There is accordingly in his treatment a confusion between two distinct ideals of the art, one traditional and northern, the other congenial to an Italian painter of the sixteenth century. According to the first ideal of the art, that on which it was founded and nurtured north of the Alps, it depended for its effect upon coloured glass, that is upon the varied tints of pieces of glass stained in the mass with metallic oxides, and called by the moderns ‘pot-metal.’ These different coloured pieces were so arranged and so treated as to give the appearance of figures or ornaments, and to this extent the effect was pictorial, but such a window would depend for its beauty far more on the sumptuous display of coloured light than on any delineation of figures or objects.
The sort of window which would present itself to the Italian of the Renaissance, as representing his ideal of the art, is rather a transparent picture painted on glass, in which delineation is the chief part of the effect. This is the view that Vasari has in mind, when, in § 101, he insists on transparency in the glass employed. The old glass worker of Chartres or the Sainte Chapelle would hardly have known what to do with transparent uncoloured glass, for this, save in pearl borders, was not an element with which he worked. Vasari however starts with the idea of clear glass and imagines it coloured in such a way as to produce a transparent picture. There were two methods for this colouring. The only satisfactory one was to paint in transparent enamel colours which were afterwards burnt in on to the glass. This was a process specially developed in the fifteenth and sixteenth centuries in Flanders, whence it was probably introduced into Italy.
The other method was to employ transparent pigments, such as were used for ordinary painting, and to fix them on the glass by means of gum or varnish. This method is of course a mere pis aller to which no self-respecting worker in glass would like to have recourse, and must be regarded as merely a cheap imitation of true glass-painting in enamel colours. That is to say, it did not precede, as Vasari suggests in § 101, but followed as an imitation, the development of true enamel painting. That the two processes were in use in Italy in Vasari’s time is shown by a contract printed by Gaye, Carteggio, II, 446, in which certain windows to be executed at Arezzo in 1478 are to be ‘cotte al fuoco,’ ‘burned in the fire,’ and not merely to have the colours ‘messi a olio,’ ‘laid on in oil paint.’
The earlier glass workers of the palmy days of the craft, from the twelfth to the fourteenth century, in France, England, Flanders, or Germany, aimed at different effects altogether, and their technique is explained by Vasari in § 102, (ante, p. [268]), where the whole character of the work envisaged differs from the painted work previously in contemplation. As is indicated in a footnote to the text, the description of the work, which starts it will be noticed with ‘bits of red, yellow, blue, and white glass,’ not with a clear pane, is almost exactly what we find in Theophilus, though a little less simple, and represents the early tradition of the mediaeval masters. Their work was the development of an Early Christian technique. Coloured glass, which it must be remembered is really easier to procure than glass perfectly clear, was first used in little rounds or squares for insertion in the holes pierced in marble or plaster slabs that filled window openings. Such window fillings are to be seen in mosques and Byzantine churches. The next stage was a mosaic of pieces of coloured glass arranged on a certain scheme and perhaps displaying geometrical patterns. No specimens of early windows of this kind seem to have survived, but they are referred to in contemporary documents, as in the Liber Pontificalis, where it is said that Leo III, 795–816, in Old St. Peter’s, ‘fenestras de apsida ex vitro diversis coloribus conclusit.’ It is not clear whether such mosaics, or something more pictorial, is referred to by Abbot Gozbert of Tegernsee about the year 1000 A.D. as ‘discoloria picturarum vitra,’ but about this same epoch we find it stated of Archbishop Adalberon of Reims, who died in 989 A.D., that he supplied his church with ‘fenestris diversas continentibus historias,’ which certainly implies something more than the kaleidoscope effect of a mere conjunction of coloured pieces. Theophilus, whose treatment of the process shows that it was fully established at the time of his writing, say about 1100 A.D., makes it clear wherein the innovation consisted. The new invention was that of a pigment, of a brown colour when fused, with which could be painted any details or shading required for representing the forms of objects. A mere patch of pale flesh-coloured glass the shape of a face would tell nothing, but when the features, the locks of the hair, and the like, were painted in with this pigment then the patch became a human countenance. In the same way a piece of red or blue glass with some lines and shading on it became a garment, and so on with the representation in a simple and summary fashion of the objects necessary to constitute the sort of pictorial representation suitable to the technique. The coloured glass remained throughout the essential element in the effect. All the finest glass windows of the twelfth and thirteenth centuries were executed with these simple media. The later history of the art shows as usual a progressive advance in the importance of the pictorial element, till by the sixteenth century coloured glass is scarcely needed, and the pictorial effect desired may be gained by fusing pigments on to clear glass, in the way Vasari contemplates in § 101 (ante, p. [267]).
Of this Italian stained glass of the Renaissance period very good examples are to be seen in the Laurentian Library at Florence, and a specimen is shown on Plate XVII.
Plate XVII
PAINTED GLASS WINDOW IN THE LAURENTIAN LIBRARY, FLORENCE
Chapter XIII of Charles Heath Wilson’s Life and Works of Michelangelo contains a good critical notice of the decorative work at the Laurentian Library. The windows, which were not executed till long after the death of Clement VII whose name appears on the glass, he thinks may be mainly from the designs of Vasari’s friend Francesco Salviati, a pupil of the glass painter Guglielmo da Marcillat. He writes of them:—‘These windows both in design and colour are admirably suited to Italian architecture, and offer useful lessons at the present time. Introduced into a Library where plenty of light was indispensable, white glass prevails. There is much yellow (silver) stain, and where colour is wanted in some parts, pot metal is introduced, but there is not much of it. The shadows are vigorously painted in enamel brown of a rich tone. Unlike modern painted glass, the figures and ornaments are drawn with all the skill of an educated artist, and it is a pleasure to look at them.’
The Victoria and Albert Museum in London affords the opportunity for a comparison of all these different styles. There is some original glass of the thirteenth century from the Sainte Chapelle at Paris, made of small pieces of very richly tinted glass, coloured in the mass, the effect being more that of a very rich and beautiful diaper pattern than a picture; while close by may be seen a Flemish window of 1542, in which the pieces of glass are of large size and in many cases are white, the necessary colouring being added in different enamel pigments. The subject is the Last Supper, and a purely pictorial result is aimed at, the effect of colour being as a fact extremely poor.