I have a pretty extensive library on the history of glass and crystal making as I descend from over 6 centuries of master glass and crystal makers (maitre verriers). Here are some notes from books long out of copyright (1800s and before 1915) that concern the Rose Windows of Notre Dame: To say this fire was a travesty is such an understatement. Ancient craftsmanship that may not be able to be duplicated today went up in smoke. My heart goes out to all of my French cousins.
...but the artist of the twelfth and thirteenth centuries, as we know from Theophilus, and as was probably the case for long after, did all his work " on the bench." The most he could do would be to hold a few pieces together in his hands up to the light, but for the rest he had to trust to his experience and training. Not quite always did he succeed. Much depended upon his getting just the right quality of blue, and sometimes this seems to have failed him. I have already noticed the rather purplish blue which is found in some of the windows at Lincoln, and this occurs again at Chartres in the central lancet of the apse, and the one next it on the north containing the big angel illustrated in Plate X. This purplish blue when interspersed with red produces at a distance the effect of a rather unpleasant mauve, making these two windows less attractive in colour than the others.
This purplish blue occurs again in the north rose of Notre Dame at Paris, but there the artist has countered it by the use of a good deal of a rather sharp pale green, which completely balances it and turns the window into perhaps the most glorious of all the great rose windows of France.
(the above from page 104 of STAINED GLASS OF THE MIDDLE AGES IN ENGLAND AND FRANCE
PAINTED BY LAWRENCE B. SAINT
DESCRIBED BY HUGH ARNOLD
ADAM AND CHARLES BLACK SOHO SQUARE • LONDON • MCMXIII)
Page 133 of Windows a Book About Stained and Painted Glass by Lewis F Day. London 1897 It follows inevitably from the small scale on which these patterns are set out, and from the radiation of the coloured light, that unless very great discretion is exercised the rays get mixed, with a result which is often the reverse of pleasing. And the worst of it was that the French glaziers particularly affectioned a combination of red and blue most difficult to manage. A very favourite pattern consisted of cross bands of ruby (as above), enclosing squares or diamonds of blue, with dots of white at the intersection of the ruby bands, which persists always in running to purple. Instances of this unpleasant cast of colour are of continual occurrence, but they are never otherwise than crude and plummy in effect. The rather unusual combination of red and green mosaic diaper occurs, however, pretty frequently at Carcassonne. The diapers illustrated indicate the variety of geometric pattern to be found at Bourges, Chartres, Le Mans, and Notre Dame at Paris, and elsewhere. In proportion as there is in them a preponderance of blue and ruby the effect is that of an aggressive purple. The safest plan seems to be in associating with the blue plenty of green, or with the ruby plenty of yellow glass ; or a similar result may be obtained by the choice of a deep neutral blue and of an orange shade of red, taking care always that the two contrasting colours shall not be of anything like equal strength. and page 383: A really garish window may be beautiful as the light wanes. The great North Rose at Notre-Dame (Paris) is impressive at dusk.
From Edward Dillon’s 1907 version of the book Glass, published in both New York and London Pages 132-133:
FRENCH MEDIEVAL GLASS The composition of the window-glass of the thirteenth century is in some ways remarkable. It contained as much as from 8 to lo per cent, of alumina, which we must regard as replacing in a measure the silica, for this constituent falls to as low as 56 per cent., and we can hardly otherwise account for the high percentage of the other bases—14 per cent, of lime, 17 per cent, of potash, and often 3 or 4 per cent, of iron. The result was a tough, somewhat horny glass, hard to work in consequence of the short duration of the viscous stage during the cooling. This was one reason for the smallness of the gatherings, and the modest dimensions of the resultant discs. On the other hand, such glass resists the action of the atmosphere better than any made nowadays, and the large amount of potash present probably promoted the brilliancy of the colours. From the earliest times the blue colouring was given by cobalt, and this was never of a richer and purer tint than in the twelfth century ; already in the thirteenth copper was added to correct a tendency to purple. The famous ruby red, which became rarer after the thirteenth century until in the seventeenth the secret was entirely lost, was produced by the partial reduction of a small quantity of suboxide of copper, but in this case the colour is only developed on reheating the glass. The more purplish tint given by a somewhat similar treatment with gold was not known to the mediaeval glass-maker.^ Manganese was of course the source of the purple—the colour was used for fleshtints in the twelfth century ! The green was made by a mixture of the ces ustum or copper scale with a native oxide of iron, the latter often known as ferretto—of this the best came from Spain. Finally, the yellow was given either by the sesqui-oxide of iron kept well oxidised by the presence of bin-oxide of manganese, or (where the surroundings favoured a reducing action) by a mixture of sulphur and some sooty material which probably yielded an alkaline sulphide. But in the older glass the yellow colour was never very brilliant ; at a later time a fine yellow was obtained by a cementation process from silver, which was applied as a chloride or a sulphide to the surface of the glass
