158 NEW YORK STATE MUSEUM 



ill lime, on the other hand, possess a strong tendency to crystalHze 

 and only consolidate in glassy form when quickly cooled. The 

 crystallizing property becomes more marked with increase in the 

 lime and is very strong in anorthite. This feature, of course, 

 operates against the use of the more basic feldspars in pottery 

 wares. 



Uses of pegmatite. The products of the local pegmatite quarries 

 include feldspar of different grades, quartz, mica and unsorted 

 crushed pegmatite. 



The uses of feldspar are various. The principal demand for 

 high-grade potash spar is in the pottery industry, parti'cularly in 

 the manufacture of porcelain, semiporcelain and china tablewares, 

 and porcelain sanitary wares and electrical supplies. The feldspar 

 for such purposes should contain no more than a mere trace of 

 iron, and very little muscovite or other mineral impurities except 

 quartz, which is allowable up to a certain extent. In such wares 

 it performs a double function, being employed to bind together 

 the quartz and kaolin that constitute the body and also as a con- 

 stituent of the glaze when this is required. The proportion of 

 feldspar used in the body of vitrified wares ranges from 10 to 35 

 per cent and in glazes from 30 to 50 per cent. Bastin states ^ that 

 the requirements in regard to allowable percentages of free quartz 

 differ among individual potteries ; a few manufacturers of high- 

 grade wares demand a feldspar with less than 5 per cent of free 

 quartz, but most potters perhaps use the " Standard " ground spar 

 carrying 15 to 20 per cent of admixed quartz. 



Manufacturers of enamel ware, glazed brick and terra cotta con- 

 sume consideralDle quantities of feldspar. In enamel ware, the re- 

 quirements are perhaps not so strict in regard to iron as in pottery 

 manufacture, but the spar must be fairly free of quartz, as the 

 latter tends to raise the melting point. Among enamel ware and 

 terra cotta manufacturers, a preference is shown for albite over the 

 potash varieties owing to its lower fusing point. Little of this 

 mineral is found in the New York pegmatites, but it occurs in 

 quantity in eastern Pennsylvania and in Maryland. Another use 

 for the local feldspar is in the manufacture of opalescent glass. 

 This requires a material of about the same quality as that for 

 enamel ware, but may contain more quartz. 



A large quantity of feldspar is employed as an abrasive, es- 

 specially in the form of scouring soaps and powders. For that 



^Feldspar Deposits of the United States, U. S. Geol. Sur. Bui. 420, 1910, 

 p. 19. 



