The Process of Creating Stained Glass Windows
The making of stained glass windows in total is a very involved process from beginning to end, which is why they are so expensive.
The first stage in the production of a stained glass window is to make, or acquire from the architect or owners of the building, an accurate template of the window opening that the glass is intended to fit.
The subject matter of the window needs to be determined to suit the location, a particular theme, or the whim of the patron. A smaller design called a Vidimus is prepared which can be shown to the patron.
A traditional narrative window has panels which relate a story. A figurative window could have rows of saints or dignitaries. Scriptural texts or mottoes are sometimes included and perhaps the names of the patrons or the person as whose memorial the window is dedicated. In a window of a traditional type, it is usually at the discretion of the designer to fill the surrounding areas with borders, floral motifs and canopies.
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A full sized cartoon is drawn for every “light” (opening) of the window. A small church window might typically be of two lights, with some simple tracery lights above. A large window might have four or five lights. The east or west window of a large cathedral might have seven lights in three tiers with elaborate tracery. In Medieval times the cartoon was drawn straight onto a whitewashed table, which was then used for cutting, painting and assembling the window.
The designer must take into account the design, the structure of the window, the nature and size of the glass available and his or her own preferred technique. The cartoon is then be divided into a patchwork as a template for each small glass piece. The exact position of the lead which holds the glass in place is part of the calculated visual effect.
Each piece of glass is selected for the desired colour and cut to match a section of the template. An exact fit is ensured by grozing the edges with a tool which can nibble off small pieces.
Details of faces, hair and hands can be painted onto the inner surface of the glass in a special glass paint which contains finely ground lead or copper filings, ground glass, gum arabic and a medium such as wine, vinegar or (traditionally) urine. The art of painting details became increasingly elaborate and reached its height in the early 20th century.
Once the window is cut and painted, the pieces are assembled by slotting them into H-sectioned lead cames. The joints are then all soldered together and the glass pieces are stopped from rattling and the window made weatherproof by forcing a soft oily cement or mastic between the glass and the cames.
Traditionally, when the windows were inserted into the window spaces, iron rods were put across at various points, to support the weight of the window, which was tied to the rods by copper wire. Some very large early Gothic windows are divided into sections by heavy metal frames called ferramenta. This method of support was also favoured for large, usually painted, windows of the Baroque period.
From 1300 onwards, artists started using silver stain which was made with silver nitrate. It gave a yellow effect ranging from pale lemon to deep orange. It was usually painted onto the outside of a piece of glass, then fired to make it permanent. This yellow was particularly useful for enhancing borders, canopies and haloes, and turning blue glass into green glass for green grass.
By about 1450 a stain known as Cousin’s rose was used to enhance flesh tones.
In the 1500s a range of glass stains were introduced, most of them coloured by ground glass particles. They were a form of enamel. Painting on glass with these stains was initially used for small heraldic designs and other details. By the 1600s a style of stained glass had evolved that was no longer dependent upon the skilful cutting of coloured glass into sections. Scenes were painted onto glass panels of square format, like tiles. The colours were annealed to the glass and the pieces were assembled into metal frames.
In modern windows, copper foil is now sometimes used instead of lead. For further technical details, see Lead came and copper foil glasswork.
In the late 19th and 20th centuries there have been many innovations in techniques and in the types of glass used. Many new types of glass have been developed for use in stained glass windows, in particular Tiffany glass and Slab glass.
The color-materials of the glass-maker are chiefly metallic oxides. In their natural state they would not suggest to the layman what glories of tint lie hidden in them, to be brought forth by the heat of the melt.
Of the oxides, the oxide of iron, or plain, common iron rust, is a veritable mother of colors. The colored canyons of the West are largely painted by nature’s iron rust and we get browns, greens, blues, yellows, reds, all from the self-same oxide, either by itself or in combinations.
It is this same oxide of iron that has produced much of the glory of the great cathedrals whose arched and rose windows bring something of Heaven’s sublimity near to man.
The glass-maker produces his wonderful reds by mixing with the batch in the melting-pot a combination of oxide of iron, sub-oxide of copper, a little gold, and silicate of sodium, all in varying proportions.
For blues, he introduces an addition of cobalt, zaffre, and copper.
His greens are won by using various oxides of iron, peroxide of copper, and chromium oxide.
By adding oxide of manganese, oxide of uranium, and perhaps some antimony and silver, he makes the chemistry of heat give him glowing violet. And with oxide of iron, antimony, and a few other chemicals he produces the tints of orange, ranging through all the sunset hues of that gorgeous color.
