CLAYS OF NEW YOKK 553 



^]Sr = s"uin of rLondetrimentals, or silica, alumina, titanic acid, 



water, moisture, and carbonic acid 

 D = sum of detrimental impurities, or tlie iron, lime, magnesia, 



alkalis, sulfuric acid,, sulfur, etc. 

 D^= sum of alkalies which. Wheeler supposes to have twice the 



fluxing value 

 = 1, when clay is coarse-grained and specific gravity exceeds 



2.25 

 = 2, when clay is coarse-grained and specific gravity from 



2-2.25 

 = 3, when specific gravity ranges from 1.75-2 

 = 2, when clay is fine-grained and specific gravity above 2-25 

 = 3, when clay is fine-grained and specific gravity from 2-2.25 

 = 4, when clay is fine-grained and specific gravity from 1.T5-2 

 This formula gives better, but still not regular results. The in- 

 sertion of a term to account for fineness of grain is perfectly 

 rational, but the specific gravity is dependent on the mineral com- 

 position of the clay and therefore indirectly connected with the 

 chemical constitution. 



Determination of fusibility. The temperature at which a clay 

 fuses is determined either by means of test pieces of known com- 

 position, or by some form of apparatus or mechanical pyrometer 

 whose principle depends on the expansion of gases or solids, thermo- 

 electricity, spectro-photometry, etc. 



When test pieces are used, there are two methods for determining 

 the fusibility of a clay, the direct and the indirect. 



The direct method is that of Soger, who devised the test pieces 



known as " Soger cones ". 



These consist of a series of mixtures of clay with fluxes, so 



graded that they represent a series of fusion points, each being but 



a few degrees higher than the one next to it. The materials used 



in making them are such as would have a constant composition, and 



consist of washed Zettlitz kaolin, Rorstrand feldspar, ISTorwegian 



quartz, Oarrara marble, and pure ferric oxid. 



