§ 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.