MINERAL INDUSTRIES FULLERS EARTH. 29 



apparently from the physical condition of the clay and can be 

 detected only by a filtering test by which its practical utility in 

 clarifying oils is determined. 



In testing an earth for clarifying a mineral oil the earth is 

 dried, powdered and placed in a tube. The mineral oil is then 

 passed througii the tube and will be more or less perfectly clarified, 

 depending upon the quality of the earth. A different test is neces- 

 sary for a vegetable oil. In testing vegetable oils according to 

 Wesson* a weighed amount of the oil and the fullers earth are 

 stirred together for a regular period at a temperature of lOO de- 

 grees C. The oil is then filtered and compared with other known 

 fullers earth treated under exactly the same conditions. 



Various other properties are assigned to fullers earth, but all, 

 aside from the actual bleaching tests, are so variable, or are com- 

 mon to such a variety of clays as to be of only secondary value 

 as a means of identification. Non-plasticity is often given as a 

 property of fullers earth, but it appears from the investigations of 

 Porterf and others that some of the fullers earths are distinctly 

 plastic when mixed with a large proportion of water. Some of the 

 fullers earths will disintegrate in water, although others are little 

 affected thereby. Most fullers earths on account of their porosity 

 when dry will adhere firmly to the tongue, but some other clays will 

 do the same. In color fullers earth is as variable as other clays, and 

 while buff and blue clays predominate, others are brown, gray or 

 almost white. As a rule fullers earths are light in weight owing 

 to their porosity, although there are exceptions, and the specific 

 gravity is much the same as that of other clays. These secondary 

 properties, althoug^h of value in tracing any particular bed after 

 this has been located, are not to be relied upon as a complete test. 



MINING FULLERS EARTH. 



All the sedimentary deposits of fullers earth are mined by the 

 open pit method, the overburden being removed by steam shovel 

 in the larger mines, and by team and scraper or by pick and 

 shovel in the smaller mines. The depth of overburden that can 

 profitably be removed is variable, depending as it does upon the 



*Bleaching of Oils with Fullers Earth, by David Wesson, Trans. Amer. 

 Inst, of Chemical Engineers, Vol. iii, 1910, pp. 327-332. 



tProperties and Tests of Fullers Earth, by John T. Porter, U. S. Geol. 

 Sur. Bull, 315, pp. 268-290, 1907. 



