CHEMICAL SCIENCE. 198 



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of cobalt, the ore is wetted and spread over the sole of the reverberatory 

 furnace, in a layer five or six inches deep : it is then cautiously heated for 

 six hours ; when the ore becomes red-hot the operation ends. The 

 eand which was separated in the dressing is sometimes mingled in 

 certain proportions with the ore in roasting, and the product thus ob- 

 tained is the zaffre or saffre of commerce, a crude product. Smalt, on 

 the contrary, is a valuable and carefully prepared vitreous compound, 

 a rich blue glass in fact, to be afterwards reduced to powder and elab- 

 orated in the manner now described. Silica and potassa, both very 

 carefully prepared, calcined, sifted, and preserved from moisture, are 

 mixed with oxide of cobalt to form smalt, the proportions varying 

 according to the commercial variety of the article required. The in- 

 gredients are intimately mixed and then transferred to the melting 

 pots, which are built up in a furnace heated to the proper temperature, 

 each pot being first charged with an inferior blue glass in powder 

 called eschcl, the effect of which is to give an interior vitreous lining to 

 the pots. The smalt mixture is poured into the pots by means of iron 

 ladles with long handles, and in about eight hours it fuses. "When the 

 pots appear at white heat, their contents are quite fluid, and the chem- 

 ical combination of the materials has been effected. The pure glass is 

 taken from the glass-pot in iron ladles, and, as the object of subsequent 

 processes is to reduce the glass to powder, that process is facilitated by 

 emptying the ladles into vessels of water, the water being constantly 

 renewed. The glass being at a red heat when it first comes in contact 

 with the water, is thus rendered, like Prince Rupert's drops, exces- 

 sively brittle, granular, and easy to pulverize. The next process in the 

 manufacture of smalts is the apparently simple one of reducing the 

 blue glass to powder. But if we try the experiment of grinding to 

 powder a portion of blue glass, we shall find that the substance, which 

 by transmitted light had appeared so beautiful, is reduced, in its dis- 

 integrated state, to a light dingy powder ; yet who can doubt that the 

 same amount of coloring matter is present in the powder as in the 

 glass ? Therefore there are difficulties to be overcome in converting a 

 sheet of blue cobalt glass into a powder of an intensely blue color, and 

 in obtaining all those shades and varieties of blue which are found in 

 our manufactures. The glass, after being crushed and sifted to the size 

 of ordinary sand, is transferred to lar^e vats full of water, and, in the 

 course of a very few minutes, a separation of particles takes place in the 

 powder. The heaviest, being those which are richest in cobalt, sink 

 to the bottom, and this deposit constitutes one of the commercial vari- 

 eties of smalts, known as azure, coarse blue and streablau. The water 

 which holds the finer particles of the powdered blue glass in suspension, 

 is drawn off into other vats, where it is allowed to subside for three 

 quarters of an hour or more, according to the variety of smalt intended 

 to be produced ; this second deposit is called farbc, the German word 

 for color. The water drawn off from this second deposit is poured into 

 vats and allowed to remain for an indefinite time ; its deposit is called 

 escliel or blue sand. But the colors thus obtained are all again sub- 

 jected to the action of water before they are fit for the market. The 

 glass of cobalt apoears to be a mixture of the less fusible silicates, in 



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