418 



SMELTING PEOCESSES 



of it, and instead of the immovable brick vault over the hearth a movable iron cover, which, by moans of an attach- 

 ment to a windlass, can be hoisted and moved to one side, as is the case with the cupollal .ion furnace. 



It is necessary to prepare the material for its hearth solo at least two days before. It must be sifted as fine as 

 possible and moistened with water. This mass is spread upon the bard burnt (ire brick, and after being pounded 

 hard is scraped into a basin form, the lowest point being in the middle, and the sole being here 1J to 2 inches in 

 thickness. Tu the centre of this basin a hemispherical pot is made in order to allow the silver to collect, and to render 

 the ladling out more easy. 



After all these preliminaries have been attended to, a small fire of coke is built upon the hearth in order to dry 

 it, whereupon the cover is put, on and luted with clay. Before the Blick Silver can bo placed in the furnace, the 

 hitter must be not only entirely dry, but shortly before this must have been brought to the highest possible melting 

 heal, as otherwise numerous cracks would appear in the hearth, and the silver would be more, difficult to melt. 

 The hearth must, therefore, be brought, to a bright red heat just before the Blick Silver is put in. 

 For this purpose, 8 or !) hours before the Introduction info the furnace of the Blick Silver, a light fire of stone 

 coal is kindled in the blast furnace, and this is gradually Increased until just before the Blick Silver is put in, the 

 furnace has reached the highest possible temperature. In placing the blocks of Bliok Silver into the furnace, care is 

 taken to bring them near the lire bridge, where they get the greatest amount of beat. If all has not, been put in at 

 once, fhi' remainder is added after the silver already in the furnace has been melted. The working door is thou 

 closed by an iron slab, and it, as well as all other apertures luted with clay, and the (ire increased. 



If after two or three hours the silver is melted, the heat, must, still be continued, and a, weak stream of air must 

 be admitted to the silver bath from the tuyeres on both sides of the lire bridge. From time to time the molten metal 

 is stirred up with the stirring iron, in order to bring all impurities which maybe contained in the silver to the 

 surface. Finely pulverized hearth-mass is strewn over the surfaoe, and aids in absorbing the liquefied oxides of 

 bismuth and lead. It is soraped from time to time out of the working door. After several hours, during 

 which the temperature has been maintained uniformly, and the molten metal well stirred, a. small quantity is taken 

 out in the assay spoon to ascertain bow far the silver is refined. Only when the fracture shows satin lustre and per- 

 fectly white color is the silver of the requisite fineness. Next to these appearanoes is that of the reflecting power of 

 the molten bath to be observed. When any of the implements are held over the surface of the silver, a perfect 

 image must be observed in the latter. Very often it is found that, though the reflection is perfect, the fracture is 

 gray and granular, a proof that the silver has absorbed too much oxygen from the blast. As the silver in this con- 

 dition is brittle, and not suitable for the purpose of coining, the absorbed oxygen must be got rid of by strewing 

 small lumps of coke of tin; si/.e of a walnut upon the surface, whereby the. oxygen is driven oil' in carbonic acid. As 

 ...on as it is evident that the silver is line, and can be drawn out without exhibiting cracks at the edges, it is sub- 

 jected to another high melting beat, in order to bring it, into the proper condition to ladle out. Before ladling out, 

 the blast is stopped, and a granular test, is taken—*'. «., a little of the molten silver is taken out in the assay spoon and 

 poured over the moistened ends of a brich broom into water, when it granulates. This is assayed to ascertain the 

 degree of fineness. This silver is given to the Director of the Works for assaying. After this the silver is poured 

 into iron pans holding 25 to :!() lbs., which must previously be coated with a, layer of clay, as must also the ladle 

 itself. The hitter is heated red hot before being dipped into the silver, to prevent particles of the precious metal 

 from adhering to it. 



In this manner 20-25 centners of blick-silver can be refined in ten to twelve hours, with an expenditure of eight to 

 nine bushels of slaty coal. 



If it is desirable to refine more silver, it can be introduced, into the same furnaces without changing the hearth. 

 After hulling out, the oxygenated condition of the silver shows itself in the phenomenon of spurting. Silver possesses 

 the curious power of absorbing oxygen in its molten state, and then suddenly giving it, up just a moment before it 

 congeals. This gives rise to a spurting of the silver, and to prove that during this phenomenon oxygen is really set 

 free from the silver, it is only necessary to strew coal dust on the surface of the metal. At, the, moment of the spurt 

 this coal takes fire, and burns up with great rapidity. The blocks of silver are then turned out of the moulds, and 



