The classification before given may be made clearer by the following explanatory remarks. Glass-painting is distinguished especially from other illuminating processes in that the colors and the foundation on which they are laid must, in this art, be fused together in the kiln.
Now, some few colors combine with the surface of the glass, at the temperature of fusion, without further previous preparation than the simple laying on, wherefore these give to the glass only a coloring cementation or stain. Others, on the contrary, in consequence of their peculiar nature, can only be made to combine with the glass by fusing them upon its surface, into another thin sheet or layer of colored glass. This is done by means of the flux, a vitreous compound, which fuses more easily (i. e. at a lower temperature) than the foundation, the glass plate.
The Process of Laying the Colors on the Glass. The manipulation and the process of laying the colors on the glass varies, in some measure, according to the different kinds of glass-painting, which therefore call for the first explanation.
Either the colors may be laid upon a single sheet of glass, upon which the whole figure with all its principal colors and intermediate tints are burned in (Peinture en apprêt); or,
The figure may be composed of various pieces of pot metal (glass already colored in its manufacture), and only the outlines and shadows painted on, the glass pieces giving the colors for the peculiar places where they are inserted (Mosaic glass-painting); or, both these methods may be combined in one and the same picture, by composing it partly of pieces of colored pot metal and partly of white and painted glass, fixed together,
Peinture et Apprêt. For painting on a single sheet of glass, the following rules must be observed.
A pure white glass must be chosen for the purpose, free from air specks or bubbles, and especially difficult of fusion, as the whole labor would be lost if it were attempted to burn in the colors upon a ground which fused as easily as themselves. It is practicable, as the examples of the ancients show, to paint on what would appear the commonest glass with a good result, provided that it does not contain too much lead, and thereby become too easily fusible.
Before the operation of painting, the glass plate must be rubbed to a sufficient extent with pure lime, slaked by exposure to the air, in order to clean it perfectly.
The ground or foundation must then be laid over the whole surface of the plate, which may be done in two different ways. Some artists simply dip a piece of clean linen cloth, or a flat camel-hair pencil, in oil of turpentine, and brush the pane of glass with it equally over its surface, while others give to the whole a thin clear ground of black glass-painting color, in such manner as not to destroy its transparency, but at most to give it the form of a dead ground glass. Both methods answer the purpose of covering the glass with a viscous surface, which takes the design and the colors better than a polished ground; the latter prepares the glass at the same time for the painting effects which are to be obtained upon it.
In both cases the ground which has been laid on must be most carefully leveled over, and brought to as thin a coat as possible with a large hair pencil, and must be dried quickly, taking great care to preserve it from dust, etc.