THE CLAYS. 653 
when heated a second time, even if the water is all expelled by the first 
heat, yet it is practically impossible to fuse it. Buta 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 non- 
shrinking body. Many clays contain sand enough naturally to shrink 
little or none on heating, and some are so sandy as to actually expand, 
though usually at the expense of soundness of structure ; 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, re- 
fractoriness, 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 clay or kaolinite base and its main 
impurity, quartz. Both are infusible, taken separately in the highest 
heats obtained in metallurgical practice. Both are fusible in the 
oxyhydrogen blow-pipe flame, but the kaolin is the more infusible of 
the two. 
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 readily 
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 impurities most dreaded in a refractory clay are iron and 
potash ; it is hard to state which is most to be feared. 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 clayware, 
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 pres- 
ent in the usual small amounts it produces an incipient vitrification 
