Silver when applied to the surface of glass produces a beautiful yellow color and it has been widely used in this manner. It has little coloring effect in glass, because it is so readily reduced, resulting in a metallic black. Uranium produces a canary yellow in soda and potash-lime glasses, which fluoresce, and these glasses may be used in the detection of ultra-violet rays. The color is topaz in lead glass. Both sulphur and carbon are used in the manufacture of pale yellow glasses. Antimony has a weak effect, but in the presence of much lead it is used for making opaque or translucent yellow glasses. Chromium produces a green color, which is reddish in lead glass, and yellowish in soda, and potash-lime glasses.
Iron imparts a green or bluish green color to glass. It is usually present as an impurity in the ingredients of glass and its color is neutralized by adding some manganese, which produces a purple color complementary to the bluish green. This accounts for the manganese purple which develops from colorless glass exposed to ultra-violet rays. Iron is used in "bottle green" glass. Its color is greenish blue in potash-lime glass, bluish green in soda-lime glass, and yellowish green in lead glass.
Cobalt is widely used in the production of blue glasses. It produces a violet-blue in potash-lime and soda-lime glasses and a blue in lead glasses. It appears blue, but it transmits deep red rays. For this reason when used in conjunction with a deep red glass, a filter for only the deepest red rays is obtained. Nickel produces an amethyst color in potash-lime glass, a reddish brown in soda-lime glass, and a purple in lead glass. Manganese is used largely as a "decolorizing" agent in counteracting the blue-green of iron. It produces an amethyst color in potash-lime glass and reddish violet in soda-lime and lead glasses.
These are the principal coloring ingredients used in the manufacture of colored glass. The staining of glass is done under lower temperatures, so that a greater variety of chemical compounds may be used. The resulting colors of metals and metallic oxides dissolved in glass depend not only upon the nature of the metal used, but also partly upon the stage of oxidation, the composition of the glass and even upon the temperature of the fusion.
In developing a glass filter the effects of the various coloring elements are determined spectrally and the various elements are varied in proper proportions until the glass of desired spectral transmission is obtained. It is seen that the coloring elements are limited and the combination of these is further limited by chemical considerations. In combining various colored glasses or various coloring elements in the same glass the "subtractive" method of color-mixture is utilized. For example, if a green glass is desired, yellowish green chromium glass may be used as a basis. By the addition of some blue-green due to copper, the yellow rays may be further subdued so that the resulting color is green.
The primary colors for this method of color-mixture are the same as those of the painter in mixing pigments—namely, purple, yellow, and blue-green. Various colors may be obtained by superposing or intimately mixing the colors. The resulting transmission (reflection in the case of reflecting media such as pigments) are those colors commonly transmitted by all the components of a mixture. Thus,
| Purple and yellow | = red |
| Yellow and blue-green | = green |
| Blue-green and purple | = blue |
The colors produced by adding lights are based not on the "subtractive" method but on the actual addition of colors. These primaries are red, green, and blue and it will be noted that they are the complementaries of the "subtractive" primaries. By the use of red, green, and blue lights in various proportions, all colors may be obtained in varying degrees of purity. The chief mixtures of two of the "additive" primaries produce the "subtractive" primaries. Thus,
| Red and blue | = purple |
| Red and green | = yellow |
| Green and blue | = blue-green |
Although the coloring media which are permanent under the action of light, heat, and moisture are relatively few, by a knowledge of their spectral characteristics and other principles of color the expert is able to produce many permanent colors for lighting effects. The additive and subtractive methods are chiefly involved, but there is another method which is an "averaging" additive one. For example, if a warm tint of yellow is desired and only a dense yellow glass is available, the yellow glass may be cut into small pieces and arranged upon a colorless glass in checker-board fashion. Thus a great deal of uncolored light which is transmitted by the filter is slightly tinted by the yellow light passing through the pieces of yellow glass. If this light is properly mixed by a diffusing glass the effect is satisfactory. These are the principal means of obtaining colored light by means of filters and by mixing colored lights. By using these in conjunction with the array of light-sources available it is possible to meet most of the growing demands. Of course, the ideal solution is to make the colored light directly at the light-source, and doubtless future developments which now appear remote or even impossible will supply such colored illuminants. In the meantime, much is being accomplished with the means available.