Glass making at the Duomo construction site went hand in hand with the other aspects of construction and the question of how to complete the windows with glass arose already with the completion of the first architectural structure, the 'aquilonare' or north sacristy.
The first solution put forward (1397) was to insert coloured glass, but in 1403 it was decided that historiated stained glass should be installed in the windows.
The reason for this decision was the ease with which the stained glass could be understood, since the panes were decorated in such a way as to provide a visual account by which God shows himself to his people, the image of the “true light / that lighteth every man” (1 John 1:5), that is Jesus Christ, son of the living God.
A very original style of stained glass windows was created in the Duomo in the second half of the 15th century; it was able to express the perspective boldness inspired by Mantegna and the School of Ferrara by using glass tesserae in a range of colours based on shining and cold tones.
In the first half of the 16th century, it became necessary to repair and put in order all the glass work. In this period, various commissions were given to Pietro da Velate, the Flemish artists Giorgio da Anversa and Dirck Crabeth and the personality of Giuseppe Arcimboldi emerged to create numerous cartoons for the windows, including the one depicting Santa Caterina d’Alessandria. The work of this original artist and of Pellegrino Pelegrini (Four Crowned Saints Window) marked the beginning of a new way of working which clearly distinguished the design phase, entrusted to an artist valid as a painter, but inexpert in glass making techniques, from the production phase, carried out by a skilled master glass-maker.
The processing technique remained almost unchanged until the 19th century when, with the cultural renewal brought about by Lombard Romanticism and the “Gothic Revival” in which Palagi, Sanquirico and Hayez were active, interest in the stained glass in Milan's Cathedral considerably increased.
In this climate of renewed enthusiasm, Giovanni Battista Bertini and his sons worked using a new technique, that of enamel decoration, based on the earlier model of historiated stained glass. The Bertini family made no less than eleven large stain glassed windows from scratch, including the two apse windows.
During the war, all the stained glass windows were taken down and placed in a crypt to protect them from the bombings. Immediately after World War II, the stained glass windows for the facade were made: the Church, the Synagogue, the Trinity, were designed by Giovanni Hajnal, also commissioned to design the stained glass window dedicated to the beatified Cardinals Andrea Carlo Ferrari and Ildefonso Schuster in 1988.
In 1968, the cartoons for the stained glass window on the subject of Maria Mater Ecclesiae and the Conciliar Messages were drawn by various artists from Lombardy (De Amicis, Longaretti, Panigati, Filocamo). A very demanding restoration task was performed very recently (1962-92), that of cleaning, consolidating and repairing the windows, work which has been carried out by the Fabbrica since the 17th century, when the work of adapting and restoring the existing windows began.
Glass deteriorates both due to the effects of human action (fine dust, pollutant and aggressive gases, acid rainfall, vibrations caused by traffic) and due to natural causes (vibrations and corrosion produced by wind, rain and fog, earthquakes…) and attack by a flora of fungus, lichens and moulds as well as colonies of bacteria.
Preservation and restoration of the windows, for obvious reasons dictated by organisation of the work site and safety, was carried out at the same time as restoration of the marble of the walls where the window concerned is located. This long and meticulous restoration work has been performed in full compliance with the protocol signed, after execution of a sample window, between the Fabbrica and Milan's two Superintendency offices. In thirty years (1962-1992), over 1700 sq. m of historiated stained glass windows, plus around 800 sq. m of decorated glass were restored.
For further information, see:
E. Brivio, Le vetrate del Duomo di Milano. Un itinerario di fede e di luce, Milan, NED, 1998