THE NONMETALLIC MINERALS. 343 



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. 



Testing clays. 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 con- 

 fidence, 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. Expe- 

 rience soon renders one proficient in judging of clays by this test. 1 



A method of testing the fusibility of clays by comparing them with 

 samples of known composition and fusibility has of late years come 

 into extensive use. These prepared samples, known from their inventor 

 and their shape as Seger's pyramids, consist of mixtures in varying 

 proportions of kaolin and certain fluxes, so prepared that there is a 

 constant difference between their fusing points. When such pyramids, 

 together with the samples to be tested, are placed in a furnace or kiln, 



colored brick are made from red clays. In nearly or quite all cases, whatever the 

 original color of the clay, the brick are reddish when partially burned. The expla- 

 nation seems to be that at a comparatively moderate temperature the iron constit- 

 uent is deprived of its water and fully oxidized, and is therefore red, while it ia 

 only at a relatively high heat that the union with the lime and magnesia takes place, 

 giving rise to the light color. The calcareous and magnesian clays are, therefore, a 

 valuable substitute for true aluminous clays, for they not only bind the mass together 

 more firmly, but give a color which is very generally admired. They have also this 

 practical advantage, that the effects of inadequate burning are made evident in the 

 imperfect development of the cream color, and hence a more carefully burned pro- 

 duct is usually secured. It is possible to make a light-colored brick from a clay 

 which usually burns red by adding lime. The amount of lime and magnesia in the 

 Milwaukee brick is about 25 per cent. In the original clays in the form of carbo- 

 nates they make up about 40 per cent. Geology of Wisconsin, I, 1873-79, p. 669.) 

 1 Geological Survey of Ohio, Economic Geology, V, pp. 652-655. 



