FIRE CLAYS AND FIRE-BRICK INDUSTRY. 32 1 



tire brick. In Chapter IV, under Fusibility, several arbitrary 

 limits have been suggested. 



Other properties of fire clays. — As mentioned at the beginning 

 of this chapter the term fire clay does not signify the presence of 

 any other character than refractoriness. Fire clays may vary 

 widely in their plasticity, shrinkage, texture, color, tensile 

 strength and other physical properties, all of which affect the 

 behavior of the clay during the process of manufacture, but none 

 of which can be used as a guide in determining its probable re- 

 fractoriness. Color may be an aid under certain conditions, since 

 pure white clays and light-yellowish clays are often at least semi- 

 refractory, and sometimes highly refractory. Some fire clays are 

 tinged a deep yellow, or yellowish red, as though they contained 

 considerable ferric oxide, and yet they possess considerable heat- 

 resisting power. If the clay is black or bluish black, there is no 

 means of telling from mere inspection what its heat-resisting 

 qualities are, for under these conditions both a clay with very 

 little iron oxide and one with much iron oxide will sometimes 

 outwardly appear the same. There is consequently no sure means 

 of determining the refractory character of a clay without testing it 

 in a furnace or making a chemical analysis of it. 



Two kinds of fire clay are recognized in the field, viz., plastic 

 fire clays and flint clays. The former are plastic when wet, the 

 latter are hard and flint-like, with a smooth, shell-like fracture 

 and dense texture. They develop no plasticity, even when ground 

 very fine, but are usually highly refractory and show little air 

 or fire shrinkage ; none are found in New Jersey. 



Plasticity has little or no direct relation to refractoriness, al- 

 though H. Seger has pointed out that of two clays of unequal 

 refractoriness, the one of lower fire-resisting qualities may with- 

 stand the action of molten materials better, if it is of high plas- 

 ticity, as this makes it burn to a very dense body at a compara- 

 tively low temperature. The result of this is that the pores are 

 closed and the clay resists the corrosive action of a fused mass 

 better than the more refractory clay, which does not burn dense 

 at as low a temperature as the first one, and which, therefore, per- 

 mits the fused mass to enter the pore spaces between its grains. 



21 cl, G 



