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 mud 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 Brown 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 calk, 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.
Concerning the Black's.
Black colours are the most difficult to be obtained very beautiful. There is no metallic oxide, which, singly, affords a fine black. Manganese gives the best ; iron, an opaque, dull, blistered black, which easily turns to red. The makers of colours have therefore combined se veral metallic oxides, which, singly, do not afford blacks, and they have obtained a very beautiful colour, but it is subject to scale and become dull.
The oxides are, those of manganese, the brown oxides of copper, and a little of that of cobalt. Grey is obtained by suppressing the quantity of copper and increasing the quantity of flux.