The Craft of Stained Glass

Few art forms depend so completely on their surroundings as stained glass. A painting hangs on a wall and looks much the same whatever the weather, but a stained-glass window is nothing without light passing through it. In the dim interior of a great church, the same window may glow with a soft radiance on a cloudy morning and blaze with colour when the sun breaks through. This dependence on light gives stained glass a changing, almost living quality, and it is one reason the craft has been prized for so many centuries as a way of transforming the inside of a building.

The colours of stained glass do not come from paint applied to the surface. Instead, the glass itself is coloured all the way through while it is still molten, by adding small quantities of certain substances to the mixture. Different additions produce different colours: some metals give shades of blue, others produce greens, reds or yellows. Because the colour is part of the glass itself rather than a coating, it does not fade or peel away as a painted surface might, which helps explain why windows made many hundreds of years ago can still shine as brightly as when they were new. Controlling these colours was a matter of skill and experience, and the exact methods used by early workshops were closely guarded.

Making a window was a demanding process that began not with glass but with a drawing. The designer first prepared a full-size plan showing every line and shape of the finished window. Sheets of coloured glass were then cut into the many small pieces this plan required, a delicate task in an age before modern cutting tools. Details that could not be created simply by shaping the coloured pieces, such as the features of a face or the folds of a garment, were added by painting fine dark lines onto the surface of the glass. The painted glass was then heated in a furnace, or kiln, so that the paint fused permanently into the surface and would not wash away. Only after this were the many pieces ready to be joined together.

The joining was done with strips of a soft metal, lead, which could be bent easily around the edges of each piece. Grooved lengths of lead were fitted between the pieces of glass and then joined at their meeting points, holding the whole design together rather like the lines of a drawing. These dark lines were not merely practical; skilful designers used them as part of the composition, letting them outline figures and separate areas of colour. A completed panel was strengthened and made weatherproof, then fixed into the stone framework of the window opening, often supported by iron bars to help it bear its own weight and resist the wind.

For much of its history, stained glass served above all to tell stories. Many of the people who gazed up at these windows could not read, and the great sequences of glowing images acted as a kind of picture book, illustrating tales and lessons for all to see. A single large window might be divided into many small scenes to be read in order, guiding the viewer through a narrative. In this way the windows were not only beautiful but useful, teaching and reminding a congregation of the stories at the heart of their faith while flooding the interior with coloured light.

The craft rose to extraordinary heights during the medieval period, when new styles of architecture allowed builders to replace heavy stone walls with soaring expanses of glass. Later centuries saw changing tastes, and for a time the art declined in some places, with older techniques half forgotten and many fine windows damaged or destroyed. In the nineteenth century, however, a wave of renewed interest led to a revival, as artists studied surviving medieval windows, rediscovered old methods and created ambitious new works of their own. The craft has continued to evolve since then, with modern makers experimenting with abstract designs and new materials while still relying on the same basic principle their predecessors understood: that coloured glass and passing light together can turn a plain opening in a wall into something that seems almost alive.