4 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The crucible. — The ideal crucible for work on allo> s would be one 

 made from thin metal, if we could find some metal that would not 

 diffuse, for we could then readily study the effects of chilling, as the 

 crucible would be a good heat conductor. But since all metals diffuse, 

 even in the solid state, we cannot hope for such a crucible. The best 

 substitute would be one made from thin metal and lined with a thin 

 coating of lime. 



The need for such a crucible has been felt by all who have attempted 

 to make anything like an accurate study of alloys, and attempts have 

 been made to perfect one, but until now without success. The author 

 has finally succeeded in lining a steel crucible with a thin coating of 

 lime. These linings are fairly durable so long as they are kept free 

 from moisture, but if left in a damp place the lime slacks and cracks 

 off. With a lining two tenths of a millimeter in thickness I have made 

 seventeen melts, each time chilling in an ice-bath, and at the end of 

 this time the lining was almost intact. 



These linings are made from a mixture of finely powdered quick- 

 lime and calcium nitrate, about four parts by weight of the lime to one 

 by weight of the nitrate. Care must be taken that the powders are 

 thoroughly mixed. Such a mixture fuses to solid lime when sufficient 

 heat is applied to drive off all the nitric acid. 



This last fact is not new. It is often made use of in the chemical 

 department of Harvard University for preparing small boat- shaped 

 crucibles suitable for melting small portions of metal. But one meets 

 with considerable difficulty in keeping the powder in place on the steep 

 sides of the crucible while it is being fused. 



The method finally employed is to use a cylindrical-shaped crucible. 

 This is clamped, open end outward, in the chuck of a high-speed lathe 

 and then set spinning. The mixed powder is then carefully blown 

 inside the crucible where the centrifugal force holds it firmly on the 

 sides in the form of an even coating. The spinning crucible is then 

 heated with a blast lamp until all the acid is driven off. It is then 

 removed from the lathe, set in an upright position, and the bottom 

 sprinkled with a coating of the powder. This is finally fused on, the 

 force of gravity keeping the powder in place. 



By this method it is possible to give the crucible a very even coating 

 of almost any desired thickness, though the first attempts are usually 

 discouraging. I have found it possible to chill an alloy very satisfac- 

 torily through these thin linings. 



For work on weakly magnetic alloys, especially for such as fuse above 

 eight hundred degrees centigrade, a factor of safety is added by using 

 for the crucible some weakly magnetic metal such as platinum, as traces 



