Stained Glass Window Film: Creating stained glass windows

Glass production

From the 10th or 11th century, when stained glass began to flourish as an art, glass factories were set up where there was a ready supply of silica, the essential product of glass manufacture. Glass was usually coloured by adding metallic oxides to the glass while in a molten state in a clay pot over a furnace. Glass coloured in this way was known as pot metal. Copper oxides were added to produce green, cobalt for blue, and gold was added to produce red glass. Much modern red glass is produced using ingredients less expensive than gold and giving a brighter red of a more vermilion shade.

Cylinder glass This glass was collected from the pot into a molten ball and blown, while being continually manipulated until it formed a large cylindrical bottle shape of even diameter and wall-thickness. It was then cut open, laid flat and annealed to make it stable. This is the type of glass most commonly used for ancient stained glass windows.

Crown glass This glass was partly blown into a hollow vessel, then put onto a revolving table which could be rapidly spun like a potter's wheel. The centrifugal force caused the molten material to flatten and spread outwards. It could then be cut into small sheets. This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process. The center of each piece of glass received less force during the spinning, and thus produced was a thicker piece. These centres were for the special effect created by their lumpy, refractive quality. They are known as bull's eyes and are feature of late 19th century domestic leadlight and are sometimes also used with cathedral glass or quarry glass in non-pictorial church windows of that date.

Table glass This glass was produced by pouring molten glass onto a metal table and sometimes rolling it with a large metal cylinder. The glass thus produced is heavily textured by the reaction of the glass with the cold metal. Glass of this appearance is commercially produced and widely used today, under the name of cathedral glass, although it was not the type of glass favoured for stained glass in ancient cathedrals. It has been much used for lead lighting in churches in the 20th century. Modern glass made by this technique is often heavily patterned by the use of an engrave metal roller.

Flashed glass Red pot metal glass was often undesirably dark in colour and prohibitively expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten cylinder of clear glass was dipped into a pot of red glass so that the red glass formed a thin coating. The laminated glass thus formed was cut, flattened and heat annealed.

There are a number of advantages to this technique. It allows a variety in the depth of red ranging from very dark and almost opaque, through ruby red to pale and sometimes streaky red that was often used for thin border pieces. The other advantage was that the red of double-layered glass could be engraved or abraded to allow light to shine through the clear glass underneath. In the late Medieval period, this method was often employed to add rich patterns to the robes of Saints. The other advantage, much exploited by late Victorian and early 20th century artists, was that sheets could be flashed in which the depth of colour varied across the sheet. This was applied to a range of colours. Some stained glass studios, notably Lavers, Barraud and Westlake in England, made extensive use of large segments of irregularly flashed glass in robes and draperies.

Modern production of traditional glass There are a number of glass factories, notably in Germany, USA, England, France, Poland and Russia, which produce high quality glass by traditional methods. Such glass is produced primarily for the restoration of older windows from 1920s and before. The production of new windows in traditional Victorian, Arts and Crafts and early 20th century styles often uses traditional glass. Modern stained glass windows also often use a variety of these different types of glass, or employ commercially made glass.

Creating stained glass windows

The first stage in the production of a 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 to fit.

The subject matter of the window is determined to suit the location, a particular theme, or the whim of the patron. A small 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.

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 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.