608 NEW YOEK STATE MUSEUM 



alumina, occurring as six sided plates or irregular scales usually 

 of a dark color. As it decomposes easily with tlie formation of 

 ii'cn oxid, it is not as coinmonly found in clays as muscovite. The 

 latter is sometimes called potash mica, though it also contains a 

 small amount of iron and magnesia,. It is of a silvery white or 

 light brown color. 



As before mentioned, mica by itself is rather refractory, but in 

 the presence of other minerals may serve as a flux at high tem- 

 perature. Even in burned bricks the mica scales are often per- 

 ceptible. This is frequently the case when the brick has been 

 burned at a temperature of 2500° F. 



Siderite. This is perhaps a more common constituent than is 

 usually imagined. It generally occurs in the form of opaque, 

 rounded masses, and effervesces on the addition of warm muriatic 

 acid. In the burning of the clay siderite or iron carbonate is 

 converted into iron oxid. 



Pyrite. This is a combination of iron and sulfur. It has a 

 metallic luster and yellow color, and is a very common constituent 

 of many fire clays, occurring either in the form of small yellow 

 metallic grains or concretionary masses of yellow crystals. In the 

 burning of clay it may be changed to. sulfate, of iron and if. the 

 clay is heated tO' vitrification, it will serve as a flux. The brick 

 makers' common name for pyrite nodules is " sulfur-balls ". 



Marl or limestone fragments. The action of these is the same 

 as that of calcite. Their presence may be detected by treatment 

 with muriatic acid. 



Dolomite, the double carbonate of lime and magnesia, and mag- 

 nesite, the carbonate of magnesia, may both occur in clay, either 

 as earthy grains or as rhombohedral crystals. Either alone is highly 

 refractory and in this condition is used as linings for furnaces, but 

 when present in clay may serve as a flux, their action being similar 

 to that of lime. 



Iron oxid. This is perhaps the most common impurity of clay 

 next to quartz. It may occur in the form of earthy grains, or 



