344 O n lb* Colours obtained from the metallic Oxides, 



bafis metallic oxides ; but all the metallic oxides are not 

 proper for this purpofe : befides, as they are not verifiable 

 by themfelves, they can fca.rcely ever be employed alone. 



Highly volatile oxides, and thofe which adhere little to 

 the great quantity of oxygen they contain, either cannot be 

 employed in any manner, as the oxide bf mercury and that of 

 arfenic, or are employed only as agents. 1 he colour they 

 prefent cannot be depended on, fince they mud lofe it in the 

 flighted heat by lofing a part of their oxygen : fuch are the 

 puce-coloured and red oxides of lead, the yellow oxide of 

 gold, &c. 



Oxides in which the proportions of oxygen are fufceptible 

 of varying with too much facility are rarely employed : the 

 oxide of iron, though black, is never employed for that co- 

 lour; and the green oxide of copper is, under many circum- 

 ftances, very uncertain. 



I have faid that oxides alone are not fufceptible of fufion : 

 however, as they are deftined to be applied in thin (irata on 

 vitrifiable fubftances, they may be attached to them by a 

 violent heat. But, except the oxides of lead and bifmuth, 

 they would give only dull colours. The violent heat, often 

 necefTary to fix them, would change or totally deltroy the 

 colours. A flux then is added to all metallic oxides. 



This flux is glafs, lead, and filex ; glafs of borax, or a mix- 

 ture of both. 



Its general erTecl: is, to give fplendour to the colours after 

 their fufion; to fix them on the article which is -painted, by 

 promoting more or lefs the foftening of its furface; to en- 

 velop the metallic oxides, and to preferve their colour by 

 fheltering them from the contact of the air : in a word, to 

 facilitate the fufion of the colour at a low temperature not 

 capable of deftroying it. 



The obfervation which proves the latter ufe of fluxes is 

 taken from the delicate colours, fuch as carmine obtained 

 from gold : thefe colours require much more flux than others. 

 Metallic oxides are employed fometimes dire&ly, and merely 

 mixed with their flux, without being previoufly fufed with it: 

 thefe colours are thofe which a ftrong heat, or heat too often 

 repeated, would alter. It may be readily conceived that a 

 ftronger and longer continued heat is neceflary to fufe a cru- 

 cible full of coloured glafs, than a ftratum of colour not the 

 tenth part of a millimetre in thicknefs. 



I (hall return to this fubjccl when I come to fpeak of the 

 reds obtained from gold. 



In many circumstances, oxides are fufed previoufly with 



their 



