874 COBALT BLUE 



being frequently stirred with an iron rod in order to render it perfectly homogeneous ; 

 when this point is attained, the stirring is discontinued, and the speiss is allowed to 

 settle. In 6 or 8 hours, the glass is sufficiently fluid, and it is then ladled out with 

 iron ladles, and thrown into water. After the contents of two pots have been 

 removed, the working doors are closed, and the fire is urged for some time, in order to 

 bring the contents of the remaining pots which have somewhat cooled down to a 

 proper degree of fluidity. 



Grinding the smalts. The blue glass, rendered brittle by sudden cooling, is stamped 

 or crushed, sifted, and ground wet, under hard stones. The finely-divided glass 

 from the mill is then passed through a system of washing-vats ; the first three give a 

 coarse sand which is usually returned to the mill for further grinding, the finer slime 

 is allowed only a few minutes to deposit in No. 4, about a quarter of an hour in No. 

 5, then it filters more slowly through No. 6, and finally into a slime-pit, where the 

 whole of the suspended matter is deposited. The darkest coloured smalts, known as 

 King's blue or azure, are found in vats Nos. 4 and 5. No. 6 gives the lighter coloured 

 tub smalts. The pale blue powder from the slime-pit is mostly returned to the melting 

 furnace. 



SiO." Al a O. FeO. CoO. KO and NaO. 



I. Norwegian high coloured . 70*9 0*4 0'3 6'5 21-4 /"with some car- 

 II. German pale coloured . . 72-1 1'8 1-4 2-0 20'0 \bonateoflime. 



Smalt is therefore a kind of glass coloured by oxide of cobalt. One part of oxide 

 of cobalt communicates a distinctly bluish tinge to 240 parts of glass ; with more 

 than 18 per cent, the glass becomes black. In practice a potash-glass is always 

 employed, and hence smalt may be regarded as a double silicate of potash, and prot- 

 oxide of cobalt. Soda cannot be substituted for the potash, as it produces an inferior 

 colour, and the same remark applies to some other oxides. The following are the 

 principal facts observed with regard to the influence of the oxides of other metals 

 on the colour of smalts. Baryta heightens the tone, giving an indigo tinge ; soda, 

 lime, and magnesia, have a decidedly lowering effect, and produce a reddish shade ; 

 alumina has no effect on the purity of the colour, but lowers its intensity ; iron pro- 

 duces a blackish-green dull tinge, which is very prejudicial to the higher coloured 

 varieties ; manganese communicates a violet shade, as does also nickel, but in a much 

 higher degree. Copper, zinc, bismuth, and antimony, all give a dull dirty shade, 

 especially to the higher colours. Oxide of lead has no effect on the colour, but as it 

 increases the specific gravity of the glass, and causes the ground smalts to subside 

 too rapidly in the washing vats, producing an incomplete separation, its presence in 

 the mixture is not desirable. 



In commerce, smalts are classified both according to their contents in cobalt, and 

 the size of the grain, the following being the chief marks : 



F.C. Fine colour. F.C.B. Fine Bohemian colour. F.E. Fine Eschel. 



M.C. Middling colour. M.C.B. Middling Bohemian colour. M.E. Middling Eschel. 

 0.0. Ordinary colour. O.C.B. Ordinary Bohemian colour. O.E. Ordinary Eschel. 



In the above scheme, the words fine, middling, and ordinary, indicate the relative 

 quantities in cobalt, while colour, Bohemian colour, and Eschel, express the state of 

 division. The two former are composed of angular fragments, while tho latter are 

 the finest-rounded grains deposited in the lowest washing vats and the slime-pits. 

 Smalts that are darker in colour than F. are indicated by multiples of that letter, 

 the highest being FFFFC ; while, on the other hand, those that are lower than are 

 distinguished by exponents following the second letter, thus OC 2 , 00* contain respec- 

 tively only J and as much cobalt as 0.0. 



Zaffre, or Safflor, is the name given to a mixture of roasted ore and quartz, similar, 

 in fact, to that employed in smalt-making, but without potash. It is used either for 

 pottery purposes, or for bringing up the colour in smalts made from low-classed ores. 



Formerly smalts were used to a great extent in colouring paper, but at present 

 artificial ultramarine is almost exclusively used for this purpose (see ULTBAMABINE). 



For an account of the method of assaying cobalt ores, see NICKEL. 



The imports of cobalt can no longer be given ; the Custom-house authorities now 

 returning the cobalt ores as ' Ores unenumerated.' 



COBAliT BXiOOMi Hydrous arsenate of Cobalt commonly occurring as a 

 peach-blossom coloured incrustation on arsenical ores of cobalt. It is also found in 

 crimson or peach-red crystals, and is known mineralogically as Erythrine. When 

 abundant it is used in the preparation of cobalt colours. See COBALT. 



COBAliT BXiTTE, or Thenard's Blue, is prepared by precipitating a solution 

 of sulphate or nitrate of cobalt by phosphate of potash, and adding to the resulting 



