C H A P. X.
Of the method of painting on glals by burning, or with tranfparent colours that vitrify.
SECT. I. Of the general nature of painting on glafs with vitreous .
THE art of painting on glafs with colours
that vitrify has been efteemed, as far
as regards the compofition and burning of the
colours, a myftery known perfectly in the
former ages; but loll in a great degree to the
prelent times. It will appear, however, on
due examination, that the cafe is far otherwife :
that from default of artifts who cultivate this
manner of painting, which probably would not
fend many patrons at prefent, the dexterity or
experience of making an advantageous ufe of
die colours fo as to form good pictures is wanting; but that as to the knpwledge of the preparation of the colours, and the method of burning them, we poflefs them from the modern improvements of chemiftry, in a much more ex- tenfive degree than the former times: and that if any able painters were to apply themfelves to this way of working, undoubtedly much better pictures would be now produced than thofe We fo much value as remains of an art, of which we miftakenly fuppofe the methods of execution at prefent loft. For the fake, therefore, of thofe, who, from views -either of profit or amufement, may chufe to apply themfelves to the reviving this fpecies of painting, I will give fuch lights into the nature of the fubjeCt, and the manner of perfuing it practically, as may enable any who can paint in oil, water, or other vehicles, foon to become mafters of every thing peculiar to this art.
The painting with vitreous colours on glafs depends intirely on the fame principles, as painting in enamel; and the manner of executing it is likewife the fame; except that in this the tranfparency of the colours being indifpen- fibly requifite, no fubftances can be ufed to form them but fuch as vitrify perfectly; fince without fuch vitrification, there can be no tranfparency. In other words, the whole my- ftery'confifts, in finding a fet of colours, which are conftituted or compofed of fuch fubftances, as, by the admixture 6f other bodies, that may promote their vitrification and fufion, are ca-pable of being converted into glafs j and melting, when in that ftate, with lefs heat than wUl melt fiich other kinds of glafs as may be chofen for the ground or body to be painted ; in tempering thefe colours, foas to make them proper to be worked with a pencil j and in burning, or reducing them by heat, to a due ftate of fufion without injuring or melting the glafs which conftitutes the body painted.
The circumftances of this art are fo analogous to thofe of the art of enameling, that the fame means will, as I have before intimated, ierve for almoft every particular purpofe that occurs in the profecution of it: and I have, therefore, but little occafion to enter into thé detail, either of the preparation of the colours, or the ufe of them ; as it will be fuffi- oient to refer to what has been before laid j and only to fhew how the methods there taught are applicable to this intention.