Stained glass windows have been an integral part of religious architecture for centuries. The techniques for making them came perilously close to being lost, but have been revived in recent times and now modern stained glass can be found in many public and private buildings. The following is an overview of the glass itself, the process of putting together a window, and the evolution of artistic styles.

The Glass

Making glass is a complex and finicky process. Just the right proportions of ingredients and a steady high temperature are needed. The techniques for making glass evolved over time. The earliest glass often contained imperfections such as uneven tinting, odd colour hues or embedded impurities. Sometimes the artists took advantage of these flaws for artistic effect. For example, purple was very difficult to make, but the uneven waves of colour made a royal robe look like folds of cloth.

The window construction crews set up shop near the site and usually close to a forest so they would have an ample supply of wood for the ovens. There were two ways of making the pieces.

Crown Glass

This is the older of the two techniques:

• Take a glob of glass on the end of a stick, called a punty or pontil.

• Smoosh the glob on a flat surface.

• Twirl the stick.

• You are left with a small round piece (about 500mm in diameter) with thin rounded edges. Cut the stick from the centre of the round piece, leaving a sort of belly button or bull's eye. The surface may have a slight rippling shape, and any bubbles that can be seen appear in a curved pattern. Both sides are shiny because they have been reheated with no contact with another surface. Some small windows were made entirely of these thick perfectly round pieces.

Cylinder Glass

• Blow a hollow tube.

• Cut it down the centre.

• Flatten out the tube into a flat pane.

The result has two shiny surfaces, and any bubbles are stretched out and parallel - this is called a 'seed'. The cut edge may remain sharp but the other edges are fire-rounded. The size of the pane is limited only by the height and strength of the men handling it.

Coloured Glass

The glass was usually dyed in small batches, since it was difficult to consistently dye a large batch. The earliest dyes were different metallic oxides, ground glass from previous windows, and ground mosaic tiles. The monk Theophilus described in exacting detail the methods used to make windows and glass in his treatise Schedula diversarum atrium, written circa 1100 AD. He listed cobalt oxide for blue, copper was used to produce both red and green, yellow was derived from antimone, and purple came from manganese.

The Window

Turning the pieces of glass into a window was a fairly long process.

First, the bay was carefully measured. There was no room for error as the pieces would fall out or crack, should temperature changes make a window expand and contract. A typical bay can be roughly divided into two parts. Long columns at the bottom which usually depict a scene or tell a story, then a large round shape at the top (often a flower) surrounded by smaller decorative shapes to fill in the space. Many variations of shapes and designs exist.

The funding for windows came from many sources. In some respects it was similar to corporate sponsorship today. The nobility, clergy, and even trade unions would contribute money and in return they would be symbolically represented on the window. This could range from a family coat of arms or a likeness of a major donor. Since stained glass windows were largely to be found in churches, the designs usually had a religious theme, and in some cases could portray complex Biblical scenes in an almost comic-book format. They also described historical events such as the Crusades.

Once the design was finalized, a sort of blueprint called a cartoon was painted on a whitewash table that marked the shapes of the pieces and the location of the metal supports. The glass was then layered over it and the design showed through. This process was probably replaced by paper blueprints later on. The pieces were cut and numbered to mark their location in the window.

The artists carefully selected panes of glass for their thickness, colour quality, and any other desirable traits. The designs for each piece were traced onto the glass. The earliest tools were sharp and/or hot metal; later on they used diamond-tipped tools¹. The piece was then gently snapped off and any rough edges smoothed out with a 'grozing iron': a pair of small pincers used to nibble the jagged edges off. After the 14th Century, diamond and other hard stones were introduced but only used on gentle curves. Grozing irons are still used for intricate designs.

Depending on the desired effect, the glass could be etched with acid or painted with a type of enamel. The enamel was made of iron oxide or sometimes copper, glass as flux, gum arabic and wine or urine as a binder. It could be applied either diluted or full strength, and several different methods were used to show gradations of tone:

• Back painting - Painting on back of glass that reinforced it, used especially in dark areas.

• Trace lines - For the thick outlines matting; even colour, black on the reverse.

• Smear shading - Even washes of thin solutions of the paint.

• Stipple shading - A mid-14th Century technique of dabbing with the end of a brush; much quicker than other techniques.

Next, the glass was baked to fix the paint. This was one of the least precise stages of the process. A temperature between 600 - 620°C was needed. If the temperature was too cool, the paint would not fix properly. If it was too hot, the glass would warp. They tended to err on the side of too cool, so many designs wore away quickly due to exposure to the elements.

The metal lines (called ferrament) connecting all the pieces were made of lead since it's malleable and doesn't rust. The ferrament look like 'I' beams with different shapes on the surface. They can be rounded, flat, or peaked. The glass was inserted between the top and bottom ridges. Broken pieces could be easily replaced by peeling back a ridge; this way they didn't have to take down the whole window to repair a missing or cracked piece later on.

Grout sealed the glass pieces inside the lead supports.

A window was subdivided into many panes held together by thick metal frames; each pane contained many smaller pieces held together by the lead lines. The panes could only be about a square metre maximum or else their weight made them unstable and unwieldy. The panes were fitted into the bay with a system of pins to form a complete window.

Windows in History

Antiquity

No one is really sure when stained glass windows first appeared; they were fragile and often didn't survive natural or man-made disasters. Perishable plaster or wood could have been used as supports.

The Romans used glass in windows as one of several translucent materials, along with mica, alabaster (which could be any white stone) and shell. Roman glass probably had the imprint of sand casting on one side. The first decorative windows appearing in Christian churches around 348 - 410 AD are mentioned by Prudentius. They are also mentioned by St Augustine of Hippo, who lived between 354 - 430 AD. There are no surviving windows from before the Sixth Century, however there is literary evidence of them.

Stained glass was reintroduced into Britain via Gaulish churches. To date, the earliest example found goes back to 540 AD.

Ninth Century

Most evidence in existence today dates from the ninth to the 11th Century AD. These pieces are of a high enough standard to suggest a longstanding tradition. However, medieval glass was prone to corrosion and pitting due to the high alkali content caused by the beech-ash mixed in with the sand. Rainwater and condensation allowed chemical attacks to take place so it's difficult to find medieval windows in good condition.

Twelfth Century

The earliest full windows that still exist date from this time period and can be found in places such as Le Mans, Chartres, St Denis, Poitiers, and Angers in France. They usually portray Biblical scenes. Around this time, windows became so ornate that there was a backlash in the religious community. Some religious orders felt they were decadent and banned them and used clear or white glass instead. However, the geometric and floral designs used in the white glass became so intricate that they became just as beautiful and decadent, in their own right.

In most windows from this time, you will see very bright colours, particularly crimson and a deep blue. They let in a lot of light, and often had large, wide, ornate borders. The faces and other details of the human figures were done in three thicknesses of paint: undiluted for the outlines of facial features, then medium to faint for flesh tones. The designs often featured very complex iconography. Since the vast majority of people in these times were illiterate, the windows were a teaching tool to instruct people in spiritual matters and were packed with symbolism.

Thirteenth Century

Gothic architecture came into being at this time and had a profound influence on stained glass windows. The main feature that emerged from of this style of architecture was flying buttresses which were attached to the side of the building and took the weight of the roof. Since the walls no longer had to support all the weight of the roof, large chunks of the walls could be taken away, leaving enormous bays available for windows. The vivid colours continued, with the addition of more realistic flesh tones thanks to a reddish-brown paint that was made from ground rocks that contained iron and rust. The borders became narrower, with simpler geometric or heraldic designs. The nobility had their coats of arms worked in more or less discreetly into the designs, and around this time living people (mostly clergy and nobility) were featured as well as Biblical figures. The human body was also portrayed more realistically than in previous centuries.

Forteenth Century

At this time you can see more sophisticated painting techniques. For example, words could be incorporated into the designs, and the start of rendering a perspective could be seen. As glass making techniques improved, the quality of the glass was more consistent so the colours were even. The colours were more realistic but muted. In particular, a new kind of yellow paint called jaune d'argent made from silver nitrate of silver sulphide debuted. It was used for flesh tones when diluted or gold accents at full strength. Unfortunately, very few windows survive from this time period.

Fifteenth Century

Stained glass windows started to appear in civil buildings such as government offices and trade halls. The style of the windows increasingly resembled paintings since glass dyed in batches was gradually replaced with painted surfaces. New paint colours were developed, including green, blue, pink, and violet used for highlighting and detail work.

Sixteenth Century

The window-making industry by this time was tightly regulated but thriving. This was the peak period of virtuosity in painting on glass, due largely to the discoveries of perspective and other artistic advances during the Renaissance. Most of the existing windows today date from this period, which is surprising considering that many were destroyed during the Reformation. Just as in the 12th Century, the windows were seen as decadent and extravagant, often serving to gratify the ego of the donor rather than glorify God.

Seventeenth and Eighteenth Centuries

Religious wars and the overall expense of producing a window led to a slow decline in the production of stained glass and a different style emerged during this time period. One central image, a person, a coat of arms, or a vignette from the Bible, was surrounded by white glass². This new style was practical since it was cheaper to produce and it let in more pure white light, but it also achieved an aesthetic ideal, trying to reduce the extravagance that the church was widely criticized for. Artists started leaving their signatures embedded in their work. The techniques that were honed during the previous centuries were no longer passed on. Many windows in France were destroyed during and just after the French Revolution in an attempt to obliterate all the symbols of the nobility, including the coats-of-arms embedded in the windows.

Nineteenth Century

By this time, the techniques for making stained glass windows were almost forgotten. Luckily, this century saw a renewed interest in making stained glass windows. Existing ones were repaired or restored as best as possible, and new ones were commissioned again. However, many windows were damaged because they were too harshly cleaned: the replaced pieces often stand out because the colours do not match up well with the surrounding pieces.

Twentieth Century

Stained glass windows made a strong comeback, this time finding their place in all sorts of civil buildings including banks, hotels, theatres, train stations, and apartment buildings. New schools cropped up to teach glass and window making techniques, and artists experimented with deliberate flaws and variations in the glass to add texture and realism to the designs. They also began moulding shapes in bas relief on the glass and marbling colours. New art forms such as art nouveau and art deco were incorporated into the designs, with an emphasis on grey, beige, and yellow tones.

Sadly, in this century as in all previous ones, storms, vandalism, the two World Wars, negligence and pollution all took their toll on windows.

Reading a Window

Appreciating windows can be as simple as gazing at it in full sunshine and admiring its beauty, or approaching it as a very scholarly research project. Some of the factors that historians look at when studying a window include:

• A chemical analysis of the glass to determine when and how it was made.

• Researching the history of the building to discover who sponsored the window and why.

• Taking into account where the window is within the building since this often corresponds to the type of scene depicted. For example, east-facing windows were often done in red to catch the light from the rising sun, and a prominent window might show a scene from the life of the patron saint of the church.

• What the scene is and who is in it.

• The sequence of the panels, especially for windows that tell a story. The panels can be read from top to bottom, or from bottom to top, and it can be difficult to figure out what way the sub-panels surrounding a scene fit into the story, or if they even fit at all or are purely decorative.

• If there is any writing or signatures.

• If there are any restorations or damage already done and what restorations or repairs are needed.

Due to painstaking research, craftsmanship, and advances in technology, they can make almost exact replicas to fill in broken or faded areas. Today windows are recognised as a precious part of historical record and as an architectural and cultural heritage.

Sources

• Brisac, Catherine and Alliou, Didier - Regarder et Comprendre un Vitrail
• Perrot, Françoise and Granboulan, Anne - Vitrail: Art de Lumière

Suggested Links

• Stained Glass by Robert Oddy highlights a modern artist who makes fascinating windows, demonstrating that the art of stained glass is alive and well today.