ULTIMATE CHEMICAL COMPOSITION.
By ultimate chemical composition is meant the percentage by weight of the several oxides of the elements that occur in clay irrespective of their original state or combination. Ordinary chemical analyses are re ported in terms of so much silican oxide, aluminium oxide, calcium oxide, etc. All are more or less familiar with such analyses, and not a few brick manufacturers have had repeated analyses of their clays made by chemists. The reports they received are what is known as the "Ultimate Chemical Analysis," in contradistinction to the rational analysis, that gives the supposed approximate percentage of clay sub stance, feldspar and quartz in the clay, instead of the individual ox ides of which these substances are composed.
The persistent belief in the value of an ultimate chemical analysis on the part of layman and scientist alike is a not wholly unwarranted compliment to the science of chemistry. It cannot be denied that there is some slight foundation for this unflinching confidence in the value of an ultimate chemical analysis. but it is equally true that even after these many years of constant research by scientists the world over, very little advance has been made in ability to interpret the facts that ought to be disclosed by such analyses. Because so many chemists, as well as laymen, do not seem to understand the difficulties that attend the interpretation of such an analysis, a brief review of a few of the recorded facts will be given.
In 1868, Richterst in his classic work entitled "Refractoriness of Clays," promulgated laws in regard to the fluxing effect of the various elements in simple mixtures at high • heats that are now known as the Richters laws. In 1895, E. Cramer published in the "Thon-Industrie Zeitung" a review of Richter's work confirming his laws in every re spect. The fluxing behavior of the various bases according to Richter's laws are shown in the following curves. Figs. 18 to 21.
From Richters' and Cranier's investigations it is learned that the order of fusibility of the different oxides in simple clay mixtures is as shown in the first column of the following table. In the presence of free silica the order is changed somewhat, as is shown by contrasting the order given in the third column with that given in the first. Go
ing to the other extreme of silicate mixture, that of glazes, the order of the fluxing effect of the various oxides is, according to Segeri as given in the fifth column.
Seger further says: "The law established by Richter and Bischof, concerning the fusibility of clays, `that equivalent proportions of fluxes exert an equal influence on the fusibility,' and which appears to be ap proximately correct for the very high temperatures employed in clay testing, and for the very small quantities of the fluxes coming into ac tion in the clays, has no bearing on the glasses and glazes, far richer in fluxes and melting at far lower temperatures." Ludwig' having made similar studies with more complex mixtures summarizes his results as follows : "First—Richters' law is a special case of the general law of dilute solu tions.
Second—This law is restricted by the following correlations: a. It applies only to very dilute solutions, that is, clays with a small amount of fluxes and not to brick clays or glazes.
b. Is assumes intimate mixtures.
c. Iron shows a different effect, due to its two stages of oxidation, since one molecule of ferric oxide corresponds to two molecules of ferrous oxide. A given percentage of iron contains fewer molecules of ferric oxide than of ferrous oxide, since the former has a higher molecular weight. On changing to the ferrous oxide the number of molecules is doubled, and hence the fluxing action is doubled.
Third. The analysis of a fire clay is of great importance in estimating its refractioriness.
Fourth. The estimation of refractioriness by means of the percentage of alumina and fluxes leads to erroneous results." From the above citation it must be concluded that the fluxing power of a given oxide is affected very materially by the kind and number of oxides present, as well as their chemical combination, degree of hydra tion, oxidation, Etc. The facts gleaned from a study of the fluxing effect' of a single oxide in a simple mixture do not necessarily hold true in the same degree in complex mixtures. It is well known in glaze manufacture, for instance, that a mixture of several fluxes pro vokes greater fusibility than a mixture of any two or them. What is true of glazes is likewise true of clays.