53. Mosaic Glass, Assisi.
There are two very workmanlike ways in which white and colour may be obtained in one piece of glass. If the glass is not coloured throughout its thickness, but only a part of the way through, the coloured part may be eaten away in places by acid (it used formerly to be tediously abraded); and so a pattern of white may be traced upon a ground of blue, for example, or, as is more common, ruby.
A piece of white or pale coloured glass may further be stained, but only, so far, of one colour, yellow. The window [opposite] is all in white and golden-yellow. This result is produced by the action of silver upon it, which, at a sufficient temperature, develops a tint varying from lemon to orange of beautiful quality, and as imperishable as the glass; but one cannot be quite certain always as to the precise shade it will take in the fire. On blue it gives green, and so on.
By the combination of these two processes three tints may be obtained, or even four upon the same piece of glass—say white, green, and yellow all upon a blue ground.
There is a third method of avoiding lead glazing. If little jewels of coloured glass be cut out of various sheets and placed upon white glass they become fused at a sufficient heat in the kiln, and adhere more or less firmly to the glass on which they are laid; but this process of “annealing” is not very safe. Still less to be depended upon is the fourth process of “enamelling.” In that case the coloured glass is applied in the form of a paint upon a sheet of white. Fusing at a comparatively low temperature, it rarely gets quite firmly fixed. Nor has it the depth of pot-metal colour. The three processes of staining, annealing, and enamelling, entail, it will be seen, the burning of the glass. Literally this is the limit of what can be done in stained glass.
54. WINDOW IN WHITE AND STAIN, WARWICK CASTLE.
The term stained glass, however, is generally used to include painting, which from the first has been associated with it. This painting (not to be confounded with the above mentioned enamelling) is a second process, which the glass undergoes after it is cut and before it is fired. It is not in the least what a painter understands by painting. It is, in the first place, a means of giving in solid brown pigment, which effectually stops out the light, detail smaller than mere glazing would permit, such as the features of a face or the veining of a leaf: it gives the foils of the foliage, and marks the individual berries in the border overleaf. In the next it is used partially to obscure the glass, so as to give shading. The pigment is not used as colour, but for drawing and shading only. Local colour is represented by the pieces of pot-metal glass employed; the painting fulfils precisely the part of the engraving in a print coloured by hand. The various methods of painting are explained on [pages 45], [64], [89]. In some respects they have more affinity with line drawing, mezzotint, and etching than with oil or water-colour painting.
It is extremely difficult to get delicacy of modelling or high finish at one painting—to all but a consummate glass painter impossible. Many a time the work has to be painted several times over, each painting being separately burnt in, always at some risk. Painting that is not sufficiently fired peels off in time. If it is fired too much it may be burnt quite away.