This was also found true in the manufacture of porous white ware bodies, such as are used in jardinieres and art wares, and no doubt is en tertained but that the same would be found to hold true to a large ex tent in the manufacture of vitreous china. In these cases, however, rational analysis, i. e., the determination of the proportional quantity of clay substance, feldspar, and free'silica finds value in that these sev eral minerals have decided effect on the expansion and contraction of the blended pottery body, and,. consequently, upon the proper fitting on it of a glassy coating (the glaze.) The only instance in which chemical analysis is of positive aid, aside from the explaining of some observed phenomenon, is in the execution and study of a systematically planned series of experiments. Seger's classical studies that resulted in the invention of the pyrometric cones would probably never hare been carried out had he not followed closely the chemical analyses of the raw materials and planned his series on chemical formulae. Following him, there has been much of exceedingly great value resulting from researches in pottery mixtures that would have been impossible on any other than a strictly chemical basis. In the study of paving brick clays here reported the fact has been discovered that the best paving clays contain a relatively high content of magnesia. Such a discovery has been and would have been impossible from an analysis of an isolated sample. Further, this fact would not have been noted had no systematic researches on a chemical basis been made with pure clays, minerals, and magnesia compounds, showing that mixtures containing a comparatively high content of magnesia have a long fusion range, for, as will be seen later, the value of clays for paving brick man ufacture, or even their fusion range, do not always correlate with high magnesia content.
The suggestion made by Dr. Seger in the paragraphs quoted in the
introduction to this chapter, that a chemical analysis of the several sub divisions of the particles according to size would be of value, is possibly true. In fact it is obvious why such should be the case. The time and trouble involved in making a thorough mechanical analysis of a clay into several groups having different ranges in size of particles in quantities sufficient to make accurate and especially duplicated analyses of each group, places such a determination out of consideration as a commercial test on clays. But for a scientific purpose it is believed that the re sults obtained would justify the expense and trouble involved. Such a study was made by Grout on a composite sample of West Virginia clays. His results are cited and discussed on pages 179 to 180 of this report. So far as the writer is aware, Grout was the first to make such an analy sis, and it is hoped that the deductions drawn from his results show justification for the making of similar studies on single clays.
Notwithstanding the fact that up to date it would be but a matter of chance that an interpretation of the results of a chemical analysis would agree with the observed working properties of a clay, it should not be concluded that our chemists may not in the near future devise a method of analysis that will meet the requirements of the case. In fact, so strongly do many believe that this will come to pass, that they see justification in making and reporting chemical analyses of clays by geological surveys, as has been their wont in the past. The writer firmly believes, however, that as a forerunner to such an event, many carefully executed and systematic physical and chemical researches on each type of clay must be made with parallel observations on synthetical mixtures of pure minerals. A few such observations will be made in subsequent paragraphs.