"Two brick clays may have exactly the same composition and still differ in every respect, because the complete analysis, for example, gives us no idea whatever as to how much silicic acid, alumina and flux belong to the clay substance, to the rock dust, and to the sand individually; for instance, in the one case all or the greater part of the flux may belong to the clay substance, in another case to the constituents which make clays lean, and accordingly the properties of the compound be subject to the greatest varia tion with the same percentage composition. In the one case it may be the clay, in another the rock dust or sand, which, with the same percentage composition of the whole, is the most fusible constituent, as admixed iron oxide or carbonate of lime, which according to the manner of distribution are inclined to have the strongest effect sometimes on the clay, sometimes on the rock dust and sand, and thereby produce a number of variations which and not the slighest explanation by a simple chemical analysis.
"If we conclude from this that chemical analysis can claim only a limited value for the discovery of the properties of brick clay, such a judgment would be highly one-sided and inaccurate. . . . For our purpose it is especially the physical properties of clay that are of greatest importance in judging the same, and the chemical properties only as far as they supple ment the former. Here, therefore, to express it in a few words, it will be the task of the chemical analyst to determine the composition of the con stituents that are physically alike, that of the clay, rock dust, sand, and accessory constituents, separately and singly, and to make possible a com parison of these with each other. In this way we are able to get a good idea of the properties of the components,) whereas an analysis of the whole mass would' be of little use. We are thus convinced of the necessity of physical analysis of clay simultaneously with, or rather before the chemical, as far as the investigation is made for the purposes of 'pottery ware, and especially for the manufacture of bricks. Even though scientific men have repeatedly referred to the importance of the mechanical and physical inves tigations, this direction of the investigation has not been pursued with such vigor that the results obtained from it show any real use for the brick industry." In the foregoing statements Dr. Seger has very forcibly set forth the same doctrines that the writer has come to thoroughly believe as a re sult of observations in the factory and laboratory. In subsequent writ
ings Dr. Seger set forth the value of what is known as the "Rational analysis" in which "clay substance," feldspar and free silica are differ entiated. He cited many cases in which, with the aid of the Rational analysis, he was able to obtain more clearly an idea of the constitution and properties of clay than he could from any other method of analysis. In this, however, he was no doubt over zealous, for later studies by other chemists proved that not only does the "Rational analysis" fail to sharply differentiate between the "clay substance," feldspar and free silica, but that the analysis is. of value only in the purest clays, viz.: China and ball clays. The writer has made `rational" analysis of many types of clays, and, barring those used in the vitreous pottery wares, he is compelled to state that not once has he been able to obtain a clearer insight into the actual constitution of the clays than he could from the gross or ultimate analyses. Predictions concerning a clay based on a rational analysis in the great majority of cases, go very far wrong. After considerable pains and labor in the execution of the analysis the operator is compelled to make guesses that are much less scientific and accurate than he would if he had merely burned a piece of the same clay in a small muffle furnace, and noted the rate of change in color and dem sity with increasing intensity of heat treatment, a test that ought not to require more than three hours time, and can be made by any one who has access to a kiln.
It was shown in a preliminary report on the fire clays of Illinois' that fire clays having the same ultimate chemical composition behave very differently in burning. Indeed the chemical composition and ulti mate fusion period were very often found to coincide in clays which, on the one hand, would remain open and porous through sufficiently long and severe heat treatments to make them fit for use in fire brick, or, on the other, would be nearly vitrified under the heat treatment used in burning stoneware and sewer pipe. Such phenomena are discussed and illustrated in this report under the title of pyro-physical and chem ical properties of clays.
In the manufacture of vitreous floor tile the writer learned by prac tical experience that particular effects either in color, vitreousness, ulti mate fusibility, or any other physical property requisite in the produc tion of floor tile, could not be duplicated on the basis of chemical com position.