§ 102 The Technique of the Stained Glass Window271
For painting on glass, we must first have a cartoon on which are drawn the outlines of the figures and of the folds of the drapery. These show where the pieces of glass have to be joined; then the bits of red, yellow, blue, and white glass must be picked out and divided according to the design for the flesh parts and for the draperies, as occasion demands. To bring each piece of glass to the dimensions traced on the cartoon, the said pieces are laid on the cartoon and the outline marked with a brush dipped in white lead, and to each piece is assigned its number in order to find it easily when joining them together; when the work is finished the numbers are rubbed off. When this is done, in order to cut the pieces to measure, the workman, having first drawn an emery point over the upper surface of the glass along the outline, which he damps with saliva, takes a red-hot pointed tool and proceeds to pass the point along the outlines, keeping a little within them; as he gradually moves the tool, the glass cracks and snaps off from the sheet. Then with the emery point he trims the said pieces, removing the superfluous part, and with a tool called ‘grisatoio’ or ‘topo’ (grozing iron) which nibbles the traced edges, he makes them exact and ready to be joined all round.
In this manner then the bits of glass fitted together are spread on a flat table above the cartoon, and the artist begins to paint in the shadows over the draperies, using for this the ground scales of iron and of another rust272 found in iron pits, which is red, or else hard red haematite finely ground, and with these pigments he shades the flesh, using alternately black and red, according to need. To produce the flesh tints it is necessary to glaze all the glasses with this red, and the draperies with the black, the colours being tempered with gum,273 and so gradually to paint and shade the glasses to correspond with the tints on the cartoon. When this process is finished, the worker, desiring to put in the brightest lights, takes a short thin brush of hog’s bristles and with it scratches the glass over the light, and removes some of that coat of the first colour that had been given all over, and with the handle of the brush picks out the lights on the hair, the beard, the drapery, the buildings, and the landscapes as he sees fit. There are great difficulties however in this work; and he who delights in it may put various colours on the glass, for example, if he trace a leaf or other minute object over a red colour, intending it to come out in the fire a different tint, he removes from the glass a scale the size of the leaf, with the point of a tool that pares away the upper surface of the glass. This must be the first layer and not more; by so doing, the glass remains white274 and can be tinged afterwards with that red275 made of many mixtures, which when fused by heat becomes yellow. This can be done with all the colours, but the yellow succeeds better on white than on other colours; when blue is used to paint in the ground, it becomes green in the firing, because yellow and blue mixed make a green colour. This yellow is never used unless at the back of the glass where it is not painted,276 because if it were on the face it would mingle and run, so as to spoil and mix itself with the painting; when fired however the whole of the red remains on the surface, this, when scraped away by a tool leaves the yellow visible.277
After the glasses are painted they must be put into an iron muffle with a layer of sifted cinders mixed with burnt lime, and arranged evenly, layer by layer, each layer covered with these ashes; they are then put into the furnace, in which at a slow fire they are gradually heated through till both cinders and glasses begin to glow, when the colours thereon become red hot and run and are incorporated with the glass. In this firing the greatest care must be taken, because a too violent heat would make the glasses crack and too little would not fix the colours. Nor must they be taken out till the pan, or muffle, in which they are placed is seen to be red hot, as well as the ashes, with some samples laid on the top to show when the pigment is liquefied.
After this the leads are cast in certain moulds of stone or iron. The leads have two grooves; that is one on either side, within which the glass is fitted and pressed tight.278 The leads are then flattened and made straight and fastened together on a table. Bit by bit all the work is leaded in many squares and all the joinings of the lead soldered by means of tin soldering irons. Across it in parts are iron rods bearing copper wires leaded in to support and bind the work, which has an armature of irons that do not run straight across the figures, but are twisted according to the lines of the joinings, so as not to interrupt the view of the figures. These are rivetted into the irons that support the whole, and they are made not square but round that they may interfere less with the view. They are put on to the outside of the windows and leaded into holes in the walls, and are strongly bound together with copper wires, that are soldered by means of fire into the leads of the windows. And in order that boys and other nuisances should not spoil the windows, a fine network of copper-wire is placed behind them. These works, if it were not for the too fragile material, would last in the world an infinite time. But for all this it cannot be said that the art is not difficult, artistic, and most beautiful.