To the rule that a separate piece of glass must be used for every change of colour, there are, in later work, two exceptions. The most important, which was discovered early in the fourteenth century, is the use of silver stain. It was then found that if white glass is painted with a preparation of silver—either oxide or chloride of silver will do—and then "fired" in the kiln used for the enamel painting, it will be stained a clear and indelible yellow, varying from pale lemon to deep orange, according to the strength of the painting.
Abrasion.
The other exception was Abrasion, effected by the use of what is called "flashed" glass. Flashed glass is glass so made that instead of being coloured all through, it consists of a thin film or "flash" of colour on a backing of white. With this glass it is possible to chip with a burin, or grind away, the coloured film in places (we do it now with hydrofluoric acid) so as to get white and colour on the same piece of glass.
PLATE II
PART OF CRUCIFIXION WINDOW, POITIERS
Late Twelfth Century
In the Middle Ages only red and certain shades of blue were made in this way, so the use of the process was very much restricted. The invention of silver stain, on the other hand, by enabling the artist to decorate his white glass and make it interesting, led him at once to use a larger quantity of white in his window, and so, as will be seen later, had a considerable influence on design.
These, however, are the exceptions which prove the rule, and, broadly speaking, a stained-glass window must consist, to the eye, of flat patches of colour, large or small, worked on with dark monochromatic line work and shading. These patches of colour must each be separated from the next by a black line—the leading—varying from a quarter to half an inch in thickness, and crossed at intervals by still thicker black lines—the iron bars.
Limitations of the art.
It follows from this that anything like illusion is impossible in stained glass, and no artist with any sympathy for the medium would attempt it. Unwise persons in decadent times have wasted much ingenuity in the endeavour, but the result has always been disastrous and ridiculous. Apart from its higher mission,—the expression of ideas and emotions,—which it shares with every other branch of art, the mission of stained glass is to beautify buildings and nothing else. It is the handmaid of architecture, and can only justify itself by loyal service of its mistress. The ideal of the stained-glass artist must not be a picture made transparent, but a window made beautiful.