A variety of colored lights are in demand for theatrical effects, displays, spectacular lighting, signaling, etc., and there are many superficial colorings available for this purpose.  Few of these show any appreciable degree of permanency.  Permanent superficial colorings have recently been developed, but these are secret processes unavailable for the market.  For this reason colored glass is the only medium generally available where permanency is desired.  For permanent lighting effects, signal glasses, colored caps, and sheets of colored glass may be used.  Tints may be obtained by means of colored reflectors.  Other colored media are dyes in lacquers and in varnishes, colored inks, colored textiles, and colored pigments.

Inasmuch as colored glass enters into the development of permanent devices, it may be of interest to discuss briefly the effects of various metallic compounds which are used in glass.  The exact color produced by these compounds, which are often oxides, varies slightly with the composition of the glass and method of manufacture, but this phase is only of technical interest.  The coloring substances in glass may be divided into two groups.  The first and largest group consists of those in which the coloring matter is in true solution; that is, the coloring is produced in the same manner as the coloring of water in which a chemical salt is dissolved.  In the second group the coloring substances are present in a finely divided or colloidal state; that is, the coloring is due to the presence of particles in mechanical suspension.  In general, the lighter elements do not tend to produce colored glasses, but the heavier elements in so far as they can be incorporated into glass tend to produce intense colors.  Of course, there are exceptions to this general statement.

The alkali metals, such as sodium, potassium, and lithium, do not color glass appreciably, but they have indirect effects upon the colors produced by manganese, nickel, selenium, and some other elements.  Gold in sufficient amounts produces a red in glass and in low concentration a beautiful rose.  It is present in the colloidal state.  In the manufacture of “gold” red glass, the glass when first cooled shows no color, but on reheating the rich ruby color develops.  The glass is then cooled slowly.  The gold is left in a colloidal state.  Copper when added to a glass produces two colors, blue-green and red.  The blue-green color, which varies in different kinds of glasses, results when the copper is fully oxidized, and the red by preventing oxidation by the presence of a reducing agent.  This red may be developed by reheating as in the case of making gold ruby glass.  Selenium produces orange and red colors in glass.