§ 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.