Stained glass is not one of the arts in which the method of production reveals itself at the first glance. Indeed, so few people when looking at a stained-glass window, whether a gorgeous and solemn one of the thirteenth or fifteenth century, or a crude and vulgar one of the nineteenth, realize the long and laborious process by which the result, good or bad, has been obtained, that a short description of that process as finally perfected some five hundred years ago may not be out of place here.
One hears it so often spoken of as "painted glass"--Mr. Westlake calls his book _A History of Design in Painted Glass_--that it is not surprising that there should be a good deal of misconception on the point. It must be clearly understood then that the colour effects which are the glory of the art are not directly produced by painting at all, but by the window being built up of a multitude of small pieces of white and coloured glass--glass, that is, coloured in the making, and of which the artist must choose the exact shades he needs, cut them out to shape, and fit them together to form his design, using a separate piece for every colour or shade of colour.
In twelfth and thirteenth century windows many of these pieces are only half an inch wide and from one to two inches long, and few are bigger than the palm of one's hand; so the reader can amuse himself, if he wishes, in trying to calculate the number of pieces in one of the huge windows of this date in the Cathedral of Canterbury, York, or Chartres, and the labour involved in this, the initial stage of the process.
When the window is finished these pieces are put together like a puzzle and joined by grooved strips of lead soldered at the joints, just as any "lattice" window is put together (and until glass was made in large pieces this was the only way of filling a window); but before this is done the details of the design--features, folds of drapery, patterns, and so on--are painted on the glass in an opaque brownish enamel made of oxide of iron and other metals ground up with a "soft" glass (_i.e._ glass with a low melting-point). This is mixed with oil or gum and water in order to apply it, and then the glass is placed in a kiln and "fired" till the enamel is fused on and, if well fired, becomes part of the glass itself. This is the only "painting" involved in the production of a stained-glass window, and its effect, in the hand of an artist, besides enabling him to express more than could be done merely with glass and lead, is to decorate and enrich what would otherwise be somewhat crude and papery in effect.