Page:The American Cyclopædia (1879) Volume IV.djvu/661

 CLAY 649 physical properties, depending on the admix- ture of other bodies, such as sand, lime, mag- nesia, alkalies, oxide of iron, and organic mat- ter. Dry clay is readily separated into la- minae, which, however, especially in the more tenacious varieties, break with a conchoidal fracture. It is non-crystalline, opaque, and of varying degrees of hardness, the harder kinds being nearly of the consistency of chalk. It has a strong affinity for water, which is not destroyed by moderate baking. A new clay pipe taken into the mouth sticks to the tongue and lips by absorbing the moisture from these parts. When moistened, clay emits a peculiar smell, called argillaceous, and also has a pecu- liar taste, which may also be so called. Mixed with considerable water, the clays are more or less plastic, the degree depending on their purity and peculiarities of composition. When subjected to a white heat they become very hard, so that they will strike fire with steel. Those clays which have a high degree of plas- ticity are said to be fat or long; and when possessing but little plasticity they are called lean, meagre, or short. They all shrink con- siderably in drying and burning, the amount depending on the quantity of water which they contain. Pure clay or silicate of alumina is infusible, but when mixed with the alkalies or alkaline earths it becomes fusible in propor- tion to the admixture. The principal varieties of clay used for ceramic manufacture may be divided into refractory, to which belong por- celain clays, and fusible, as potter's clay. To these may be added limey and ochrey clays. Porcelain clay, or kaolin (a word derived from Kao-ling, the name of a hill in China whence the material for making porcelain is obtained), is rather softer and more crumbly than many other kinds of clay, does not form so stiff a paste with water on account of the free silica which it contains, and adheres but slightly to the tongue. It has a specific gravity of about 2 '2. When separated from free silica, it has an average composition of 47 per cent, of silica, 40 of alumina, and 13 of water, having the for- mula A1 4 3, 2SiO a + 2aq (or Al a O 3 , 3Si0 3 + 2 aq). It may be considered as derived from potash feldspar, or orthoclase, which has the formula K 2 OiAl 4 O 3, 6SiO 2 , the transformation taking place by the loss of all the potash and two thirds of the silica, and a union with two equivalents of water. Some varieties, how- ever, have a different composition, as the ka- olin of Passau in Bavaria, which contains 43-65 per cent, of silica, 35-93 of alumina, 1 of sesquioxide of iron, 0-88 of carbonate of lime, and 18-5 of water ; represented by the formula 4Al 4 O s 9SiO a + 12aq. Porcelain clay from Gu- tenberg, near Halle, contains, according to Bley, 39 '02 per cent, of silica, 45 of alumina, 0-07 of carbonate of lime, 3-32 of carbonate of magnesia, 0-19 of sesquioxide of iron, and 10 of water, having approximately the formula 2A1 4 O S, 3SiO 2 + 3aq. The Chinese and Japan- ese kaolins contain about twice the proportion of silica to alumina that is found in the above European specimens, and are whiter and more unctuous to the touch. The Cornish kaolin of England, however, has an unctuous feel, and is very white and evidently formed by the decomposition of feldspar. Potters' clay, or plastic clay, contains lime and magnesia with more or less oxide of iron, either bluck or red. Mixed with water, it becomes exceedingly, plastic; it is the common modelling material of the sculptor. The finer varieties are used for white earthenware, and the coarser for inferior ware 'and for drain pipes. Pipe clay is a pure variety of potters' clay, approaching somewhat in its character to kaolin, although containing less silica, and remaining more po- rous after baking. Fine pipe clay is found at Poole in Dorsetshire, England, and in the isle of Purbeck. It is semi-fusible, allowing the stems to be bent while hot. Beds of fine kaolin are found at Brandon, Vt., which is used in the manufacture of paper to give weight, and the colored varieties are used in the adulteration of paint. In New Jersey, near Woodbridge, and also at South Amboy, beds of clay are worked to great extent for the manu- facture of stone ware, which also furnish an excellent material for fire brick. For this pur- pose they must be burned twice; that is, a cer- tain amount of clay is burned, and then ground up with another portion to make it sufficiently plastic for moulding into forms, which are again baked. Similar deposits also compose the banks of the Delaware river between Bor- dentown and Burlington. They all belong to the series of upper secondary rocks, underlying the greensand beds. Common brick clay is usually obtained from alluvial deposits. The color of the brick depends upon the quantity of oxide of iron in the clay. Clay beds found in Wisconsin near Lake Michigan are so free from this coloring matter that the bricks are of a straw color. Clay for fire brick is obtained for the most part from the coal formation. Each coal bed, with very rare exceptions, rests upon a stratum of bluish gray clay, which appears of the solidity of structure of stone, though it is found to be easily cut with a knife. It often contains some carbonaceous matter lining its seams and giving them a black color, and it abounds with the fossil stigmaria roots, which are all converted into the same material as the rest of the beds. When ground to powder in a mill, and mixed with quartz sand, obtained from the sandstone beds of the same formation, it makes an excellent quality of fire brick. Old, hard-burned brick, broken up and mixed with the clay, improves its quality, causing it to retain better its shape after moulding. Such a mixture is well adapted for the manufacture of crucibles. The following are analyses of a number of the best foreign fire clays, that of Gross Almerode being the material of which Hessian crucibles are made, with the addition of an equal weight of sand, and the Stour- bridge clay that of the fire brick of this name :