A PRELIMINARY REPORT ON CLAYS OF FLORIDA 105

They should be dense-burning, have a low shrinkage, and be free from warping. The absence of soluble salts is an essential property. Most terracotta is made from a blend of several clays and is commonly fired at about cone G or 8. Gray or buff-burning fire clays are commonly used, but in most cases, an artificial color is applied to the surface of the ware.
 Sewer-pipe Clays-Clay for sewer-pipes should have a low shrinkage, high strength, and be free from warping and cracking. It should be vitrified, hence a clay high in fluxes is best, but at the same time it should have a long firing range. Vitrification should be reached at cone 3 or 4, but some clays used vitrify at a lower temperature. This type of ware is commonly covered with a salt glaze. Clays suited for sewerpipe manufacture are similar to those used for paving brick. A darkcolored sewer pipe is most commonly called for on the market.
 Brick Clays-Clay for common brick is usually impure, often sandy, high in fluxes, of medium plasticity, variable strength, color and porosity. They are in most cases red-burning but sometimes gray, cream, buff, or yellow. Common brick-clays are widely distributed.
 Face brick, or sometimes called front or pressed brick, requires a better grade of clay than common brick. They must have a uniformity of color in burning, freedom from warping or splitting, absence of soluble salts, good hardness after firing and low porosity. The shrinkage and transverse strength are variable. Front brick may be red, white, cream or buff after firing. Such clays are commonly fired from cone 1 to 9. Clay for face brick should be more plastic than that for common brick so that the ware may retain its form better and present smooth faces and square corners. Plasticity, however, is not an important factor in dry-press methods. The clay should have a good firing range and should be burned so hard that it cannot be scratched with a knife.
 Clay for paving brick should have the same general properties required of a sewer-pipe clay. Ordinarily such clay is burned to the point of vitrification but from some clays the toughest product is obtained before vitrification is reached. Such clays should have fair plasticity and good transverse strength. The firing range should be at least 150* C.
 Hollow block, fire proofing, and drain tile all require clay of the same characteristics; in fact, they may all be made from the same kind of clay. The properties of clay suitable for these products is quite variable. They should have a fair transverse strength, burn to a hard but not vitrified body at a comparatively low cone (03-1), and should have sufficient plasticity to flow smoothly through the peculiar shape of die necessary to form them.