On the whole subject of the glass-painting craft see the Note on ‘The Stained Glass Window,’ postea, p. 308, where the curious confusion of two different processes, between which Vasari’s treatment oscillates, is elucidated.
[269]. The significance of Vasari’s demand for transparency in glass is explained in the Note, postea, p. 308.
[270]. It is somewhat remarkable that the Venetians, who practised the art of glass mosaic from about the ninth century, and in the thirteenth began their famous glass works, never achieved anything in the technique of the stained glass window. Venetian glass vessels, like the glorious lamps from the Cairo Mosques, owe much of their beauty to the fact that the material is not clarified but possesses a beautiful warm tone. It is indeed more difficult to get clear glass than tinted.
[271]. For the most part this description, with the exception of the part about scaling-off glass in order to introduce a variety in colour, corresponds closely with the technical directions which Theophilus gives so fully and clearly in his Schedula Diversarum Artium of about 1100 A.D. It is pretty clear that Vasari is telling us here what he learned from William of Marcillat who would have inherited the traditions of the great French glass-painters of the thirteenth century.
[272]. The ‘scaglia’ is the thin scale that comes off heated iron when cooling under the hammer, and is collected from the floors of smithies. Vasari thinks of it as a ‘rust’ ‘ruggine,’ because rusty iron scales off in much the same way, the cause in both cases probably being oxidization. Hence the expression ‘another rust.’
[273]. The pigments or pastes that are to be fused on to the coloured glass, to modify its hue or to indicate details, are powdered and mixed with gum for convenience in application. The gum is not to serve as permanent binding material as the pastes are subsequently fused and burnt in on the glass.
[274]. It will be understood that the glass subjected to this treatment is not coloured in the mass, or what is called ‘pot-metal,’ but has a film of colour ‘flashed’ or spread thinly on a clear sheet. This is done with certain colours, such as the admired ruby red, because a piece coloured in the mass would be too opaque for effect. Economy may also be a consideration, as the ruby stain is a product of gold.
[275]. The composition, which when fused stains the glass yellow, may before fusion be of a red hue. As a rule the yellow stain on glass is produced by silver. Vasari does not say what his composition is.
[276]. The red film is what Vasari understands by the ‘painting.’ This might fuse and run with the heat required to fuse the yellow.
[277]. That is, the space where the yellow leaf is to come may be cleared of the red film after the yellow leaf has been painted on the back, as well as before that process. The process Vasari describes of introducing small details of a particular colour into a field of another hue is a good deal employed by modern workers in glass, but it was not known to Theophilus, or much used in the palmy days of the art, the twelfth and thirteenth centuries.