and fixed hy Fujion on different vitreous Bodies. 34$ 



their flux, and afterwards ground. In fpeaking of colours in 

 particular, I (hall point out thofe which experience this kind 

 of fufion. 



Thefe general principles arc fo Ample that it is not necef- 

 fary to enlarge on them further. . 



I (hall fpeak here only of the application of metallic colours 

 to vitreous bodies or to vitreous lurfaces. 



Thefe bodies may be divided into three claffes, very diftin& 

 by the nature of the fubftances which compofe them, the 

 effects produced on them by the colours, and the changes 

 they experience. Thefe claffes are : 



ift, Enamel, foft porcelain, and all crufts, enamels, or 

 glafs, that contain lead in a notable quantity. 



2d, Hard porcelain, or porcelain which has a cruft of 

 feld-fpar. 



3d, Glafs in the compofition of which no lead enters, fuch 

 as common window-glafs. 



1 mall here examine in fuccefiion the principles of the 

 compofition of thefe colours, and the general phenomena 

 thev exhibit on thefe three kinds of bodies. 



Colours for painting in enamel are thofe moft antiently 

 known : the recipes found in the works I have already men- 

 tioned, all relate to thefe colours. 



It is well known that enamel is glafs rendered opake by 

 the oxide of tin, and exceedingly funble by the oxide of lead. 

 It is the oxide of lead, in particular, contained in it that gives 

 it properties very different from thofe of the other excipients 

 of metallic colours. Thus all glafs and glazing that contain 

 lead will participate in the properties of enamel ; and what 

 We (hall fay of one' may be applied to the reft with very 

 trifling differences. 



Such are the white and tranfparent glazing of flone ware, 

 and the glazing of porcelain called foft glazing. 



This porcelain, which was the nrft made in France, and 

 particularly at Sevres, where it was a long time manufac- 

 tured almoft exclufively, has for its bafe a vitreous fritt ren- 

 dered 'almoft opake, and fufceptible of being worked with 

 clay, and is glazed with an exceedingly diaphanous glafs, 

 into the compofition of which there enters a great deal of 

 lead. 



The colours emploved in it are thofe which ferve for paint- 

 ing in enamel : confequently whatever changes thefe colours 

 experience on enamel they nmft experience on this kind of 

 porcelain, flnce the caufes of this change, which we (hall 

 mention hereafter, are the fame. 



Knamel or foft porcelain colours require lefs flux than 



others, 



