This assay is presented here, not with the purpose of giving the student an accurate and handy means of estimating copper in its ores, but to give him some practice in manipulation, which, on the small scale, somewhat resembles metallurgical methods. In all modern mining fields the fire-assay of copper is replaced by the Volumetric or Electrolytic methods, both of which have been given (except the Lake Superior method).
The ore taken for assay is a clean sample of copper pyrites or copper matte.
Method.—The sulphur in the ore or matte is eliminated by roasting. The oxide thus formed is reduced to metallic copper by fusion with potassium cyanide (Ricketts and Miller).
Apparatus.—The usual scales aud furnaces for roasting and fusion (Hessian crucibles).
Reagents. —Ammonium carbonate, charcoal, borax glass, potassium cyanide.
Details.—Weigh in duplicate 10 gms. finely powdered (80 sieve) ore or matte. Carefully roast as before described. Remove and mix with 1 gm. ammonium carbonate. Re-roast. Remove. Mix with 1 gm. charcoal. Re roast. Remove. Cool. Mix with 30 gms. KCN and 3 gms. borax glass. Place in a No. 5 Hessian crucible, and cover with 10 gms. KCN, then with salt. Fuse in a bright red fire for about half an hour. Remove, and pour the charge. Cool, collect, and weigh the copper.
Calculation, Accuracy, etc.—The result, multiplied by 10, gives the percentage of copper. With clean mattes (containing no antimony, lead, etc. etc.), results accurate within 2% may be obtained ; still, unless the exact nature of the matte be known, this method is too uncertain, and, as before remarked, must give place to other more accurate methods.