322 CLAYS AND CLAY INDUSTRY. 



Fire clays are of variable tensile strength, and some of the 

 highest grades mined in New Jersey have so little tensile strength 

 and crack so badly in burning, when used alone, that it is neces- 

 sary to add a certain quantity of more plastic clay as a binder,, 

 even though it may be of less refractory character. 



MINERAL IMPURITIES. 



The analysis of a true fire clay shows but a small percentage of 

 injurious substances or mineral impurities. This is not due al- 

 ways to their absence from the deposit, but because they are fre- 

 quently segregated in. lumps which can be thrown out in the 

 mining of the clay, and consequently an analysis of the marketable 

 clay does not show them. One of the most abundant of these is 

 pyrite, the compound of sulphur and iron already referred to 

 (p. 46). This forms lumps which are more or less widely dis- 

 tributed through the clay, but not uncommonly predominate in 

 certain layers, or in certain parts only of one or several layers. 

 In these cases they can be avoided or separated in mining. In 

 New Jersey pyrite is not uncommon in the Woodbridge fire clay, 

 but is more abundant in the South Amboy fire clay. Layers of 

 iron-stained sands, or sandstone layers cemented by iron, may 

 also occur here and there in the bank, or again nodules of iron 

 oxide are sometimes found. Lignite or carbonized wood is often 

 seen in the New Jersey fire clays, and is an almost invariable ac- 

 companiment of pyrite, the latter having been usually deposited 

 on or around the former. Pyrite, as mentioned on a previous 

 page (75), may on weathering yield alum, which forms whitish. 

 spots or coatings in the clay, and in some stoneware clays of the 

 Amboy district causes considerable trouble. Where the impuri- 

 ties are scattered throughout the deposit, hand sorting may not 

 always eliminate them, and washing is necessary. 



USES OF FIRE CEAY. 



The main use of fire clay is for the manufacture of fire brick. 

 These are made in many different shapes and sizes, the density 



