Painting on one sheet requires only one pattern drawing or cartoon, which, however, may be used in two ways. Either the glass sheet, grounded and dried as above directed, may be laid upon the drawing, and the outlines, as seen through the glass, traced lightly with a fine pencil, and with black or other glass color corresponding to the ground. Or the drawing may be placed reversed on the sheet, and all the outlines marked over with a steel or ivory style. If this latter method is used upon a ground of simple turpentine, the back of the drawing must previously be rubbed over with black lead, so that the traced lines may appear dark on the light ground.
In both cases, the drawing, whether it is placed upon or under the glass, must, for the sake of convenience, be fastened to it with pieces of wax at the four corners.
For properly carrying out the process of laying on the colors, a desk or easel is necessary, which should be capable of being placed in an inclined position by means of props, and should be formed by fixing a glass plate in a wooden frame, so that the light may pass through the painting. Sometimes during the progress of the work, the glass which is being painted may be removed from the easel and laid upon a sheet of white paper, in order better to show the effect of certain colors.
The vehicle with which the pigments are laid on is generally oil. Some artists use exclusively water, but this alone is an insufficient medium for binding the metallic bodies to the glass, particularly if, as in the case of fused colors, they are somewhat coarse in their nature, and require to be laid on in thick layers. They then easily loosen from the plate before the firing, and render the process of laying on much more difficult. It is an important advantage, that with oil the edges are more sharply defined, and the parts already painted may be again touched over when dry without danger of loosening the ground.
It must be understood that when it is wished to make use of water, the plate must either not be grounded at all, or only with a glass-painting color worked up with water.
The most suitable kind of oil for the purpose is rectified oil of turpentine, somewhat thickened by standing, and to which a little oil of lavender is added. This preparation gives the mass the necessary degree of viscosity, and also prevents the color on the palette from drying up and thickening too quickly.
The palette should be of thick sheet glass, ground rough by rubbing with a glass muller and fine sand.
Preparatory to mixing with oil for laying on, those colors which require a flux must (unless a different process is specially indicated) be ground fine in water with the flux, and again dried. But the fused colors, i. e. those in which the oxide has already been vitrified with the flux into the state of a transparent glass, should for the purpose of laying on, only be coarsely granulated; for the finer these are ground the more likely is their transparency and perfection to be impaired when burnt in.
Those pigments which are laid on in their simple combination with an earthy vehicle, and without flux, as for example the yellow and red colors prepared from silver, form an absolute exception to the use of oil, and must, for laying on, be stirred up with water to the consistency of a thick cream.
The first of these three kinds of pigments should, as a general rule, be laid on in a thin, the latter two in a pasty, state. The depth of tone of the color depends, with all three, upon the degree of thickness in which the pigments are laid upon the glass.