica, ; alumina, 42.58.
While the refractory nature of the clay is due, to a very great extent, to its chemical composition, it is not due to it alone. There are, probably, no two beds of clay in the world, or even different parts of the same bed, that have exactly the same composition, and yet they may be very nearly of the same quality. The power to resist heat is, undoubtedly, owing to the molecular condition of the particles, a subject which has been but little studied and is but little understood. Many clays, which would be rejected from chemical analysis alone, are sometimes found in practice to be excellent refractory ma terials. It has been found that the refractory nature of the clay depends also to a great extent on the mechanical arrange ment of the particles, for of two materials having exactly the same chemical composition, one being coarse and the other fine, the coarse may be practically infusible, while the fine may be more or less easily fusible. The more porous the same sub stance is, the more infusible it will be. It may be said in gen eral terms that the value of a given refractory clay will be inversely as its coarseness, and as the amount of iron contained. When the amount of iron reaches 5 per cent., the material, as a rule, becomes worthless. This is true, however, only in gen eral, for Pettigand cites an excellent clay from Spain in which there is 25 per cent. of iron. This is, however, an exception.