Material—Calcium carbonate, hydrates of silica, alumina, and iron, as well as zeolitic compounds, when first precipitated or formed, are in the majority of cases in extremely fine grains. The fluxing behavior of any substance is materially different when thoroughly disseminated in minute grains, especially in the colloidal form, than when present in coarser grains. Iron, for instance, has been found to enter into chem ical combination with silica as a ferric silica when the iron is precip itated on flint and as a ferrous silicate, if at all, when the two are mixed as dry powders. The vast difference between the fluxing action of ferrous and ferric oxides and compounds need not be discussed at this tinge. The important fact in this connection, is that it depends to a very large extent on the form and manner in which the iron is dis seminated through the clay, as to whether it will combine as the lower or higher oxide. What is true of iron in this respect is true to a degree of other fluxes.
Summary of Factors Affecting Manner of Fusion of Clays—First The manner in which the several constitutent elements are combined, one with another, very materially affects the fluxing behavior of a clay.
Second—The size of grain of the several mineral constitutents is an important factor in determining the fusing behavior of clays.
Third—The amount, form, and character of the volatile constitutents of clay does not directly affect the thermo-chemical reactions, but the difference in physical condition and structure of the clay, and the sta bility of the non-volatilized compounds, caused by the expulsion of these substances, does materially affect the manner in which fusion takes place.
Fourth—The importance of the role that absorbed salts play in the fusing behavior of clays is little appreciated. The evidence on the manner in which they operate is so indirect that definite statements or conclusions are impossible. That they are important factors, however, there is no doubt.
Fifth—Concerning precipitated materials, we have evidence from syn thetical experiments that prove beyond doubt that they mist be con sidered as most potent in affecting the fusion of clays.
From the above, it is evident that the writer has but little confidence in the efficiency of an ultimate analysis of a clay as a means of fore telling its burning properties. The combination, size and character of grain, solubility, volatility, and dissemination of the several salts, and, lastly, the manner in which the uncombined oxides are introduced into the clay are more effective factors than the total ultimate composition.