68 



LEAD 



The size of the fire-place varies with the other, dimensions of the furnace, but is 

 usually nearly square, and in an apparatus of ordinary size may be about 2 feet x 2 

 feet 6 inches. This is separated from the body of the furnace by a fire-bridge 18 

 inches in breadth, so that the flame and heated air pass directly over the surface of 



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the cupel, and from thence escape by means of two separate apertures into the main 

 flues of the establishment. The cupel or test consists of an oval iron ring, about 5 

 inches in depth, its greatest diameter being 4 feet, and its lesser nearly 3 feet. This 

 frame, in order to better support the bottom of the cupel, is provided with cross-bars 

 about 4J inches wide and inch in thickness. In order to make a test, this frame 

 is beaten full of finely-powdered bone-ash, slightly moistened with water, containing 

 a small quantity of pearlash in solution, which has the property of giving consistency 

 to the cupel when heated. 



The centre of the test, after the ring has been well-filled with this mixture, and 

 solidly beaten down, is scooped out with a small trowel, until the sides are left 2 inches 

 in thickness at top, and three inches at the bottom, whilst the thickness of the sole 

 itself is about 1 inch. 



At the fore part or wide end of the test the thickness of the border is increased to 

 6 inches, and a hole is then cut through the bottom, which communicates with the 

 openings or gates by which the fluid litharge makes its escape. 



The test, when thus prepared, is placed in the refinery furnace, of which it forms 

 the bottom, and is wedged to its proper height against an iron ring firmly built into 

 the masonry. When this furnace is first lighted, it is necessary to apply the heat very 

 gradually, since if the test were too strongly heated before it became perfectly dry, it 

 would be liable to crack. As soon as the test has become thoroughly diy, it is 

 heated to incipient redness, and is nearly filled with the rich lead to be operated on, 

 which has been previously fused in an iron pot at the side of the furnace, and beneath 

 which is a small grate where a fire is lighted. 



The melted lead, when first introduced into the furnace, becomes covered with a 

 greyish dross, but on further increasing the heat, the surface of the bath uncovers, 

 and ordinary litharge begins to make its appearance. 



The blast is now turned on, and forces the litharge from the back of the test up to 

 the breast, where it passes over the gate, and falls through the aperture between 

 the bone-ash and the ring into a small cast-iron pot running on wheels. The air, 

 which is supplied by a small ventilator, not only sweeps the litharge from the sur- 

 face of the lead towards the breast, but also supplies the oxygen necessary for its 

 formation. 



In proportion as the surface of the lead becomes depressed by its constant oxidation, 

 and the continual removal of the resulting litharge, more metal is added from the 

 melting pot, so as to raise it to its former level, and in this manner the operation is 

 continued until the lead in the bottom of the test has become so enriched as to render 

 it necessary that it should be tapped. The contents of the test are now so far reduced 

 in volume that the whole of the silver contained in the rich lead operated on remains 

 in combination with a few hundred-weights only of metal, and this is removed by 

 carefully drilling a hole in the bone-ash forming the bottom of the test. The reason 

 for the removal of the rich lead, is to prevent too large an amount of silver from 

 being carried off in the litharge, which is found to be the case when lead containing 

 a very large amount of that metal is operated on. 



When the rich load has been thus removed, the tapping hole is again closed by a 

 pellet of bone-ash, and another charge immediately introduced. 



As soon as the whole of the rich lead has been subjected to cupellation, and has 

 become thus further enriched, the argentiferous alloy is itself similarly treated, either 



