ASSAYING. 125 



in a charcoal draft furnace, which would admit four No. 10 French cruci- 

 bles at once. The fuel employed was charcoal made from the piflon pine, 

 and it was scarcely inferior in heating power to coke. The heat was kept 

 as nearly as possible at a point a little below the melting point of cast iron, 

 and experience showed that after two houi-s the silver obtained no longer 

 increased nor yet perceptibly diminished. With a higher temperature it 

 may be that the time might be shortened, but taking into consideration the 

 volatility of silver at high temperatures this expedient cannot be considered 

 advisable. When the melting was less prolonged the maximum amount of 

 silver was never obtained; indeed, in a series of careful experiments made 

 to determine the best time of melting, it was found that some assays which 

 had been kept at a melting heat too short a time, though thoroughly melted, 

 yielded less silver than the litharge alone was known to contain. 



cupeiiation and cupels. — The lead buttons were cupelled in a small muffle 

 furnace, the heat of which could be easily regulated, the fuel employed 

 being also charcoal. Usually but two cupellations were carried on at one 

 time and great care was taken to reduce the loss by cupellation to a mini- 

 mum by keeping the heat at the lowest temperature consistent with the 

 oxidation of the lead. In order that the heat should be maintained at the 

 lowest possible point, the cupels used were made from one part of fine 

 leached wood ashes and two parts of bone ashes. The ashes were those 

 resulting from the burning of cedar wood, the most available wood con- 

 taining little silica. The cupels were prepared in the following manner: 

 The mold was filled with the requisite amount of the moistened mixture 

 of the two ashes and the mass was pressed into shape by the punch. Then 

 a coating of dry elutriated bone ash was spread over the top of the cupel, 

 the punch again inserted and driven home. In this way a cupel was ob- 

 tained which had great absorbing power, allowed the lead to be cupelled 

 at an exceedingly low temperature, and because of its smooth surface pre- 

 vented the small silver button from being engulfed in the coarse material 

 of which the cupel was composed. ' The button, too, could easily be re- 

 moved bv the point of a knife without retaining any of the bone ashes. 



loss by cupeiiation. — Many experiments with a view to determining the loss 

 by cupellation have been made by Hauibly, Klasek, Plattner, and others; 



