The effect of paint in the form of shading is naturally to obscure the glass. Up to a certain point there is not much harm in that; it counts for nothing as compared with the facilities of expression it affords. But that point is soon reached. Then it becomes a question of the relative value of, on the one hand, purity and translucency of glass colour, and, on the other, of pictorial qualities. The problem is to get the utmost of modelling or expression with the minimum of obscuration. Much depends upon the method of painting adopted. So long as the light is allowed to get through it, one may indulge in a fair amount of shading, but a deep even tint, leaving none of the glass clear, is inevitably heavy. The more one can represent shadows by deeper tinted glass the more brilliant the result will be.
55. Auxerre.
This painting, although, strictly speaking, in brown enamel, is not, as was said, what is usually meant by enamel painting: that is described on [page 77]. A window may be painted altogether in enamel; and, when the mosaic method went out, designs were painted in enamel upon panes of plain white glass; but, for the most part, since the pieces had to be connected by lead, it was found convenient to use pot-metal for some of the stronger colours. In recent times, however, owing to the introduction of large sheets of thicker glass, to improved glass kilns, and also to more accurate knowledge of the chemistry of enamel colours, it is possible to paint a picture-window on one sheet of glass. That has been done with extraordinary skill at Sèvres. You may see really marvellous results in this kind in the Chapel of the Bourbons at Dreux. If you want neither more nor less than a picture upon glass, and are content with a picture in which the shadows are opaque and the lights transparent, that is the way to get it. You will not get the qualities of glass. Within the last two or three years there seems to have been very considerable improvement in the purity, translucency, and depth of enamel colours. How far they are lasting remains to be proved. Anyway, brilliant as they are, they have not by any means the intensity of pot-metal glass, and it does not seem, humanly speaking, possible that a film of coloured glass upon a sheet of white can ever compete in strength and volume with colour in the body of the glass itself.
If, therefore, we want the qualities of deep, rich, luminous and translucent colour, which glass better than any other medium can give, we must resort to the use of pot-metal—that is to say, to glazing—assisted more or less by brown paint, used, not to get colour, but to stop it out, or to tone it down.
According to the more or less of your dependence upon paint your method may be described as mosaic or pictorial.
Starting upon the mosaic system, you rough out your design in coloured glass (or what stands for it upon paper), and then consider how, by use of paint, as above mentioned, you may get further detail, shading, harmony of tone.
Starting upon the pictorial system you sketch in your design, shade it, and colour it, and then bethink you how you can get the glass to take those lines.
In either case you have, of course, from the first, a very distinct idea as to the assistance you will get from the supplementary process; but it makes all the difference whether you think first of the glass or of the painting. Upon that will depend the character of your window. If you want all that glass can give in the way of colour, begin with the mosaic. If you want pictorial effect, think first of your painting. If you want to get both, balance the two considerations equally in your mind from the first. Only, to do that, you must be a master of your trade.