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.