Mosaic glass-painting requires two cartoons. One of these, a finished and colored one, is used by the artist as a pattern, and serves to determine the arrangement of the piece of glass according to their several colors, and the manner of introducing the leaden ribs to fasten them together, according to the outlines of figures. Each piece of glass proposed to make part of the picture, must be distinguished by a separate number.
The other cartoon, which consists only of the black outlines of the lead jointing, and whose several parts are numbered to correspond with the first, is to be cut up in pieces according to the outlines, and the size of each piece diminished all round by one-half the thickness of the lead bar of the jointing, so that the pieces of glass may be exactly cut to the proper dimensions.
The cutting of the glass may either be done by the diamond, or by tracing the line of division with a red-hot iron, after having made a small incision at its commencement, or by cutting with scissors under water, which, however, is not a safe process.
With overlaid glass, i. e. pot metal, several sheets or layers laid upon each other from the frit, as for example, red and white, blue and white, etc., it is possible to produce many effects of shading by removing more or less of the colored glass sheet, according to the outline, by grinding with emery. Or the colored sheet may be ground through to the white glass, and thus colored ornaments may be given on white ground, especially for the representation of damasked materials. Also, the white parts thus exposed may have a color given them at pleasure on the opposite side, in order to produce many kinds of effects, or to avoid the necessity of using many pieces when the introduction of another color in that of the pot metal is indispensable for the effect required.
The colored pot metal may be painted with intermediate tints of its own principal color, or even in order to produce certain effects, may be covered on one of its surfaces with another color. Thus, a fiery red may be obtained by covering a red overlaid glass on its white surface with the yellow silver color, and burning it in, or a shade of green by a similar use of the same pigment on a blue overlaid glass. In these operations the widest latitude is left to the talent and practice of the artist.
GILDING THE BORDERS OF GLASS,
IN TRANSPARENCY, ANTIQUE,