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. [Sidenote: The two parts of the process.] The process thus consists of two parts. The cutting and putting together of the glass is called _glazing_, and it is this that gives the window colour; while the enamel work is spoken of as _painting_, and gives detail, richness, and texture. I shall presently show that the glazing and painting are really two separate crafts, having separate origins and development, and that stained glass as we know it, or as it should be called in strict accuracy "stained-and-painted" glass, is the product of their union. There is another method, far inferior in the beauty of its results, by which pictures can be produced in glass, which is to paint on white glass with transparent coloured enamels. As, however, this method was not used till the seventeenth century, and is now once more almost wholly abandoned, it does not concern us here. The softness of lead which makes it the only practicable metal for joining pieces of glass of complicated shapes, has the disadvantage that a stained-glass window when leaded up has a considerable degree of flexibility, and, if held by the edges alone, would be quite unable to resist the pressure of the wind, which on a big window is enormous,--think of the power even of a fresh breeze on a boat's sail. [Sidenote: The iron-work.]