Enamels.—An enamel paint may be either an exceedingly fusible glass, coloured by some metallic oxide, and rendered opaque by the presence of arsenio 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 arc, therefore, usually found to be injured and distorted when removed from the kiln. To obviate this defect, it has become oustomary to fix the paint by means of a glass very much more fusible than the glass used for glazing. Such a glass 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 deooration, 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 earbonie 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 reabsorbs moisture, and becomes hydrated 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 . . 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 oolour, for outlines and shading. This is prepared by carefully grinding and mixing with the powdered fusible glass a proportion of ferric oxide, cupric oxide, or black oxide of cobalt. The oxide of iridium 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 one of three 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.
The kilns used for burning-in stain and _enamel are represented in Fig. 802. 1, 2, 3 may be regarded as the same kiln in different conditions ; S is the _ (3r door ; , the grate ; o, cast iron screens, placed one upon the other, with protruding open ings 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 muffie, upon which the pieces of glass rest ; the inside of the muffie 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 f' into the main flue F ; X, soot doors.
Mosaic windows may be divided into two classes, namely, pattern and picture windows ; in the former, the pattern may be entirely represented by the shapes and colours of the glass of which it is composed, or may to a certain extent depend upon the use of enamel colour ; in the latter, the use of enamel colour is absolutely necessary. In the manufacture of a picture window, or of a pattern window, whose effect partly depends upon painted outlines or shadows, the pieces of glass must pass from the cutter's hands into those of the artist. The artist places the pieces of glass over the cartoon, and traces the outlines with enamel colour. The process of tracing upon dark-cnloured glass is facilitated by the use of a transparent desk. After the outline has been traced, the pieces of glass are fastened to a glass easel, by means of wax, in such a way that the artist can obtain an idea, as the work proceeds, of the ultimate effect of the pattern or picture. Whilst the glass remains upon the easel, the artist introduces shadows and high lights, and applies stain wherever it may be required. The pieces of glass pass from the artist to the kilns ; and from the kilns, to the glazier and cementer.