All France was talking of the new works at St. Denis. Never before had been such a gathering of skilled masons and sculptors, of goldsmiths and glassmakers. St. Denis' school was to direct the glassmakers' art through the second half of the XII century. Little is known of the origin of that art; the early basilicas of Christian Gaul had made use of pieces of colored glass framed together, and in the X century figures were represented. No work, however, previous to the XII century has survived. For the earlier fenestration the term "painted glass" is a misnomer, since each piece was colored in the mass, and only a few black lines were applied to denote the features, or the folds of the draperies. The artists of St. Denis obtained their relief effects by a skilled juxtaposition of tones; intensity of hue was increased by the employment of thick rough leaves of glass. Scarcely any white was used; in the ancient windows no spots spring out unpleasantly.
To St. Denis' school succeeded that of Chartres, which predominated during the first part of the XIII century, while its second half was ruled by the school of Paris, when windows of the Sainte-Chapelle type were the rule. Gradually the craftsmen gave up their sound tradition that a window should be a transparent mosaic, subordinate to its architectural setting. They began to treat a window as an isolated picture and the art declined.
Abbot Suger's school of glassmakers carried their art to its zenith. Not all the wonders of XIII-century fenestration equaled the unfathomable vibrant blue in the background of XII-century windows--a fugitive mystery whose secret has been entirely lost. The popular fancy was that Suger ground down sapphires to obtain his magic color.
All over the land the church builders desired windows like those of St. Denis. Suger's own craftsmen went to Chartres to make the three big lancets in that cathedral's western front. The St. Denis school influenced the superb Crucifixion window in Poitiers Cathedral, and others in the cathedrals of Angers and Le Mans and in the Trinité at Vendôme, also the Tree of Jesse window in York Cathedral. And, had the choir glass of Notre Dame at Paris survived, it would have been of the school of St. Denis.