344 REPORT OF NATIONAL MUSEUM, 1899. 



they begin to soften as the temperature is raised, and as it approaches 

 their fusion point the cones bend over until the tip is as low as the 

 base. When this occurs the temperature at which they fuse is con- 

 sidered to be reached. 1 



Uses. Clay when moistened with water is plastic and sufficiently 

 firm to be fashioned into any form desired. It can be shaped by the 

 hands alone; by the hands applied to the clay as it turns with the pot- 

 ter's wheel, or it can be shaped by moulds, presses or tools. When 

 shaped and dried, and then burned in an oven or kiln, it becomes firm 

 and solid, like stone; water will not soften it, it has entirely lost its 

 plastic property, and is permanently fixed in its new forms, and for 

 its designed uses. These singular and interesting properties are 

 possessed by clay alone, and it is to these it owes its chief uses. It is 

 used (1) for making pottery; (2) for making refractory materials; (3) 

 for making building materials; (4) for miscellaneous purposes. 



Pottery. Pure clay worked into shapes and burned, constitutes 

 earthenware. The ware of itself is porous, and will allow water and 

 soluble substances to soak through it. To make it hold liquids, the 

 shaped clay before burning is covered with some substance that in the 

 burning of the ware will melt and form a glass coating or glazing 

 which will protect the ware in its after uses from absorbing liquids, 

 and give it a clean smooth surface. The color of the ware depends 

 on the purity of the clay. Clays containing oxide of iron burn red, the 

 depth of color depending on the amount of the oxide, even a small 

 fraction of 1 per cent being sufficient to give the clay a buff color. 



Clay containing oxide of iron in sufficient quantity to make it par- 

 tially fusible in the heat required to burn it, when made into forms 

 and burned, is called stoneware clay. The heat is carried far enough 

 to fuse the particles together so that the ware is solid and will not 

 allow water to soak through it; and the fusion has not been carried so 

 far as to alter the shapes of the articles burned. The oxide of iron 

 by the fusion has been combined with the clay, and instead of its 

 characteristic red, has given to the ware a bluish or grayish color. 

 Stoneware may be glazed like earthenware, or by putting salt in the 

 kiln, when its vapor comes in contact with the heated ware and makes 

 with it a sufficient glaze. 



Clay which is pure white in color and entirely free from oxide of 

 iron, may be intimately mixed with ground feldspar or other minerals 

 which contain potash enough to make them fusible, and the mixture 

 still be plastic so as to be worked into forms for ware. When burned, 

 such a composition retains its pure white color, while it undergoes 



'See Dr. Ries's paper on North Carolina clays, already quoted, and also his 

 numerous contributions on their subject in the volumes of the United States Geolog- 

 ical Survey relating to mineral statistics. 



