IOWA ACADEMY OF SCIENCES. 41 



about the proportion of the admixture. So that clays con- 

 taining more than about 3 per cent of lime can not be made into 

 good brick from this fact, and that the calcium carbonate being 

 reduced to calcium oxide by heat will slack by moisture and 

 the brick then crumble. However, by burning at a higher 

 temperature than is usual the injurious effect of lime can be 

 greatly overcome unless it is in so great quantity as to lower 

 the fusing point too much. The amount of combined water in 

 a clay is a very important item in determining its adaptability 

 for good brick. In a pure hydrated silicate of aluminum so much 

 water will be given off by burning that the brick in going 

 through the sweating process become too soft and run 

 together, or else crack so as to be made much inferior. So all 

 pure clays for brick must be mixed with sand, powdered quartz, 

 powdered brick, gangue, or some such material, in order to 

 alleviate this difficulty. In loams a certain per cent, of lime or 

 similar material needs to be added to act as a flux, as too much 

 sandy material makes the brick brittle. Marls in this country 

 have been, it appears, but little used for brick making, as the 

 lime is supposed to be detrimental. Yet in Europe a very fine 

 malm is made from marls having as high as 40 per cent or more 

 of calcium carbonate. They simply heat the brick probably 

 200 degrees higher than the ordinary brick. This gives the 

 brick a white color instead of red, the iron and calcium being 

 united with the aluminum as a ferric- aluminum calcic silicate. 



Of the Indianola brick clays, analyses of two samples will be 

 sufficient for our purpose of comparison. The brick are made 

 from a certain small deposit of blue clay, taken probably 

 twenty feet below the surface, mixed with a much larger pro- 

 portion of a darker colored clay immediately above this blue 

 layer. 



The lower strata gave the following analysis from the air 

 dried samples: 



Si O2... 66.779 



AljOg. 19.525 



Fe^ O3 - 72 



Ca O trace 



Loss dried at 100° 8.08 



Loss by ignition 5.48 



Total 100.584 



