160 APPLIED SCIENCE 



causes the feldspar to form an insoluble silicate and a soluble 

 silicate (sodium or potassium silicate). The soluble part 

 is washed away and the insoluble portion which remains 

 particles of mica, quartz, carbonate of lime and magnesium, 

 and iron is called clay. The greater part of the clay is pure 

 aluminum silicate (H 4 Al2Si 2 O 9 ). 



182. Properties of Clay. The principal property of 

 kaolin, or clay, is that it becomes slightly soft (plastic) when 

 wet and may be molded into various shapes. When clay 

 is heated it shrinks and in cooling becomes very hard. 

 The color of clay, which is due to the presence of iron and 

 other impurities, varies from gray to red. 



183. Porcelain. Porcelain is a glazed material used for 

 insulators, etc. It is made by mixing kaolin, fine sand, and 

 powdered feldspar, shaping the mass, and then heating it 

 to a high temperature. The surface is glazed by being coated 

 with a mixture of salt and heated. The heat causes the glaze 

 to melt and penetrate the surface. 



184. Earthenware. Impure plaster clay, when wet, 

 shaped, and heated to a moderate temperature may be used 

 for tiles, etc. 



185. Bricks. Many materials used in building construc- 

 tion, such as bricks, drain pipes, etc., are made from impure 

 clay by wetting, molding, and then heating the mixture 

 sufficiently to harden it. The red color in bricks is due to 

 the iron oxide in the compounds of the clay. 



186. Mortar. To make mortar a thick paste is formed 

 by mixing lime, sand, and water. This paste is placed 



