IRON, ALUMINUM, ETC. 147 



Questions, (i) Does the alum cause a precipitate ? (2) Of what 

 is the precipitate composed ? (3) How would alum act in the di- 

 gestive tract in the presence of soluble albuminous compounds ? 

 (4) Why is alum an undesirable ingredient in baking-powders and 

 foods. 



186. Pottery. Pure clay or kaolin is used for the 

 manufacture of the best grades of porcelain and pottery. 

 The plastic clay is modeled into the desired form and 

 then dipped into a bath containing feldspar and other 

 materials which, when fused, form the glaze. Ordinary 

 earthenware is made from impure clay which contains 

 compounds of iron and other elements. Brick and tile 

 are also made from clay, the physical properties, as color, 

 hardness, weathering properties, etc., depending upon 

 the amounts of iron, lime, magnesia, and alkalies present- 

 As ordinarily found in the soil, clay is mechanically asso- 

 ciated with a large number of other substances, many of 

 which contain the elements essential to plant life as po- 

 tassium and calcium. Pure clay itself contains no plant 

 food, but clay soils are usually among the most fertile 

 because, along with the disintegration of feldspar and 

 other rocks, various minerals that impart fertility are 

 made available and are associated with the clay. 



