Green and Blue Ultramarine. 539 



From the increasing production of ultramarine, and its im- 

 portance in trade and industry, it appeared interesting to explain 

 by chemical analysis the nature of the blue compound; and by 

 the kindness of one of the most celebrated vdtramarine makers 

 in Germany, I obtained very beautiful samples of ultramarine, 

 blue as well as green. 



Porcelain clay, or a similarly constituted artificial silicate, is 

 mixed with soda and sulphur, and ignited without access of air 

 until the mass has caked together ; it is then ground and washed. 

 The resulting powder is cither again ignited with soda and sul- 

 phui-, or gently heated with access of air ; the blue colour then 

 appears. 



The different sorts of blue ultramarine exhibit different phy- 

 sical properties, while their chemical relations are the same. 

 The colour varies from a delicate cserulean to a fiery dark blue 

 with a tinge of red. The lighter sorts form a compact, dense 

 powder ; the darker kinds are looser and velvety. Green ultra- 

 marine has no fiery colour ; its shades vary from apple-green to 

 blue-green. 



Ultramarine is not moistened by water, but it is by alcohol, 

 even greatly diluted. When treated with water, some sulphate 

 of lime is dissolved ; but neither a sulphite, hyposulphite, nor a 

 sulphide can be detected in the solution. 



Sulphuretted hydrogen is evolved when ultramarine is treated 

 with acids ; even with dilute acetic acid the colour disappears 

 more or less rapidly. The most beautiful shades are the soonest 

 decomposed. ]\Iany ultramarines resist the action of acids more 

 energetically than others ; the green are the easiest decomposed. 



If strong hydrochloric acid in excess be poured on ultra- 

 marine, an odour is evolved which irritates the eyes, similar to 

 that observed in the preparation of polysulphuretted hydrogen 

 when polysulphide of calcium is treated with excess of strong acid. 



When ultramarine is decomposed by acid, a whitish gelatinous 

 fluid is obtained, which does not filter clearly. The cloudiness 

 arises from finely divided sulphur; it is more decided in the 

 blue than in the green ultramarine, and shows the presence of a 

 higher sulphide which is present in ultramarine, and contributes 

 essentially to the colour. Sulphur, clay, and silica remain on 

 the filter ; the filtrate contains the chlorides of aluminium, iron, 

 and sodium, and sulphate of lime. 



When strongly ignited in air, both ultramarines lose their 

 colour, which becomes first dull and dirty, and then quite disap- 

 pears. If green ultramarine be heated with pentasulphide of 

 sodium, and the mass washed out and gently heated in the air, 

 it bccomea blue. The reason of this change will be found in the 

 different constitution of blue and green ultramarine. 



