Modern Gasworks Practice

194 MODERN GAS WORKS PRACTICE in the presence of carbonic acid and water may be chemically expressed as follows :— Al2O3.K2O.2(3SiO2) +2H2O + CO2 = 4SiO2 + K2CO3 + Al2O3.2SiO2.2H2O (kaolin) These kaolin clays are highly refractory but are practically devoid of plas-ticity ; they exist, however, in relatively small quantities, and the larger deposits of fireclays of service to the gas engineer are the result of incomplete or mtermittent weathering. It should be stated here that although, as a general rule, the refractoriness of a substance may be gauged by the proportion of silica it contains, this proposition is by no means beyond question, and definite combinations of silica and alumina are more highly infusible than when tlie latter gives way to silica. In faet, certain experimenters have shown that alumina is rather more refractory than silica, and that by admixing the two so as to give the composition Al2O3.2SiO2 th.c most infusible product is obtained. Ordinary fireclays may in general be de-noted by the formula— Al2O3.6SiO2 but in addition to the silica and alumina they contain varying quantities of impurities, such, as oxides of calcium, magnesium, titanium, potassium, sodium and iron. These impurities, owing to their low fusibility, exert a most harmful influence on the ultimate refractoriness of the clay, and in most cases their total content should be limited to 4 or 5 per cent. Much depends, however, on the type of clay in question, and some highly refractory materials now used contain as mueh as 8 per cent, of iron alone. Chief objection to impurities lies in the faet that once they themselves have set up fusion by the formation of fusible silicates the remainder of the material may fall a victim to their influence, even though the prevailing temperature is con-siderably below the normal fusing point. Many recent investigators into the refractoriness of clays have emphasized the faet that ultimate Chemical analysis cannot be taken as a final indication of serviceability ; in faet, instances have been quoted where clays having chemically a model composition have been known to give defeetive articles. For this reason it has been suggested that the analysis should be directed towards the presence of the various mineralogical constituents on a subdivision such as the following :— (a) True clay substance. (6) Quartz. (c) Remnants of the original mother rocks. The last-named component, containing the original alkaline impurities, is, per-haps, of chief importance, and for effeetive results should be in the neighbourhood 2% per cent. In the less siliceous clays the desirable kaolin will average about 72 per cent., the remainder being accounted for by quartz, which has a varying in-fluence, chiefly dependent upon the quantity of impurities present. If the impurities are on the low side the tendency of the quartz will be to aid infusibility. An im-