THE NONMETALLIC MINERALS. 341 



The expulsion of the combined water in a clay is. nearly always 

 accompanied by a diminution in volume, which varies directly as the 

 water, or the purity of the clay. Pure kaolin shrinks as much as one- 

 fourth of its bulk, it is stated, sometimes even more. The sandy clays 

 used in making sewer-pipe and stoneware shrink from the tempered 

 state from one-ninth to one-sixteenth, usually about one-twelfth. The 

 shrinkage of the raw clay would be very much less, probably not 

 over 3 or 4 per cent. 



A clay, when all the water of crystallization is expelled, will not 

 shrink any more at red heat, but with increased heat will shrink more 

 and more up to the moment of fusion. A pure kaolin apparently 

 shrinks when heated a second time, even if the water is all expelled 

 by the first heat, yet it is practically impossible to fuse it. But a good 

 flint clay containing some sand will lose all shrinkage on being once 

 calcined at white heat. Such clay is then used to counteract shrinkage 

 in a body of green clay, as this effect is obtained by mixing in sand or 

 some nonshrinking body. Many clays contain sand enough naturally 

 to shrink little or none on heating, and some are so sandy as to 

 actuall} T expand, though usually at the expense of soundness of struc- 

 ture; for the particles of clay will shrink away from the grains of 

 sand and this renders the structure very friable. 



The qualifications of a clay for common pottery and building mate- 

 rial are simple, viz., plasticity when wet, and solidity and hardness 

 when burned, but those products involving the highest qualities of clay, 

 refractoriness, require much sharper tests. 



The first requisite is purity, at least purity within limits, and 

 though the other points, density, plasticity, and non-shrinkage add 

 greatly to the value of a pure clay, they can in no degree supply its 

 place. 



Infusibility in clays rests in the aluminous base and the quartz. 



Long and intense heat applied to an intimate mixture of clay and 

 silica is apt to result in a silicate of another ratio of base to acid, and 

 which is likely to be fusible. But the great trouble with free silica in 

 clay, in a fine state of division, is the fact that any fluxing agent read- 

 ily unites with it, and makes a fluid slag; and in a refractory body the 

 fusing of any one part is the beginning of the end. 



The constituents tending to make a clay fusible are iron, the alkalies 

 soda and potash, and lime and magnesia. It is hard to state which is of 

 the most consequence. Of the first two, iron is not so powerful a flux 

 as potash, which is the worst of all the common elements; but the iron 

 is present in larger amounts than potash in most clays, and consequently 

 does as much harm, if not more. 



The effect of the iron is detrimental to the appearance of clay ware, 

 and consequently has a direct bearing on the price of goods, while 

 potash shows no more on the surface than on the inside, and when 



