Stained-glass windows have been admired for their utility and beauty since ancient Rome, when pieces of colored glass were assembled into patterned window frames. In Europe, the art of stained glass reached its height between 1150 and 1500, when magnificent windows were created for great cathedrals.
Most of what is known about medieval stained-glass making comes from a twelfth-century German monk who called himself Theophilus. An artist and metalworker himself, Theophilus described in his text, On Diverse Arts, how he carefully studied glaziers and glass painters at work in order to provide detailed directions for creating windows of “inestimable beauty.”
The basic ingredients for making glass are sand and wood ash (potash). The mixture is melted into liquid which, when cooled, becomes glass. To color the glass, certain powdered metals are added to the mixture while the glass is still molten. Molten glass can be blown into a sausage shape, then slit on the side before being flattened into a sheet; it can also be spun with a pontil iron into a round sheet (crown). A window’s pictorial image is created by arranging the different pieces of colored glass over the design drawn on a piece of board. If fine details such as shadows or outlines are required, the artist paints them on the glass with black paint.
To assemble the window, pieces of colored and painted glass are laid out on the design board, with the edges of each piece fitted into H-shaped strips of lead (cames). These cames are soldered to one another so that the panel is secure. When a panel is completed, putty is inserted between the glass and the lead cames for waterproofing. The entire composition is then stabilized with an iron frame (armature) and mounted in the window.