ENAMELLING. 



count of the lead contained in this yel- 

 low. When these colours are once 

 made, they do not change : they disap- 

 pear almost entirely in the porcelain fire- 

 yellows. 



These cannot be applied to glass ; they 

 are opaque and muddy. That employed 

 by the ancient painters on glass is, on the 

 contrary, beautifully transparent, very 

 brilliant, and of a, colour approaching 

 gold. The processes they give indicate 

 that it contains a mixture of silver; but 

 when exactly followed they afford no- 

 thing satisfactory. Citizen Meraud suc- 

 ceeded in making it as beautiful as the 

 ancient painters on glass, by employing 

 muriate of silver, oxide of zinc, white 

 clay, and the yellow oxide of iron. These 

 colours are applied to glass simply 

 ground, and without flux. The oxide of 

 iron gives the yellow nearly the same 

 tinge as it ought to have after the baking, 

 and contributes, with the clay and oxide 

 of zinc, to decompose the muriate of sil- 

 ver without disoxydating the silver itself. 

 A powder remains after baking, which 

 does not penetrate the glass, and may be 

 easily cleared off. 



This yellow when employed in greater 

 quantities affords deeper shades, and pro- 

 duces a reddish colour. 



Concerning the Blues. 



These are known to be obtained from 

 the oxide of cobalt ; their preparation is 

 known to every chemist. The superiori- 

 ty at Sevres, so justly reputed for the 

 superiority of its blues, is owing mere- 

 ly to the care taken in its fabrication, 

 and to the quality of the porcelain, 

 which appears more proper to receive 

 it, on account of the violent fire it can 

 support. 



Brougniart observed one fact respect- 

 ing the oxide of cobalt, which is, perhaps, 

 not known to every chemist. It is vola- 

 tile in a violent heat ; to this property 

 must be attributed the bluish tint which 

 the white (bordering upon blue) always 

 receives. A white piece was purpose- 

 ly put in the same case next to a blue ; 

 the side of the white piece which was 

 turned towards the blue became very 

 bluish. 



The blue of hard porcelain, prepared 

 for what is called a blue ground by 

 strong fire, is fused with feldspar ; the 

 solvent for tender porcelain is silex, pot- 

 ash, and lead ; it is not volatilized like 

 the preceding, because the fire is much 

 inferior to that of the hard porcelain. 



These colours, being previously fus^ 

 ed, do not in the least change when ap- 

 plied. 



The blues for glass are the same as for 

 tender porcelain. 



Concerning Greens. 



The greens employed in painting are 

 made with the green oxide of copper, or 

 sometimes with a mixture of yellow and 

 blue. They must be previously melted 

 with their flux ; without this precaution, 

 they would become black : but they do 

 not change after the first fusion. 



They must not be treated with a vio- 

 lent fire, or they would totally disappear. 

 The green grounds by strong heat are 

 made with the oxides of cobalt and nick- 

 el, but it is only a brownish green. 



The bluish greens, named sky-blue, 

 formerly a colour very much in esteem, 

 can only be used on tender porcelain ; 

 they always scale off from hard porcelain, 

 because there is potash in their composi- 

 tion. These greens cannot be used on 

 glass, because they afford a dirty colour ; 

 it is necessary to put a yellow on one side, 

 and a more or less pale blue on the other, 

 in order to produce a green. This co- 

 lour may likewise be fabricated, by mix- 

 ing a blue with the yellow oxide of iron. 

 Brougniart hoped to obtain a green from 

 the oxide of chrome ; and the experi- 

 ments he made promised to be attended 

 with success. The pure chromate of lead, 

 fixed on porcelain by means of a strong 

 fire, afforded him a very deep and very 

 fixed blue, of considerable beauty. 



Concerning Bistres and JJrovvn Reds. 



These are obtained by mixtures of dif- 

 ferent proportions of manganese, brown 

 oxide of copper, and the oxide of iron, 

 called umber. They are likewise pre- 

 viously fused in their solvents, so that 

 they do not in the least change on ten- 

 der porcelain, lead not having the same 

 action on the oxide of manganese as it 

 has on that of iron. This colour may be 

 employed very well on glass. 



The brown red, ground by strong heat, 

 known by the name of fonds caille, are 

 made in the same manner : feldspar is 

 their flux. There is no titanium in their 

 composition, though generally asserted in 

 books. Titanium was not known at 

 Sevres when Brougniart first came to that 

 manufactory. He treated this singular 

 metal in various ways, and never ob- 

 tained any grounds but a slight obscure 

 yellow, and very uncertain in its quality. 



