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I have said that stained-glass work is the product of the union of two crafts, the glazier's and the enameller's. The glazier's work being the groundwork of a window, I will take it first.

Glass-making.

What, to begin with, is glass? It is sand melted and run together. The best sand for the purpose is that which is most largely composed of the substance called silica, such as sand formed of powdered quartz, or flint. For some reason, the silica when melted does not recrystallize on cooling as might have been expected, but forms an even transparent substance, plastic while still hot. Think of the tremendous effect this one natural fact has had on the architecture, dress, and, probably, the physique of the nations of northern Europe.

Given sufficient heat, glass can be made by this means alone, but the heat required is so great that it has only been done in recent years for special purposes by means of the electric furnace. Failing this, the sand must be induced to melt at a lower temperature by means of a flux, for which either potash or soda may be used, and to which lime or lead must be added, to enable the glass to resist moisture. (Theophilus, describing the process in his Treatise, certainly no later than the thirteenth century, recommends the use of beech twigs calcined in an earthen pot, whence the name "Pot-ash.")

Colouring.

Glass may be coloured or "stained" while in a molten condition by the admixture of various substances, mostly metals, gold, copper, manganese, and so on, the result depending on the temperature to which it is subjected and on the exact composition of the glass as well as on the colouring matter used.

Blowing.

Glass is made into vessels, as most people know, by "blowing." The workman takes a dab of molten glass on the end of a long metal pipe, and putting his mouth to the pipe blows the glass—soap-bubble fashion—into a hollow bulb. Then by a rapid and dexterous series of alternate and repeated heating, blowing, and spinning, and manipulation with tools, most fascinating to watch, he shapes it into the form required. If a flat sheet be required for window glass it may be run out flat when liquid, or blown as described above, and worked into a cylindrical form, split open, and unrolled. This is called "muff glass." But glass can also be formed, by rapidly spinning it while soft, into a large flat disc, called a "crown," and is then known as "crown glass." It was by these last two methods, the "muff" and the "crown," that all the material of the windows we have to consider in this book was made.

Cutting.

When cold, the sheet or disc of glass may be cut to the shape required, either, as in the old days, by running a hot iron slowly along the proposed line of fracture, in which case a crack will follow the iron, or by scratching it with a diamond and then bending it so as to break along the line of the scratch. The latter is a comparatively modern invention, and has in its turn been superseded by the use of a little steel wheel with a sharp edge.

Pliny's story.

Pliny gives a story of the invention of glass which, if false, is still so picturesque that I cannot resist quoting it here.

A certain merchant-ship touched on the coast of Syria, and the crew landed near the mouth of the river Belus, on a beach of fine white sand which, Pliny says, was still in his day of great repute for glass-making. The ship's cargo consisted of natron—a natural alkaline crystal which was much used in ancient times for washing,[1]—and the crew having lighted a fire on the sand used lumps of it from the cargo to prop up their kettle. What was their surprise to find afterwards a stream of molten glass running down from their camp-fire. In this case the natron acted as a flux and enabled the sand to melt in the heat of the camp-fire, which, however, must have been a very large and hot one.

Egyptian glass.

Now, whether this story is true or not, it cannot have been the beginning of more than a local industry, for the art of glass-making was known in Egypt from very early times indeed.

Its earliest use seems to have been in the imitation of precious stones, and perhaps for this reason it seems from the first to have been made in colours as well as in white; but the art of blowing it into vessels was certainly known in the fourth dynasty, and in some of the paintings in the tombs the process is actually represented.

PLATE III

METHUSELAH,

CANTERBURY, ORIGINALLY IN CHOIR CLERESTORY

Twelfth Century

It was not, however, till the first century of the Christian Era that any one seems to have thought of using glass to fill windows. In Egypt naturally the climate made it unnecessary, and even in Italy, where it can be cold enough in winter, civilization had evolved a style of architecture independent of glass.

Roman windows.

Nevertheless it was introduced in Rome under the first emperors. Caligula had his palace windows glazed, and Seneca mentions it as one of the luxuries which had been introduced into life in his time, but which did not really add to a philosopher's happiness. Its introduction was, however, very gradual, and even two centuries later its use was still quoted as evidence of excessive luxuriousness.

Remains of these Roman windows have been found at Pompeii and elsewhere. At Pompeii they are in the form of small panes of glass held, in one case in a wooden, and in another in a bronze lattice.[2] It must be remembered that large panes were not available. Another method seems to have been to set panes of glass directly into small openings in stone-work.

When coloured glass was first used in windows we have no evidence to enable us to say. As, however, the manufacture of coloured glass was already a flourishing art it cannot have been long before the idea came of using it to decorate windows.

St. Sophia.

Whether the windows of St. Sophia at Constantinople originally had colour in them or not, is not quite certain. That they were glazed we know, from the description of the church by Paul the Silentiary, an officer of Justinian's court, but his language about them is tantalizingly vague. From his enthusiasm at the effect of the sunlight through them I am inclined to suspect that they were coloured, though he does not definitely say so. Of this glass, which seems to have been fixed in small rectangular openings in a slab of alabaster, nothing, I believe, remains; but similar work—coloured—is to be seen in other mosques, the only difference being that the openings in the slab are formed into patterns and kept very small. (I have already mentioned the necessity, when dealing with clear coloured glass, of keeping the pieces small and contrasting them with plenty of solid dark.)

Mahom­medan windows.

This was as far as stained glass in the East ever got. The Mahommedan conquerors seem to have taken the art as they found it, and continued it down without much change almost to modern times. Their religion debarred them from any attempt to represent living forms, so that the art as it stood sufficed for the needs of their architecture. Visitors to Leighton House may see some of these pierced and glazed lattices from Damascus, and very beautiful they are. In them the pieces are not much larger than a penny, and are set in holes cut in plaster slabs, bevelled on the inside, the glass being set at the outer edge of the hole. The glass is not really of very good quality, but treated in this way even thin poor glass looks rich and jewel-like.

Glass in the West.

What course the glazier's art first followed in the West it is impossible to say, for nothing of it remains earlier than the eleventh century, if as early. Nevertheless, in spite of repeated barbarian invasions, it seems never to have quite died out.

The Church, the refuge of the arts and civilization in the general debacle, sheltered it, and from being the luxury of the Roman millionaire it became the ornament of the house of God. From time to time we get allusions to glazed windows, but never a description that can throw much light on their construction or design. Enough is said, however, to show that coloured glass was sometimes used. For instance, we read that St. Gregory of Tours placed coloured windows in the Church of St. Martin in that city in the sixth century.

One or two facts, however, lead me to think that whereas, in the East, glass was set in stone or plaster, in the West it was usually set in metal. At Pompeii, as we have seen, panes of glass are set in a bronze lattice and fixed with nuts and screws. As colour was introduced it is probable that from the necessity, already spoken of, of keeping the pieces small, several bits would be joined together with lead to fill one opening of the rigid lattice, and so patterns could be formed. Leo of Ostia says his predecessor, the Abbot Desiderius, filled the windows of the Chapter-House at Monte Cassino in the eleventh century with coloured glass, "glazed with lead and fixed with iron"; and certain it is that the earliest existing windows consist of a large rigid lattice of massive rebated iron bars, in which leaded panels have been placed separately, and held there by light cross-bars passed through staples and keyed with wedges.

If this conjecture is correct, we may assume that the art of the glazier had for some time been perfected, and had progressed as far as was possible for it unaided, when its union, probably in the tenth century, with that of the enameller gave birth to the art of "stained-and-painted" glass—that is, stained glass as we know it.

The opaque enamel.

Without the use of enamel the glazier's craft must always have been strictly limited to patterns in glass and lead, or, as we now call it, "plain glazing." What was needed to convert it into the art as we know it was the addition of painting in the black or brown monochrome enamel described in the first chapter.

Only one who has worked in glass, and seen his work grow from a map-like combination of white and coloured glass to the finished glass painting, knows the power the enamel gives him of controlling, softening, and enriching his effects of colour. The power it gives of suggesting form is only one, and not the most important, of its functions, and it was as vital to the work of the twelfth and thirteenth as of the fifteenth century. With its introduction the glorious windows of the Middle Ages became possible.

Exactly when and where the application of the enameller's craft to glass windows first took place it is impossible to say with certainty; but there is some reason to suppose that it was in France, and not earlier than the tenth century.

Venetian enamellers.

Enamel—the art of painting on metal with an easily fusible glass ground to powder, which is then fused on to its groundwork in a furnace—was of ancient invention, and had been carried to a high state of perfection in Constantinople in the eighth and following centuries. Thence by way of Venice it had come to France, where a colony of Venetian craftsmen had established itself before the end of the tenth century.

Monk­wearmouth.

France was already famous for its glaziers: for instance, when in A.D. 680 the Abbot, Benedict Biscop, glazed the windows of the monastery at Monkwearmouth, we read in Bede that "he sent messengers to Gaul to fetch makers of glass (or rather artificers) till then unknown in Britain. … They came, and not only finished the work required, but taught the English nation their handicraft"; and it is probable that the French glaziers, chafing under the limitations of their art, called in the aid of the Venetian enamellers. It is noteworthy that no attempt seems to have been made to use transparent coloured enamel on glass. That mistake was reserved for the decadence of the art seven hundred years later. Perhaps experiment convinced them that enamel colour could never hope to rival the depth and richness of coloured glass, and the glazier would realize that what he wanted of the enameller was not colour but black, to modify and enrich the colour which his glass already gave him in full measure. In this book, therefore, the word "enamel," when used in connection with glass, must be understood to refer, unless coloured enamel is specifically mentioned, to this brown opaque enamel or "paint," as glass-workers call it.

Cloisonné.

But the enameller's art had another influence on that of stained glass. A form of enamelling developed at Constantinople and practised at Limoges was that known as "cloisonné." In this, narrow strips of metal are soldered edgeways to the groundwork and the spaces between are filled with differently coloured enamel, the different colours being thus separated by strips of metal.

When the enameller's attention was first turned to glasswork, in which different coloured pieces of glass were separated by strips of lead, he must have been struck with the similarity of the two arts, and have perceived that the style of design already developed in enamel could be applied with little change to glasswork.

This probably explains not only the apparently sudden birth of the art fully formed, but the strongly Byzantine character of the design in the earliest work, the enameller's art having been brought, as we have seen, from Constantinople by way of Venice.[3]

What, then, is the oldest "stained-and-painted" glass in existence? At Brabourne in Kent there is a small window, of which a coloured tracing may be seen in South Kensington Museum, which may belong to the eleventh century. It consists of a simple pattern of white glass and leading, with small pieces of colour inserted at intervals. Some of these latter, however, have been formed into rosettes of simple design by means of opaque enamel, which is the only painting in the window at all. Whatever the actual date of the window, I think it is not unlikely that it shows the manner in which enamel painting and glazing were first combined.