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.
Department of Medieval Art and The Cloisters, The Metropolitan Museum of Art
As a material stained glass is glass that has been colored by adding metallic salts during its manufacture. The colored glass is crafted into stained glass windows in which small pieces of glass are arranged to form patterns or pictures, held together (traditionally) by strips of lead and supported by a rigid frame. Painted details and yellow stain are often used to enhance the design. The term stained glass is also applied to windows in which the colours have been painted onto the glass and then fused to the glass in a kiln.
Stained glass, as an art and a craft, requires the artistic skill to conceive an appropriate and workable design, and the engineering skills to assemble the piece. A window must fit snugly into the space for which it is made, must resist wind and rain, and also, especially in the larger windows, must support its own weight. Many large windows have withstood the test of time and remained substantially intact since the Late Middle Ages. In Western Europe they constitute the major form of pictorial art to have survived. In this context, the purpose of a stained glass window is not to allow those within a building to see the world outside or even primarily to admit light but rather to control it. For this reason stained glass windows have been described as "illuminated wall decorations".
The design of a window may be abstract or figurative; may incorporate narratives drawn from the Bible, history, or literature; may represent saints or patrons, or use symbolic motifs, in particular armorial. Windows within a building may be thematic, for example: within a church – episodes from the life of Christ; within a parliament building – shields of the constituencies; within a college hall – figures representing the arts and sciences; or within a home – flora, fauna, or landscape.
Stained glass is still popular today, but often referred to as art glass. It is prevalent in luxury homes, commercial buildings, and places of worship. Artists and companies are contracted to create beautiful art glass ranging from domes, windows, backsplashes, etc.