of colour, there are, in later work, two exceptions. The most important, which was discovered early in the fourteenth century, is the use of silver stain. It was then found that if white glass is painted with a preparation of silver—either oxide or chloride of silver will do—and then "fired" in the kiln used for the enamel painting, it will be stained a clear and indelible yellow, varying from pale lemon to deep orange, according to the strength of the painting.

Abrasion.

The other exception was Abrasion, effected by the use of what is called "flashed" glass. Flashed glass is glass so made that instead of being coloured all through, it consists of a thin film or "flash" of colour on a backing of white. With this glass it is possible to chip with a burin, or grind away, the coloured film in places (we do it now with hydrofluoric acid) so as to get white and colour on the same piece of glass.

PLATE II
PART OF CRUCIFIXION WINDOW, POITIERS
Late Twelfth Century

In the Middle Ages only red and certain shades of blue were made in this way, so the use of the process was very much restricted. The invention of silver stain, on the other hand, by enabling the artist to decorate his white glass and make it interesting, led him at once to use a larger quantity of white in his window, and so, as will be seen later, had a considerable influence on design.

These, however, are the exceptions which prove the rule, and, broadly speaking, a stained-glass window must consist, to the eye, of flat patches of colour, large or small, worked on with dark monochromatic line work and shading. These patches of colour must each be separated from the next by a black line—the leading—varying from a quarter to half an inch in thickness, and crossed at intervals by still thicker black lines—the iron bars.

Limitations of the art.

It follows from this that anything like illusion is impossible in stained glass, and no artist with any sympathy for the medium would attempt it. Unwise persons in decadent times have wasted much ingenuity in the endeavour, but the result has always been disastrous and ridiculous. Apart from its higher mission,—the expression of ideas and emotions,—which it shares with every other branch of art, the mission of stained glass is to beautify buildings and nothing else. It is the handmaid of architecture, and can only justify itself by loyal service of its mistress. The ideal of the stained-glass artist must not be a picture made transparent, but a window made beautiful.

Let no one suppose, however, that the artist is hampered by these limitations on the higher side of his work. On the contrary, they set him free to tell his story his own way. Ruskin—poor Ruskin, out of date, ridiculed, forgotten—pointed out long ago, in writing of Giotto's frescoes, the advantage which the pure colourist has over the chiaroscurist in his power of telling a story. In our times the fact has been rediscovered with a flourish of trumpets by the Post-Impressionists. I have no great enthusiasm, I confess, for the way in which they have carried out their principles, but I do know two perfect tellings of the story of the Creation. One is in mosaic on the ceiling of the narthex of St. Mark's at Venice, and the other is in the upper part of the east window of York Minster; and in each case the language used consists of flat forms and colour only.

II
THE BEGINNINGS OF STAINED GLASS

II
THE BEGINNINGS OF STAINED GLASS

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