Fine grained clays, as will be seen later, have not proved to be good paving brick clays. It cannot be said, however, that all coarse grained clays are good paving brick clays. Indeed, although evidence is lack ing, there is no obvious reason for believing that any hard and fast rule can at present be laid down in regard to either fine or coarse grained clays.
When the history of a few paving brick plants in various parts of this country reveals the fact that experienced paving brick manufactur ers have so misjudged a deposit of clay as to erect an extensive plant upon a particular site and soon find that they must abandon the idea of attempting to make any other than a building brick, it must be in ferred that even a burning test as ordinarily conducted by ceramic engineers, surveys and brick machine manufacturers likewise. often gives evidence that is untrustworthy. By what means then can the suitability of a clay for paving brick purposes be ascertained? It was with hopes of obtaining evidence upon this problem that the Survey undertook a study of the properties of the clays and burned bricks of several of the leading paving brick manufacturies in the middle west, together with several samples of clays from various parts of this state, that are not now being used for paving brick manufacture.
For many years scientists have been devising methods with which to determine the cause and effect of the various properties of clay, but they have not made much progress. For instance, the reason .why a kaolin and a ball clay, having similar chemical composition and size and apparently character of grain, should differ so widely in plasticity, is still an open question. The refractoriness of a clay is still 'incal culable from analytical data, although exhaustive researches have been made to determine the pyro-chemical effect of inorganic acids and bases, singly, collectively, and in mixtures, with standard clays and com pounds. While from these pyro-chemical studies it has been shown that the fluxing power of the bases is roughly proportional to their mole cular weight, and that the several acids operate in a definite manner, so that synthetical mixtures can be made with assurance that each compon ent will operate in a given manner, and that the resultant effect of the mixtures will in general be as presupposed, similar natural mixtures, known as clays, exhibit properties that are in the large majority of cases entirely contradictory to those of synthetical mixtures, due no doubt to differences in the physical properties of the minerals as well as to variation in mineral content.
Many theories have been advanced concerning the geological history of clays, and general statements can be made as to the probable conditions that cause the breaking down of the parent rock, the character of the resi dual debris, the agencies sorting and transporting this debris, and the conditions under which it can be deposited in different grades of fineness and purity. Geologists can state with considerable accuracy, the effect of
vegetable growth and of ground water, the cause for the precipitation of salts from solutions, the cementing value of various compounds under different conditions, etc. They can establish the fact that there is a cycle of rock decomposition, residual deposition, and rock formation going on constantly en masse, as well as in the small grains of which clay and soils are composed. Yet, after all, neither geologists nor chemists are able to determine the exact stage of breaking down or building up, nor the exact combination of several ingredients existing in a clay at the time of examination. It certainly seems patent that until we can determine the exact mineralogical condition and chemical aggregation of a given clay it will be impossible to use the analytical data obtained by ordinary phy sical and chemical tests as ground for predictions concerning its probable pyro-chemical behavior.
In the process of any chemical analysis known to the writer, the character and exact identity of the clay as a whole, as well as its con stituent parts, are destroyed by the disintegration or unlocking of the natural combinations, making an exact or complete determination of the chemical conditions originally present, a mere supposition. In fact all we know or can learn from a study of the origin and mode of forma tion of clays, and of the alterations in their composition constantly go ing on under varying conditions, as well as by attempts to unlock the combinations or separate the ingredients by chemical methods is, that we are arresting the changes of transition in the clay from one state to an other, but are not able to ascertain the forms or conditions existing at that time. From these considerations it should be plain that two samples of clays having similar origin and chemical constitution, may differ rad ically in their mineralogical make-up. The kind, size and composition of the several minerals affect so materially the pyro-chemical properties of the clay as a whole, that until mineralogists can find means of determin ing the kind, quantity and relative size of the several mineral ingredi ents, ceramists cannot predict, even with a fair degree of accuracy, the behavior of a clay in burning.
Several of the so called physical properties of raw clay may, however, be measured and their effect on the behavior of the clay described. In this, physical conditions of the clay, such as hardness, fineness of grain, plasticity, etc., are by custom regarded as properties.
The properties of clays may be classified under three general heads : Physical, chemical, and pyro-chemical.