62 CLAYS AND CLAY INDUSTRY. 



Many erroneous statements are found in books, regarding the 

 allowable limit of lime in clays, some writers putting it as low 

 as 3 per cent. ; still a good building brick can be made from a 

 clay containing as much as 20 per cent, or 25 per cent, of lime 

 carbonite provided it is in a finely divided condition, 1 and a 

 vitrified ware is not attempted. If, however, that much lime is 

 contained in the clay in the form of pebbles, then much damage 

 may result from bursting of the bricks, when the lumps of 

 burned lime slake, by absorbing moisture from the air. 



Clays containing a high percentage of lime carbonate are 

 used in the United States, especially in Michigan, Wisconsin and 

 Illinois, for making common bricks, common earthenware, roof- 

 ing tile, and some terra cotta. 



Effect of lime-bearing silicates. — The effect of these is much 

 less pronounced than that of lime carbonate. They contain no 

 volatile elements, and hence do not affect the shrinkage as lime 

 carbonate does. They serve as fluxes, but do not cause a rapid 

 softening of the clay. 



Effect of gypsum. — Lime, if present in the form of gypsum, 

 seems tO' behave differently from lime in the form of carbonate, 

 although few clays contain large percentages of it. 



Gypsum as already shown 2 is a hydrous sulphate of lime. In 

 calcining gypsum for making plaster of Paris, the chemically 

 combined water is driven off at 250 C, and it has probably been 

 usually taken for granted that the sulphuric acid was driven off 

 at a red heat. In order to determine what amount of loss actually 

 occurred, and at what temperature this took place, the writer 

 made a mixture consisting of 75 per cent, of a white-burning clay 

 and 25 per cent, of white gypsum, which was nearly chemically 

 pure. The two were ground together in a ball mill to> mix them 

 thoroughly, dried until stiff enough to mold, and then formed 

 into bricklets which were dried in a hot-air bath at ioo° C. The 

 white clay contained 13.24 per cent, of chemically combined 

 water. The gypsum contained 46.6 per cent, sulphur trioxide, 



1 For analyses and uses of calcareous clays see H. Ries, Clays and Shales of 

 Michigan, Mich. Geol. Sur., Vol. VIII, Pt. I; and E. R. Buckley, Clays and 

 Clay Industries of Wis., Wis. Geol. Sur., Bull. 2, Economic Series. 



2 Chap. Ill, Minerals in Clay. 



