One or two facts, however, lead me to think that whereas, in the East, glass was set in stone or plaster, in the West it was usually set in metal. At Pompeii, as we have seen, panes of glass are set in a bronze lattice and fixed with nuts and screws. As colour was introduced it is probable that from the necessity, already spoken of, of keeping the pieces small, several bits would be joined together with lead to fill one opening of the rigid lattice, and so patterns could be formed. Leo of Ostia says his predecessor, the Abbot Desiderius, filled the windows of the Chapter-House at Monte Cassino in the eleventh century with coloured glass, "glazed with lead and fixed with iron"; and certain it is that the earliest existing windows consist of a large rigid lattice of massive rebated iron bars, in which leaded panels have been placed separately, and held there by light cross-bars passed through staples and keyed with wedges. 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. [Sidenote: The opaque enamel.] 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. [Sidenote: Venetian enamellers.] 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. [Sidenote: Monkwearmouth.]