310 JOSEPH STRUTHERS 



ble, while if too fine it will be apt to crack when heated, be- 

 cause of its great density. The clay is selected, not on ac- 

 count of its refractoriness, but because of its plasticity. The 

 sand is free from fluxing impurities, such as iron oxide, lime, 

 magnesia, etc., and is sifted through a 40 mesh screen. 



In combining the constituents, the clay is made into a 

 thin paste with water, and the graphite and sand are thor- 

 oughly mixed in by repeated treatments in an ordinary pug 

 mill. The mass is kept moist for several weeks, in order to 

 insure the expulsion of any air bubbles which may have been 

 formed in the mixing. This stage of the work is called tem- 

 pering. A portion of the tempered mixture of the proper 

 weight for a crucible of the desired size is molded into shape 

 on an ordinary potter's wheel. This method is superior to 

 machine molding, because the rotary motion and external 

 pressure cause the flakes of graphite to become arranged 

 nearly parallel to the sides of the molded shape, thus bind- 

 ing the mass together more strongly. The freshly molded 

 crucibles are inclosed for several hours in a close fitting section- 

 al mold of plaster, which absorbs a portion of the moisture; 

 they are then dried for a week or ten days at a temperature 

 of from 70° to 80° F., and finally are fired for several days 

 in a common pottery kiln. The use for graphite crucibles 

 of a fusible clay of great plasticity instead of one of high 

 refractoriness is based upon the fact that for the combination 

 of two substances by fusion mechanical contact is absolutely 

 essential, so that, whether the clay be refractory or not, the 

 presence of an inert material between the particles of clay 

 and sand prevents fusion. Furthermore, as soon as the 

 crucible is placed in the kiln the graphite on the outer surface 

 is burned away by the oxidizing gases, permitting contact 

 between the particles of clay and sand; these, reacting upon 

 each other, fuse and form on the outside of the crucible a 

 glaze which protects from oxidation the graphite flakes be- 

 neath the surface. 



The quantity of the mineral used for making lead pencils 

 is comparatively small, but the quality must be of the best. 

 For use as a lubricant a high grade of the crystalline product 

 is essential. But material of an inferior grade is employed in 



