414 



SMELTING PROCESSES 



The point to which it is possible to carry the separation by this method is about 2 ; ] per cent, of silver, according 

 to experiments conducted at these works. Here the formation of crystals becomes slower and slower, and the still 

 molten mass shows an unmistakable tendency to congeal and harden into a perfectly homogeneous mass. The explana- 

 tion of this is, that up to this percentage the melting point of the alloy is lower, but above this it is higher than that 

 of lead. Certain it is, that the melting point of an alloy of equal parts silver and lead (or even three parts of lead 

 and one part of silver) is higher than that of pure lead. 



The process can be undertaken either with or without intermediate crystals. 



1. The Two-THIBD Rystkm without INTERMEDIATE Ckystals. — The following is the manipulation of the 

 two-third system without intermediate crystals : — 



After the lead in the kettle has been melted, and the scum forming on the top (copper, arsenic and lead) has been 

 removed, the fire is almost entirely raked away, and the fire-door under the kettle opened. Water is then carefully 

 sprinkled on the surface of the molten lead bath. This causes the lead near the edge of the kettle to solidify. These 

 solid lumps are broken off and stirred into the rest of the lead, by which means the temperature of the whole mass is 

 evenly lowered. When this lowering of temperature has reached a certain point, the mass becomes viscous from the 

 beginning of the formation of crystals (which are small octahedra). Two workmen now grasp tho ladle, and sink 

 it into the kettle, along the sides and bottom, over which it is scraped to collect the crystals deposited 

 there. It is then raised by making a fulcrum of the rim of the kettle. Here it gets two good shakes, to force out tho 

 liquid contents of the ladle through tho holes. Tho ladle and its contents are then slid over to the next kettle to tho 



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right, on a movable traveller, and the crystals are turned into it. This kettlo has been already heated, so that they 

 melt Immediately on being placed in it. When two-thirds of the original contents of the kettle have been transferred 

 to the kettle to tho right, the remainder is ladled in a liquid state into tho kettle to the left. This latter one-third is, 

 compared to the original mass in the first kettle, rich in silver, and tho two-thirds which have gone in solid crystals 

 to the right comparatively poor Those two are now treated in the same way, the two-thirds crystals from the kettle 

 on the left being ladled into tho middle kettle in which tho operation began, into which comes also tho one-third 

 mother-liquid from the kettle on the right, while one-third still richer mother-liquid goes into the next kettle to the 

 left, and two- thirds still poorer lead crystals go to the kettle still further to the right. Soon the whole battery is in 

 operation, the alternate kettles, at the close of any one operation, being empty, and the same kettles, at the close of 

 the next step, full. Ill order to fill up the kettles into which respectively one-third and two-thirds of a charge have 

 been put, refined lead, of various percentages of silver, is stamped, assayed and distributed to the different kettles 



