LEAD 7.5 



late, by a damper, the heat of the larger apparatus, it is often found advantageous to 

 be able to produce a high temperature. A furnace of this kind should be connected 

 with a chimney of at least twenty feet in height, and be supplied with good coke, 

 broken into pieces of the size of eggs. 



The air furnace (see COPPEK, fig. 533), is well adapted for the assaying of lead ores. 

 The ores should be dried in shallow, flat metal vessels, powdered and sifted through 

 a sieve of from 40 to 60 holes to the linear inch, previous to being submitted to 

 assay. The balance employed should carry 1,000 grs., and turn with ith of a 

 grain. 



The tongs (see COPPER, fig. 536) may be used for the assay with earthen crucibles. 

 For working with iron crucibles tongs of stronger and stouter make are desirable. 

 The iron mould (Jig. 534) is used for receiving the assay products, or one containing 

 a deep conical cavity may be substituted for it. The fluxes should be employed in 

 the dry state. 



OEES OF THE FIRST CLASS. The assay of ores of this class is a simple operation, 

 care being only required that a sufficient amount of carbonaceous matter be added to 

 effect the reduction of the metal, whilst such fluxes are supplied as will afford a 

 readily-fusible slag. When the sample has been properly powdered, 400 or 500 

 grains are weighed out, and well mixed with 500 or 600 grains of carbonate of soda, 

 and from 40 to 60 grains of finely-powdered charcoal, according to the richness of the 

 mineral operated on. This is introduced into an earthen crucible, of such a size as 

 not to be more than one-half filled by the mixture, and on the top is placed a thin 

 layer of the flux. The crucible is then placed in the furnace and gently heated, care 

 being taken to so moderate the temperature, that the mixture of ore and flux, which 

 soon begins to soften and enter into ebullition, may not swell up and flow over. If 

 the action in the crucible becomes too strong, it must be checked by a due regulation 

 of the heat by means of a damper. When the action has subsided, the temperature 

 is again raised for a few minutes, and the assay completed in about half an hour. 

 During the process of reduction, the heat should not exceed dull redness ; but in order 

 to complete the operation, and render the slag sufficiently liquid, the temperature 

 should bo raised to bright redness. When the contents have been reduced to a state 

 of tranquil fusion, the crucible must be removed from the fire, and the assay either 

 rapidly poured, or, after being tapped against some hard body to collect the lead in 

 a single globule, be set to cool. When the operation has been successfully conducted, 

 the cooled slag will present a smooth concave surface, with a more or less vitreous 

 lustre. When cold, the crucible may be broken, and the button extracted. To remove 

 from it the particles of adhering slag, it is hammered on an anvil, and afterwards 

 rubbed with a hard brush. Instead of empleying carbonate of soda and powdered 

 charcoal, the ore may be fused with 1 times its weight of black flux, and the 

 mixture covered by a thin layer of borax. Good results are also obtained by mixing 

 together 400 grains of ore with an equal weight of carbonate of soda and half that 

 quantity of crude tartar. These ingredients, after being well incorporated, are placed 

 in a crucible, and slightly covered by a layer of borax. Each of the foregoing 

 methods yields good results, and affords slags retaining but a small proportion of 

 lead. 



OBES OF THE SECOND CLASS. This class comprehends galena, which is the most 

 common and abundant ore of lead, and also comprises sundry metallurgic products, as 

 well as the sulphates and arsenates of lead. 



The assay of galena and other substances is variously conducted ; but one of the 

 following methods is usually employed for commercial purposes : 



Assay in an earthen crucible with metallic iron. Mix 400 or 500 grs. of the ore 

 with about an equal weight of carbonate of soda, and, after having placed it in the 

 earthen crucible, of which it should occupy about one half the capacity, insert with 

 their heads downward threo or four tenpenny nails, and press the mixture firmly 

 around them. On the top place a thin layer of borax. The whole is now introduced 

 into the furnace, and gradually heated to redness ; at the expiration of ten minutes 

 the temperature is increased to bright redness, and the heat continued until the fused 

 fluxes present a perfectly smooth surface. When this has taken place the pot is 

 removed from the fire, and the nails are separately withdrawn by the use of a small 

 pair of tongs, care being taken to well cleanse each in the fluid slag until free from 

 adhering lead. When the nails have been thus removed, the pot is gently shaken, to 

 collect the metal into one button, and laid aside to cool; after which it maybe broken, 

 and the button removed. Instead of first allowing the slags to cool, and then breaking 

 the crucible, the assay may, if preferred, after the withdrawal of the nails, be poured 

 into a mould (see COPPER, fig. 534). Hoop iron or iron rod may be used, instead of 

 the iron nails. The assay should be completed in from 25 to 30 minutes. 



Assay in an iron pot with fluxes. Instead of adding metallic iron to the mixture of 



