It would not even be able to support its own weight for long, and so it follows that it must be held up by a system of short metal bars fixed firmly into the stone-work. Naturally the design of the window must be so arranged that these bars either do not interfere with it or form an integral part of it. In early windows, especially those of the twelfth and thirteenth centuries and even to some extent in those of the fourteenth, the bars are sufficiently important to form the governing factor in the design. It must not be thought that stained glass loses in beauty by the presence of these black lines of lead and iron. On the contrary it gains enormously. Large pieces of unrelieved colour in windows are thin in effect and trying to the eye, which needs the continual contrast of the solid black of the lead all over the window to enable it to appreciate the colour and brilliance of the glass. The painting when rightly used is directed to the same end, for it may be said that the smaller and more divided the spaces of clear glass, the more brilliant and jewel-like is the effect. [Sidenote: Silver stain.] 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. [Sidenote: 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. [Illustration: 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.