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.

The preparation of ‘calcedonio in tutta perfezione’ is next described, and I may note that the presence in it of salts of iron and copper, to say nothing of silver, mercury, and azurro, would point to some variegated mixture resembling the schmelz of later days rather than to the opalescent glass to which this name was subsequently given (cf. p. [206]).

Of greater importance than any of these little treatises is the work that Antonio Neri published in 1612. In fact, having regard to the influence of this book on future writers on the subject, especially upon those who sought to make glass by Venetian methods in England and elsewhere, it may without doubt be given the premier place as the most important work that has ever appeared on the preparation of glass. We know very little of the author except that he was born in Florence towards the end of the sixteenth century, that he was a priest, and that he spent some time at Antwerp, where it would seem that his attention was first directed towards the manufacture of glass. When, after the death of the Grand Duke Ferdinand in 1609, the manufacture of the soft-paste Medici porcelain was abandoned, we are told that in its place glass-works were established at Pisa, and with these works we may perhaps connect Neri’s little treatise. I have, however, already gone over most of the ground covered by this book in my quotations from Biringuccio and others, and I will postpone the consideration of what little further is to be gleaned from it until I come, in the account of our English glass, to speak of the translation of Neri’s book made by Merret in 1662.

CHAPTER XIV
THE FRENCH GLASS OF THE RENAISSANCE

In the history of European glass the culminating point is perhaps reached in the Venetian glass of the first half of the sixteenth century—I am speaking, of course, from the artistic point of view. For a century or more after this time our history is concerned with little else than the spread of the Italian methods of manufacture and decoration over the west of Europe. After the middle of the seventeenth century the interest becomes more and more centred in the technical and economical improvements in the manufacture. The invention of plate-glass by the French, in England the use of coal instead of wood in the glass-furnace, and the adoption of a heavy fusible type of glass containing lead (an indirect consequence, perhaps, of this change of fuel)—these are the really notable points in the history of the first century of industrial advance. After the middle of the eighteenth century England takes a more and more important position, and the prominent question was the production of a glass of high technical excellence at a greatly reduced price. Preoccupied as we were at that time with the absorbing interest of this industrial revolution, less attention was given in this country to the artistic side in the manufacture of glass.

In the sixteenth century the interest of our subject centres in the story of the emigration of skilled glass-workers from Venice and from L’Altare, and in the more or less complete replacement of the old methods, as these Italians found their way into nearly every corner of Western Europe. It was technically the victory of the carefully prepared cristallo over the old mediæval verre de fougère or wald-glas. From another point of view the revolution was but one phase in the spread of the Italian renaissance. In fact, in one respect it was distinctly a renaissance, for the glass of Venice in composition differed little from that made during the Roman domination: it belonged essentially to the great Mediterranean family of soda-lime glass, prepared, if not from sea-weed, at least from maritime herbs. On the other hand, the indigenous glass which the cristallo replaced was almost without exception of forest origin, a potash glass made from the roughly lixiviated ashes of beechwood or bracken.