782 NEW YORK STATE MUSEUM 



many Peimsjlvania mines there often occurs considerable iron 

 oxid, and iron carbonate, which, are known as ore-balls.^ The iron 

 oxid concretions are generally near the surface, while the' iron car- 

 bonate is confined to the interior of the bank. In the mining' of the 

 clay, these concretions have to be picked out. This collecting of the 

 iron by natural processes has therefore served to purify the clay 

 mass; where it has not taken place the material loses its value as 

 a refractory clay. Flint clays are hard, they lack plasticity, have 

 a conchoidal or shell-like fracture, and sometimes a very faint 

 luster. At times they occur in beds inter stratified with other rocks, 

 or again they may occur as basin-shaped deposits, in which case 

 they seem to have been formed by chemical precipitation. 



The color of a fire clay is an indication of its refractoriness only 

 to a limited eixtent. Many fire clays are bluish in color, while 

 others are light gray or yellowish white. A given amount of iron 

 will color a sandy clay more strongly than a less silicious one; the 

 same is true of organic matter. The latter might even mask the 

 color of iron. The condition of oxidation of the iron would also 

 cause a difference in color, ferric compounds being red or yellow, 

 while ferrous ones may be blue or g^ay. 



Whatever the color of the fire clay in its unburned condition, in 

 the fired state it is always buff, unless vitrified, when it may bo* 

 come red. 



The refractoriness of a fire clay is dependent almost entirely 

 on its chemical 'composition. It can be said in general that the 

 m.ore powerful fiuxing impurities, such as ferric oxid, lime, alkalis, 

 and magnesia should not exceed 1^ if possible. Many fire brick 

 manufacturers do not seem to recognize the fact that silica acts 

 as a flux at high temperatures. 



The recent experiments of H. 0. Hofman tend to indicate that 

 in the case of fire clays at least the size of grain has no effect on 

 the fusibility of the mass. The tests which he carried out are 

 described under " Fusibility of clays" p. 563. 



' Annual report Pa. state college. 1897. p. 53. 



