THE CLAYS. 655 
The fusion produced by the alkalies as explained, is not of a kind 
to excite attention, but when a piece of brick vitrifies to a “ body” on 
burning, it is a sure sign as to its probable action in an exposed position. 
The amount of potash which a clay can contain and keep its fire 
properties is variously put by different authorities. As with iron, pure 
kaolin will stand a good deal when no other base is present, but a multi- 
plicity of bases makes fusion easy. ‘Titanic acid is regarded as neutral 
to fire qualities; the form in which it is present being infusible. 
The statement of the tendencies and comparative power of the 
dangerous impurities of clay would lead us to believe we could use 
predictions as to their result in a given clay with some confidence, but 
the best practice does not yet trust to analysis alone. 
The most complete test of a clay now known would be obtained by 
use of such analysis as has been described, coupled with a fire test made 
especially to develop such points as the analysis indicates to be weak 
ones. Fire tests are of two kinds—one is subjecting the clay to absolute 
heat without the action of any accompaniments, and the other is in 
putting the clay through the course of treatment for which it is designed 
to be used. ‘The former develops the absolute quality of the clay as 
good or bad, the latter proves or disproves the fitness of the clay for 
the work. The latter is better of course as a business test wherever it 
is practicable to use it. The former can be made only in a specially 
adapted furnace. The clay is cut into one-inch cubes with square edges, 
and is set in a covered crucible resting on a lump of clay of its own kind, 
so that it touches no foreign object. The heat is then applied and its 
effect will vary from fusing the mass to a button to leaving it with edges 
sharp and not even glazed on the surface. Experience soon renders one 
proficient in judging of clays by this test. 
The statements now made cover the leading facts in regard to the 
composition and structure of clays. | 
