346 EEPORT OF NATIONAL MUSEUM, 1899. 



fully, and to allow no impurity to get in while handling the day or 

 working the components together. 



Fire bricks intended, in addition to their refractory qualities, to 

 retain their size and form under intense heat without shrinkage, have 

 been made to some extent. The English Dinas bricks are of this 

 kind, and the German and French "silica bricks." The Dinas bricks 

 are of quartz sand or crushed rock, and contain very little alumina 

 and about one per cent of lime. They stand lire remarkably- well, the 

 lime being just enough to make the grains of sand stick together 

 when the bricks are intensely heated. In the other "silica bricks," 

 fire clay to the amount of 5 or 10 per cent is mixed with the sand, and 

 this plastic material makes the particles of the sand cohere sufficiently 

 to allow of handling the bricks before burning. They have met the 

 expectation of those who made them, and are extensively used.^ 



Under the head of "Miscellaneous uses of clay," p. 317, Cook 

 gives the following, which may well be incorporated entire: 



Paper clay. — Clay which is pure white and that also which is discolored and has 

 been washed to bring it to a uniform shade of color, is used by the manufacturers 

 of paper hangings, to give the smooth satin surface to the finished paper. It la used 

 by mixing it up with a thin size, applying it to the surface of the pieces of paper, and 

 then polishing by means of brushes driven by machinery. The finest and most 

 uniformly colored clays only are applicable to this use, and they are selected with 

 great care. Clay is also used to some extent by paper manufacturers, to give body 

 and weight to paper. 



Heavy wrapping paper, such as is used by the United States Post- 

 Ofiice Department, must, according to specifications, contain 95 per 

 cent of jute butts and 5 per cent of clay. The cheaper forms of con- 

 fectionery, particularly such as is sold from carts upon the streets, is 

 very heavily adulterated with this material. 



Alum clay. — A large quantity of clay is sold every year to the manufacturers of 

 chemicals, for making alum. A rich clay is needed for this purpose, but those con- 

 taining lignite or pyrite which renders them inapplicable for refractory materials, 

 do not spoil them for this use. Alum is made by digesting the clay in sulphuric 

 acid, which forms sulphate of alumina, then dissolving out the latter salt from the 

 silica and other impurities, and forming it into alum by the addition of the necessary 

 salt of potash, soda, or ammonia, and crystallizing out the alum. 



The white clay of Ga}- Head and Chilmark, Marthas Vineyard, 

 Massachusetts, was at one time used extensively for alum making, 

 according to Edward Hitchcock.^ 



As a substitute for sand in making mortar and concrete clay is per- 

 haps the best material to be found. For this purpose the clay is burnt 

 so that it is produced in small irregular pieces that are very hard and 

 durable. These pieces are then ground to a fairly fine powder, which 

 is used to mix with the lime or cement just as sand would be. The 



^ Geological Survey of New Jersey, Report on Clay Deposits, pp. 307-312. 

 ^ American Journal of Science, XXII, 1832, p. 37. 



