A book, entitled " L'Origine de Part de la Peinture stir Verre " published at Faris in the year 1693, and " Le Traite de l'art de la Verriere," by Neri and Kunckel, seem to be the first works con taining complete descriptions of this art. Those published since, even the great work of Leviel, which constitutes part of " Les Arts et Metiers," of theilFrench academy, and of the " Encyclopedic Me thodique," are only compilations from the two former works.
It is somewhat remarkable, that if we follow the processes exactly as they are described in these works, as our author has done with some of them, the colours of which they pretend to give the re ceipt, would never be fabricated. They only serve to show an able practitioner the method, and leave it to him to cor rect or make additions. This was found to be the case by Citizen Meraud, who was engaged to prepare them for the manufactory of Sevres. He was obliged to make the colours for painting on glass rather from his own experience, than from the instructions in the work just mentioned.
The materials and fluxes which enter into the compositions of the colours for painting on glass are, in general, the same as those applied to porcelain. They vary only in their proportions ; but a great number of the colours used for enamel and porcelain cannot be applied on glass; many of them, when seen by transmitted light, entirely change their aspect, and exhibit an obscure tint, which can be of no use when deprived of the white ground which throws them out. We shall point out these when we treat of the colours in particular. Those co lours which can be used on this body sometimes change in the baking, and ac quire a great transparency. They are generally beautiful only when placed be tween the eye and the light, and they answer the purpose intended in painting glass.
There is more difficulty in baking plates of coloured glass than is common ly thought. The bending of the piece and the alteration of the colours are to be avoided. All the treatises we have con sulted recommend the use of gypsum. This method sometimes succeeded with Brougniart, but generally the glass be came white, and cracked in all direc tions. It appears, that the glasses which are too alkaline, and which are far the most common in clear white glasses, are attacked by the hot sulphuric acid of the sulphate of lime. He was able with ease
to bake much larger glasses than any before painted, by placing them on very smooth plates of earth or unglazed por celain.
Concerning the several particular Colours.
After having collected the several phenomena which each class of vitrifiable colours offer, with regard to the bodies on which they are placed, we must skew the particular and most interesting phe nomena, which every principal species of colours employed on tender porcelain, on glass, and in the fire that bakes the porcelain, present.
Concerning the Reds, Purples, and Violets, obtained from Gold.
The carmine-red is obtained from the purple precipitate of Cassius. It is mix ed with about six parts of its flux, and this mixture is directly employed without being first fused. It is then of a dirty vi olet, but acquires the beautiful car mine by baking. It is however very deli cate ; a little too much heat or carbonat ed vapours easily spoil it ; yet it is more beautiful when baked with charcoal than with wood.
This colour, and the purple, which dif fers little from it, as well as the shades which are obtained from their mixture with other colours, really change in all porcelains, and in the hands of all opera tors. But this is the only one which changes on hard porcelain. It may be replaced by a substitution of rose-co lour from iron, which does not change ; so that by excluding from the pallet the carmine made from gold, and substitut ing the rose-coloured oxide of iron here spoken of, we have a pallet composed of colours, none of which are subject to any remarkable change. The rose-coloured oxide of iron has been long known, but was not employed on enamel, because it is then subject to considerable change. Or, perhaps, when the painters on ena mel became painters on porcelain, they continued to work according to their an cient method.
It might be supposed, that by previous ly reducing the colour flamed carmine, already mixed with its solvent into a vitreous matter, the last tint would be obtained; but the fire which must be used to melt this vitreous mass destroys the red colour. Besides, it is found, that, to obtain this colour in perfection, it is necessary to pass it through the fire as little as possible.