The Sevres manufactory is the only one which has as yet produced beautiful blacks with a strong fire. This is more owing to the quality of the biscuit, than to any peculiarity of process. It is by a mixture of blue with the oxides of man ganese and iron that they make this very brilliant black.
The blacks for opaque glass are made the same as for painting, by giving dif ferent doses of solvent.
After the display of the principles of fa bricating each principal colour, it is clear, that by mixing these colours all possible shades may be obtained : and also that care in the preparation, choice of mate rials, and just proportions of doses, must exhibit very sensible differences to the experienced eye of a painter. A know ledge of the composition of colours does not give the requisite care and neatness in making them up.
On recapitulating the facts here just stated, in order to present them in a ge neral view, we see, first, that amongst the colours usually employed for hard porcelain, one only is susceptible of change, namely, the carmine : and this may be replaced by the reds of iron, and then no colour changes.
M. Brougniart presented to the Insti tute an unbaked head made in this man ner, and a painting of two roses, the one baked, and the other in its first state.
There was not any difference between them.
Secondly, That amongst the colours of soft? porcelain and enamel, several change considerably, particularly the reds of iron and gold, with the yellows, greens, and browns. None have been substituted instead of them, this species of painting being almost abandoned.
Thirdly, That several of these colours change likewise upon the glass by be• coming perfectly transparent, particular ly the yellows and violets.
Fourthly, That neither an additional calcination, nor an additional fusion, as has been suspected, will prevent them from changing : fbr this method alters the colours that change, and does no thing to the rest. The change which se veral colours undergo on tender porce lain, and on glass, does not therefore re late to the nature of their composition, but rather to that of the body on which they are placed. Consequently, by sup pressing the carmine of gold from the colours of hard porcelain, we shall have a series of unchangeable colours.