Glass used as a semi-transparent sealing for windows, and other openings, was already widespread in ancient times. Fragments of glass have been found in Pompeii, Herculaneum, and Rome and in other cities of the Roman Empire. The technique for the creation of stained glass, unchanged until the 12th Century, is described in the “Diversarum atrium schedulo” written by the monk Teophilus, which explains in minute detail, the procedures to follow and how to make stained glass, ovens and crucibles. To make the drawings, filings and iron oxide were applied to glass. When the mixture was set, the glass was covered with lime and then placed in a wood-fired oven, heating it to a temperature just below its melting point; in the joints/seals between the glass and lead, a putty was applied, consisting of ash and linseed oil, to stop rain water from getting inside.
With the development of new materials and technologies in the 19th and 20th Centuries, the process for making stained glass changed extensively. Nevertheless, great craftsmen still use all the ancient techniques which until now neither machinery nor modern manufacturing systems have been able to replicate, never mind improve.
Paint and materials:
- Grisaille: vitreous paints, usually black or brown in colour. Made up of iron oxide, copper and a flux, borax.
- Enamels: very bright, transparent colours
- Lead: assembly and containment material for the glass.
The restoration of stained glass requires research and close attention to details such as: the era, the techniques used to make the piece, the materials, the setting, the provenance and current condition of the piece. The most common procedures are: strengthening, restoration and cleaning.
Strengthening is carried out on the framework or on the frame supporting the stained glass and lead profiles: after a thorough preliminary examination, we begin strengthening the welding, only replacing the lead profiles that have been irreversibly damaged, because keeping the original lead profiles is an important part of the historical record.
Strengthening is also carried out on the glass and grisaille.
Cleaning is also very important: the layers of dirt and old paint from previous restorations are removed using scalpels, brushes, paint brushes and moistened cotton.