Page:The American Cyclopædia (1879) Volume X.djvu/266

 260 LEAD CONSTITUENTS. I. II. III. IV. V. VI. VII. VIII. Lead 96-69 99-9660 99-27 86-53 52-84 99-95200 99-98720 0-20 trace 0-2240 0-12 traces traces 0-03180 0-00710 Antimony 0-71 0-0260 0-6970 0-57 11-29 47-16 0-00590 0-00250 1-30 Iron 0-06 0-0039 0-0045 0-04 0-84 traces o-ooiio o-66ii6 0-0041 0-00870 0-00080 Silver 0-53 0-00500 0-00180 trace P I. Freiberg furnace lead. II. Bleiberg (Carinthia) furnace lead. III. Upper Hartz furnace lead. IV. English hard lead. V. Lead from dross of calcination of IV. VI. Lead from dross of calcination of V. VII. Eefined English lead. " W. Black- ett, best selected." VIII. Kefined Upper Hartz lead (Clausthal), desilverized by zinc. the hearth is composed, and is caught in iron moulds. The interior of the masses thus formed cools slowly, and has generally a red color, and is then sold as such. The rest of the yellow litharge is reduced to metal in fur- naces by itself. That which forms during the latter part of the operation contains consider- able silver, and the lead obtained from it is again cupelled, after concentration of the sil- ver, by one of the processes presently to be described. When the operation is nearly com- pleted the thin layer of molten litharge cover- ing the silver breaks away, leaving the bright surface of the silver exposed. The silver is then said to " blick," and the mass is called blick silver. This still contains from 8 to 10 per cent, of impurities, and must be further re- fined. This could be effected on the hearth of the cupellation furnace, by simply continuing the operation at an increased heat ; but as the loss of silver would be large under these cir- cumstances, the silver is removed and placed on a small hearth especially prepared for it, and exposed at a high temperature to a strong blast, with occasional stirring, until it is " fine." In this operation it absorbs oxygen, which on cooling is often violently expelled, giving rise to the phenomenon of " spitting." In some cases a large amount of lead is cu- pelled until the silver is concentrated to the extent of 10 per cent, of the lead, when it is tapped off and further cupelled on another hearth. The cupellation of large quantities of lead at one operation is especially adapted to those cases where the lead is impure, as the foreign matters are then all removed in the abstrich, the litharge subsequently formed being pure.- In Freiberg, ' Saxony, where the amounts mentioned above are cupelled at one operation, there are formed 400 to 600 Ibs. of abzug, but no abstrich, as the lead is previously refined, from 32,000 to 33,000 Ibs. of litharge, and 520 to 530 Ibs. of silver ; the loss of lead is from 8 to 10 per cent. Ore very rich in sil- ver is often directly submitted to cupellation by placing it under the pigs of lead in charg- ing the furnace. In the English system an el- liptical frame of wrought iron, about 4 ft. long and 2| ft. wide, is filled with moistened bone ash, firmly packed down, and then scooped out to the depth of about 3 in. This "test "is placed in a furnace, having a large fireplace on one side of the ellipse ; at one end is the nozzle supplying the blast, and at the other is the Silver is extracted from lead by exposing the melted argentiferous lead to the action of a blast of air at a temperature above the melt- ing point of litharge. The lead is converted into oxide, while the less oxidizable silver re- mains unaffected. This process, called cupella- tion, is of great antiquity. In the opinion of Percy it is plainly indicated in Jer. vi. 29, 30 : " The bellows are burned, the lead is consumed of the fire ; the founder melteth in vain; for the wicked are not plucked away. Reprobate [refuse] silver shall men call them, because the Lord hath rejected them." " In this pas- sage all the essential points are mentioned: the artificial blast, the oxidation or consump- tion of the lead, and the 'reprobate silver,' sil- ver dross, or litharge. There is either no resi- due of precious metal, or what remains is con- taminated with impurities, so tenaciously ad- herent as not to admit of being separated by the oxidizing and solvent action of molten litharge." The operation may be performed in a variety of ways. The two prominent sys- tems are the German and the English. The German cupelling furnace consists of a large shallow hearth, somewhat elliptical in shape, about 8 or 9 ft. in diameter, made of marl or a mixture of limestone and clay, firmly stamped down, but retaining a certain degree of poros- ity and absorbent power for molten litharge. On one side is a fireplace for either wood or coal, and at right angles to it are the nozzles, generally two, for supplying the blast. Near- ly opposite to the blast, and near the fireplace, is an opening for the removal of the litharge. The top or head of the furnace is movable, and is let down and luted on when the operation is ready to begin. From 35,000 to 37,000 Ibs. of lead are treated at one operation, of which 13,- 000 are introduced in the form of pigs at the beginning, and the rest after the litharge has begun to flow. When the pigs first introduced are melted, there remains a sand-like scum (Abzug) on the surface, composed of impuri- ties in the lead, some metallic lead and oxide, and particles from the hearth. This is re- moved, and the surface of the metal exposed. The first litharge which forms is pasty, and contains largely the impurities in the lead, principally antimony, and is called black lith- arge (Abstric7i) it gradually becomes lighter in color, and passes into pure litharge. This is allowed to flow from the hearth as fast as formed by cutting notches in the marl of which