The famous "Chartres blue," seen here in the image of April, from a cathedral window, 12th century

"Chartres blue" could just as well be called "Saint-Denis blue" because it was perfected at these two sites (and a few others) in the middle of the 12th century. This luminous blue glass was made from a sodic flux and colored with cobalt. An extraordinarily stable glass, it has survived the centuries almost unchanged, while reds and greens from the same period have been greatly altered.

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