The virtrifying clays then are to be looked on as in all cases a mix ture of refractory and non-refractory ingredients with enough plasticity to enable them to be molded into desirable forms and which may be burned to a tough impervious mass without losing their shape. These properties may be found in high as well as in low-grade clays, but of course are much more rare in the former than in the latter. The man ufacturer of china finds it necessary to make his mixtures artificially by incorporating ground feldspar, flint, Cornwall stone and other in gredients with his kaolin, and in Holland and Germany, and at a few places in this country as well, fluxes and even low-grade clays are mixed with fire clays for the manufacture of pavers. Vitrified ware may be manufactured from any clay if it is properly prepared, the necessary ad mixtures made and carefully fired, but whether any given clay can be so used commercially is a question for each manufacturer to decide for himself.
There seems no good reason for the commonly held opinion that shales are better suited to the manufacture of vitrified ware than other clays except that the manufacturers of such wares in this country have adapted their methods to the use of shales and that consequently these methods do not lend themselves kindly to the use of clays of other grades. Practically every grade of clay has been successfully used for the man ufacture of these goods on a commercial scale.
Terra-Cotta Clays.—§ 43. To the ceramist the term terra-cotta sig nifies porous unglazed as distinguished from faience or porous glazed ware, but in common use the term is restricted to high-grade artificial or ornamental wares having the above characteristics.
Clays for these purposes must be even-grained and smooth-working, fairly plastic with good binding power, and capable of being burned to a uniform pleasing color. These qualities call for a clay of uniform com position and texture which may be produced by weathering of residual masses, but is more usually formed by decomposition from bodies of water which have a slight but uniform current. As natural deposits having all these characteristics are not abundant, the manufacturer usually reaches his ends by washing and mixing several clays of uniform texture which by their combination will give the other qualities desired.
Some of the most progressive manufacturers of these goods now use vitrifying clays and burn them until the wares are. impervious, thus re moving them from this group entirely.
Brick Clays.—§ 44. This group is hard to describe because the re quirements for brick are passing through a transition. Formerly any
plastic clay with reasonable binding power fired to a heat somewhat above the end of the water-smoking period would meet all requirements. Then came the demand for face brick with definite shape, smooth surface, and uniform, well-defined colors, which made requirements the same in kind and nearly as exacting as those placed upon the manufacturer of terra-cotta. Now the trade begins to ask that these same wares shall be vitrified and so rendered non-absorbent. This last requirement has probably come to stay, and in a short time only vitrifying clays will be used for the manufacture of brick. Then only clays which have sufficient plasticity to enable them to he molded into definite forms, sufficient bind ing power to give the necessary strength, such openness of structure as will permit them to dry without cracking, such coloring ingredients as will cause them to take on a uniform and pleasing tint when properly burned, and such chemical and physical characteristics as will give them a wide vitrifying range will be used for the manufacture of brick. Such clays will be found among the somewhat impure residual deposits and the better grades of alluvial and diluvial clays.
Drain Tile Clays.—§ 45. The requirements for a drain tile clay are sufficient plasticity to permit its being readily molded into pipes, suffi cient binding power to give it such strength as will withstand handling without breakage, and such openness of structure as will enable it to dry quickly without cracking and give it after-it is burned such porosity as will enable water to pass through it readily.
These requirements call for the lowest gracles of clays and shales usually carrying considerable quantities of lime carbonate or sand or both, and often a considerable admixture of soil to give the ware sufficient porosity. Of course any clays of higher grades which possess the prop erties designated above could be used if it seemed desirable to do so.
Gumbo Clays.—§ 46. These are composed of exceedingly fine clay-like material which packs so closely that water passes through it but slowly. It was laid down in still water, the streams which brought it having deposited all sediment coarser than the finest sand and clay, because of loss of velocity, before reaching this point. It is very plastic and possesses great binding power, but dries slowly and cracks badly. Such Clays when black are called gumbo, but when lighter in color, hard pan. The black color is due to the admixture of a considerable amount of organic matter.