54^ On the Colours oltahtedfrom the metallic Oxides, 



others, becaufe the glafs on *hich they arc applied become* 



fufficieiuly loft to be penetrated by them. 

 ' This flux may be either glafs of lead and pure filex, called 

 rocail/t, or the fame glafs mixed with borax. 



Montamy aflerts that glafs of lead ought to be banifhed 

 from among the enamel fluxes; and he employs only borax, 

 lie then dilutes his colours in a volatile oil. 



On the other hand, the painters of the manufactory of 

 Sevres employ only colours without borax, becaufe they di- 

 lute them in gum ; and borax does not dilute well in that 

 iubftance. T have found that both methods are equally good; 

 and it is certain that Montamy was wrong to exclude fluxes 

 of lead, mice they are daily ernploved without any inconve- 

 »ience, and as they even render the application of colours' 

 cafier. 



I have laid that in the baking of thefe colours, the cruft, 

 foftened by the nre ; fuffers itfelf to be eafily penetrated by 

 them. This is the fit ft eaufe of the change which they ex- 

 perience. By mixing with .the cruft they become weaker, 

 and the firft heat changes a figure which appeared to be 

 finifhed into a very light iketch. 



The oxide of lead which the cruft contains is a fecond but; 

 much more powerful caufe of the confiderable changes which 

 colours experience. \ he deftruclive action which that metal 

 exercifes chiefly on iron reds is Angularly remarkable. I fliall 

 foon give an account of fomc experiments which prove it in 

 a very evident manner. 



It has been already feen, that the two principal caufesof 

 the changes which colours on enamel and foft porcelain are 

 fufceptible of experiencing do not depend in any manner 

 on the compofition of thefe colours, but on the nature of the 

 glafs to which they are applied. When it is laid that the 

 colours of porcelain change in a confiderable degree, it ought 

 to be added, .that it is thofe of foft porcelain; a kind almoft 

 neglected. 



it follows from what has been faid, that painting on foft 

 porcelain has need of being feveral times retouched, and of 

 leveral heats, in order that it may be carried to the neceflary 

 degree of Itrength. Thefe paintings have always a certain 

 faintnefs'; but they are conftantly more brilliant, and they 

 never are attended with the inconvenience of detaching them- 

 felves in frale's. 



Hard porcelain, according to the divifion which I have 

 eftabiiflud, is the fecohjd fort' of excipient of metallic colours. 

 This porcelain, as is well known, has for its bafe a very white 

 clay -.alio! U:lu<, mixed with a iuiccou, and calcareous flux/ 



