The popular name for the glass used in the making of coloured windows. The term is a misnomer, as stained glass is only one of the glasses so employed. It is more the result of a process than a glass per se, as it is produced by painting upon any glass, clear or coloured, with the oxide of silver, which penetrates the glass when subjected to heat and gives a yellowish reaction. In building a coloured window a variety of glass can be used, but usually there is only one kind employed, viz.: pot- metal, a glass that is coloured throughout its substance while in a molten state. This is used either directly or after it has been toned, or ornamented, or made a background for a figure subject by painting the same upon it with vitrifiable pigments, fused to its surface or incorporated with its substance by the means of heat. Nevertheless, although the word stained-glass is inaccurately used, usage has so fixed its erroneous meaning in the public mind that in all probability it will continue for all time to be applied in naming coloured windows and their glass.

Documentary, and, far more, monumental history, demonstrates that glass has been in use from the most remote ages; that the ancients were familiar with it; moreover, that its origin or discovery, or invention is lost in the twilight of fables. In many cases where china and metal are now employed the ancients used glass: they blew, cast, and cut into it thousands of objects with which they furnished tombs and temples, palaces and private houses; and adorned their persons, their garments, and their buildings. It is indeed doubtful if there was any branch of the art of glass-making and the utilization of its products that was not known to them, a fact proved by the fragments of innumerable articles found to- day in countless numbers among the ruins of Egypt, Chaldea, Phoenicia, Greece, and Rome. It is true, however, that the glazing of window openings with glass cannot be traced back beyond the year 306 B. C. At this early date in the Far East coloured windows were made by arranging small gem- like pieces of pot-metal in perforated wooden or stone panels. This kind of window, still in use in the Orient, found its most notable development after the advent of Christianity; but it was not until the birth of Gothic architecture, with its large window-openings, that the full value of glass as a transmitter of light and a polychromatic decorative material was fully appreciated. Gothic window-openings called for a filling strong enough to keep out the weather, yet transparent enough to admit the light; on the other hand, as, in this form of architecture, the wall-spaces were necessarily small, the windows offered the only opportunity for the decorator's art in so far as it depended upon colour. As glass at that time was to be had only in small pieces, the glazier was compelled, in order to fill the window-openings, to make his lights a mosaic, that is a combination of pieces of glass of various sizes and colours worked to a given design by placing them in juxtaposition. These pieces of glass had to be kept in place by some other material, and the best medium for the purpose was found to be lead, applied in strips made with lateral grooves for the reception of the edges of the glass.

The early windows were purely ornamental transparent mosaics; later, when figure subjects were portrayed, the artist, on account of the limitations of the mosaic method, was compelled to use paint in order to get the proper effect, painting directly upon the glass with ordinary transparent pigments; but as this was not durable, when exposed to atmospheric changes, he protected the painted portion by covering it with another piece of glass which was held in place by means of leads, and thus insured its preservation, at least as long as the superimposed glass remained intact. This imperfect method was not long in use before a great discovery was made at Limoges in France, where a Venetian colony of glass-workers had settled as early as the year 979. The new process, which revolutionized the art, consisted in painting with metallic pigments which could be fused into the glass, the painting being thus made as lasting as the glass itself. Not the first, but one of the first, to employ this permanent process of painting on glass to any considerable extent was the great twelfth-century promoter of all things ecclesiological, the Abbot Suger. Recognizing the value of the invention, he caused the windows of the Church of St. Denis at Paris to be executed in this way, and they were so successful that picture-windows became thereafter a necessary constituent of every ecclesiastical edifice.

The oldest painted picture-window that has survived the action of time is one representing the Ascension in the cathedral of Le Mans, which is believed by many antiquarians to be a work of the late eleventh century. The glass composing it is very beautiful, more particularly the browns, which are rich in tone, the rubies, which are brilliant, streaked and studded with gemlike blobs of black, and the blues, which are of a greenish azure hue, while the general colour treatment is extremely oriental. The drawing of the figures is most effective, although simple in line, and Byzantine in character, differing in this point from those at St. Denis, which are Romanesque. The painting is peculiar in that the hair of the figures is rendered in solid black, and not in lines. Although Le Mans was one of the first places where windows made by the new process were used, yet it did not become the centre of work; the city of Chartres took the lead, and became the greatest of the schools of medieval glass-painting, and from it the art slowly made its way to Germany and England, keeping always its essentially French character. Even to-day the Chartres windows are the most beautiful in existence.

At the very beginning -- the eleventh and twelfth centuries -- there were two methods of work: one school of artists freely employed paint in their windows, the other avoided its use, striving to obtain the result sought by a purely mosaic method, a system destined to be revived and developed in after ages; but the former school almost at once gained the mastery and held it for eight hundred years. Examples of the early work of these rival schools can best be studied by comparing the painted windows erected at Le Mans with those at Strasburg, which were built in accord with mosaic motives. In many of the first windows the figure subjects were painted upon small pieces of glass imbedded in a wide ornamental border, a large number of these medallions entering into the composition of a single window, and each section held in place by an iron armature -- a constructive necessity, as the window-openings were without mullions. The medallions were all related to one another through their colour key, depicting various incidents in the same history or a number of points in a theological proposition. This form of window, peculiarly adapted to a single light, continued in fashion from the twelfth century until the introduction of tracery, and in some parts of France long after the single light had given way to the mullioned window. Contemporaneous with these medallion windows there were two other kinds: the canopy and Jesse windows. In the first there was a representation of one or two figures, executed in rich colours on a coloured or white ground within borders and under a low-crowned, rude, and simple canopy, usually out of proportion to the figure or figures it covered. The second variety, of pictorial genealogy of the Redeemer, consisted of a tree or vine springing from the recumbent form of Jesse, lying asleep at the foot of the window, the branches forming a series of panels, one above another, in which kings and patriarchs of the royal house of the Lion of Juda were pictured.

In the future, as in the past, the proper field for this art is an ecclesiastical one. It therefore behooves the artist in glass, if he hopes to reach a high degree of perfection, to study the principles which govern Christian art, and ever to bear in mind that the glazier's art is but an auxiliary to the architect's.

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