In addition to their use in coloured glass, certain copper compounds play an important part in the production of stained glass, a craft in which the English, Germans, Italians and French of mediaeval times excelled. Despite the ravages of war, some magnificent examples of stained glasswork still survive in the cathedrals of Western Europe. The process differs from the manufacture of coloured glass in which pigments are introduced in the original melt; in stained glass the picture is painted on the glass surface. For this purpose a vitreous paste is employed, copper being one of the important constituents. The product is then heated so that the design and the glass fuse into one. It is a highly skilled and very delicate art, and it seems impossible to get quite the same effect by modern methods, because only age can bring out the brilliance of the colours at their very best.
From glass to windows is a simple transition and it is interesting to note that even Homer mentions a bronze window frame. With stained glass windows, the leading of the panels was held in place by copper wire.