Glass Staining.

Scientific American 12, 12.12.1846

Painting on glass properly so called, that is to say, the application of colored enamels to uncolored sheets of glass, was little known to the ancient artists, and it is only in our own day that the progress of chemistry has advanced this art to any degree of perfection.

Painting on uncolored glass was executed in 1800 by Dihl; it consists in tracing the same design on two sheets of plain glass, which are submitted to the action of fire, and then the faces on which the designs are drawn are laid upon one another.

To fix by heat the colors on glass without altering its form, or fusing it, it is necessary to add vitreous matters, which are readily fusible, fluxes, which vary according to the nature of the colors.

Silicate of lead is employed with or without borax, miniur and very fine sand are fused together, and different portions of calcined silex and quartz. For instance, take quartz 3 parts, minium 9 parts, borax calcined 1 1-2 parts, or borax calcined 5 parts, quartz 3, minium 1.

The quantity of flux required for each color, so that it may have the required fusibility and clearness is very variable; the necessary proportion is in general three or four parts. All colors are not adapted for the same flux; the purple of gold, the blue of cobalt, require an alkaline flux; the minium injures these substances, while other deep colors are not injured by fluxes into which lead enters.

Some substances require to be vitrified with the flux proper to them, before they can be employed in painting, as the feeble heat to which they are subsequently subject is not sufficient to develop the color properly. The deutoxide of copper, and the yellows, blues and violets, are among these substances. With purple of gold and oxide of iron, on the contrary, great precautions are necessary to prevent the injury of the color by too great heat. The colored enamels when prepared are reduced to powder, and preserved from the action of moisture.

All kinds of glass are not suitable for painting. Excess of alkali is destructive; preference is therefore given to the hardest glass, which contains a great deal of silex, and which does not attract moisture, as the Bohemian glass for instance.

Before applying the colors with the brush they are mixed on a palette with turpentine. When the painting is finished the colors are fixed by heat, an operation which requires great care and experience. Pots of fire clay closed by a cover of the same substance are placed in a support of iron, so that they can be enveloped on all sides by the flames; the method adopted in France for cooling glass is to put it on separate furnaces heated by charcoal. The plates of glass are laid one on another on clay slabs, supported on props of the some material. The heat is judged of by trial pieces, which are introduced with the rest of the glass into the furnace, and are withdrawn with a spatula. When the colors are well vitrified, the plates are put in the annealing oven and gradually cooled. It is necessary that this last operation should be conducted very gradually, to insure the permanence of the colors.

The color communicated to glass by protoxide of copper is, as has been observed, too intense to be employed alone, for it causes the metal to appear opaque of a deep brown. It is necessary, for procuring a transparent red, that the glass should be extremely thin. Consequently, the only means of getting red glass of a proper thickness is by covering plain glass by a thin layer of red. The plated glass has the advantage of allowing the partial removal of the red layer, in order to obtain white figures, or add other colors. The glass of the middle ages shows that this method was adopted by the ancients.

In order, that, when the red and white glass are blown together, they may be well united, and do not separate during cooling, (as happened in some of Engelhardt's experiments,) the metal of both must be the same, or at least analogous. It is best to make the red a little weaker than the white; the latter most net contain any oxydising substance, which would injure the red color.

Great care is required to avoid air bubbles in the glass. The red and white must be ready at the same time, in order to work together well. The beauty of the glass depends also materially on the skill of the workman, for it is easy to understand that the colored glass is always thicker near the orifice of the blowing iron than at a distance. It is on this account that the glass is seldom of a uniform color, except in the middle of the plate; at the extremity of it the red layer is sometimes so thin that all trace of color is lost. Dr. Engelhart has procured several ancient specimens, in which this gradation from a deep to a light color, has been made use of in a very happy manner to produce striking effects. — After a certain degree of practice, the workman is able to obtain a tolerably uniform color.

It is sometimes necessary, when the glass has once been painted and the colors fixed by baking, to add a second coat of painting; and as it is then necessary that the glass should be again subjected to heat, the coloring matter most be rendered so fusible. by an additional proportion of flux as to avoid all risk of fusing the colors first painted.