Who were these men and where did they come from? Some would have it that there was a great central school at Chartres, but there is little evidence for it. When Abbot Suger built the great abbey of St. Denis, which was dedicated in 1142, he filled it with glass, "painted," says his secretary, Monk William, "with exquisite art by many masters of _divers nations_" ("de diversis nationibus"). Does this mean that some of them were English or Germans, or only men from other provinces than the Ile de France? No one can say; but they must have been working together to produce the results they did. One statement of the Monk William's leads me to think that the work was done on the spot. He says the work was very costly, because they "used sapphires to colour their glass." Now this is an obvious misunderstanding, due to the practice, in those days, of describing coloured glass by the name of the precious stone it resembled, and such a mistake is most likely to have been made in conversation with the artists themselves. It is, of course, always possible that they had no permanent headquarters but took up their abode in whatever city their chief work was for the time being, there erected their furnaces, which the description of Theophilus shows to have been simple affairs, and remained there till their work was completed--which must have taken some years in every case--and then moved on to the next work. Much has been made of the fact that a window in Rouen bears the signature of one Clement of Chartres,--"Clemens vitrearius Carnutensis me fecit,"--but that window is a hundred and forty years later than St. Denis. By that time the whole Cathedral at Chartres had been filled with glass, a task which extended over thirty years; and Clement may well have learned his trade and passed from apprentice to master there. No other artist of the school has signed his name anywhere, nor has Clement anywhere else. One can tell pretty well the order in which the most important of their work which remains was done. First Chartres and St. Denis,--so near together that one cannot say which came first,--then Canterbury, Sens, and then back to Chartres again, where a fire had destroyed all but the west windows. This, however, probably represents only a small portion of their labours, of which the rest has disappeared. For instance, a few fragments set among later work in York Minster have all the characteristics of this school; and we know that Prior Conrad's choir at Canterbury, which was completed in 1130 and destroyed in 1175, was renowned for the splendour of its glass, which may have been their work too.