The art of stained glass is the translucent offspring of such earlier art forms as mosaic and enamelling. From the mosaicist came the conception of composing monumental images out of many separate pieces of coloured glass. Cloisonné enamelling probably inspired not only the technique of binding these pieces together with metal strips but that for treating the strips themselves as a positive design element. From the enamellers must also have come the near-black vitreous enamel made from rust powder and ground glass that was mixed with a mild water-based glue to form a paint. This could be used to render more or less opaquely onto glass the details of figures, ornaments, and inscriptions.

The technique of making stained-glass windows is first described in the Schedula diversarum artium, a compendium of craft information probably written between 1110 and 1140 by the monk Theophilus (tentatively identified as the 12th-century goldsmith Rugerus of Helmarshausen). First, a full-sized cartoon, or line drawing, of the window was painted directly onto the top of a whitewashed table, showing the division of the various colour areas into individual pieces of glass. Next, sheets of glass of the appropriate colours were selected and from these pieces were cut, or, more accurately, cracked away with a red hot iron. By applying the hot iron to the edge of the sheet it was possible to start a crack that could then be guided more or less in the direction in which the iron was moved, thus enabling the glazier to break away from the sheet of glass a piece of approximately the right shape and size. This he would then further shape by “grozing,” or crumbling away bits of glass from its edges with a notched tool known as a grozing iron. When all of the pieces were thus accurately cut to shape, with due allowance between pieces for the leads that would join them together, the details of the design were painted onto the glass wherever necessary with vitreous enamel. The pieces were then placed in a kiln and fired at a temperature just hot enough to fuse the enamel to the glass. This done, the windows were ready for assembly with grooved strips of lead that look in cross section like the letter H. The glazier would begin by butting together on his workbench two long strips of lead, to form a corner of the panel. He would then set the corner piece of glass in place between these two leads and cut another strip of lead just long enough to surround the rest of the piece. Against this lead he would then be able to set the next piece of glass, and so on across the panel, until it was completely assembled on the glazing bench. The joints between the leads were then soldered, the panel was waterproofed by rubbing a putty compound under the leads, and it was ready for installation.

Because of the flexibility of the leading it was found necessary to divide all but the very smallest windows into a series of separate leaded panels and to insert iron framing members, or armatures, between the panels. In the earliest single-figure lancet windows, such as the Prophets in Augsburg Cathedral, the divisions tend to be purely functional. Very soon, however, more ambitious windows became much too large to be handled in this manner. Whereas the Augsburg Prophets measure only about 12 square feet (1.1 square metres) in area, the Poitiers Cathedral Crucifixion window contains approximately 175 square feet (16.3 square metres) of stained glass, and the Life of Christ in Chartres contains more than 250 square feet (23.2 square metres). A much more elaborate system of subdivisions in the window opening, consisting of vertical as well as horizontal members, was developed. These systems of supports often formed a geometric pattern that was incorporated in the overall design of the window. In fact, it was the ingenious conversion of this structural necessity into a positive design element that set the stage for the creation of the medallion windows of the great Gothic cathedrals. By utilizing these armatures to delineate the principal ornamental subdivisions of the windows, as in the Chartres Good Samaritan, the glass painters were able to fuse a complex didactic imagery and an austere architecture into one of the most compelling artistic unities of Western art. At the same time, particularly in the upper levels of a church, stone mullions began to be employed for the same purpose. The most spectacular examples are the great rose windows, in which masonry is so literally dissolved into fenestration, and the individual window opening so completely absorbed into the overall pattern, as to defy any meaningful distinction between window and wall. This perfect fusion of image, ornament, and structure, with each deriving strengths from the others that none would ever have alone, was one of the most significant turning points in the history of stained glass. From this point on the relation between stained glass and architecture begins to decline. The aims, techniques, and achievements of the stained-glass artist begin to resemble those of the fresco and easel painters, and it is by the standards applicable to the latter that the stained glass of the 14th, 15th, and 16th centuries must be judged.

Developments in the 14th century

The first significant developments in the glass painter’s craft appear to have been made more or less simultaneously in the early years of the 14th century. Glass in a range of previously unavailable secondary colours—smoky ambers, moss greens, and violet—becomes generally available for the first time. The technique of staining glass yellow by painting it with silver salts is discovered. The glass painters also begin to develop a number of techniques for shading or modelling forms with vitreous enamel by applying translucent matts of halftone to the whole surface of the window and delicately brushing it away where highlights are desired. Darker shading is sometimes reinforced by painting on the outer as well as the inner surface of the glass. The uses of line also become increasingly refined and versatile, especially in the 15th century.

To these refinements of the craft was added one wholly new technique, the abrasion of flashed glass. Ruby glass, whose unique composition made this technique possible, was a laminated glass, although it appears to be coloured intrinsically throughout like all of the other glass in the early windows. Because the metallic agent used to produce its colour was so dense, all but the thinnest films of ruby were opaque. To obtain sufficient translucency, either the glassmaker had to suspend striations of ruby in a clear glass, thereby creating the “streaky rubies” of the early 13th century, or the glass was “flashed”; that is, clear glass while still pliant was dipped into molten coloured glass, thus coating its surface with a thin film of colour. Detailed effects, unhindered by intricate leading, could then be achieved by grinding away portions of this coloured film, first on ruby glass and then on other colours deliberately “flashed” for this purpose. To these colours could now also be added the silver salts stain in tones of yellow ranging from the palest canary tint to a deep fiery amber, depending on how heavily the stain was applied and how thoroughly it was fired. The whole gamut of more or less translucent tonalities that could be created with vitreous enamel were also used. Taken altogether, these techniques when used in combination represented a considerable liberation of stained glass from what was increasingly considered to be the “tyranny” of the lead line.

The technique of grinding flashed glass was first practiced in the late 13th and early 14th centuries; one of the earliest extant examples is in the church at Mussy-sur-Seine in France, where the windows have a blue groundwork covered all over, or diapered, with ruby roses with white centres, each rose being a single piece of glass. This type of work, however, was not common until the 15th and 16th centuries.

Later developments

At the end of the 15th century a whole new range of vitreous enamels was developed, and by the middle of the 16th century the technique of painting in enamel colours on glass began to be of major importance. In this method, granulated coloured glass of the desired colour is mixed with a flux of clear ground glass and fired onto the surface of the glass. Enamel painting was not altogether successful either technically or aesthetically, since the colours thus created were translucent rather than transparent, generally pallid, and of uncertain durability. Political disturbances in the mid-17th century created a scarcity of coloured glass throughout Europe, and gradually the traditional use of coloured glass was replaced by the new technique.

Between the 16th and 20th centuries the developments in the craft of making stained-glass windows were purely utilitarian. In the 16th century the diamond glass cutter was invented, and in the 18th century hydrofluoric acid was introduced as a means of etching flashed glass. In the 19th and 20th centuries, gas and electric kilns and soldering irons were used, as were plate-glass easels upon which stained-glass panels could be temporarily mounted for painting before they were leaded. The largest palette of glass—the widest range of colours, textures, and thicknesses that the art has ever known—was also developed in the 20th century. Contemporary technical innovations include the slab glass and concrete windows developed in France about 1930, where glass set in concrete provides an alternative to leading. In the mid-20th century such experimental techniques as bonding glass to glass with transparent resin glues were developed. Measured purely by technical standards, contemporary stained glass has never been rivalled in its versatility as an instrument of artistic expression.