The laying on of the fused colors is accompanied with more difficulty than that of the other kinds. The latter are simply laid on with the pencil, in the same manner as with other kinds of painting, and the only care necessary is that the coat may be perfectly even and regular, therefore for large surfaces a wide smooth pencil or driver is usually employed. The colors prepared from silver must be treated differently, and laid on the glass at least to the thickness of the back of a knife.
But the fused colors must be brought upon the surfaces to be covered in the state of a thick flowing mass, moist enough to run, but consistent enough to lie upon the glass. For this purpose small portions must be laid on and spread out with a pencil or small spoon, and made to flow to the circumscribing outlines, by inclining the sheet in the proper directions. If any part of the surface thus covered is required to take a darker tone of color, the plate must be kept for some time at an inclination in the corresponding direction, so that the color may thus accumulate thicker on that part. By this process many gradations of tone may be obtained from one and the same pigment.
The remaining rules for the laying on of the pigments are those which principally result from the different methods of painting on one sheet, of which there are principally three.
Either the whole picture may be brought out in its outlines and shadows, on one side of the sheet, with black, brown or gray color, and illuminated with the proper colors in the proper places on the other side.
Or simply the manner of ordinary oil painting may be adopted with the glass colors, and the picture treated as by an artist in oil.
Or, as is now most customary, both methods may be united, the artist making use of each in certain places, according to the requirements of the object he has in view.
For these three methods the following common rules will serve.
The shadows and dark colored outlines, and that which is called in oil ‘under painting,’ should be drawn on the front side of the glass, or that which is turned towards the spectator.
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.