The illuminating colors, especially the principal ones, should be laid on the back or reversed side.
Intermediate tints, and gradations by shading, should generally be placed on the front side, but sometimes, when they alternate with each other, necessarily must lie on both; as they cannot be put in contact on one and the same side without danger of running into each other, and making a false color.
The silver yellow and red colors, before alluded to, must always be placed on the back or reversed side.
In some particular cases colors may be laid on corresponding places on both sides of the glass, in order to produce certain effects by the light falling through the two together. Thus, purple on one side and gold yellow on the other, give a magnificent fiery scarlet; blue and yellow, according to their respective intensities, give different shades of green ; the latter, again, with blue on the opposite side, serve for excellent distance colors. And finally, by the mixture of several colors, the most diversified intermediate tints may be obtained, so that glass-painting in its present state may be brought to assimilate with oil painting in its power of producing varied effects.
In order to put a new tone of color on a surface already marked with outlines, etc., it must first be dried by a gentle and equal heat, (to avoid the warping of the glass), and again painted immediately after it has cooled. Or the black lines first laid on may be at once burnt in, and where possible, with these any yellow shades also which may be required, after which the painting, then fixed, may be further worked upon without danger of damage. The residuum of the unfluxed yellow color may be removed after burning, and again used. This color must never be put over any other, nor over dark shadows, unless these are previously burnt in, but always require a carefully cleaned surface of glass to lie upon; otherwise it would combine with the flux of the under color, whereby the earthy residuum would be fixed, and the transparency and beauty of the whole destroyed.
All pigments must be laid on somewhat darker than in other kinds of painting, as they lose in depth by burning.
When a pigment has overrun its outline, the superfluous quantity must be removed, when dry, with a knife.
By taking away the ground with a style of fine grained wood, pointed in front and smooth at the back, (a tool used in etching), the most effective lights may be obtained.
Should the colors not appear quite dull and dry, but shining and greasy, after the drying of the picture, this is caused by the misuse of the oil, which is always dangerous to the beauty of the pigments in firing.
It is neither necessary nor advisable to allow more than one day for the drying of the colors; the burning in should be proceeded with at the expiration of the time named.
Lastly, during the work, the greatest cleanliness must be observed throughout, the pencil and palette must be kept perfectly clean, and the painting preserved from dust, etc., for which reason it is not advisable to paint in a laboratory or melting room, where the presence of vapor, dust, and impurities of many kinds cannot be avoided.
Mosaic Glass -Painting. The before mentioned rules for laying on the colors will apply also to the method of forming designs with colored pieces of pot metal, or partly with these and partly with painted white glass. It remains to say something more in reference to the employment of the cartoons, and the cutting and arrangement of the glasses in this branch of the art, which, however, is but little practiced, since the leaden bars in a picture calculated for a near view are detrimental to the effect.
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.