stylistic:tone
Windows and stained glass

Increasing the amount of light in the interior was a primary objective of the founders of the Gothic movement. Abbot Suger described the new kind of architecture he had created in the east end of the Saint-Denis: "a circular ring of chapels, by virtue of which the whole church would shine with the wonderful and uninterrupted light of most luminous windows, pervading the interior beauty."

Religious teachings in the Middle Ages, particularly the writings of Pseudo-Dionysius the Areopagite, a 6th-century mystic whose book, De Coelesti Hierarchia, was popular among monks in France, taught that all light was divine. When the Abbot Suger ordered the reconstruction of choir of the abbey church at Saint-Denis, he had the builders create seventy windows, admitting as much light as possible, as the means by which the faithful could be elevated from the material world to the immaterial world.

The placement of the windows was also determined by religious doctrine. The windows on the north side, frequently in the shade, had windows depicting the Old Testament. The windows of the east, corresponding to the direction of the sunrise, had images of Christ and scenes from the New Testament.

In the Early Gothic period, the glass was particularly thick and was deeply coloured with metal oxides; cobalt for blue, copper for a ruby red, iron for green, and antimony for yellow. The process of making the windows was described in detail by the 12th-century monk known as Theophilus Presbyter. The glass of each colour was melted with the oxide, blown, shaped into small sheets, cracked with a hot iron into small pieces, and assembled on a large table. The details were painted onto the glass in vitreous enamel, then baked in a kiln to fuse the enamel on the glass. The pieces were fit into a framework of thin lead strips, and then put into a more solid frame or iron armatures between the panels. The finished window was set into the stone opening. Thin vertical and horizontal bars of iron, called vergettes or barlotierres, were placed inside the window to reinforce the glass against the wind.

The use of iron rods between the panels of glass and a framework of stone mullions, or ribs, made it possible to create much larger windows. The three rose windows at Chartres (1203–1240) each were more than 12 m (40 ft) in diameter. Larger windows also appeared at York Minster (1140–1160) and Canterbury Cathedral (1178–1200)

The stained glass windows were extremely complex and expensive to create. King Louis IX paid for the rose windows in the transept of Notre-Dame de Paris, but other windows were financed by the contributions of the professions or guilds of the city. These windows usually had a panel which illustrated the work of the guild which funded it, such as the drapers, stonemasons, or coopers.

The 13th century saw the introduction of a new kind of window, with grisaille, or white glass, with a geometric pattern, usually joined with medallions of stained glass. These windows allowed much more light into the cathedral, but diminished the vividness of the stained glass, since there was less contrast between the dark interior and bright exterior. The most remarkable and influential work of stained glass in the 13th century was the royal chapel, Sainte-Chapelle (1243–1248), where the windows of the upper chapel, 15 m (49 ft) high, occupied all of the walls on the three sides, with 1,134 individual scenes. Sainte-Chapelle became the model for other chapels across Europe.

The 14th century brought a variety of new colours, and the use of more realistic shading and half-toning. This was done by the development of flashed glass. Clear glass was dipped into coloured glass, then portions of the coloured glass were ground away to give exactly the right shade. In the 15th century, artists began painting directly onto the glass with enamel colours. Gradually the art of glass came closer and closer to traditional painting.

One of the most celebrated Flamboyant buildings was the Sainte-Chapelle de Vincennes (1370s), with walls of glass from floor to ceiling. The original glass was destroyed, and is replaced by grisaille glass. King's College Chapel (15th century), also followed the model of walls entirely filled with glass.

The stained glass windows were extremely complex and expensive to create. King Louis IX paid for the rose windows in the transept of Notre-Dame de Paris, while other windows were often financed by the contributions of the professions or guilds of the city. These windows usually incorporated a panel which illustrates the work of the guild which funded it, such as the drapers, stonemasons, or barrel-makers.

In England, the stained glass windows also grew in size and importance; major examples were the Becket Windows at Canterbury Cathedral (1200–1230) and the windows of Lincoln Cathedral (1200–1220). Enormous windows were also an important element of York Minster and Gloucester Cathedral.

Much of the stained glass in Gothic churches today dates from later restorations, but a few, notably Chartres Cathedral and Bourges Cathedral, still have many of their original windows