Hence it follows, that the paintings of porcelain require to be several times re touched and burned, in order to possess the necessary strength. Though these paintings have always a certain softness, they are constantly more brilliant, and never subject to the inconvenience of scaling off.
Hard porcelain is the second species of ground or excipient for the metallic colours. It is known that the base of this porcelain is a very white argil, call edkaolin, mixed with a siliceous and calcareous solvent, and the glaze is no thing but feldspar fused without an atom of lead.
This porcelain, which is that of Saxony, is of a much later date at Sévres than the soft or tender. The colours employed are of two kinds ; the first, used for re presenting different objects, are baked with a very inferior fire to that required for baking the porcelain itself. They are very numerous and varied.
The others, which require to be fused at as great a heat as that for baking the porcelain, are laid on the general surface. They are much less numerous.
The colours for painting are made up very nearly of the same materials as those for tender porcelain ; they only contain more flux. This flux is compossd of the glass of lead (called rocaille) and of bo rax. M. Brougniart asserts, that he has not met with any work that treats of the composition, use, and effects of these co lours. In fact, it has no where been as serted, in print, that all these colours, except one, are unchangeable in fire; whereas it has been often asserted, in books, that paintings in enamel are sub ject to considerable change.
When the porcelain is put into the fire to bake the colours, the feldspar glaze dilates and opens in pores, but does not become soft. As the colours do not penetrate it, they are not subject to the changes they undergo on tender porce lain. It must, however, be observed, that they lose a little of their intensity by ac quiring the transparence given them by the fusion.
When works of little importance are made, they need not be retouched ; but this is necessary when a painting is to be highly finished. This retouching is not more distinguishable in paintings on por celain, than in that of any other species of painting.
One of the great inconveniences of these colours is, that they scale or fly off when the fire is often applied.
This has been particularly remarked at Sévres, on account of the solidity and infusibility with which porcelain is there manufactured. But these qualities cause it to resist the alterations of heat and cold for a longer time, and give its ground a more brilliant colour. On the other hand, the porcelains of Paris being more vitre ous, transparent, and of a bluish cast, generally crack, if boiling water is fre quently poured into them.
In order to remedy this evil, without altering the quality of the body, Broug niart softens the glaze a little, by intro ducing more siliceous or calcareous flux, according to the nature of the feldspar. This method succeeded, and for twelve months then past the colours had past two and three times through the fires, without cracking, provided there were not too mitch flux, and they were not laid on too thick.
It has been remarked, that when soda awl potash have been introduced, the co lours scaled, so that they cannot be used as fluxes. These alkalies, being vola tilized, abandon the colours which can not adhere to the glaze by themselves.
It has been observed, that other co lours are likewise prepared, which be ing laid upon the general surface, are fused by the same fire as bakes the por celain. These colours are but few, be cause there are few metallic oxides that can support such a fire without being vo latilized or discoloured. Their solvent is the feldspar. As they incorporate with the glaze they nevercrack, and are more brilliant.
The third receptacle of metallic vitri fiable colours is glass without lead.
The application of these colours con stitutes the art of painting upon glass ; an art much practised in former ages, but which was, till lately, supposed to be lost, because out of fashion. It, however, too immediately depends on the art of painting on enamel and porcelain to be lost. Descriptions of the processes may be found in different books.