The carmine of tender porcelain is made of fulminating gold, gently decom posed, and muriate of silver; there is no tin in it, which proves it is not necessary, for the fabrication of a purple colour, that the oxide of this last metal and that ofgold should be combined.
Violet is likewise obtained from the purple oxide of gold. This colour pro ceeds from having a greater quantity of lead in the flux, and it is nearly of the same tint, whether crude or baked.
These three colours totally disappear in the strong fire necessary to bake por celain.
Carmine and purple afforded, upon glass, only tints of dirty violet. The vio let, on the contrary, has a beautiful ef fect, but is subject to change to blue.
Concerning the Red, Rose, and Brown Colours, obtained from Iron.
These colours are made from red ozydated iron, prepared with nitric acid. The oxides are calcined still more by exposing them to the action of fire. If too much heated, they change to a brown.
Their flux is composed of borax and minium in small quantity.
These are the oxides which afford the rose and red colours, which may be sub stituted instead of the same colours made from oxide of gold. If properly applied on hard porcelain, they never change. Brougman made roses with these colours, and there was no difference between the flower, before and after baking, except the brilliancy which colours naturally receive from fusion.
The colours may either be previously fused or not, at pleasure.
In a violent fire, they either partly disappear, or produce a dull and brick dust red colour, which is not at all agree able.
Their composition is the same, either for tender porcelain or for glass. They do not change on the latter, but on the former they almost entirely disappear by the first fire ; and they must be laid on very heavily, in order to have any part visible.
It is to the presence of lead in their glaze that this singular effect must be at tributed. Brougman ascertained this by a very simple experiment. He placed this colour on window glass, and fired it very strongly, and it did not change. He then covered some parts of it with minium, and again exposed it to the fire. The colours totally disappeared in those places where the red oxide of lead had been applied. When this experiment
was performed on a larger scale, in a closed vessel, a large quantity of oxy gen gas was disengaged.
This observation seems clearly to prove the effect of oxydated lead as a dis colourer of glass. We see that it does not operate, as has been supposed, by burning combustible impurities in the glass, but by dissolving, discolouring, and volatilizing the oxide of iron, which may affect its clearness.
Concerning the Yellows.
Yellows are colours which require much precaution in fabricating, on ac count of the lead they contain ; which, sometimes, by approaching to the metal lic state, produces black spots.
The yellows of hard and tender porce lain are the same. They are composed of oxide of lead, white oxide of antimony, and sand. Oxide of tin is sometimes added ; and when it is required very lively, and resembling the colour of mari gold, red oxide of iron is added, the very deep colour of which disappears during the previous fusion they undergo, on ac 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.