How Stained Glass Shapes Light and Colour

Coloured glass can be made by adding small amounts of metal compounds or other materials during production. The colour is not simply paint sitting on the surface. Light passes through the glass and is filtered, so some wavelengths reach your eyes more strongly than others.

Artists select pieces for hue, transparency, bubbles, streaks and thickness. A single red area may contain several shades because handmade glass is not perfectly uniform. These variations make the surface feel active when light moves across it.

Details can also be painted onto the glass with special pigments and then fired so they bond to the surface. Faces, folds, lettering and shadows may use this method. Silver stain can produce yellow tones, which is where the English name stained glass partly comes from.

Traditional windows are assembled from separate glass pieces joined by strips of lead called cames. The lead forms a flexible network that holds the design while allowing small movements caused by heat, wind and the building itself.

These dark lines are structural, but artists also use them as part of the drawing. A lead line can define a figure, divide clothing, create rhythm or guide your eye through the scene. Good designs respect the material instead of pretending the joints are invisible.

Large windows need additional support. Iron bars, frames and stone tracery help carry the weight and resist pressure. The architecture therefore influences the scale and shape of each image. A circular rose window invites a different composition from a tall narrow panel.

A stained-glass window changes with weather, season and time of day. Direct sunlight can make colours intense, while cloudy light produces a softer effect. From outside, the same window may appear dark because the main view is designed for transmitted light from behind.

Historically, many windows communicated religious stories, symbols, donors and community identities. Viewers did not always read them like simple picture books. Their placement, repeated symbols and glowing colour helped shape the atmosphere of the whole space.

When studying a window, notice:

• •Which colours dominate each area.
• •Where painted detail appears.
• •How lead lines organise the image.
• •How architecture divides the composition.
• •How changing light alters the effect.

A traditional window begins as a full-size drawing, called a cartoon, which shows the design at its real scale. From it the maker produces a cutline, a drawing that marks exactly where each piece of glass and each lead line will fall. Glass is chosen sheet by sheet and cut to match the pattern by scoring the surface and snapping it along the line. Painted details are then applied, and the pieces go into a kiln so the paint fuses to the glass. Some pieces are painted and fired more than once.

Once all the glass is ready, the pieces are fitted into the lead cames, starting from one edge and working across the panel. Where the lead strips meet, the joints are soldered on both sides. A putty-like cement is then pushed under the edges of the lead to fill gaps, make the panel weatherproof and stiffen it. Finished panels are fixed into the window opening with supporting bars. Because lead slowly fatigues, old windows are periodically taken down, cleaned and releaded by conservators, who try to keep as much of the original glass as possible.

A popular story claims that old windows are thicker at the bottom because glass is a slow-moving liquid that has flowed downward over centuries. It is a myth. At room temperature glass is so stiff that any flow over a few hundred years would be far too small to measure, and ancient Roman glass and old telescope lenses show no such sagging. If medieval glass really ran like that, much older glass would have melted out of shape entirely.

The real explanation lies in how the glass was made. Medieval glassmakers produced flat sheets by spinning a blob of molten glass into a disc or by blowing a cylinder and slitting it open, and neither method gave an even thickness. When cutting and fitting the pieces, glaziers often placed the heavier edge at the bottom, which is more stable, but in some windows the thick edge sits at the side or top. Uneven thickness is also part of the beauty: the varying depth of colour makes old windows sparkle differently from modern uniform sheet glass.

Red was the hardest colour for medieval glassmakers. Glass coloured with copper turned out so dark that a full-thickness piece let almost no light through. The solution was flashed glass: a sheet of clear or pale glass coated with a very thin layer of red, made by dipping the gathered molten glass into red glass before it was blown into a sheet. Light passed through easily while still glowing deep red. Flashing had a bonus. By grinding or scraping away parts of the thin coloured layer, a craftsperson could create pale patterns or lettering within a single piece, without adding more lead.

The reason copper and gold give such strong reds was only explained in modern times. The colour comes from metal particles far smaller than a wavelength of light, scattered through the glass. Particles this tiny absorb and scatter particular wavelengths depending on their size, an effect now studied in nanotechnology. A famous Roman example, the Lycurgus Cup in the British Museum, looks greenish when lit from the front but glows red when light shines through it, because of tiny particles of gold and silver in the glass. Its makers achieved the effect by craft and experiment, centuries before anyone could see what caused it.

In the late nineteenth century, American artists changed what coloured glass could do. John La Farge and Louis Comfort Tiffany developed opalescent glass, which is milky rather than clear and streaked with swirls of several colours in one sheet. Because the glass itself varied in tone and translucency, it could suggest folds of cloth, clouds, water or petals with little or no painting. Tiffany's workshops also made textured sheets, such as glass rippled while still soft to imitate drapery. Many of their windows look almost like paintings in light, a deliberate break from the medieval method of clear coloured glass plus painted detail.

The workshops also popularised a different way of joining pieces. Instead of fitting glass into lead cames, a worker wrapped the edge of each piece in thin copper foil, laid the pieces together and soldered along the seams. The joints could be much finer than lead, so tiny pieces and tight curves became possible, along with three-dimensional shapes such as the famous domed lampshades. Research on letters published in the 2000s showed that many of the best-known Tiffany lamp designs were created by Clara Driscoll, who led a department of women glass cutters, although for decades the work was credited simply to Tiffany.

Stained glass is built from coloured material, painted detail, lead structure and transmitted light. The joints that support the window also shape its drawing, while the building controls its scale and setting. Look at both the image and the light passing through it, and the artwork becomes a changing part of the architecture.