60 



Dr. Stenhouse on Artificial Ultramarine. 



The last chemist who has examined Ultramarine is Dr. Eisner, who 

 has published a very elaborate paper upon it in the 23d number of 

 Erdmann's Journal for 1841. The first part of Dr. Eisner's paper is 

 historical, and contains an accounf of the accidental discovery of arti- 

 ficial Ultramarine by Tassart and Kuhlman in 1814, and of the labours 

 of subsequent chemists. He then gives a detailed account of his own 

 experiments, which have been very numerous, and from these he de- 

 duces the following conclusions: — 1st, That the presence of about one 

 per cent, of iron is indispensable to the production of Ultramarine ; he 

 supposes the iron to be in the state of sulphuret. 2d, That the green 

 Ultramarine is first formed, and that as the heat is increased it passes 

 by degrees into the blue. The cause of this change is, he affirms, that 

 part of the sodium absorbs oxygen from the atmosphere, as the opera- 

 tion is conducted in only partially closed vessels, and combines with 

 the silica, while the rest of the sodium passes into a higher degree of 

 sulphurization. Green Ultramarine therefore, contains simple sul- 

 phurets and blue, polysulphurets. 



Dr. Eisner's paper does not, however, furnish any details by which 

 Ultramarine could be manufactured successfully on the great scale. 

 Thus, for example, in regard to the necessary degree of heat, perhaps 

 the most important circumstance in the process, he gives no directions 

 whatever. We know, however, from other sources, that it should be a 

 low red heat, as at much higher temperatures both native and artificial 

 Ultramine soon become colourless. Dr. Eisner, indeed, does not affirm 

 that he was able to procure Ultramarine in quantity of a uniformly good 

 colour. In fact the process of Robiquet, published nearly ten years 

 ago, is the best which scientific chemists possess, though undoubtedly, 

 the manufacturers have greatly improved upon it. Robiquet's process 

 consists in heating to low redness a mixture of one part porcelain clay, 

 one and a half sulphur, and one and a half parts anhydrous carbonate 

 of soda, cither in an earthenware retort or covered crucible, so long as 

 vapours are given off. When opened the crucible usually contains a 

 spongy mass of a deep blue colour, containing more or less Ultramarine 

 mixed with the excess of sulphur employed, and some unaltered clay 

 and soda. The soluble matter is removed by washing, and the ultra- 

 marine separated from the other impurities by levigation. It is to be 



