Stained glass is one of the most enduring artistic expressions of Western civilisation, combining the craft of glassmaking with the art of painting with light. For over a thousand years, stained glass windows have served as both decoration and narrative medium, teaching biblical stories to largely illiterate congregations and filling sacred spaces with transcendent colour.
The Chemistry of Colour: Metallic Oxides
The colours in stained glass are produced by adding metallic oxides to the molten glass batch. Each metal produces a characteristic colour through its absorption spectrum. Cobalt oxide yields the most intense blue in glassmaking, so powerful that as little as 0.1% by weight produces a rich sapphire blue. The cobalt-stained blue of Chartres Cathedral's twelfth and thirteenth century windows remains one of the most celebrated achievements in glass history.
Copper oxide can produce either green or ruby red depending on the furnace atmosphere. Under oxidising conditions, copper yields a turquoise or emerald green. Under reducing conditions, where oxygen is limited, metallic copper nanoparticles precipitate within the glass and produce a deep ruby red. This ruby glass, known as flashed glass, was often made by fusing a thin layer of red glass onto clear or pale glass, because full-thickness ruby was so dark as to be nearly opaque.
Iron oxide produces colours from pale aqua to deep olive green, and in the presence of manganese can yield amber to brown tones. Gold, in the form of gold chloride, produces a pink to cranberry red, a technique used since Roman times. Selenium combined with cadmium produces yellows and oranges, while pure selenium yields a pink similar to gold but at far lower cost.
Medieval Cathedral Windows
The great era of stained glass was the Gothic period, from roughly 1150 to 1500. Chartres Cathedral, located about 80 kilometres southwest of Paris, contains the most extensive and best-preserved collection of medieval stained glass in the world. Of its original 186 stained glass windows dating from 1200 to 1235, approximately 152 survive, covering roughly 2,500 square metres.
The windows of Chartres are famous for what art historians call "Chartres blue," a distinctive cobalt blue of extraordinary depth created using a soda-rich glass formula. This blue was so prized it became known as "the blue of Chartres" and was widely imitated through the medieval period. The windows tell a comprehensive Christian narrative, from Genesis scenes in the north transept to the life of Christ in the choir, the Last Judgment in the west rose window, and the lives of saints throughout the nave.
Notre Dame de Paris, constructed between 1163 and 1345, contained three great rose windows in the north, south, and west facades. The south rose, dedicated to the New Testament and measuring 12.9 metres in diameter, was a gift of King Louis IX in 1260. Remarkably, all three rose windows survived the devastating fire of April 2019, though some of the smaller lancet windows beneath were damaged.
Other significant medieval cycles survive at Canterbury Cathedral, Sainte-Chapelle in Paris, and York Minster. Sainte-Chapelle, consecrated in 1248, contains 15 towering windows each 15 metres high, with 1,113 narrative scenes depicted in glowing colour.
Tiffany Glass and the Copper Foil Method
Louis Comfort Tiffany, son of the founder of Tiffany and Company, revolutionised American stained glass in the late nineteenth century. Dissatisfied with the limited palette of commercially available coloured glass, Tiffany developed his own formulas and opened furnaces at his Corona, New York studio to produce what he called Favrile glass.
Tiffany's most significant technical innovation was the copper foil method, patented in 1881. Traditional stained glass used lead cames, H-shaped channels into which the glass pieces were slotted. This restricted the artist to relatively simple shapes, as each piece had to be surrounded by continuous lead lines. Tiffany's method wrapped each individual piece of glass around its edge with a thin copper foil tape, then soldered the foiled pieces together with tin solder. The copper foil technique allowed for much smaller, more intricate pieces and three-dimensional assemblies. It made the celebrated Tiffany lampshades possible, enabling complex curved surfaces that lead came construction could not achieve.
Equally important was Tiffany's development of opalescent glass. By adding bone ash or other opacifying agents, he produced glass that was translucent rather than transparent, with swirling internal colour variations that mimicked natural landscapes and fabrics. This glass did not need to be painted on the surface the way traditional cathedral glass did; the colour and texture were inherent in the material.
Modern Architectural Stained Glass
Stained glass has expanded far beyond ecclesiastical settings in the twentieth and twenty-first centuries. Modern artists such as Marc Chagall, Gerhard Richter, and Sigmar Polke have designed major stained glass installations. Chagall's windows for the Hadassah Medical Center synagogue in Jerusalem, installed in 1962, depict the twelve tribes of Israel in luminous, floating colours. Richter's window for Cologne Cathedral, installed in 2007, contains 11,500 squares of mouth-blown antique glass arranged in a grid generated by a mathematical randomisation process.
In contemporary architecture, stained glass techniques merge with structural glazing systems. Dichroic glass, which changes colour depending on the angle of view through thin-film coatings, creates dynamic facades. Fused glass panels allow photographic images to be embedded within laminated glass for large-scale public art installations. The medium that illuminated medieval cathedrals continues to evolve.