Fire clays may or may not have iron oxides in them, but they must be free or nearly free from fluxing materials, such as lime, magnesia and the alkalies (sodium and potassium compounds). They may be more or less plastic, the essential quality being their ability to withstand high temperatures without fusing. Silica (as sand) tends to diminish the refractory quality; so that a clay otherwise suitable, if it has sand in it, becomes at best a second grade fire clay. In coal mining sections it is customary to term those beds of clay either above or below the coal, “fire clay”; but this is an unfortunate designation, for though some of them are true fire clays, the most of them are not. Stone-ware clays are those with considerable sand and up to five per cent of fluxing materials. They must be plastic enough to be readily worked, and then burn to a dense body at comparatively low temperatures. Sewer-pipe clays must be plastic, and carry a considerable amount of fluxing material, as the surface of the pipe is expected to vitrify in the burning. Brick clays are low grade clays and vary greatly in composition. The main requisites are that they mould easily and bake hard at relatively low temperatures with as little warping and cracking as possible. As most clays shrink both in the air drying and in the baking, sand is added when the clay is being mixed. The color is mostly due to the presence of iron impurities. If there are iron oxides and little or no lime, the brick bakes to a red color, but if there is an excess of lime over the iron oxides, it bakes to a cream or buff color, which on vitrifying turns green. Paving-brick clays range from surface clays, to semirefractory clays, shale being often used. The essential component is enough fluxing material, so that the bricks shall begin to vitrify, or fuse, at not too high temperatures. Slip clays are those with a high percentage of fluxing material; so that, when baked at moderate temperatures, the surface fuses into a glassy brown or green glaze. Adobe is an impure calcareous clay, widely used in the western United States for making sun-dried bricks. Gumbo is a term applied to fine-grained plastic clays which shrink too much in the burning to be useful in manufactures. They can be burned to make an excellent ballast for railroads and highways. They are especially abundant in the Middle Western States. Loess