412 Account of certain Colours dug up at Pompe'id. 



Ammonia has no action on it. When exposed to the 

 flame of the blow-pipe, it becomes black, and forms a.Jritfe 

 of a reddrsh-brown colour, by the prolonged action of the 

 flame. When melted by the blow- pipe with borax, it gives 

 a grtenish-blue glass. 



When treated with potash on a platina saucer, it pro-" 

 duces a greenish fritte, which first becomes brown, and 

 ends in assuming the metallic colour of copper. This fritte 

 partly dissolves in water : the muriatic acid when poured 

 into the solution forms an abundant flaky precipitate, and 

 the liquor, when decanted from above the first precipitate, 

 furnishes a considerable additional quantity with oxalate of 

 amiTionia. 



The nitric acid dissolves with effervescence the residue 

 left undissolved by the alkali ; the solution is coloured 

 green. Ammonia forms a precipitate in it, which it re- 

 dissolves when we pour it in excess, and the solution thea 

 becomes blue. 



This colour therefore seems to be composed of oxide of 

 copper, lime, and alumine; it resembles Saunders blue in 

 the nature of its principles, but diflfers from it in its che- 

 inical properties; it seems to be the result, not of a preci- 

 pitaiion, but the effect of a commencement of vitrification, 

 or railier a ixne fritte. 



The process by which the ancients obtained this colour 

 seems to be now lost : all that we know, from consulting 

 the annals of the arts, is, thai the use of this colour is of 

 a more ancient date than the destruction of Pompeia. 

 M. Descostils has rematked a lively shining and glass blue 

 on the hieroglyphical paintings of an Egyptian monument, 

 and he ascertained that this colour was owing to the pre- 

 sence of copper. 



Setting out from the nature of the constituent principles 

 of this colour, we can compare it to nothing so well as the 

 Saunders blue of the moderns : when we consider it with 

 respect to its utility in the arts, we may with advantage 

 oppose it to idtramarinc and azure, particularly since 

 M. Thenard has made known a preparation of the latter, 

 which admits of its being employed with oil. But modem 

 Saunders blue has neither the lustre nor the solidity of co- 

 lour conspicuous in that of the ancients ; while azure and 

 ultramarine are much higher in price than a composition 

 the three elements of which are of little value. It would 

 therefore be of some importance to discover the processes 

 by which this blue colour was prepared. 



No. 6. is a pale-blue saad, mixed with some small 



whitish 



