302 Our Surroundings 



is called a sandy soil. One in which there is a combination of 

 clay, sand, and humus is called loam. A loam soil is most desir- 

 able since it contains, in better proportions, the materials needed 

 by plants for making food. 



Uses of Soil. The soil is very useful to man for other 

 purposes than for growing plants. This is especially true of 

 soils containing a large proportion of clay or silica, for they are 

 ;used in making pottery, bricks, and glass. 



Pottery. Many common objects of our everyday life are 

 :made from clay. Clay, for example, is the basis of china and 

 porcelain. In ancient times the clay was roughly shaped by hand 

 and sun baked. Today the process is more complicated. The clay 

 must be entirely free from iron or any other substances which 

 would tend to color or stain the finished material. The selected 

 clay is thoroughly dried and then ground to a fine powder. 

 The powder is made pliable by the addition of water. The 

 material is then shaped by hand or by machinery into the 

 forms desired. These are dried and then are subjected to a high 

 temperature, or firing, for many hours and permitted to cool very 

 slowly. The resulting plates or vases or other articles are still in 

 a porous state, although hard. They are then treated with a mate- 

 rial to close the pores and to give them, after another baking, a 

 glass-like finish, or glazing. If the plain pottery is to be deco- 

 rated, the design is then placed on the pieces and they are again 

 fired. Certain finer potteries require even additional processes. 



Bricks. From ancient times bricks have been made from clay. 

 The early bricks were generally made by molding the original clay 

 as found in beds and by baking the forms in the sun. Tn many 

 cases, the bricks did not dry uniformly and consequently they 

 tended to warp and crack. Our modern bricks are made of clay, 

 often with the addition of small amounts of sand, ashes, or marl, 

 which is itself a combination of clay, sand and limestone. Some 

 marls contain sufficient iron to give bricks, when baked, their 

 common red color. Other marls, lacking this material, produce 

 the common light brick. 



As in the case of pottery, the clay and other materials are 

 mixed in proper proportions. The resulting mixture is moistened 



