Stained glass is considered one of the most vibrant and radiant media in the history of human artistic expression. Particularly popular in the Gothic era, the gem-like windows created an ideal of the church as a metaphor for the New Jerusalem, which is described in the Bible as being composed of precious stones. Stained glass has been produced for around two thousand years, though the early development of stained-glass windows as an art form is somewhat hazy.
There are two basic techniques used in the formation of stained glass. The first method is called the “pot-metal” technique, in which glass is melted in a ceramic pot and metal oxides are added to produce color. The pot-metal method has two basic types: hand-blown and rolled. The second method is called the “flash” technique, which involves fusing a layer of colored glass onto a clear glass pane.
Hand-blown stained glass can be made in one of two ways. The first type is called “cylinder” glass, which involves using a blow-pipe to blow an air bubble into a glob of molten glass and, using metal tools and water-soaked wooden molds, reshaping the glob into a long cylindrical shape. The bottom of the cylinder is removed and one side opened, and it is then flattened and cooled at a controlled rate (called “annealing”) for stability. The second type is called “crown” glass, which is placed on a spinning wheel (much like a potter’s wheel) and flattened using centrifugal force after the air bubble is blown into it. Crown glass is known for its concentric, curving waves and the thicker “bull’s-eye” at the center where the centrifugal force was lowest. Hand-blown cylinder and crown glass were the primary glass types used in older stained-glass windows.
Rolled glass (or “table glass”) is produced by placing a glob of molten glass onto a hard table made of metal or graphite and rolling it like a pie crust using a large metal cylinder. Glass can also be double-rolled, which involves flattening it between two spinning cylinders; this is done when a specific thickness is desired. After flattening, while it is still molten, it can be pushed and tugged to create special textures; in the case of double-rolled glass, the cylinders can have a design engraved on them which will imprint onto the glass while rolling. The technique of rolling glass wasn’t used commercially until the mid-1830s, but it is still widely used today.
To withstand the pushing and pulling of typical winds, architectural glass must be at least 1/8 inch thick. However, with certain colors of glass in which the coloring ingredients must be of a high concentration, such as red glass, 1/8 inch is too thick to transmit adequate light and glass that thick will appear black. The “flash” technique was invented in order to counteract this problem by laminating a small layer of colored glass onto the surface of glass that is either clear or lightly tinted. This method involves taking a glob of molten clear or tinted glass and dipping it into molten glass of the desired color. Then it is flattened using either the cylinder or crown technique. The result is glass that is thick enough to withstand wind loads but still clear enough to transmit light. Another great advantage of flashed glass is that the outer colored layer can engraved or abraded to reveal the clear or tinted glass below, thus allowing rich details to be added to the design without adding more lead lines. At first this was done by grinding off the layer, then hydrofluoric acid was used to remove the flash chemically, and then sandblasting was used from the 19th century onward. Once this technique was discovered for red glass, artists began experimenting with other colors as well.
Ordinary glass is clear to the naked eye (or lightly tinted green when thick enough). In order to achieve color, metallic oxides must be added while the glass is in a molten state. Green glass is typically made using Iron(II) oxide, with the addition of chromium to enrich the color. Tin oxide and arsenic are added to produce an emerald-green color. Blue color was achieved in medieval times using cobalt; since then, however, nickel has been found to produce blue, violet, and black glass at various concentrations, and copper oxide produces turquoise. The rich blue colors of the South Rose Window in Notre Dame, as well as in Creation of the World in Galway Cathedral, show the extensive use of these different elements to create varying shades of blue. Originally, red glass was made by adding gold in very small concentrations. Pure metallic copper has been found to produce a dark, opaque red that is useful for flashed glass, and selenium is useful for lighter red and pink glass.
Yellow glass is made primarily using silver compounds (typically silver nitrate) to produce a rich stain that is flashed on. Different shades and types of yellow are produced using sulphur with carbon and iron salts (amber), titanium (yellowish-brown), cadmium with sulphur (deep, rich yellow), and even uranium (fluorescent yellow or green). Purple glass was originally made using manganese to give glass an amethyst color, which is the oldest known coloring technique; now the correct concentration of nickel is used in to produce violet glass in addition to manganese. Tin oxide with antimony and arsenic oxides will produce white glass, which is highly translucent and even opaque. This was likely the technique used by Bernini for the white dove in Dove of the Holy Spirit.
There are three stages in the production of a stained-glass window. The first stage involves making, or acquiring from the architect, an accurate template of the window opening and then selecting and drawing the subject matter on a full-sized cartoon. This cartoon is usually done on a whitewashed table with then serves as a pattern for cutting and arranging the glass pieces. The positioning of the lead strips that hold the glass pieces together is also noted. The second stage is the selection and painting of the glass. Each piece of glass is chosen with the desired color and cut to match the template cartoon, and the finer details in the design are painted on the inner surface of the glass using a special glass stain. The final stage is assemble and mounting. Once the glass is cut and painted, the pieces are assembled by placing them in the lead strips (called “came”) which are then soldered together. The glass is held in place and kept from rattling by forcing soft, oily cement between the glass and the lead strips. Once the window is constructed it is mounted, sometimes with iron rods placed across it to support its weight. The East Window in Westminster Abbey is a good example of this technique in practice. Each angel, the figure of Mary, the various Nativity pieces, and the symbols of purity on either side of the central window were all first laid out as cartoon drawings.
The exact origins of stained glass and the art of constructing stained-glass windows are uncertain. Colored glass has been produced since ancient times with the Egyptians and Romans excelling at making small, colored glass objects. An earlier form of the stained-glass technique used alabaster, which was revived for Dove of the Holy Spirit; 4th and 5th century Christian churches still contain alabaster windows in wooden frames. Evidence of stained glass in Britain dates back as early as the 7th century, and the production of colored glass in Southwest Asia existed by the 8th century. Stained glass use became even more common during the Romanesque period and peaked during the Gothic era.
As an art form, stained glass reached its height during the Middle Ages as a major pictorial form used to illustrate Bible narratives to a largely illiterate populace. It was particularly popular in Gothic churches whose pointed arches and flying buttresses allowed for thinner walls, which accommodated larger and larger windows as Gothic architecture developed. Like the exterior architecture, the stained-glass windows instructed people through their figurative imagery, but unlike the outer architecture, the stained glass added a spiritual, almost mystical, element. It is estimated that the light outside a cathedral is about a thousand times brighter than inside, so as a person’s eyes adjusted to the dark, the radiant light of the windows stands out with a rich glow that casts beams of light throughout the interior. The circular form known as the “rose window” emerged from France during the Gothic era; it is still used today, such as for The Mysteries of the Rosary in Galway Cathedral.
The stained glass art continued into the Renaissance, the oldest known plan being the six-window scheme devised by Lorenzo Ghiberti for the Florence Cathedral, which includes The Resurrection by Paolo Uccello. During the Reformation, many stained-glass windows in England were destroyed and replaced with plain glass, and the art form was lost in Britain until the early 19th century with a revival of Gothic-style church building. A similar “Gothic revival” occurred in both France and Germany, with older churches being restored and surviving windows being refurbished. Around this same time, the practice of making stained-glass windows travelled to the United States, and newer innovations in the art form continued to develop all the way into the 20th and 21st centuries.
A Window Panes A Thousand Words 