VALUE OF CHEMICAL ANALYSES.
Because, in private correspondence and on every public and semi-public oc casion that has afforded opportunity, the writer has taken a stand against the large absolute value and importance of chemical analysis of clays that is contrary to views popularly held by practical clay. workers and encouraged by many of the State Survey reports, the discussion of the chemical properties of clays is intro duced by liberal quotations from the most eminent of ceramic chemists. Dr. Her mann Seger.
"Thee demands which the cement and the ceramic industries make on the qualities of clay are as different as the purposes which these industries pursue.
"In the manufacture of Portland cement we have in mind the obtaining of a product of a definite chemical composition, and, since the character of clay as such must completely vanish in this, the mutual relation of the indi vidual constituents is to be considered above all things, and the physical condition in which these are found be considered only as far it opposes greater or less difficulties to the destruction of the clayey character.
"The clay industries, on the other hand, pursue a quite different purpose in the treatment of their raw material. The limits within which the chem ical constitution of clay may vary are very wide, and, since the clayey char acter of the material is to be preserved, its physical qualities and those of its essential and accessory constituents are to be placed in the foreground. While for such a purpose, the chemical composition of clay, as a whole, ap pears more indifferent and accidental, inasmuch as it depends on the mutual relation of clay, rock flour, sand, accidental admixtures and their chemical constitution, the physical properties of the same, the grain and its form, cap illarity, plasticity, fusibility, etc., are of greater importance, and the chemi cal constitution of each of these constituents is to be considered only as far as it permits us to infer the physical properties of the whole. It is surely a serious mistake to treat material so heterogeneous chemically and me chanically, as the clays and earths used in the ceramic industries, like sub stances chemically and physically homogeneous, as for example glass, and to base conclusions with regard to their properties on their chemical composi tion.
"The chemical changes which the materials of the ceramic industries suffer in the course of manufacture, step into the background, with the exception of the loss of chemically bound water, which has as a consequence the loss of plasticity, and must not be produced in the same degree as in the manufacture of cement and glass, or the material will lose its earthy character. In fact, it seems advisable to drop the investigation of the chem
ical composition of clay as a whole, and put in its place a deeper study of the composition of the essential and accidental constituents, in order to to infer the properties of the whole from the properties of the compounds thus obtained. For example, we need not ask how much pure clay and silicic acid we have in clay, but what part of the clay and silicic acid be longs to the sandy constituents, what part to the silty, or the clayey con stituent, and . what physical properties must we, according to these data, ascribe to the sand, rock dust, clay, etc., individually.
"It cannot be denied that in the examinations of clays scrupulously accur ate analyses of the material have heretofore been made, but that little has been learned concerning structure, condition of plasticity, power of absorbing water, shrinkage on drying and burning, form and size of grains of sand, and rock dust, concerning the pecularities of the concretions, and concern ing eflorescences and incrustations. In the consideration of the properties of clay for the purposes of the clay industries we must put ourselves more upon a physical than a chemical standpoint.
"Ifl chemical analysis has discovered a fixed relation between alumina, silicic acid and flux, we know that these constituents belong essentially to a single well-characterized combination, so that we can take the degree of refractioriness from the laws established by Bischofand Hitchers with a greater or less assurance, according as this substance is present in a greater or less degree of purity. However, if we should proceed in a similar way in the investigation of brick clays, we would get a theoretical result so very different from the practical result,2 that it would have no value what ever in regard to the knowledge of the material. The chemical analysis gives us only an average of the composition of the components forming the clay, differing very widely in their chemical composition and their physi cal properties. Since the clay, after burning, preserves its earthy charac ter, and the various constituents act only superficially on each other, the chemical analysis gives us absolutely no clue for the deduction of definite properties of the whole.