Cennini was essentially a writer of this class, but in his _Trattato della Pittura_ there are only a few casual references to glass. The three little treatises found by Gaetano Milanesi in the Florentine archives, and published by him in 1864, are chiefly concerned with the preparation of glass for mosaics. They may probably be attributed to the first half of the fifteenth century, and we thus have in the recipes which fill these books the earliest documentary evidence for the composition of Venetian glass. I will quote from the first of these little works a section (xxiii.) which treats of ‘the placing of glass on the surface of glass.’ The writer, it should be noted, is concerned with the preparation of the _piastre_ or slabs from which were cut the little cubes for mosaic work; this question of the various ways in which a leaf of gold may be included between two sheets of glass is one which has already interested us. ‘♃ The glass to be about as thin as an eye-glass. Cut the leaves of the gold to the length of the glass, and put the gold upon the glass with white of egg; then place above this gold the other upper glass, and dry the whole. Then put them in the small ovens (_fornelli_), and let them be on a level so as not to slope, in order that the glass may not run. When they have become red-hot, load them with an iron so that they may grow together and unite. Then place them over the arch of the _fornacetta_ (probably the fritting-oven), and let them cool little by little.’ The next section treats of the preparation of _lattimo bianco_ by calcining four parts of tin and two parts of lead, and then mixing the resulting powder with ten parts of Syrian soda. But as is the case with all the treatises of this class, the majority of the sections are concerned with the preparation of the various ingredients by means of which glass may be coloured—the _colori da ismalti_. The green and opaque red are both obtained from copper-scale, the purple and crimson from various mixtures of manganese[162] (so spelt in the text), and the yellow either from iron-scale or from a mixture of resin and tartar. As for the fine blue—the _zaffiro_—it should be noted that the pigment employed is described as _azurro da vetro_,[163] probably a preparation of cobalt—similar to what in later times was known as smalt—which the glass-workers obtained ready-made from Germany. In the early sections of the third of these little treatises[164] the preparation of the soda is described in some detail. Much importance appears to be attached to the frit, for the third section is headed ‘_Questa si è la pratica di fare la fritta, ciò è li pane del cristallino. Nota ed impara._’ In the composition of this frit there enters not only soda and the white pebbles from the Tecino, but a considerable amount of gromma or tartar, a substance containing potash, and perhaps lime also.