Granted that it would be possible to make a fairly accurate mineral ogical analysis of a clay, it is doubtful if our present knowledge would enable us to predict its working qualtities or even its fusibility with accuracy. When it is considered that a mixture of 40 per cent quartz and 60 per cent feldspar has approximately the same pyrometric value as feldspar taken alone, and that both have like effect on the green prop erties of clay, some idea of the complexity of the problem is apparent. What is true of a mixture of feldspar and flint is true of a large number of pairs of other minerals. What is true of minerals when considered in pairs, is true to a larger degree when taken in a multiple combin ation. It does not require much imagination to see where one would be led if it should be required to predict the fusion behavior or a hetero geneous mixture of a large number of minerals.
This sort of a study is of value and in fact is now looked upon as a necessity in the compounding of artificial mixtures of clays and min erals for pottery purposes, but in these cases the operator is dealing with substances the mineral character of which is to a large degree known, and he is mixing these minerals in predetermined proportions. He has in this case a synthetical mixture of known mineralogical con stitution adn of comparative simplicity (containing at the most not more than four or five different minerals, and, therefore, his practical experience ought to enable him to predict its physical and pyro-chem ical behavior. In the case of nature's mixtures, however, man has at
present no way of determining their mineralogical constitution, and must depend upon an actual test for obtaining a knowledge of the working properties of the mixture.
To illustrate these difficulties reference might be made to the fire clays, which are comparatively pure clay substance or at least rela tively simple mixtures of mineral ingredients. It has been shown that in plotting the position that indicates the relative fusibility of the clays on the basis of their alumina-silica ratio in reference to the position oc cupied by a synthetical mixture having a similar alumina-silica ratio, no concordant relation existed between them. Further the difference between the No. 1 and the No. 2 fire clays of the usual clay workers' classification having practically the same ultimate fusion point, but differing from one another in the manner of vitrification is no doubt explainable either on account of difference in mineralogical composi tion or character of grains. What is true in the case of simple mineral mixtures like the fire clays would be greatly exaggerated in the case of the exceedingly complex mixtures, such as most of the shales and sur face clays.