If this conjecture is correct, we may assume that the art of the glazier had for some time been perfected, and had progressed as far as was possible for it unaided, when its union, probably in the tenth century, with that of the enameller gave birth to the art of "stained-and-painted" glass--that is, stained glass as we know it.
Without the use of enamel the glazier's craft must always have been strictly limited to patterns in glass and lead, or, as we now call it, "plain glazing." What was needed to convert it into the art as we know it was the addition of painting in the black or brown monochrome enamel described in the first chapter.
Only one who has worked in glass, and seen his work grow from a map-like combination of white and coloured glass to the finished glass painting, knows the power the enamel gives him of controlling, softening, and enriching his effects of colour. The power it gives of suggesting form is only one, and not the most important, of its functions, and it was as vital to the work of the twelfth and thirteenth as of the fifteenth century. With its introduction the glorious windows of the Middle Ages became possible.
Exactly when and where the application of the enameller's craft to glass windows first took place it is impossible to say with certainty; but there is some reason to suppose that it was in France, and not earlier than the tenth century.
Enamel--the art of painting on metal with an easily fusible glass ground to powder, which is then fused on to its groundwork in a furnace--was of ancient invention, and had been carried to a high state of perfection in Constantinople in the eighth and following centuries. Thence by way of Venice it had come to France, where a colony of Venetian craftsmen had established itself before the end of the tenth century.
France was already famous for its glaziers: for instance, when in A.D. 680 the Abbot, Benedict Biscop, glazed the windows of the monastery at Monkwearmouth, we read in Bede that "he sent messengers to Gaul to fetch makers of glass (or rather artificers) till then unknown in Britain.... They came, and not only finished the work required, but taught the English nation their handicraft"; and it is probable that the French glaziers, chafing under the limitations of their art, called in the aid of the Venetian enamellers. It is noteworthy that no attempt seems to have been made to use transparent coloured enamel on glass. That mistake was reserved for the decadence of the art seven hundred years later. Perhaps experiment convinced them that enamel colour could never hope to rival the depth and richness of coloured glass, and the glazier would realize that what he wanted of the enameller was not colour but black, to modify and enrich the colour which his glass already gave him in full measure. In this book, therefore, the word "enamel," when used in connection with glass, must be understood to refer, unless coloured enamel is specifically mentioned, to this brown opaque enamel or "paint," as glass-workers call it.