COLOURS OBTAINED FROM METALLIC OXIDES. 105 



Lattly, I thought it would be advantageous to the progrefs Exhibition of 

 of the art, and that it was incumbent on the national inanufac- c J, oursatS £ vre8> 

 tory at Sevres to difclofe the pretended fecret of the compo- 

 sition of colours for porcelain which do not change in the fire. 

 Thefe colours were prefented to the Inftitute in the year 6, by 

 a manufacturer of porcelain, juftly admired for the beauty of 

 the works produced at his manufactory. I fliould not have 

 publiflned this fecret had it been given to me in confidence, 

 but as it is known, I hope I mail no longer be fufpe&ed of 

 the leaSl breach of integrity. 



From what has been faid, it will be feen that my object is 

 not to give t]ie exact composition of all the vitrifiable colours 

 At full length. Such a work could not be the Subject of a Sim- 

 ple memoir. 



It is known that metallic oxides are the bales of all vitrifia- Vitrifiable co- 

 ble colours ; but fome of the metallic oxides are not proper for ^metf oxides, 

 this ufe ; and as they are not vitrifiable without fome admix- 

 ture, they can feldom be employed alone. 



The very volatile oxides, and thofe that do not adhere Not thofe which 

 Strongly to the oxigen they contain in abundance, either can- ^wh^nge* 

 not be employed in any manner, as is the cafe with the oxides their oxigena- 

 of mercury and arfenic, or elfe they can be employed only as tlon# 

 agents. The colour they afford cannot be depended on, be- 

 caufe they lofe it, in proportion as they loCe their oxigen by 

 the flightefl heat. Such are the puce and red coloured oxides 

 of lead, the yellow oxide of gold, &c. 



Thofe oxides, in which the proportion of oxigen is capable 

 of being varied too eaSily, are feldom employed. The black 

 oxide of iron is never employed for that colour. The green 

 oxide of copper is in many circumstances very uncertain in its 

 effea. 



I have remarked that the oxides do not melt alone ; though Oxides cannot 

 when placed in thin layers on vitrifiable fubftances they can e a one » 

 be made to adhere by means of a violent fire ; but, with the 

 exception of the oxides of lead and bifmuth, they afford only 

 dull colours. The very violent heat which is frequently re- 

 quired to fix them, changes or totally destroys their colours. 

 Some flux is therefore added to all the metallic oxides. 



This flux is either glafs of lead and Silex, or glafs of borax, but require a 



c , ,, flux. Lead and 



or a mixture of both. filex> or ]ead> 



Its /ilex, and borax. 



