The variety-names enumerated above in the order of the actual contents of the materials in true clay substance (“colloidal clay”), are partly based upon that fact, partly upon the degree of plasticity attained by that substance, and essentially upon the nature and amount of foreign admixtures associated with it. Thus, porcelain clay is chalky kaolinite, sometimes associated with enough of pure white plastic clay to render it workable in the potter’s lathe, but more commonly requiring to be molded in porous molds; it is very refractory to heat. Pipe-clay is also white, but more plastic and usually less refactory. Fire-clay is a refractory pipe-clay commingled with some coarse infusible material, such as quartz sand (or the same clay burnt and crushed), in order to prevent excessive contraction and change of shape in drying and burning. Potters’ clay is a much less pure, and from that cause more fusible clay, which when burnt forms at a moderate heat a semi-fused, more or less hard mass, such as crockery and pottery ware. Brick-clay is a still more impure clay, or loam, containing considerable sand and usually iron oxid, and largely falls already within the limits of tillable soils or subsoils, rendered fusible by the presence of relatively considerable amounts of iron, magnesia and lime. _Iron_ colors natural clays either red, yellow, green or blue; the latter two colors turning to yellow or red on exposure to the air, and to red on burning. Black color is usually due to carbon, such clays often turning white on heating. Clays containing much lime are usually of a gray or whitish tint, and like the soft crumbly limestones are often called marls, and are used as such for land improvement. But it should be understood that the colors of clays, mostly derived from some iron compound, have little to do with their uses in the arts, except that no deeply colored clay (black excepted) is refractory in the fire. _“Colloidal” Clay._[17] In connection with soils, clay may be defined, in the most general terms, as being the substance which imparts plasticity and adhesiveness to soils when wetted and kneaded, and which, when heated to redness, loses this property completely and permanently, becoming hard and coherent in proportion to the degree of heat to which it is exposed.