EFFECT OF PHYSICAL AND CHEMICAL PROPERTIES OF CLAYS.
The effect of carbon in its different forms has been discussed. The oxidation of ferrous compounds in the presence and absence of carbon has also been considered. Certain other points need be considered un der this heading. Among these are: distribution of the carbon, fine ness of grain of the clay, iron in combination as a stable or not easily altered silicate, the structure of the clay mass, the presence of moisture, and the temperature factor.
Varied distribution of carbon—If carbon is thoroughly disseminated through the mass, it will be so surrounded by the mineral matter as to cause slow oxidation in a manner similar to the slow burning of a fire banked with earth. If, on the other hand- the carbon is concentrated in particles the size of coal dust, oxidation can take place much more readily. Anthracite screenings added to a clay either for the purpose of effecting equal distribution of heat or making the ware more pores, seldom give trouble in oxidation. The quantity of carbon, therefore, is not so important a factor in determining the oxidizing behavior of clay as its character and distribution. On this account chemical analysis has failed to give much aid in detecting the difficultly oxidizable clay.
Fineness of grain—If the clay itself is fine grained, and especially if very plastic, it will prevent oxygen getting to the carbon and will delay the expulsion of the gases formed by the burning carbon. Fur ther, in case of fine-grained clays, the carbon will not be completely oxidized, i. e., CO instead of CO. will be formed and in escaping from the center of the bricks will keep the iron in the outside portions in a reduced condition long after the carbon here has been burned out and time given to oxidize the iron to the ferric condition. Fineness of grain of a clay, therefore, plays an important role in the oxidation of clay wares.
Stable iron compounds—Iron combines with silicates in both the "ous" and "ic" condition, i. e., we have ferrous silicates and ferric silicates. The instances, however, of iron in the "ic" condition com bining with silicates are comparatively rare. This was shown in a very
forcible manner in a series of experiments in which the writer used several varieties of granite in porcelain floor tile bodies. The granites were obtained from different quarries in the form of "spalls" that are made when the rough granite is cut into shapes. These spalls were ground to powder that was as fine as feldspar and flint, as prepared by millers for pottery use. In the majority of cases the tiles were full of minute black specks with but a trace of the buff color that would be given if the iron had been in the "ic" condition. In one or two cases the iron. specks were buff instead of black, showing that the iron was either in the "ic" form in the granite or had been oxidized in the burning. From the fact that all these "granite trials" were burned at the same time and hence under the same heat treatment, it was con cluded that in these exceptional cases the iron was originally in the more highly oxidized form. This conclusion was substantiated in later experiments in which iron calcines were used.
When fusion in a clay or clay mixture has progressed sufficiently to cause the whole to be vitrified, iron, if originally present as an oxide, carbonate or hydrate, will generally combine as a lower oxide, forming ferrous silicate. The blueing of fire clays and the changing from red to chocolate in shales is evidence of this. For this reason iron oxide added to a porcelain body either as an oxide or as an ingredient of a shale will, on vitrification of the body, result in a blue tint. Iron pre cipitated into a mass of silica or alumina, and the mixture dried and calcined under oxidizing conditions. will when added to the porcelain body, produce a buff or pink, never a blue color. These experiments proved conclusively that iron can combine with alumina and silica in the "ic" condition forming ferric compounds, and, further, that when so combined fusion of the body will not result in the reduction of the iron compound.