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.

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. Uran