552 MINERALOGY 



Substances are either heated alone on coal or mixed with a flux 

 to aid the reaction. When heated alone the following phenomena 

 may occur: (1) It volatilizes, disappears as a vapor. Volatility 

 must not be confounded with (2) decrepitation, where the mineral 

 is thrown off the coal by crackling due to the evolution of gases in 

 cavities causing the mineral to explode. (3) A coat may form on the 

 cooler portion of the coal. Metallic vapors driven from the assay 

 come in contact with the oxygen of the air, are oxidized, and settle 

 on the coal, forming coats. (4) Some oxides and compounds are 

 reduced to metal when heated along on coal, as Cu, Pb, Sn, Ag, Au, 

 Pt all yield metallic globules, which again upon further heating 

 may volatilize and yield a coat, according to the metal reduced. 

 (5) Magnetism, in the case of compounds of iron, the assay may 

 become magnetic ; nickel and cobalt when reduced to metal will 

 also be magnetic. (6) Fusion, the fragment becomes rounded on 

 the edges or if easily fused forms a spherical globule. (7) Odors, 

 as of a burning match, very characteristic of sulphur dioxide ; 

 arsenic, selenium, and tellurium also yield odors. (8) Flame color- 

 ation, the vapors of some metals when burning color the flame, as 

 antimony, zinc, copper, or lead. 



Illustration. In a shallow cavity on coal place some oxide of 

 lead, then holding the coal as directed, blow a short, hot R. F., cov- 

 ering the assay with the flame. It will be seen to fuse first, then 

 little globules of metallic lead will appear, which grow in size until 

 all the oxide is reduced to metal. Change the flame to the O. F., 

 holding the globule just beyond the tip of the blue cone. The me- 

 tallic lead will boil, the vapors of lead coloring the flame an azure- 

 blue. If the O. F. is continued, the globule will volatilize entirely ; 

 the hot metallic vapor will combine with the oxygen of the air and 

 settle on the coal beyond the assay, forming a yellow oxide of lead 

 coat. 



Roasting is a metallurgical term, used when either sulphur, ar- 

 senic, antimony, or other volatile compounds are either burned or 

 driven off an ore by heat and oxidation. In blowpipe work roast- 

 ing is in some cases a necessary operation preliminary to reduction 

 on coal or testing with the fluxes. The substance to be roasted is 

 ground to a fine powder, spread out in a thin layer, in a broad shal- 

 low cavity on coal, then heated very gently with the O. F. The 

 assay should be carefully watched and not allowed to fuse, as fusion 

 will prevent the air from coming in contact with each particle. If 

 the assay has been fused, it should be powdered again, returned to 



