60 LEAD 



continued, or practised at longer intervals, the hearth quickly gets too hot, and imme- 

 diately begins to agglutinate together; rendering evident the necessity of these 

 operations to the successful management of the process. It is not difficult to under- 

 stand -why these effects take place, -when it is considered, that in smelting by means 

 of the ore-hearth, it is the oxygen of the blast and of the atmosphere which principally 

 accomplishes the reduction ; and the point to be chiefly attended to consists in exposing 

 the ore to its action, at the proper temperature, and under the most favourable cir- 

 cumstances. The importance of having the ores free from impurities is also evident ; 

 for the stony or earthy matter it contains impedes the smelting process, and increases 

 the quantity of slags. A very slight difference of composition of perfectly-dressed ore 

 may readily be understood to affect its reducibility ; and hence it is, that ore from 

 different veins, or the same vein in different strata, as before observed, is frequently 

 found to.pvork very differently when smelted singly in the hearth. It happens, there- 

 fore, that with the best workmen, some varieties of ore require more coal and lime, and 

 a greater degree of heat than others ; and it is for this reason that the forestone is made 

 moveable, so as to answer for ore which works either with a large or a small quantity 

 of brouze. 



It has been stated that the duration of a smelting shift is from 1 2 to 1 5 hours, at the 

 end of which time, with every precaution, the hearth is apt to become too hot, and it 

 is necessary to stop for some time, in order that it may cool. At mills where the 

 smelting shift is 12 hours, the hearths usually go on 12 hours, and are suspended 5 ; 

 four and a half or five bings ' of ore (36 to 40 cwts.) are smelted during a shift, and the 

 two men who manage the hearth work each four shifts per week ; terminating their 

 week's work at 3 o'clock on Wednesday afternoon. They are succeeded by two other 

 workmen, who also work four 12-hour shifts ; the last of which they finish at 4 o'clock 

 on Saturday. In these eight shifts, from 36 to 40 bings of ore are smelted, which, 

 when of good quality, produce from 9 to 10 fodders 2 of lead. At other mills where 

 the shift is 14 or 15 hours, the furnace is kindled at 4 o'clock in the morning, and' 

 worked until 6 or 7 in the evening each day, six days in the week ; during this shift 

 5 or 5i bings of ore are smelted, and two men at one hearth, in the early part of each 

 week, work three such shifts, producing about 4 fodders of lead two other men work 

 each three shifts in the latter part of the week, making the total quantity smelted per 

 week, in one hearth, from 30 to 33 bings. 



Hearth-ends and Smelter's fume. In the operation of smelting, as already described, 

 it happens that particles of unreduced and semi-reduced ore are continually expelled 

 from the hearth, partly by the force of the blast, but principally by the decrepita- 

 tion of the ore on the application of heat This ore is mixed with a portion of 

 the fuel and lime made use of in smelting, all of which are deposited upon the top 

 of the smelting-hearth, and are called hearth-ends. It is customary to remove the 

 hearth-ends from time to time, and deposit them in a convenient place until the end 

 of the year, or some shorter period, when they are washed to get rid of the earthy 

 matter they may contain, and the metallic portion is roasted at a strong heat, until 

 it begins to soften and cohere into lumps, and afterwards smelted in the ore-hearth, 

 exactly in the same way as ore undergoing that operation for the first time, as already 

 described. 



It is difficult to state what quantity of hearth-ends are produced by the smelting of 

 a given quantity of ore, but in one instance the hearth-ends produced in smelting 9,751 

 bings, on being roasted and reduced in the ore-hearth, yielded of common lead 315 

 cwts., and the grey slags separated in this process gave, by treatment in the slag- 

 hearth, 47 cwts. of slag-lead ; making the total quantity of lead 362 cwts., which is 

 at the rate of 3 cwts. 2 qrs. 23 Ibs. from the smelting of 100 bings of ore. 



Slag-hearth. The various slags obtained from the different operations of lead 

 smelting are divided into two classes. Those which do not contain a sufficient amount 

 of metal to pay for further treatment are thrown away as useless, whilst those in 

 which the percentage of lead is sufficiently large are treated by the slag-hearth. 



Figs. 1335, 1336, represent a slag-hearth, the fourneau a manche (elbow furnace) 

 of the French, and the Krummofen (crooked furnace) of the Germans ; such as is used 

 at Alston Moor, in Cumberland, for the reduction of the lead-slag. It resembles the 

 pScotch furnace. The shaft is a parallelepiped, whose base is 26 inches by 24 inches in 

 area inside, and whose height is 3 feet ; the sole-plate a, of cast iron, slopes slightly 

 down to the basin of reception, or the fore-hearth, b. Upon both of the long sides of 

 the sole-plate there are cast-iron beams, called bearers, c, c, of great strength, which 

 support the side walls built of a coarse-grained sandstone, as well as the cast-iron 

 plate d (forestone), which forms the front of the shaft. This stands 7 inches off from' 

 the sole-plate, leaving an empty space between them. The back side is made of cast 



' 1 blng=8 cwts. 1 fodder=21 cwts. 



