218 



UNITED STATES MINERAL RESOURCES 



tures used in glass manufacture are undesirable 

 impurities because unmelted fragments appear as 

 flaws in finished glass. Specifications for most glass 

 manufacture require the feldspar to be ground to 

 minus 20 or 30 mesh and not more than 10-20 per- 

 cent to be ground to minus 140 mesh. Minor amounts 

 of feldspar were used in glass as early as 1895 but 

 its use in container glass was generally adopted by 

 major producers in 1924 and became standard indus- 

 try practice about 1935. Use of feldspar in the flat- 

 glass industry was standard by 1940. A little more 

 than 110 pounds of feldspar is used in an average 

 ton of glass containers and slightly less than 100 

 pounds in a ton of flat glass. 



Feldspar is useful in ceramic batches because it 

 fuses at lower temperatures than most of the other 

 ingredients, and as a result, it cements crystalline 

 phases in some types of ceramic bodies and acts as a 

 flux in physicochemical reactions of crystalline 

 phases in whiteware. The feldspar, which imparts 

 strength, toughness, and durability to the finished 

 product, makes up 10-35 percent of the body and 

 30-50 percent of the glaze in many types of ceramics. 

 The ratio of potash to soda in the feldspar affects the 

 firing characteristics, and in general, high-potash 

 material is preferred. Feldspar products are avail- 

 able with potash-to-soda ratios varying from 5 to 1 

 to less than 1 to 1. A maximum iron-oxide content of 

 0.1 percent, specified limit for silica, and absence of 

 dark mineral particles are also required. Feldspar 

 for most ceramic use is ground to minus 200 mesh. It 

 has been used as a separate ingredient in ceramics in 

 China since the Tang dynasty (A.D. 621-945) and 

 in Europe since the 18th century (Burgess, 1949, p. 

 349). 



Uses of feldspar other than in glass and ceramics 

 are few, and consumption is small, but use as a 

 mineral filler, especially in latex, is growing. The 

 most important other uses are in abrasives and 

 scouring soaps because of physical properties such 

 as cleavage, hardness, and white color. Excellent 

 cleavage in two directions provides sharp edges ; but 

 feldspar hardness, being less than that of glass, 

 minimizes damage to glass and glazed whiteware by 

 scratching. Feldspar is also used in electric welding- 

 rod coatings as a source of potassium to improve arc 

 characteristics and to serve as a flux. It has been 

 used in roofing granules and poultry grit and for 

 other crushed-stone uses. 



In recent years, various materials have been used 

 in the glass and ceramic industries as substitutes for 

 feldspar. Chief among these are nepheline syenite, 

 aplite, talc, pyrophyllite, electric furnace slag, and 

 lithospar. Most nepheline syenites contain too much 



iron oxide for use in glass, but a deposit in Ontario, 

 Canada, that is low in iron is mined extensively for 

 this purpose. "Aplite" in the glass trade is the name 

 for a mixture of plagioclase and microcline produced 

 in Amherst and Nelson Counties, Va., by flotation 

 methods from anorthosite. Talc replaces feldspar in 

 dinnerware and wall tile; pyrophyllite is used in- 

 stead of feldspar in enamels and wall tile. Electric- 

 furnace slag is used as a substitute source of 

 alumina for glass, and lithospar is a naturally occur- 

 ring mixture of spodumene and feldspar used as a 

 flux. 



The United States is or could be self-sufficient in 

 feldspar production. In 1970 we imported 3,247 long 

 tons, mostly from Canada, and exported about 5,000 

 long tons. Reserves are large, and until recently the 

 industry operated at only 50-percent capacity. Less 

 than 1 percent of the feldspar used in the United 

 States is imported, but large amounts of nepheline 

 syenite, as a substitute for feldspar in glass and 

 ceramics, are imported from Canada. These ship- 

 ments began in 1936, and in recent years they have 

 increased more rapidly than domestic feldspar pro- 

 duction (fig. 25). The U.S. production of feldspar 



O 500- 



Q 300 



Figure 25. — Crude feldspar sold or used by producers in the 

 United States, 1900-1970, compared with imports of 

 nepheline syenite 1937-1970. Derivation of domestic feld- 

 spar is shown for 1955-70. Data from U.S. Geol. Survey 

 and U.S. Bur. Mines, Mineral Resources of the U.S. 

 and Minerals Yearbook, 1900-70. 



was about 28 percent of the estimated world pro- 

 duction in 1970 (Wells, 1972, p. 2) . 



