Making Mosaic Windows

Making Mosaic Windows

An example of how to make mosaic windows.

Craftsman Style

Here perhaps it will be as well to describe, once for all, the making of a mosaic window, and the part taken in it by the glazier and the painter respectively. It will be easier then to discriminate between the two processes employed, and to discuss them each in relation to the other.

The actual construction of an early window is very much like the putting together of a puzzle. The puzzle of our childhood usually took the form of a map. It has occurred to me, therefore, to show how an artist working strictly after the manner of the thirteenth century, the period, that is to say, when painting was subsidiary to glazing would set about putting into glass a map of modern Italy. In the first place, he would draw his map to the size required. This he would do with the utmost precision, firmly marking upon the paper (the mediaeval artist would have drawn directly on his wooden bench) the boundary line of each separate patch of colour in his design. Then, according to the colour each separate province or division was to be, he would take a separate sheet of "pot-metal" and lay it over the drawing, so as to be able to trace upon the glass itself the outline of such province or division. That done, he would proceed to cut out or shape the various pieces of glass to the given forms. In the case of a simple and compact province, such as Rome, Tuscany, Umbria (overleaf), that would be easy enough. On the other hand, a more irregular shape, say the province of Naples, with its promontories, would present considerable difficulties, difficulties practically insuperable by the early glazier, to whom the diamond as a cutting instrument was unknown, and whose appliances for shaping were of the rudest and most rudimentary.

The Way a Window is Glazed
The Way a Window is Glazed.

If with the point of a red-hot iron you describe upon a sheet of glass a line, and then, taking the material between your two hands, proceed to snap it across, the fracture will take approximately the direction of the line thus drawn. That is how the thirteenth century glazier went to work, subsequently with a notched iron instrument, or "grozing iron" as it was called, laboriously chipping away the edges until he had reduced each piece of glass to the precise shape he wanted.

It will be seen at once that the simpler the line and the easier its sweep the more likely the glass would be to break clean to the line, whereas in the case of a jagged or irregular line there would always be great danger that at any one sharp turn in it the fracture would take that convenient opportunity of going in the way it should not. For example, the south coast of Italy would be dangerous. You might draw the line of the sole of the foot, but when it came to breaking the glass the high heel would be sure to snap off (there is] a little nick there designed as if for the purpose of bringing about that catastrophe), and similarly that over-delicate instep would certainly not bear the strain put upon it, and would be bound to give way. It should be mentioned that even were such pieces once safely cut (which would nowadays be possible) the glass would surely crack at those points the first time there was any pressure of wind upon the window, and so the prudent man would still forestall that event by designing his glass as it could conveniently be cut, without attempting any tour de force, and strengthening it at the weak points with a line of lead, as has been done in the glass map opposite. There is a jutting promontory on the coast of Africa, which, even if safely cut, would be sure to break sooner or later at the point indicated by the dotted line.

The scale of execution would determine whether each or any province could be cut out of a single sheet of glass, but the lines of latitude and longitude would give an opportunity of using often three or four pieces of glass to a province without introducing lines which formed no part of the design. That, however, would be contrary to early usage, which was never to make use of the leads as independent lines, but only as boundaries between two colours. There is a reason for this reticence. You will see that in the surface of the sea, where the latitudinal and longitudinal lines come in most usefully, it is necessary to use also other leads, which mean nothing but that a joint is there desirable. These constructional leads, when they merely break up a background, are quite unobjectionable, they even give an opportunity of getting variety in the colour of the ground, but when some of the leads are meant to assert themselves as drawing lines and some are not, the result is inevitably confused.

All that the glass gives us in our mosaic map is the local colour of sea and land, the sea, let us say, dark blue, the countries, provinces, and islands each of its own distinctive tint. When it comes to giving their names, it would be possible indeed on a very large scale to cut the letters out of glass of darker colour, and glaze them in as shown in the title word "Italy". That would involve, as will be seen, a network of connecting lead lines. On a much smaller scale there would be nothing for it but to have recourse to the supplementary process, and paint them. The words Germany, Austria, Turkey, Naples, Sicily, and the rest would have to be simply painted in opaque colour upon the translucent glass.

But, once we have begun to use paint, there are intermediate ways between these two methods of inscription, either of which would be adopted according to the scale of the lettering. These are shown in the names of the seas. In the word "Mediterranean" each separate letter would be cut out of a piece of glass, corresponding as nearly as possible to its general outline or circumference, and its shape would be made perfect by " painting out"that is to say, by obscuring with solid pigment that part of the glass (indicated by dots in the drawing) which was meant to retire into the background. Presuming this wording to be in a light colour and the background darkish, this amount of painting would, as a matter of fact, be quite lost in the dark colour. In the lesser descriptions "Tyrrhenian" and "Adriatic Sea", each separate word, instead of each letter, would be cut out of one piece of glass (or perhaps two in the longer words), and the background would be painted out as already described.

Paint would further be used to indicate the rivers, the mountains, the towns, or any other detail it was necessary to- give, as well as to mark such indentations in the coastline as were too minute to be followed by the thick lead. As a matter of practice, it is usual to paint a marginal line of opaque colour round the glass representing just a little more than that portion eventually to be covered by the flange of the lead, so as to make sure that that will not by any chance cut off from view what may be an important feature in the design,

For example, the mere projection of a lead which too nearly approached the delicate profile of a small face might easily destroy its outline. The glazier's lead, it should be explained, is a wire of about a quarter of an inch diameter, deeply grooved on two sides for the insertion of the glass. Imagine the surfaces exposed to view on each face of the window to be flattened, and you have a section very much like the letter H, the uprights representing the flanges, and the cross-bar the "core", which holds them together and supports the glass mosaic.

The process of painting employed so far is of the simplest; it consists merely in obscuring the glass with solid paint. This is laid on with a long-haired pencil or "tracing brush". The paint itself may be mixed with oil or gum and water, or any medium which will temporarily attach it to the glass and disappear in the kiln ; for the real fixing of the paint is done solely by the action of the fire. The pigment employed consists, that is to say, of per-oxides of iron and manganese ground up with a sufficient amount of powdered flint-glass or some equivalent silicate, which by the action of the fire is fused with the glass (reduced to very nearly red heat), and becomes practically part and parcel of it.

Whenever a glass painter speaks of painted glass that is what he means viz., that the colour is thus indelibly burnt in. After the middle of the sixteenth century various metallic oxides were used to produce various more or less transparent pigments (enamel colours as they are called to distinguish them from the pot-metal colours), but in the thirteenth century transparent enamel colours were as yet unknown to the glass painter, and he confined himself to the solid deep brown pigment already spoken of, and enamel also, strictly speaking, but by no means to be confounded with the enamel colours of later centuries. Those were colours used for colour's sake; this is simply an opaque substance used solely on account of its capacity to stop out so much of the colour of pot-metal glass as may be necessary in order to define form and give the drawing of detail; and in effect the brown, when seen against the light, does not tell as colour at all but merely as so much blackness. The only colour in the window is the colour of the various component pieces of glass. Thus in the case of an early figure (Early Mosaic Windows) the face would be cut out of a sheet of pinkish glass and the features painted upon it in brown lines; each garment would be cut out of the tint it was meant to be, and the folds of the drapery outlined upon the pot-metaL In like manner a tree would be cut out of green glass, its stem perhaps out of brown, and only the forms of the leaves, and their veining, if any, would be traced in paint. In the execution of the map there is no occasion for further painting than this simplest and fittest kind of work, little more than the glazier would himself have done had his means allowed him. And in the very earliest glass the painter was almost as sparing of paint as this: he did, however it was inevitable that he should use lines, whether in drawing the features of a face or the folds of drapery, which were not quite solid, and which consequently only deepened the colour of the pot-metal, and did not quite obscure it: he went so far even as to pass a smear of still thinner colour, a half tint or less, over portions of the glass which he wished to lower in tone. He began, in fact, however tentatively r to introduce shading. Happily he was careful always to use it only as a softening influence in his design, and never to sacrifice to it anything of the intrinsic beauty and brilliancy of his glass.

The glass duly painted and burnt, the puzzle would be put together again on the bench, and bands of lead, grooved at each side to admit and hold the glass, would be inserted between the two pieces. These would be soldered together at the joints where two leads met; a putty-like composition or " cement" would be rubbed into the interstices between lead and glass to stiffen it, and make it air - and water-tight; and, that done, the window was finished.

It would only remain (what would in practice have been done before cementing) to solder to the leads at intervals sundry loose ends of copper-wire, eventually to be twisted round the iron saddle bars let into the stone framework of the window to support it; it would then be ready to be fixed in its place.

In contradistinction to the mosaic method of execution adopted by the thirteenth century glazier, a glass painter of the eighteenth century, and perhaps of the seventeenth, would, even though there were no necessity for longitudinal and latitudinal lines, cut up his window into oblong pieces of convenient size, only, of course, parallel and at right angles to one another.

The sea he might or might not glaze in blue glass; here and there perhaps, but not necessarily at. all, an occasional province might be leaded in with a piece of pot-metal; but for the most part he would use panes of white glass, and rely for the colour of the provinces upon enamel. He would have no need to separate his enamel colours by a line of lead, and where he wanted a dividing line he would just paint it in opaque brown. This method of glass painting forms an altogether separate division of the subject, not yet under discussion. It is referred to here only by way of contrast, and to emphasise the fact that, though we are in the habit of using the term stained glass rather loosely though a stained glass window is almost invariably helped out to some extent by painting (unless it be what is technically known as "leaded glass" or "plain glazing"), and though a painted window is seldom altogether innocent of glass that is stained there are, as a matter of fact, two methods of producing coloured windows, the mosaic and the enamelled; and that however customary it may be to eke out either method by the other more or less, windows divide themselves into two broad divisions, according as it is pot-metal or enamel upon which the artist relies for his effect.

Between these two widely different ideals there are all manners and all degrees of compromise, and methods were employed which, to describe at this point, would only complicate matters. It will be my purpose presently to describe in detail the steps by which mere glazing developed into painted glass, and how painting came to supersede glazing; to show in how far painting was a help to the glazier, and in how far it was to his hurt; to describe, in short, the progress of the glass painter's art, to better and to worse; and to distinguish, as far as may be, the principles which govern or should govern it.

This is Making Mosaic Windows. is Copyright © 2005-12 by International Styles