392 PURPLE DYEING, ANCIENT AND MODERN. 



sion of air, and wlicn it ceases to glow we have still sulphuric acid present, and 

 the green color is thus self-changed into a beautiful blue. 



As to this process also of transmuting the green color into blue the French 

 and Germans have their peculiar methods. The Germans use t:5mall iron cyl- 

 inders for roasting ; the French small hearth ovens, into which, however, the 

 flame cannot enter. Hitherto cylinders of potter's clay have not been adopted, 

 though we doubt not that they would serve just as well, and be even more 

 durable. The cylinder being filled with from twenty-five to thirty pounds of 

 green ultramarine, a vane (Flugelvvelle) is set in motion so that the contents of 

 the cylinder may not be burnt without being first thoroughly roasted within. 

 A pound of sulphur is now passed through an upper opening into the cylinder, 

 and while the wind-vane continues in motion the sulphur is gradually consumed. 

 The addition of sulphur may be continued as long as the color improves in 

 purity and brilliancy, but care must be taken not to continue it too long. After 

 the color has been thus roasted it must once more be washed, dried, ground, 

 and sifted. 



The French method of roasting possesses this advantage, that, by allowing 

 a freer accession of air, the green mass is the more speedily transmuted into 

 blue. But, on either the French or German method, a large quantity of sul- 

 phuric acid escapes, which renders the factory a nuisance to its neighbors, 

 while, were that quantity of sulphuric acid preserved, it would suflice for the 

 production of all the Glauber salt used in the manufacture. 



The quality of the green color is the rule and test of that of the blue ; but 

 something of its intensity also depends upon the manner in which it is ground — 

 the finer it is powdered the brighter and clearer it becomes. But not all kinds 

 ai'e of equal beauty ; lighter tones of color are frequently obtained without there 

 having been any appreciable difference in the mixture of the raw materials. 

 From these lighter and darker shades a medium kind is obtained by mixing 

 them together, and adding other white or light materials. Where this admix- 

 ture is resorted to, equal tones of color are out of the question ; the shades vary 

 from the softest sky blue to a glowing, almost ruddy, dark blue — the former 

 generally forming a more compact powder, the latter a more loose and smooth 

 one. 



The principle upon v/hich the blue color of the ultramarine is dependent is 

 as undecided now as it was in the time of Gmelin. It is much to be regretted 

 that his analysis of the Lapis lazuli, which so much conduced towards the manu- 

 facture of artificial blue ultramarine, has not been repeated and followed up. 

 The foundation which he laid in scientific experiment has been built upon only 

 in the way of the merest empiricism; and the success Avhich has thus, in a 

 merely monetary point of view, been obtained by the manufactures, has led 

 not a few of them to imagine — how vainly we need not say — that henceforth 

 they are quite independent of science. They forget that practical men, how- 

 ever ably they may profit by what science has taught them, do literally nothing 

 towards clearing up what science itself has yet to learn. It was the science of 

 Gmelin which alone laid the foundation for the manufactui'e of ultramarine as 

 it at present exists ; but who shall ])retend to limit the improvements that 

 might be made in that manufixcture could another Gmelin arise to discover 

 the principle on which the coloring of the ultramarine depends? Attempts 

 have, indeed, often been made to lift the veil from this mystery, but hitherto 

 they have been so made that it was impossible for them to succeed. Analysis 

 has followed analysis, regardless of the fiict that the ultramarine trade is 

 not a prc|Daration of determinate composition from which imiform results call 

 be obtained. However accurately the operator may have treated the clay 

 with water and sulphur, does not the color imbibe some portion of silicious 

 matter 1 Nay, has not each specimen of clay dificrent elements and different 

 proportions of elements in its own composition 1 How are we to tell, even 



