COLOURS OBTAINED FROM METALLIC OXIDES. ]()£ 



this body, fometimes change in the baking, and acquire a great 



tranfparence. They are generally beautiful only when placed 



between the eye and the light, and then they anfwer the pur- 



pofe intended in painting on glafs. 



There is more difficulty in baking plates of coloured glafs Difficulties of 



than is commonly thought. The bending of the piece, and * at ™f s,afs 



alteration of the colours, are to be avoided. All the treatifes, 



We have confulted recommend the ufe of gypfum. 'fhis me- On the ufual 



thod fometimes fucceeded, but generally the glafs became [3°tne alK" 



white, and cracked in all directions. It appears that the glafles f the glafs is at- 



that are too alkaline, and which are far the mod common in ta . cked » and the 



piece broken, 

 clear white glaiTes, are attacked by the hot fulphuric acid of 



the fulphate of lime. • We were able with eafe to bake much Porcelain bifcuit 



larger glaffes than any before painted, by placing them on very j? ' _^ better 



finooth plates of earth or unglazed porcelain. 



Concerning thefeveral particular Colours. 



After having collected the general phenomena which each Particular co- 

 clafs of verifiable colours offers with regard to the bodies on ours * , » 



which they are placed, I muft flaew the particular and moft in- 

 terefting phenomena which every principal fpecies of colours 

 employed on tender porcelain, on glafs, and in the fire that 

 bakes the porcelain, prefent. 



Concerning the Reds, Purples, and Violets obtained from Gold. 



The carmine red is obtained from the purple precipitate of Carmine re* 

 Caffius. It is mixed with about fix parts of its flux; and this from gold * 

 mixture is directly employed without being firfl fufed. It is 

 then of a dirty violet, but acquires the beautiful carmine by- 

 baking. It is however very delicate ; a little too much heat is very perifli- 

 or carbonated vapours eafily fpoil it j yet it is more beautiful 3 emthe fire > 

 when baked with charcoal than with wood. 



This colour and the purple which differs little from it, as as well as its 

 well as all the fhades which are obtained from their mixture with 

 other colours, really change in all porcelains and in the hands 

 of all operators. But this is the only one that changes on hard It is the only co- 

 porcelain. It may be replaced by a fubftitution of rofe colour n U hard tchanse$ 

 from iron, which does not change; fo that by excluding fromlainj 

 the pallet the carmine made from gold, and fubftituting the 



rofe- 



