104* COLOURS OBTAINED FROM METALLIC OXIDES. 



The flux fixes Its general efTed is to give brilliancy to the colours after 



the colour at a , 1 • r r n . ° . J 



moderate heat ; tneir tuiion, to fix them upon the piece that is painted by 

 lefs than would fattening its furface, to envelope the metallic oxides, to pre- 

 deftroy the co- fefve the ; r colour ^ defending them from the conta6l of the 



air, and particularly to facilitate the fufion of the colour at a 

 flightly elevated temperature, not capable of deftroying it. 



The obfervation which proves this latter ufe of folvents is 

 taken from delicate colours, fuch as the carmines produced 

 from gold ; thefe colours require a much greater proportion of 

 flux than the others. 

 Very tranfient Sometimes the oxides are dire&Iy employed, and fimply 

 mixed* with the m * xed wit ' n their folvents, without having been previoufly 

 flux without melted with it ; fuch are the colours which the violence or re- 

 previous fufion. petition of heat wouId a]ten It is eafy tQ conceive that a 



ftronger and more lading heat is required to fufe a crucible of 

 coloured glafs than a layer of colour, which is not the tenth of 

 a millimetre in thicknefs. 



I (hall return to this fubject when treating of the red colours 

 obtained from gold. 

 Others are fufed In many cafes the oxides are previoufly fufed with their fol- 



"rotnd ^ ^^ VentS ' and afterwards ground. When I fpeak of colours in 



particular, I ftiall mention thofe which are fubje&ed to this 



fufion. 



Thefe general principles are too fimple to require further 



explanation. 

 This treatife is I fliall only fpeak of the application of metallic colours to 

 confined to vi- vitreous bodies or furfaces. 



on elazes or Thefe bodies may be divided into three very diftinct clafFes, 



glafs. from the nature of the fubftances that compofe them, the ef- 



Thefe grounds feels produced on them by the colours, and the changes they 

 are three. undergo. Thefe are, 



i. Enamel. V& Enamel; foft porcelain, and all the glazes, enamels, 



or glafles, which contain lead in any confiderable quantity. 

 2. Hard porce- 2< ^« Hard porcelain, or fuch as is glazed with feldfpar. 

 lain. 3d. Glafs in which there is no lead, fuch as the common 



^Common window glafs . 



I fhall fucceflively examine the principles of compofition of 

 thefe colours, and the general phenomena they prefent on 

 thefe three grounds or fupporters. 



Colours 





