An enamel paint may be either an exceedingly fusible glass, covered by some metallic oxide, and rendered opaque by the presence of arsenic trioxide, or an equally fusible transparent glass, mixed with some opaque infusible powder. It is always applied as a pigment, and is fixed to the glass background by heat. It is essentially a glass, and by heat should become partially incorporated with the glass upon which it is painted. There is little doubt that, in former times, artists ground up for their paint some of the self - same glass as that with which they were glazing their windows. Paints formed in this manner require for proper fusion the actual melting - heat of the glass to which they have been applied. The pieces of glass background are therefore usually found to be injured and distorted when,removed from the kiln. To obviate this defect, it has become customary to fix the paint by means of a glass very much more fusible than the glass used for glazing. Such may be produced by the addition of a considerable proportion of borax to the raw material of fluid glass, or by the diminution of the proportion of silica in the same glass. One of the first requirements in a window is that it shall keep out the weather.

Any decoration, therefore, that happens to be on the outside of the glass, must be able to resist the action of the atmosphere. Internal decoration is at the same time exposed to the continued action of the products of human respiration, viz. moisture and carbonic acid, as well as to the moisture always present in the air. The borax contained in an enamel paint is rendered anhydrous by fusion, but after lengthened exposure, it re - absorbs moisture, and becomes hy - drated and efflorescent. The efflorescence of the borax means the decay of the glass used to fix the pigment to the background. After efflorescence has continued for some time, the pigment begins to flake off, and finally the background is denuded of ornament. Very few of the pigments sold at the present time for the decoration of glass do not contain borax. The use of such pigments upon work intended to be permanent should be carefully guarded against. Flint - glass, rendered more fusible by the reduction of the proportion of silica, is not liable to efflorescence when used as a fixative; care, however, must be taken, in preparing the fixative, that the raw materials are mixed in combining proportions. If there be an excess of any ingredient, decay must necessarily follow.

A glass formed according to the formula PbO • K20 . 4SiO2 which is the same as that of flint optical glass, will be found sufficiently fusible for use as a fixative, and will resist the action of the atmosphere. The legitimate use of enamel paint for the permanent decoration of glass is in the form of a dark - brown or red - opaque colour, for outlines and shading. This is prepared by carefully grinding and mixing with the powdered fusiblo glass a proportion of ferric oxide, cupric oxide, or black oxide of cobalt. Iridium oxide is also occasionally employed. The colour is applied to the surface of glass in the same manner as an ordinary pigment. Shadows may be represented by 1 of 3 methods, or by a combination of the same: (1) by colour applied in a mass, known as "smear shadow"; (2) by thin lines of colour interlaced, known as "cross - hatching"; (3) by a mass of colour allowed partially to dry, and then disturbed by the action of a soft - haired brush, known as "stipple" shadow. By the last method, the colour is scattered in separate particles, and a certain amount of light is allowed to pass, which gives an effect of transparency.

The effect of high light is obtained by removing, with a sharp point, parts of a smear shadow.

Fig. 128.

The kilns used for burning - in stain and enamel are represented in Fig. 128. 1, 2, 3 may be regarded as the same kiln in different conditions; S is the firing - door; G, the grate; o, cast - iron screens, placed one upon the other, with protruding openings to allow the stoker to watch the progress of the glass within; K, cast - iron casing or muffle, with iron shelves resting upon ridges projecting from the sides of the muffle, upon which the pieces of glass lie; the inside of the muffle is always carefully coated with whiting, and the shelves are covered with a layer of plaster of Paris, in which the glass is imbedded; f, openings, allowing the fire to pass from the grate, and through the arch A, in such a manner as to surround the muffle, and to pass off through /' into the main Hue F; X, soot - doors. (Powell, spons' Ency.)