ALUMINIUM 471 



rin r a .nA pftttgiy. Pure clay (kaolin) is white. It is hydro- 

 gen-aluminum silicate HAlSiCX, derived from the weathering 

 of felspar (p. 410). Common clay contains impurities such as 

 sand (silica), limestone, and compounds of iron. Both kinds are 

 plastic when wet and can be moulded. When heated strongly 

 the material shrinks (so that the products are porous) and becomes 

 hard. Bricks, and tiling for roofs and drains, are made of common 

 clay and, when red, owe their color to oxide of iron (Fe20a). 

 The firing is done with fuel gas in ovens or kilns of brickwork. 

 To glaze drain pipes and some bricks, salt is thrown into the kiln. 

 The vapor of the salt produces a more fusible sodium-aluminium 

 silicate, which fills the surface pores. Clay for fire-brick (infusible) 

 must contain free silica, but no lime. 



China and porcelain are white, translucent and non-porous. 

 They are made of pure clay to which a little of the more fusible 

 felspar is added. After firing, the articles are dipped in water, 

 in which the materals for the " glaze," namely finely ground 

 felspar anjd silica, are suspended. Having thus acquired a thin 

 coating of these substances, they are fired again at a higher tem- 

 perature and for a longer time. Colored decoration is done with 

 materials which melt (third firing) to colored enamels (p. 362). 



Ultramarine. This material is made by heating together 

 kaolin, sodium carbonate, sulphur, and charcoal, pulverizing the 

 green mass, and heating it again with more sulphur. The product 

 is used as laundry blueing, and in making blue-tinted paper. It 

 is added also to correct the yellow shade of linen, starch, sugar 

 (p. 402), and paper stock. 



Cement. Portland cemgjfa is made by heating a pulverized 

 mixture of a material rich in lime, such as limestone CaC0 3 , 

 with one in which silica, iron oxide and alumina are the main 

 constituents, such as common clay. Some natural rocks contain 

 all of the necessary elements in suitable proportions. The finely 



