THE PRODUCTS OF CLAY AND FIRE 



are the plastic clays China-clay (kaolin), "ball" 

 or "blue" clay. In the second are the glass-forming 

 materials used in the body or in the glaze. In the third, 

 flint and quartz, sometimes called "indifferent sub- 

 stances." And in the fourth, the coloring agents, 

 made of metals or metallic oxides. Most of these sub- 

 stances are natural products. Their chemical compo- 

 sition is well known, and many of them can be produced 

 synthetically in the laboratory; but good pottery can 

 not be made from these artificial products. The com- 

 position of clay, for example, is no secret, but laboratory- 

 made clay has not the peculiar plastic quality of nat- 

 ural clay so essential to the potter. 



Chemically, clay is a hydrated silicate of alumina 

 in combination with slight quantities of such substances 

 as iron, lime, soda, or potash, and is the result of the 

 decomposition of felspathic rocks. It is much richer 

 in alumina than the rocks, however, since alumina, 

 being so light a substance, is held longer in suspension 

 while the heavier materials settle to the bottom. From 

 the potter's point of view the most injurious substance 

 contained in clay is iron, owing to its coloring prop- 

 erties. If every trace of this metal is not removed, 

 the pottery as it comes from the ovens will be "off 

 color." The slightest trace, too small to be noticed 

 readily by ordinary tests, will give the disfiguring 

 stain when the ware is placed in the firing-kiln. Larger 

 quantities give the familiar red color seen in bricks and 

 flower-pots, although no such color is apparent in 

 the clay before firing. 



It should not be understood that every kind of clay 



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