CHAPTER XV. (XXIX.)
Of Glass Mosaic and how it is recognized as good and praiseworthy.
§ 94. Glass Mosaics.
We have spoken sufficiently above, in the sixth chapter on Architecture, of the nature of mosaic and how it is made, and, adding here just so much as really refers to pictures, let us say that very great mastery is needed to arrange the pieces so harmoniously that the mosaic appears at a distance a genuine and beautiful picture, seeing that this kind of work demands great experience and judgement and a profound knowledge of the art of design. For if any one in his designs obscure the mosaic with too great wealth and abundance of figures in the groups, and with multiplying over-much the pieces, he will bring it all into confusion. Therefore the design of the cartoons made for mosaic must be open, broad, easy, clear, and carried out with excellence and in admirable style.[[243]] The artist who understands the force of shadows in the design and of giving few lights and many darks, leaving in these certain vacant spaces or fields, he above all others will make his mosaic beautiful and well arranged. Mosaic to be praised must have clearness in itself, with a certain harmonious obscurity towards the shadows, and must be executed far from the eye with the greatest discretion that it may be esteemed painting and not inlaid work.[[244]] Therefore the mosaics that have these qualities, are good and will be praised by everyone; and it is certain that mosaic is the most durable picture that exists. Other painting fades through time, but mosaic continually brightens with age; other painting fails and wastes away, while mosaic on account of its long life may almost be called eternal.[[245]] For this reason we perceive in it not only the perfection of the old masters, but also of the ancients[[246]]—by means of those examples from their epoch that we recognize as such to-day, as in the Temple of Bacchus at Sant’ Agnese outside of Rome, where all that is there executed is exceedingly well done.[[247]] At Ravenna also there is some very beautiful old mosaic in many places, and at Venice in San Marco, at Pisa in the Duomo, and at Florence in the tribune of San Giovanni,[[248]] but the most beautiful of all is that of Giotto in the main aisle of the porch at St. Peter’s at Rome[[249]]—truly a miraculous thing in that kind of work—and among the moderns there is that of Domenico Ghirlandaio above the door outside Santa Maria del Fiore that leads to the Annunziata.[[250]]
§ 95. The Preparation of the Mosaic Cubes.
The pieces for mosaic are prepared in the following manner. When the glass furnaces are ready and the pans full of glass, the workers go round giving to every pan its own colour, starting from a true white which contains body and is not transparent, and carefully proceeding to the darker tints by gradual transitions, in the same manner as they make the mixtures of colours for ordinary painting. Afterwards when the glass is fused and in a fit state, and the mixtures both light and dark and of every tint are prepared, they ladle out the hot glass with certain long iron spoons and spread it on a flat piece of marble, then with another piece of marble press it evenly, making round discs that come equally flat and remain the third part of the breadth of a finger in thickness. Then some cut little square pieces with an iron tool called dog’s mouth, and others break it with a hot iron tool, cracking it as they wish.[[251]] The same pieces if too long are cut with emery and so are all the pieces of glass that have need of it. They are then put into boxes and kept arranged as is done with the pigments for fresco work, which are kept separately in various little pots so that the mixtures of the lighter and the darker tints may be ready at hand for working.
There is another sort of glass covered with gold that is used for the background and for the lights of the draperies.[[252]] When the glass is to be gilded, the workers take the glass disc which they have made, and damp it over with gum-water, and then apply the gold leaf; this done they put this gold-covered disc on an iron shovel and that in the mouth of the furnace, first covering with a thin piece of glass all the glass disc that they had coated with gold. These coverings are made either of glass bubbles or of broken bottles so that one piece covers the whole disc, and it is then held in the furnace till it becomes almost red, and quickly drawn out, when the gold at once becomes admirably set so as to be imprinted in the glass and remain there. This is impervious to water and resists every attack, and afterwards the disc is cut and disposed as the other coloured pieces described above.
§ 96. The Fixing of the Mosaic Cubes.
In order to fix the mosaic in the wall, the custom is to make a coloured cartoon, though some make it without colour, and to trace or mark the cartoon bit by bit on the stucco,[[253]] and then to proceed to arrange the pieces as many as are needed to fill in the mosaic work. The stucco, when put on in a thick coat over the wall, remains available two days and sometimes four, according to the kind of weather. It is made of travertine, lime,[[254]] pounded brick, gum-tragacanth and white of egg, and once made it is kept moist with damp cloths. Thus then, bit by bit, they cut the cartoons for the wall, and trace the design on the stucco; afterwards with certain little tongs, they pick up the bits of vitreous paste and fit them together in the stucco, and give lights to the lights, middle tints to the middle tints and darks to the darks, imitating minutely the shadows, the lights, and the half tints as they are in the cartoons.[[255]] Thus, working with diligence they gradually bring it all to perfection, and he who best succeeds in joining it so that it comes out even and smooth, is most worthy of praise and is more esteemed than the others. Some are so clever in working mosaic that they make it appear as if painted in fresco. So firmly does the glass harden into the stucco, after the latter has set, that this mosaic lasts for ever—as is testified by the antique mosaics, which are in Rome, and those also which are of the older (modern) times. In both methods of working the moderns of our days have done marvellous things.