COBALT 



873 



486 



system of flues, 6, b, in the direction shown by the arrows, into a chamber where 

 they are deposited, and may be subsequently used in the manufacture of arsenical 

 products. The charge is from 3 to 5 cwts. ; the degree to which the roasting is 

 carried varies with the composition of the ores. When many foreign metals are 

 present, they must not be roasted ' dead ' or ' sweet,' but sufficient arsenic or sulphur 

 must be left to form with the less oxidisable metals, more especially copper and 

 nickel, a speiss or regulus which separates from the blue glass produced in the fusion 

 process, and subsides to the bottom of the melting pot. The oxides of those metals 

 that are more easily oxidisable than cobalt, such as iron and bismuth, may be partially 

 removed by an addition of metallic arsenic in the fusion, whereby the latter oxide is 

 reduced and passes into the speiss with the production of arsenious acid, which tends 

 to heighten the colours produced. Earthy cobalt ores containing much iron, require 

 an addition of arsenious acid, in order to convert the protoxide of iron, which has a 

 strong colouring power, into peroxide, which is of less consequence. In some works, 

 the roasting is performed in muffle furnaces, the hearth being heated from below, so 

 that the ore does not come into contact with the flame. Ores containing bismuth are 

 subjected to the process of liquation previously to roasting. 



When the ores are very impure, 

 or when it is wished to produce 

 very deep blue colours, they are 

 subjected to the process of concen- 

 tration. Common ores, and those 

 that are rich in copper, are first 

 slightly roasted, and then fused in 

 crucibles in the smalt furnace de- 

 scribed below, with broken glass 

 fragments, charcoal dust, and car- 

 bonate of potash. When the mix- 

 ture is in a state of complete 

 fusion, the slag is partly removed 

 at intervals, and fresh quantities 

 of ore are added, until a sufficient 

 quantity of speiss is produced. In 

 this process, the oxides of iron and 

 copper in the ore form an easily 

 fusible slag with the glass, while 

 the cobalt is reduced by the char- 

 coaj to the metallic state, and unites 

 with the arsenic to form a speiss. 

 If the roasting is carried too far, 

 or if there is an insufficiency of 

 arsenic or of reducing matters in 

 the mixture, a certain amount of 

 oxide of cobalt will be taken up 

 by the slag. The concentrated 

 speiss produced is then pulverised 

 and roasted ' sweet,' leaving a 

 nearly pure basic arsenate of oxide 

 of cobalt. 



The smalt furnace, figs, 486 and 

 487, is similar in construction to 

 that used for melting flint glass. 

 The fire-place, in which wood is 

 burnt, is a long narrow rect- 

 angular chamber, with a vaulted floor perforated by draught holes, d, d ; c, the lower 

 chamber, is the ash-pit; the flame passes through the hole, a, into the melting 

 chamber above, which is a low domed structure, containing six melting pots; /,/,/, 

 are the working doors, they are faced with cast-iron frames in the ordinary way. 

 The waste flame, after heating the pots, passes through the small flues, i, i, into a 

 system of chambers, where it is employed in the accessory operations of calcining 

 quartz, or drying wood for fuel, h, h, are arches through which the pots are inserted 

 or removed, they are built up when the furnace is at work. At e, e, e, are small 

 holes, corresponding with similar ones in the pots, for removing the speiss formed 

 during the fusion. The charge consists of roasted ore, with from 1 to 16 times 

 its weight of quartz previously calcined and reduced to a fine powder, and carbonate 

 of potash to the extent of about f ths or *ths of the amount of the quartz. Each 

 pot contains about 3 cwts. ; the fusion takes place at a low temperature, the glass 



487 



