CHAPTER XVI. THE FIRE CLAYS AND FIRE-BRICK INDUSTRY. CONTENTS. Properties of fire clays. Definition. Chemical composition. Effect of silica. Effect of titanium. Other properties. Mineral impurities. Uses of fire clays. History of the fire-brick industry. Method of manufacture. Tests of New Jersey fire brick. PROPERTIES OF FIRE CLAY. Definition.-Strictly speaking a fire clay is one whose fusion point lies at least above that of cone 27, but the term is somewhat loosely used and often applied to clays of even low refractoriness. Aside from refractoriness, which is the most important property of a fire clay and the one possessed by all true ones, they vary widely, showing great differences in plasticity, density, shrinkage, tensile strength and color. Since the resistance of a fire clay to heat is governed primarily by its chemical composition, and secondarily by its fineness of grain, it may be well to consider first the former property. Chemical composition.-Fire clays (see Appendix C., Middlesex county), contain practically all the substances usually determined by the ultimate analysis, but in every good fire clay the 'See p. 49; also, pp. 315, 319, 320. 1 total percentage of certain fluxing impurities such as ferric oxide, lime, magnesia and alkalies, is small. This is necessarily the case, since, if the fluxing impurities were present in large quantities, the clay would fuse at comparatively low temperatures and could not be classed as refractory. Effect of silica.-It is found, however, that clays running low in fluxes, but high in silica, may also show poor refractoriness. If we compare two fire clays of low-flux contents, but high silica in one case, and low silica in the other, it is found that, other things being equal, the high silica clay is less refractory than the other. This indicates that a high percentage of silica, as well as a high percentage of the fluxes mentioned above, diminishes the refractoriness of the clay. We might therefore term the iron oxide, lime, magnesia and alkalies low-temperature fluxes and the silica a high-temperature flux. In any fire clay, some of the silica is combined chemically with the alumina in the form of the mineral kaolinite (p. 47), while the balance is probably there in the form of quartz.1 If kaolinite alone is heated, its refractoriness is found to be high, for its fusion point is the same as cone 36 of the Seger series (see p. 102), and the refractoriness of quartz or silica alone is nearly as high, but if these two minerals are mixed together in varying proportions, then the fusion point of the mixtures will in every case be lower than that of either silica or kaolinite alone. This fact was pointed out some years ago by Herman Seger, the German ceramic technologist, who made up a series of mixtures of alumina and silica, and kaolin and silica. In the former series of mixtures the quantity of alumina in each case was the same, but the amount of silica was increased. Starting with I part of alumina2 to one of silica by volume (91.5 of alumina to There cannot be many silicate minerals such as feldspar, mica, etc., in a fire clay, otherwise the percentage of alkalies, magnesia, lime and iron oxide would be higher than it usually is, so that the balance of the silica must be quartz. 2 * What is meant here is parts by volume, which would not be the same as parts by weight, because the 2 substances have different specific weights, hence I alumina to I silica per volume would be 91.5 per cent. alumina to 8.5 silica by weight. 8.5 of silica by weight), a mixture the fusion point of which was the same as that of cone 37, he found that the refractoriness decreased until a mixture of 1 part alumina to 17 parts of silica (10 alumina to 90 silica by weight) was reached. The fusing point of this mixture was cone 29. A further increase in the amount of silica began to increase steadily the refractoriness. This shows that silica added to alumina in certain proportions acts as a flux at high temperatures. Too: SILICA Fig 40. Diagram showing effect of Silica on the fusion point, when mixed with If now silica is mixed with kaolinite in the same manner, a similar lowering of the refractoriness of the mass takes place down to a certain point beyond which the fusion point again rises. These experiments of Seger are shown graphically in Fig. 40, in which the horizontal lines represent the different cone numbers from 26 to 38 inclusive. The divisions on the lower line represent percentages of alumina or kaolin measured above the line, 100 per cent. being at the left end, and percentages of silica measured below the line, 100 per cent. being at the right end. The solid curve represents the mixture of silica and alumina, while the dotted curve represents mixtures of kaolin and silica. An inspection of these curves, shows quite clearly how an increase in the percentage of silica up to a certain point causes a dropping of the fusion point, but that a further increase in the silica contents raises it again, although not quite as high as it originally was. It will be seen from a comparison of these two curves that the kaolinite-silica mixtures have lower refractoriness than the pure silica-alumina mixtures. This effect of silica will no doubt be at first accepted with doubt by many brick manufacturers, who have considered that silica or sand adds to the refractoriness of clay in burning, but it should be remembered that common bricks Fig.41.Diagram showing the effects of silica and titanium on the fusibility of kaolin. are burned at a much lower temperature than that at which the alumina and silica unite. In testing the New Jersey fire clays in the Deville furnace, the results obtained seemed to bear out Seger's experiments, but did not agree with them very closely, and in fact the fusion points were usually lower than would be expected from his curve. Accordingly a series of mixtures of a white-burning clay1 and finely This clay was practically free from fluxes, and, hence, had very nearly the composition of kaolinite. |