PROPERTIES OF CLAYS 



141 



changed to Fe 2 3 when the clay is heated. This formula gives 

 a good comparative value of the refractoriness of two similar 

 clays whose specific gravity does not differ by more than .2. 



When the clays to be compared differ in fineness, the formula 

 (A.) is modified by the constant C. : 



F. F. - (B.) 



C .= 1 when the clay is coarse grained and sp. gr. over 2.25. 



C = 2 when the clay is coarse grained and sp. gr. 2.00-2.25. 



C = 3 when the clay is coarse grained and sp. gr. 1.T5-2.00. 



C = 2 when the clay is fine grained and sp. gr. is over 2.25. 



C = 3 when the clay is fine grained and sp. gr. 2.00-2.25. 



C = 4 when the clay is fine grained and sp. gr. 1.75-2.25. 



The value of C is to be considered only approximate. 



The temperature of fusion of a clay is usually determined by 

 means of a pyrometer. Two kinds have been more or less used 

 for this purpose, viz., the Lunette pyrometer and Le Chatelier's 

 thermo-electric pyrometer, which is far more accurate. These 

 pyrometers have also been used to determine the temperature of 

 kilns. 



Method of analyzing clays 



By Dr. H. T. Vulte. 



One grain of the dried and finely pulverized clay is fused in a 

 platinum crucible with five to 10 times its weight of a 

 mixture of 11 parts of dry sodium carbonate and 14 parts of 

 dry potassium carbonate, the amount of fusion mixture necessary 

 depending on the more or less refractory character of the clay. 

 The fusiou is transferred to a porcelain casserole, dissolved in 

 water, and the solution acidified with hydrochloric acid ; the 

 solution is then evaporated to dryness, and the casserole with its 

 contents placed in a drying oven at 105° to 110° C, and allowed 

 to remain until all the hydrochloric acid is expelled. The silica 

 present is thus rendered insoluble. Hydrochloric acid and water 

 are now added ; the casserole is warmed for a few minutes on the 

 water bath and the solution filtered, the silica being washed with 

 hot water until the washings are free from chlorine. The silica 

 is then ignited and weighed, and as it is likely to retain small 

 quantities of alumina, it is treated with hydrofluoric and 

 sulphuric acids and heated, the silica being thus volatilized as 



