Some of the Cathedral's oldest stained glass window panes are on display In a captivating room of the Museo del Duomo di Milano, the staging of which enchantingly recreates their lighting and original colours. The pieces date from between the 15th and 16th centuries and were recovered from warehouses or made available thanks to restoration and reorganization of the stained glass. The panes are arranged in chronological order and grouped according to the window from which they came. The episodes are to be read from bottom to top, with the visit beginning at the right hemicycle.
This collection is an anthology of the Duomo's patrimony of stained glass, which over time has been subject to damage, loss, and improper relocation. Of particular importance are the panes from the San Giovanni Evangelista window, the work of Cristoforo de Mottis, donated to the Duomo in the 15th century by the "Università dei Notai". Many of those from the 16th century focus on the Old Testament, some of which are based on drawings by Giuseppe Arcimboldi (or Arcimboldo) dating from the mid 1500s and then realized by master Flemmish glass-workers. These pieces show signs of his "whimsical" style, but traces now exist only in the registries of the Veneranda Fabbrica del Duomo di Milano's Archives.
But how were these tesserae, masterpieces of glass-making, produced? What technique was used and how can they be restored today?
The windows are an arrangement of glass tesserae that interpret a drawing by an artist or master glass-maker. After the glass is coloured with metallic oxides in the semi-liquid state, it is cut to create tesserae that correspond to this drawing. Finally, the glass tesserae receive a graphic treatment that reproduces the faces, draperies, chiaroscuro effects, and details of the drawing through the apposition of a grey-brown substance known as grisaille, applied using a paint brush, that is incorporated into the supporting glass when it is cooked in a specific kind of oven (muffola) at approximately 800°.
The tesserae are later joined together according to the original drawing using lead profiles welded together with tin, thus creating a panel also known as a "capitulus" or rectangular pane or mixtilinear panel that is then inserted into a metal framework and finally positioned in the windows using clamps and hub pins.
As for the restoration, this entails a careful and meticulous analysis of the glass: dating, degree of alkalinity, impurities, bubbles and swelling, nature of the pigments, processing; its state: simple breaks, radiating breaks, cracks, flaking, formation of craters due to corrosion, moulds and other organic substances; nature and texture of the grisaille: good bonding, tendency to flake or crumble; flexibility of the connection, modifications to the layout as compared to the original drawing, past restorations which may have included the insertion of tesserae from other eras and with different colour tones, fired or cold retouches, etc.
Clearly, to proceed with these investigations, the archival documents and specifically, for the Duomo di Milano, the chapter ordinances, registries, and payment orders must be examined in order to be able to identify the precise date of creation. Furthermore, many panes have been moved and tampered with, thus altering their chronological order, the reason for which it is necessary to identify their original, or most historically reliable, positioning and to reconstruct the window's original message.
The following stage entails the appositioning of the photographic materials corresponding to each single pane, backlit in a darkroom so as to identify the potential cause of damage, often from natural circumstances, (humidity, wind, dust, temperature variations), pollution, or mechanical factors (bumps, sound waves, other materials falling on them).
This is where the real restoration work begins, and continues as we speak in the artistic glass workshops that collaborate with the Veneranda Fabbrica.