LEAD 55 



become sufficiently fused, they are, by means of rakes and paddles, made to assume 

 the required form. The charge employed, as before stated, varies in almost every 

 establishment. In the North, however, smaller charges are used than in most other 

 localities. At Newcastle, and in the neighbourhood, the charge varies from 12 to 14 

 cwts. ; in Wales, and near Bristol, 21-cwt. charges are treated ; whilst in Cornwall, 

 charges of 30 cwts. are not unfrequently worked. The time required for smelting a 

 charge varies with its weight and the nature of the ores, from 6 to 24 hours. 



In some cases the ore is introduced raw into the furnace, whilst in others it under- 

 goes a preliminary roasting previous to its introduction. Rich ores are generally 

 smelted without being first calcined ; but the poorer varieties, and particularly those 

 which contain large quantities of iron pyrites, are, in most instances, subjected to 

 roasting in a separate furnace. 



In order to understand moro clearly the operation of smelting in furnaces of this 

 description, we will suppose that a charge has just been tapped off, and that, after 

 thoroughly clearing the hearth, a fresh charge of raw ores has been introduced. 

 During the first part of the operation of roasting, which usually occupies about two 

 hours, the doors are taken off to admit free access of air, and also for the purpose of 

 cooling the furnace, which has been strongly heated at the close of the preceding 

 operation. No fuel is at this period charged upon the grate, since the heat of the 

 furnace is of itself sufficient to effect the elimination of the first portions of sulphur. 

 The ore is carefully stirred, for the purpose of constantly presenting a fresh surface 

 to oxidising influences, and when white fumes are no longer observed to pass off in 

 large quantities, a little coal may be thrown on the grate, and the temperature gradu- 

 ally elevated until the charge becomes slightly clammy and adheres to the rake. 

 When the roasting is considered as being sufficiently advanced, the smelter turns his 

 attention to the state of the fire, taking care to remove the clinkers and get the grate 

 into proper condition for the reception of a fresh supply of fuel. The furnace doors 

 are now closed, and a strong heat is kept up for about a quarter of an hour, when the 

 smelter examines the condition of his charge by removing one of the doors. If the 

 operation is progressing satisfactorily, and the lead flowing freely and passing without 

 obstruction into the tap, the firing is continued a little longer ; but when the ores 

 have been found to have taken fire, or are lying unevenly on the bottom of the fur- 

 nace, the position of the charge is changed by the use of an iron paddle. During 

 this operation the furnace becomes partially cooled, and the reduction of temperature 

 thus obtained is frequently found to produce decompositions, which facilitate the re- 

 duction of the charge. In the case of extremely refractory ores, this alternate heating 

 and cooling of the furnace is sometimes almost indispensable, whilst, in other in- 

 stances, their being once or twice raked over is all the manipulation that is required. 



We will suppose that four hours have now elapsed since the charging of the fur- 

 nace, and that the charge has run down the inclined sole towards the tap. The 

 smelter now examines the condition of the scorise, and adds a couple of shovelfuls of 

 lime and three or four shovelfuls of small coals, the amount and relative proportions 

 of these being regulated in accordance with the aspect of the slags. The charge is 

 now, by means of proper tools, again raised to the breast of the furnace, and the firing 

 continued until the charge has run down into the tap-hole. The foreman now takes 

 his rake and feels if any lumps remain in an unfused condition, and if he finds all to 

 be in a fluid state he calls his assistant from the other side, and by the addition of a 

 small quantity of lime and fine coal, makes the slag assume a pasty or rather doughy 

 consistency. By the aid of his paddle he now pushes this compound up to the oppo- 

 site side of the furnace, where it is drawn by an assistant through the back door into 

 a trough containing water. Whilst the assistant is doing this, the foreman is busily 

 engaged in tapping off the metal into the iron pan in front of the furnace, from which, 

 when sufficiently cooled, it is laded out into suitable moulds. 



The total duration of the operation may be about six hours. 



To build a furnace of the above description, 5,000 common bricks, 2,000 fire-bricks, 

 and 2J tons of fire-clay are required. In addition to this, must be reckoned the iron- 

 work, the expense of which will be much influenced by the nature of the armatures 

 employed and the locality in which the furnace is constructed. 



The amount of fuel employed for the treatment of a ton of lead ore varies not only 

 in relation to the richness of the mineral, but is also much influenced by the nature of 

 the associated matrix and the calorific value of the fuel itself. The loss of metal ex- 

 perienced during the operation is mainly dependent on the richness of the ore treated 

 and the skill and attention of the foreman. 



In the North aboiit 12 cwts. of coal are consumed in the elaboration of 1 ton of 

 ore, and the loss of metal on 60 per cent, ore may be estimated at about 12 per cent., 

 of which aboxit 6J- per cent, is subsequently recoA'ered from the slag and fumes. At a 

 well-conducted smelting works, situated in the west of England, in which the average 



