For painting on a single sheet it must be observed that a pure white glass, free from air specks or air bubbles, and hard of fusion, must be selected for the purpose. The artist's labor would be entirely lost if he attempted to burn in colors on a surface as readily fusible as the colors themselves. The example of the ancients, however, proves it practicable to paint with good results upon very common glass, free from an excess of lead, rendering it too fusible. Before being painted upon, the glass should be rubbed with pure lime, slaked by exposure to the air, which cleans it perfectly. The surface of the glass is then prepared by spreading evenly all over this surface a thin coat of common paste, or a thin, clear ground of black glass-painting color, so as not to destroy the transparency, but give it the appearance of a dead ground glass. Each method covers the glass with a viscous surface, which receives the color and design better than could a polished surface. The ground thus prepared must then be carefully stippled with a large badger, brought to as thin a surface as possible, dried quickly, and kept free from dust. The sheet of glass thus prepared, may be laid on a drawing of the figure required, and the outline, as seen through the glass, traced lightly with a fine pencil in black or other glass color like the ground. Or, the pattern may be reversed on the glass, but in this event the back of the drawing must be rubbed over with dry white or some other color, a steel or ivory point being called into requisition to transfer it to the surface of the glass. The drawing, whether placed over or under the glass, must be fastened at each corner with wax. The glass is then placed upon the easel, that the light may easily shine through it, but is sometimes, to better bring out the effect of the colors, removed from the easel and laid upon a sheet of white paper.

Fig. 79.

181. Oil is the usual vehicle with which pigments are laid on glass. Some artists, however, use water alone, but water alone is an insufficient medium for binding the metallic bodies to the glass, particularly if, as in the case of fused colors, they are coarse in texture and require to be laid on in thick layers. These applied with a water medium only, easily loosen from the plate before fixing-, and render the process of laying on much more difficult. An important advantage of the oil medium is that the edges may be more sharply defined, and the parts already painted gone over again when dry, without loosening the previous painting. Oil of turpentine, thickened by exposure, is the most suitable oil for use, giving the necessary degree of viscosity and preventing the colors from drying too quickly on the palette (which for this work should be of thick plate glass or porcelain, ground on one side).

182. There are three principal methods of painting on a single sheet of glass. The whole picture may be brought either in outline or in shadow on one side of the sheet, with black, brown, or gray color, and with appropriate tints illuminated in the proper places on the opposite side; or, the artist may make use of both methods combined, employing each in certain places, according to the requirements of the situation. The outline and shadow and the underpainting in oil should be executed on the side of the glass to be turned towards the spectator, while the illuminating colors are laid on the reverse side.

183. Stained glass must not be confounded with painted glass. The coloring in stained glass is either a superficial layer or pervades the substance of the glass, and is obtained by applying a coat of metallic oxide. The art of joining together small pieces of stained glass to form colored designs or transparent mosaics was practised in classical times. But, though the ancients were fully acquainted with the art of coloring glass, the fragments of ancient window glass hitherto discovered are all devoid of color. Glass staining may be defined as the art of applying to transparent glass, colorless or colored, in the process of its manufacture, metallic colors, afterwards burnt into the surface of the glass on which they are laid. All colors used in glass staining are oxides of metals or other metallic combinations, and may be divided into two principal classes: (1) those whose coloring basis-the oxide-is laid on the glass simply in an original combination with an earthy vehicle; (2) those whose coloring or oxide is made to adhere by the aid of a glossy body, viz., the flux, a vitreous compound fusing at a lower temperature than the foundation, or glass plate.

The colors requiring the flux may be again divided into two classes: (1) those in which the oxide requires to be vitrified by previous fusion with the flux before it is laid on the glass; and (2) those in which the oxide and flux are mixed together and fused when the color is baked into the glass during the process of firing. The first may be called fused colors, all others mixed colors.

This classification may be more readily grasped by bearing in mind that glass staining is distinguished 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.

Some few colors, without any previous preparation other than the simple laying on, combine with the surface of the glass at the temperature of fusion, and, therefore, impart to the glass a coloring cementation or stain only. Others, on the contrary, can only be brought, in consequence of their peculiar nature, to combine with the glass by being fused into another thin sheet or layer of colored glass, upon its surface. This is done by means of the flux.

The flux may be used in two ways: First, with some colors, it may be mixed before they are laid on, combining with their oxides at the temperature of fusion, uniting these, again, with the surface of the glass; second, in other cases, the flux must, before staining, have entered into a chemical combination with the oxides, i. e., by being fused together with these, to produce what is termed a fused color, which, after being pulverized, serves as a pigment. This process is rendered necessary by the difficulty of fusion in certain oxides, which, to combine with the flux and acquire the intended shades of color, demand a greater degree of heat than that available in burning colors upon the glass, without endangering the success of the operation.

184. From consideration of the nature of the colors and their combination with glass, we proceed to give the following tables: