372 SAND CLA*. 



to prevent it from falling to pieces. The ad'obies of Peru, 

 are large sun-baked bricks or blocks of clay ; and in thai 

 dry climate they are very durable. 



Clay for Fire-bricks should contain no lime, magnesia, 

 or iron, as its value depends on its being very refractory 

 There is a large manufactory in the United States, at Balti 

 more, from the tertiary clays of eastern Maryland. In Eng 

 land a slate clay from the coal series is employed. 



Potter's clay and pipe clay are pure plastic clays, free from 

 iron, and consequently burning white. The clay of Mil 

 waukie, from which the (jream-colored bricks are made 

 is much used also for pottery. 



In the manufacture of coarse pottery, the clay is worked 

 with water and tempered ; and then the required form of a 

 pot or pan is given on a wheel. The ware is dried under 

 cover for a while, and next receives the glaze in a cream- 

 like state. The glaze for the most common ware consists 

 of very finely pulverized galena, mixed with clay and water. 

 The wart, after drying again is next placed in the kiln, 

 which is very gradually heated ; the heat causes the baking 

 of the clay, and drives off the sulphur of the galena, thus 

 producing an oxyd of lead, which forms a kind of glass (or 

 glaze,) with the alumina. For a better stone ware, common 

 «alt is used, and it is put on after the baking has begun. 



For the finer earthenware, a mixture of red and white 

 .ad, feldspar, silica and flint-glass, is used for a glaze, the 

 proportions differing according to the ware. The clay foi 

 this ware is mixed with flint powder (ground flints or sand J 

 to render it less liable to contract or break, and it is worked 

 with great care, and through various processes to prepare it 

 .or moulding. The ware is usually baked to a biscuit, be- 

 fore the glazing is put on, as in the manufacture of porcelain. 



Kaolin or porcelain clay, is derived from the decomposi- 

 tion of feldspar, as stated on page 117. The foreign kaolin 

 occurs in Saxony ; in France at St. Yrieux-la-Perche, near 

 Limoges ; in Cornwall, England ; also in China and Japan 

 The kaolin used at the Philadelphia porcelain works comes 

 mostly from the neighborhood of Wilmington, Delaware. 



The name kaolin is a corruption of the Chinese Kau. 

 ling, meaning high-ridge, the name of a hill near Jauchau 

 Fu, where this material is obtained. 



In the manufacture of porcelain, the kaolin, and also the 

 other* ingredients, are first ground up separately to an im 



