On this point a recent writer says : " I have before me the results of two analyses of clays just brought up from the labor atory. These clays are very different in composition, and samples are sent five times each month for both analysis and physical demonstration. The mines are both old ones, and each concern, representing two of the largest rival concerns in the United States, has been in business a great many years, but notice the contrast : No... No. 2.
Silica 49.44 Alumina 34.26 24.0o Iron oxide 7.74 none Carbonate of lime 2.70 Carbonate magnesia I.I0 none Chloride of the alkalies . none 1.8o Organic matter none 1.o1 Moisture, etc. Total roo.00 roo.00 " These two clays are in great demand, and are very popu lar, .for precisely the same purposes, with those who use them." In connection with these analyses attention is invited to the proportion of alumina and iron contained in the two clays. It is safe to say that the greater the proportion of alumina which a clay contains, the less such a clay is affected by the presence of a large per cent. of iron, and this applies to almost as great an extent with reference to alkalies.
In order to be useful, clay should be naturally plastic or should artificially be made more or less plastic, as this prop erty is necessary to their being moulded into the shapes re quired. This plasticity is owing, first, to the fineness of the particles, to the presence of alumina, and to the water of com bination. It is diminished by the presence of iron, lime and magnesia. The refractory nature of the clays, then, is due to the presence of alumina or silica in excess, and to the absence of the fluxes, potash, soda, lime, magnesia and iron.
The characteristics of all fire-clays may be said to be that they do not effervesce with acids, that they make a paste with water, which is absorbed so rapidly as to make a slight noise. This paste can be drawn out without breaking, and is very plastic. When dry, fire-clays are solid, and break into scales when struck. They have a soapy feeling, can be scratched or polished by the nail, can be cut into long ribbons with a knife, and appear somewhat like horn. When fresh from the quarry, fire-clays have a more or less fetid odor, owing to the presence of some partially decomposed organic substances. In compo
sition they contain, as we have seen, either silica or alumina in excess. Silica in excess makes them rough, and takes away most of their plasticity and tenacity. Alumina makes them very plastic ; magnesia makes them very unctuous, and almost soapy, but does not make them fusible ; lime makes them dry and fusible. Iron and other substances change their color, and beyond certain very restricted limits make them fusible. The colors, ranging from gray and brown to black, are owing to a small percentage of bituminous material. White clays are gen erally considered the best, but there is no certainty about it, as they often crack, or even melt. It is generally an excellent sign when they leave unbroken lines when scratched by the nail. It is, however, never safe to judge by the eye or touch, as some of their chief characteristics apply equally well to materials not in the least refractory, and even those that are peculiar to them may be taken away by improperly drying them, by careless ness in storing or handling them, or by allowing them to be come mixed with other substances. A preliminary analysis gives only a general idea of their nature, but it is not always a safe guide to the manufacturer, who needs first an analysis and then an assay, for some of the most inferior clays, if we should judge by their analyses, give excellent results when used as mixtures. Analysis is necessary both before and after the assay, but there is a molecular force which seems to exert a greater influence in imparting a refractory value to the material than is exerted by the chemical composition. The greater this force, the less likely is the heat to overcome it, either to cause disintegration or chemical union. If possible to do so, all clays should undergo some process of preparation, with a view of purifying them.
Every person using clays should endeavor to get a certain knowledge of their properties by assay. There have been a number of these assays published, most of which, though they give accurate results, are too complicated for ordinary use. The two simplest and best are those prepared by Bischoff and the foil assay.