A yellow for 

 giafs equal in 

 beauty to the 

 indent. 



15Q COLOURS OBTAINED FROM METALLIC OXIDES. 



Not applicable to Thefe yellows cannot be applied to glafs, they are opaque 

 * as * and muddy. That employed by the ancient painters on glafs 



is, on the contrary, beautifully tranfparent, very brilliant, and 

 of a colour approaching to gold. The precedes they give in- 

 dicate that it contained a mixture of filver ; but when exactly 

 followed, they afford nothing fatisfaclory. Citizen Meraud, 

 whom I have before quoted, has fucceeded in making it as 

 beautiful as that of the ancient painters on glafs, by employing 

 muriate of filver, oxide of zinc, white clay, and the yellow 

 oxide of iron. Thefe colours are applied to the glafs limply 

 ground, and without flux. The oxide of iron gives the yellow 

 nearly the fame tinge as it ought to have after the baking, and 

 contributes, with the clay and oxide of zinc, to decompofe the 

 muriate of filver without difoxidating the filver itfelf. A pow- 

 der remains after baking which does not penetrate the glafs, 

 and may be eafily cleared off. 



This yellow, when employed in greater quantity, affords 

 deeper fhades, and produces a reddifh yellow. 



Concerning the Blues. 



Blues from cobalt Thefe are known to be obtained from the oxide of cobalt. 

 require nothing Their preparation is known to every chemift. The fuperiority 

 at Sevres, fo juftly reputed for the beauty of its blues, is owing 

 merely to the care taken in its fabrication, and to the quality of 

 the porcelain, which appears more proper to receive it on ac- 

 count of the violent fire it can fupport. 



I have obferved one fact refpecling the oxide of cobalt, 

 which is perhaps not known to chemifts : It is volatile in a 

 violent heat; to this property muft be attributed the bluifli 

 tint which the white (bordering upon the blue) always receives. 

 I purpolely put into the fame cafe a white piece next to a blue; 

 the fide of the white piece that was turned towards the blue, 

 became very bluifli. 



The blue of hard porcelain, prepared for what is called a 

 blue ground by ftrong fire, is fufed with feld-fpar : the folvent 

 for tender porcelain is filex, potafli, and lead ; it is not vola- 

 tilized like the preceding, becaufe the fire is much inferior to 

 that of the hard porcelain. 



Thefe colours being previoufly fufed, do not in the leaft 

 change when applied. 



The blues on glafs are the fame as for tender porcelain. 



Concerning 



Oxide of cobalt 

 is volatile. 



Blue grounds. 



Blue for glafs. 



