Scientific American 33, 6.5.1848
When certain metallic oxides or chlorides ground up with proper fluxes, are painted upon glass, their colors fuse into its surface at a moderate heat, and make durable pictures, which are frequently employed in ornamenting the windows of churches as well as of other public and private building. The colors of stained glass are all transparent, and are therefore to be viewed only by transmitted light. Many metallic pigments which afford a fine effect when applied cold on canvas or paper, are so changed by vitreous fusion as to be quite inapplicable to painting in stained glass.
The glass proper for receiving, these vitrefying pigments, should be colorless, uniform, and difficult of fusion; for which reason, crown glass, made with little alkali, or with kelp is preferred. When the design is too large to be contained on a single pane, several are fitted together, and fixed in a bed of soft cement while painting, and then taken asunder to be separately subjected to the fire. In arranging the glass pieces, care must be taken to distribute the joinings so that the lead frame-work may interfere as little as possible with the effect.
A design must be drawn upon paper, and placed beneath the plate of glass; though the artist cannot regulate his tints directrly by his palette, but by specimens of the colors producible from his palette pigments after they are fired. The upper side of the glass being sponged over with gum-water, affords, when dry, a surface proper for receiving the colors, without the risk of their runnin irregularly, as they would be apt to do, on the slippery glass. The artist first draws on the plate, with a fine pencil, all the traces which mark the great outlines and shades of the figures. This is usually done in black, or, at least, some strong color, such as brown, blue, green or red. In laying, on these, the painter is guided by the same principles as the engraver, when he produces the effect of light and shade by dots, lines, or hatches; and he employs that color to produce the shades, which will harmonize best with the color which is to be afterwards applied; but for the deeper shades, black is in general used. When this is finished, the whole picture will be represented in lines or hatches similar to an engraving finished up to the highest effect possible; and afterwards, when it is dry, the vitrifuing colors are laid on by means of larger gair pencils; their selection being regulated by the burnt specimen tints. When he finds it necessary to lay two colors adjoining, which are apt to run together in the kiln, he must apply one of them to the back of the glass. But the dew principle colors to be presently mentioned, are all fast colors, which do not run, except the yellow, which must, therefore be laid on the opposite side. After coloring, the artist proceeds to bring out the lighter effects by taking off the color in the proper place, with a goose-quill cut like a pen without a slit. By working this upon the glass, he removes the color where lights should be the strongest; such as the hair, eyes, the reflection of bright surfaces and light parts of draperies. The blank pen may be employed either to make the lights by lines, or hatches, and dots, as is most suitable to the subject.
By the metallic preparations now laid upon it, the glass is made ready fo being fired, in order to fix and bring out the proper colors. The furnace or kiln best adapted for this purpose, is similar to that used by enamellers. It consists of a muffle of arch of fire-clay, or pottery, so set over a fire-place and so surrounded by flues, as to receive a very considerable heat within, in the most equable and regular manner; otherwise some parts of the glass will be melted; while, on others, the superficial film of colors will remain unvitrified. The mouth of the muffle, and the entry for introducing fuel to the fire, whould be on opposite sides, to prevent as much as possible the admission of dust into the muffle, whose mouth should be closed with double-folding-doors of iron, furnished with samll peep-holes, to allow the artist to watch the progress of staining, and to withdraw small trial slops of glass, painted with the principal tints used in the picture.
The muffle must be made of very refractory fire-clay, flat at its bottom, and only five or six inches high, with such an arched top as to make the roof strong, and so close on all sides as to exclude entirely the smoke and flame. On the bottom of the muffle a smooth bed of sifted lime, freed from water, about half and inch thick, must be prepared for receiving the pane of glass. Sometimes several plates of glass are laid over each other with a layer of dry pulverulent lime between each. The fire is now lighted, and most gradually raised, lest the glass should be broken; and after it has attained to its full heat, it must be kept up for three or four hous, more or less, according to the indications of the trial slips; the yellow color being principally watched, as it is found to be the best criterion of the state of the others. When the colors are properly burnt in, the fire is suffered to die away, so as to annel the glass.