BUILDING BRICK CLAYS ; HUDSON RIVER CLAYS ; PAVING BRICK CLAYS ; CLAYS ; METHOD OF ANALYSIS FOR FIRE-CLAYS, FELDSPARS, KAOLIN, AND FIRE SANDS ; TERRA-COTTA CLAYS ; KAOLIN OR CHINA CLAY.
UNDER the general term clay many varieties are included Under the general term clay many varieties are included which are more or less used in the arts and manufactures. The characteristic property of clay, and one possessed by no other mineral substance, is that peculiar condition known as plastic. The ingredient of clays to which this feature is due is a hydrous silicate of alumina. When pure it is of a snow-white color ; to, the touch it has a soft unctuous feeling, and is very easily broken ; it adheres to the tongue and gives off a singular smell when breathed upon, which has been designated the argillaceous odor. When moistened with water a considerable quantity of the fluid is absorbed, and upon manipulation the clay speedily passes into the plastic condition, which enables it to be molded into many different forms.
The purest clay found in nature is known by the technical name of kaolin, or China clay.
The word " Clay " is derived from the Anglo-Saxon Claeg ; Dutch, Klei ; German, Kleben, which means " to stick." In addition to sand, there are also compounds present in clays which often have an important bearing in determining their economic value. Among these are iron pyrites, sulphate of lime, carbonate of lime, dolomite, carbonaceous and bituminous matter, oxide of iron, etc. To these are due the various colors which characterize ordinary clays, and which vary in their effect upon the material when it is applied to technical purposes. In geological works clays are divided, in accordance with their occurrence, into primary and secondary deposits. From their physical conditions of structure and chemical composition, clays are technically separated into various classes, expressed by the words fat, long, lean, short, plastic, argyllites, clay-slates, marls and foams.
From chemical and mechanical considerations it is evident that the major portion of the insoluble residue left after the de composition of feldspathic rocks cannot be pure kaolin, but must necessarily be a mechanical mixture of it, with more or less partly altered feldspar and the most easily decomposed materials of the original rock, with a certain proportion of quartz. This material in the past has been deposited in the
depressions on the earth's surface, and in changes due to the unstable condition of the earth's crust was subjected to heat and pressure and became consolidated, forming a true rock. This, in turn, was upheaved by the internal forces of the earth, and was again subjected to disintegrating influences.
Among the materials forming clay are various minerals con taining protoxide of iron ; these, under atmospheric action, become altered to higher states of oxidation, and give to the clays all shades of color from the finest yellow to the deepest brown ; they also explain why pure white kaolin is so seldom found. This disintegration of the primitive rocks and the rocks that were formed from them, going on through millions of years, has resulted in a deposition in nature's settling tanks of vast amounts of clay of more or less purity. They occur in beds of varying thickness, and follow the stratification in dips and strike of the underlying rock. These primary deposits of clay have been rearranged many times by subsequent geolog ical changes, which have had effect in redispositions of these clays, and sometimes resulted in the purification of the clay mineral proper, and at other times in its degeneration.
The power to pass, with water, into a dough-like plastic state, decreases in proportion as a sandy element is mixed with the clay. It is strongest in the " fat" and weakest in the " lean" clays. A " fat " clay dries very slowl and unevenly, V.SE and the molded objects will warp and crack in drying. Aron found that the " linear shrinkage " does not correspond with the drying of the clay, as might be expected, but ceases when followed up to a certain point, which he designates as the " limits of shrinkage." The water evaporated up to this point, he terms the "water of shrinkage ;" the remainder of the water lost, until the weight of the sample remains constant at a tem perature of 266 degrees F., he calls the "water of pores ;" the sum of both is total water.