76 LEAD 



ore and flux, it is generally better that the pot itself should be made of that metal. 

 For this purpose a piece of -inch plate iron is turned up in the form of a crucible, 

 and carefully welded at the edges. The bottom is closed by a thick iron rivet, which 

 is securely welded to the sides, and the whole then finished on a properly-formed 

 mandril. To make an assay in a crucible of this kind, it is first heated to dull red- 

 ness, and, when sufficiently hot, 400 or 500 grs. of the powdered ore, intimately mixed 

 with its own weight of carbonate of soda, half its weight of pearlash, and a small 

 quantity of crude tartar, is introduced by means of a copper scoop (see COPPER, fig. 

 535). On the top of the whole is placed a thin layer of borax, whilst the crucible, 

 which, for the ready introduction of the mixture, has been removed from the fire, is 

 at once replaced. The heat is now raised to redness, the contents gradually becoming 

 liquid, and giving off large quantities of gas. At the expiration of from 8 to 10 

 minutes the mixture will be in a state of complete fusion ; the contents of the pot are 

 now stirred with a small iron rod. Any matter adhering to its sides is scraped to the 

 bottom of the pot, and the temperature of the furnace is increased during 3 or 4 

 minutes to bright redness. 



The crucible is then seized by a strong pair of bent tongs, on that part of the edge 

 which is opposite the lip, and its contents rapidly poured into a cast-iron mould (see 

 COPPER, fig. 534). The sides of the pot are now carefully scraped down with a chisel- 

 edge bar of iron, and any adhering particles of metallic lead added to the portion first 

 obtained. When sufficiently cooled the contents of the mould are easily removed, and 

 the button of lead cleaned and weighed. By this process pure galena yields 84 per 

 cent, of metallic lead, free from any injurious amount of iron, and perfectly ductile 

 and malleable. This method of assaying, with various modifications, is that adopted 

 in almost all lead-smelting establishments, and has the advantage of affording good 

 results with all the ores belonging to the second class. 



Assay in the iron dish without fluxes. In some of the mining districts of Wales the 

 assay of lead ore is conducted in a manner somewhat different from that just described. 

 Instead of fusing the ore in an iron crucible with fluxes, the fusion is effected in a 

 flat shallow iron dish, having a hemispherical cavity of about 4j inches in diameter, 

 and about 1 inch in depth in the centre. 10 oz. of the ore are operated on, and the 

 assay is made by the aid of a blacksmith's forge-fire. The ore is placed in the iron 

 dish, provided with a sheet-iron cover, and heated for about 5 minutes, and the re- 

 duced metal poured out into a mould. The iron dish is then replaced on the fire, and 

 the heat continued for about 5 minutes, until the regulus (slurry) is fused ; the con- 

 tents are then poured out into a mould, the lead detached, and added to the first 

 portion of lead obtained ; and the whole weighed. Galena will yield as much as 82 

 per cent, of lead by this method. In some works the assay is made in an iron crucible 

 without fluxes, instead of the iron dish. 



Estimation of silver contained in lead ores. All varieties of lead ore contain silver, 

 and it is consequently necessary, in order to judge of their commercial value, to ascer- 

 tain the exact amount of this metal which they afford. This is effected by the process 

 of Cupellation. See SILVER. 



In order, therefore, to separate the silver present in buttons resulting from ordinary 

 lead assays, it is only necessary to expose them, on a cupel, to such a temperature as 

 will rapidly oxidise the lead. The oxide of lead, or litharge, produced is absorbed by 

 the porous body on which the assay is supported, and nothing but a small button of 

 silver ultimately remains on the test. These supports or cupels are made of bone- 

 ash, slightly moistened with a little water, and consolidated by being pressed into a 

 mould. The furnace employed for this purpose is described in the article SILVER, as 

 is also the muffle or D-shaped retort in which the cupels are heated. 



As soon as the muffle has become red hot, the cupels that have been drying in the 

 mouth of the opening are introduced by means of proper tongs, on the bottom of the 

 muffle, previously covered with a thin layer of bone-ash, in order to prevent its being 

 attacked in case of any portion of litharge coming in contact with it during the pro- 

 gress of the subsequent operations. The open end of the muffle is now closed by 

 means of a proper door, and the cupels are thus rapidly heated to the temperature of 

 the muffle itself. When this has been effected the door is removed, and into each of 

 the cupels is introduced, by the aid of slender steel tongs, a button of the lead to be 

 assayed. The mouth of the muffle is again closed during a few minutes to facilitate 

 the fusion of the alloy ; when the oxide begins to pass off the surface of the metallic 

 product, the assay is said to be uncovered. The lead is now quickly converted into 

 litharge, which is absorbed by the cupel as fast as it is produced, whilst at the same 

 time there arises a white vapour, which is gradually carried off by the door and through 

 the openings in the sides and end. A circular stain is at the same time formed around 

 the globule of melted metal, which gradually extends and penetrates into the sub- 

 Etance of the cupel. When nearly the whole of the lead has thus been removed, the 



