STAINED GLASS, a term that is generally understood to refer only to glass windows that have been coloured by such methods as the fusion of metallic oxides into the glass, the burning of pigment into the surface of white glass, or the joining of white with coloured pieces of glass.
The origin of stained glass is obscure. It probably came from the Near East, the home of the glass industry, and mosaic win dows of glass set in plaster work, which we know from the 17th century in Egypt and elsewhere, are probably of great antiquity. But it is not likely that the art goes farther back than the 9th century; it is doubtful if before that time glass was made in a sufficient variety of colours to suggest and produce coloured designs. The art would most naturally spread first to Italy, and Venice may have been a centre as early as the loth century. An Italian panel in the Victoria and Albert Museum, tentatively ascribed to the 13th or 14th century, is fully Italian and Roman esque in style and suggests a native tradition. Actually the earliest reference to stained glass in the accepted sense of the term (that is, windows not merely coloured, but pictorial also) is in a manuscript which records that Adalberon, bishop of Rheims from 969 to 988, rebuilt the cathedral and redecorated it with windows representing various stories (Richer, vol. ii., lib. 3). An earlier reference, in a 9th century life of St. Ludger, certainly relates to coloured windows, but it is not clear that they were pictorial. Male gives, as the earliest mention of leading, a refer ence to the Miracles of Saint Benoit where it is related that in the last years of the loth century the church of Fleury-sur-Loire was set on fire and it was feared that the leads of the windows would be melted. Which are the earliest windows extant is a matter of dispute, but the evidence, both literary and stylistic, seems to favour certain figures of prophets in Augsburg cathedral, which may date from the middle of the 11th century.
A stained-glass window is a translucent mosaic held together by lead; that is the simplest conception of its technique. Actually other considerations come into play in the design and execution of a window. The lead is not merely a connecting medium, but in all good design plays a part of its own; it outlines the main constituents of the design, giving definition and rhythm to the masses of colour. From the account given by Theophilus, we can derive a very accurate notion of the technique as practised when the art first came to its perfection in the 12th century. The general scheme for the glazing of a church was, of course, the care of the clerics, and we may assume that the artists among them, the illuminators, would prepare the first drawings. One such series of drawings, the Guthlac Roll, is preserved in the British Museum (Harley Roll, Y 6). But the actual cartoon for the glass was drawn on one end of a whitewashed board sup ported on trestles. The vacant space at the other end was used for laying out the glass and for the general business of glazing.
The cartoon was marked to indicate the various colours, and the next step was to shape a piece of glass to the outline required by the design. This was done, firstly and roughly, with a hot iron and then more carefully with a tool known as the "grozing iron," a flat piece of iron with a notch at one end, rather like a modern spanner. This must have been a slow and laborious process, but the grozing iron seems to have continued in use until about 1500, when the modern method of diamond cutting began to be em ployed. The next step, in a fully developed stained-glass window, was to paint the glass with details which the bare design of glass could not give. At first the use of paint was confined to an opaque brown (grisaille, q.v.), used, not as colour, but as a means of outlining the design in further detail, reinforcing the effect of the leads. This pigment consisted of powdered glass mixed with a metallic oxide (probably iron), to which was added sufficient gum to make the mixture adhere. If "high lights" were desired, the whole surface of the particular piece of glass was covered with a thin coating of this paint and the lines scratched through with a pointed stick. Shading effects were produced by stippling with a brush. The glass, having been painted, was next fired in the kiln, to fuse the enamels to the glass. In mediaeval times the glass was fired in a pan, which was filled to the top with alternate layers of glass and whiting. When the glass was fused sufficiently, it was brought out and cooled and then rearranged on the glaz ing table. The next step was the leading. This was (and still is) done by means of strips of lead, in section like the letter H, but with a thicker cross-bar to represent the "core" of the lead ; the upright strokes represent the "tapes." At first the leads were cast, but in the i 7th century the lead vice was introduced. This is a kind of mangle, with two toothed wheels like coins with milled edges, between which a strip of lead is squeezed, the soft metal emerging in the form already described. Sometimes the wheels had the glazier's name engraved on them, which thus became impressed on the lead as it passed through the machine, and these names are occasionally found on old leading. When the leads, cut to the required length and shape, had been inserted between the pieces of grozed glass, they were next soldered together at the points of junction, and cement or putty was rubbed into the crevices be tween glass and lead. The window was then ready to move into place, where it was fixed by means of leaden strips soldered to the leads and attached to iron saddle-bars let into the masonry. Larger windows were made in smaller units and these units fitted into an iron framework or "armature" which itself often formed a geometric design contributing to the general effect of the window.