SILVER 811 



the furnace ; 6, the sole, formed of wood-ashes, washed, dried, and beaten down ; 

 k, dome of iron plate, moveable by a crane, and susceptible of being lined two inches 

 thick with loam ; n, n, tuyeres for two bellows, s, having valves suspended before 

 their orifices to break and spread the blast ; q, door for introducing into the furnace 

 the charge of lead, equal to 84 quintals at a time ; s, fig. 1800, two bellows, like those 

 of a smith's forge; y, door of the fireplace, through which billets of wood are 



thrown on the grate : x, small 



aperture or door, for giving issue ~ ! 8 ~ 02 ~ il " '- 



to the frothy scum of the cupel- 

 lation, and the litharge ; x?, basin 

 of safety, usually covered with 

 a stone slab, over which the 

 litharge falls : in case of acci- 

 dent the basin is laid open to 

 admit the rick lead. 



The following is the mode of 

 conducting the cupellation : 

 Before putting the lead into the 

 furnace, a floor is made in it of 

 ashes beaten carefully down (see 

 fig. 1801); and there is left in 

 the centre of this floor a circular 

 space, somewhat lower than the rest of the hearth, where the silver ought to gather 

 at the end of the operation. The cupel is fully 6 feet in diameter. 



In forming the floor of a cupel, 35 cubic feet of washed wood-ashes, usually got from 

 the soap-works, are employed. The preparation of the floor requires two and a half 

 hours' work ; and when it is completed, and the moveable dome of iron plate has been 

 lined with loam, 84 quintals (cwts.) of lead are laid on the floor, 42 quintals being 

 placed in the part of the furnace farthest from the bellows, and 42 near to the fire- 

 bridge ; to these, scoriae containing lead and silver are added, in order to lose nothing. 

 The moveable lid is now luted on the furnace, and heat is slowly applied in the fire- 

 place by burning fagots of fir-wood ; this is gradually raised. Section ./fy. 1801, is in 

 the line c, D, of Jig. 1802. 



At the end of three hours, the whole lead being melted, the instant is watched for 

 when no more ebullition can be perceived on the surface of the bath or melted metal ; 

 then, but not sooner, the bellows are set a-playing on the surface at the rate of four or 

 five strokes per minute, to favour the oxidation. 



In five hours, reckoned from the commencement of the process, the fire is smartly 

 raised ; when a greyish froth (Abstrich) is made to issue from the small aperture x, of 

 the furnace. This is found to be a brittle mixture of oxidised metals and impurities. 

 The workman now glides the rake over the surface of the bath, so as to draw the 

 froth out of the furnace ; and as it issues, powdered charcoal is strewed upon it at 

 the aperture x, to cause its coagulation. The froth-skimming lasts for about an hour 

 and a half. 



After this time the litharge begins to form, and it is also led off by a small opening 

 x, its issue being aided by a hook. In proportion as the floor of the furnace gets 

 impregnated with litharge, the workman digs in it a gutter for the escape of the liqxiid 

 litharge ; it falls in front of the small aperture, and concretes in stalactitic forms. 



By means of the two moveable valves suspended before the tuyeres n, n (jig. 1802), 

 the workman can direct the blast as he wishes over the surface of the metal. The 

 wind should be made to cause a slight curl on the liquid, so as to produce circular 

 undulations, and gradually propel a portion of the litharge generated towards the 

 edges of the cupel, and allow this to retain its shape till the end of the operation. 

 The stream of air should drive the greater part of the litharge towards the small 

 opening x, where the workman deepens the outlet for it, in proportion as the level of 

 the metallic bath descends. Litharge is thus obtained during about twelve hours ; 

 after which period the cake of silver begins to take shape in the centre of the cupel. 



Towards the end of the operation, when no more than four additional quintals of 

 litharge can be looked for, and when it forms solely in the neighbourhood of the silver 

 cake in the middle of the floor, great care must be taken to set apart the latter 

 portions, becaxise they contain silver. About this period the fire is increased, and the 

 workman places before the little opening x, a brick, to serve as a mound against the 

 efflux of litharge. The use of this brick is, 1, to hinder the escape of the silver in 

 case of any accident ; for example, should an explosion take place in the furnace ; 

 2, to reserve a magazine of litharge, should that still circulating round the silver cako 

 be suddenly absorbed by the cupel, for in this dilemma the litharge must be raked 



