CHEMICAL PROPERTIES OF CLAY. 65 



It is only recently, however, that the true effects of magnesia 

 in clay have been discovered, for since it was often derived from 

 similar minerals as lime, and resembled it chemically, it was 

 thought to exert the same effect on clays. That it does not has 

 been shown in an interesting series of experiments conducted by 

 Madder. 1 As his results have been published in a German mag- 

 azine, and are probably inaccessible to many of the readers of 

 this report, it may be well to* quote from them. Madder noticed 

 that certain kinds of fireproofing, made from a calcareous clay 

 containing several per cent, of magnesia, behaved somewhat 

 differently from most products made from limy clays, and con- 

 cluded that the effects were due to the magnesia contents of the 

 material. In order to prove this point, he selected a clay, which 

 was free from lime or magnesia, and in its raw and burned con- 

 dition had the following composition : 



Analysis of clay used by Mackler in tests on effects of magnesia: 



Raiv. Burned. 



Loss on ignition, 7.07 .... 



Silica (Si0 2 ), 63.25 68.06 



Alumina ( AI2O3) , 22.97 24.72 



Ferric oxide (Fe 2 3 ), 4.98 5.36 



Lime (CaO), .... 



Magnesia (MgO), 



Alkalies (Na 2 0, K2O), 2.07 2.22 



100.34 100.36 



To one hundred parts by weight of this clay, either lime or 

 magnesium carbonate were added in the proportions given below, 

 the percentages given in parenthesis representing the quantity 

 of lime or magnesia contained in the amount of carbonate added. 

 The physical tests of these mixtures are also given below. 



1 Thonindustrie Zeitung, Vol. XXVI, p. 706. 



5 CL G 



