Conservation of stained glass.
- Steve Clare
The field of stained-glass conservation seeks to ensure the survival of historic stained glass for future generations using best practices established through collaboration with conservation scientists, art historians, conservation architects, and conservators from related disciplines such as stone and metal conservation. Strategies and philosophies for the treatment of stained glass should conform to guidelines for conservation in general which have stressed the need for minimal intervention.
See also Stained glass
1. Evaluation and preventative care.
In this traditional craft established over a thousand years ago, clear and colored glass, produced by a glassmaker, was supplied to the stained-glass artist. This glass was cut to conform to a design, originally employing a red-hot iron, and followed by a notched iron tool known as a grozing iron to achieve the desired shape. In the 16th century diamond cutters were developed, and more recently tungsten wheels have been used to cut glass. The glass was then decorated with vitreous glass paint (grisaille) which was fused to the surface by firing in a kiln. In the 14th century the use of silver-stain to produce a strong yellow color spread rapidly in Europe. The use of this revolutionary technique, in which glass was stained in the kiln with silver nitrate, allowed details such as the hair and nimbus of ecclesiastical subjects to be decorated, and the skill spread rapidly. Later in the 16th century, transparent enamels were developed and used to great effect in heraldic devices.
Kiln-fired glass was held in H-section lead “cames,” thin strips of metal which were soldered together to form portable panels to hold the decorative glass. The lead cames were originally cast in wooden or metal molds, and then machine milled to shape. The completed panels were then mounted in grooves in the stone work of the building, and pointed with lime mortar.
A detailed knowledge of past materials and methods as well as recent developments in the production of stained glass is a necessary part of the broad set of skills and knowledge encompassing the work of the modern stained-glass conservator. A high level of skill and an artistic sensibility is also required of practitioners, notably the ability to paint on glass, as well as the ability to keep meticulous records. Digital recording methods have improved the efficient production and accessibility of photographic and diagrammatic records. Where modern insertions are necessary, proven historic methods and materials, such as hand-made glasses and vitreous, kiln-fired painted detail, are employed. These additions have been routinely marked on the artifact to allow for identification by future craftsmen and researchers. Until the early 20th century, repairs often were limited to the insertion of repair leads into broken fragments of glass, and the aesthetic of mechanically repaired stained glass was valued by 19th-century connoisseurs and art historians such as William Morris and John Ruskin, and contemporary commentators such as Peter Cormack. Beginning in the mid-20th century copper foil techniques associated with Tiffany stained glass and the development of adhesives with similar refractive index to glass allowed more subtle and less visually intrusive repairs to be made while allowing the original artistic intent of the designers to be re-established.
2. Conservation treatment.
It was thought that windows had to be re-leaded at least every century due to the embrittlement of the cames, but this view has been brought into question. It is now realized that the lead matrix routinely lasts for well over a century and should be considered as part of the history of the artifact. In the conservation of the great medieval masterpiece the Tree of Jesse window from Wells Cathedral (Holy Well Glass, 2005–2014), cross-referencing of period-specific mill marks in the center of the leads with archival records provided a definitive restoration history for the window.
Conservation scientists have advised that the action of moisture, predominantly in the form of cycles of repeated condensation, exacerbated by airborne pollutants, has been an important factor in the degradation of medieval glass which incorporated potash as the flux and tends to be more unstable than the soda-lime fluxed glasses commonly used today. This has led to the development of EPG (Environmental Protective Glazing) to protect historic glass. In this system, an external, secondary screen of glass is installed. The interspace between this screen and the stained glass was normally vented to the inside of the building, allowing warmer internal air to circulate between the two layers, acting as a thermal buffer, significantly reducing or negating moisture on the historic glass which can occur in externally vented systems. In some instances systems were vented externally, which worked marginally less efficiently than internally vented examples. In best practice the need for and design of EPG has been considered on a case by case basis. The installation of environmental monitoring equipment can also assist in ensuring that optimal environmental conditions are maintained around stained-glass windows.
- Westlake, N. A History of Design and Painted Glass. London, 1881.
- Brown, S. Stained Glass: An Illustrated History. London: Studio Editions, 1992.
- Davidson, S. and Newton, R. Conservation of Glass. Oxford, 2003.
- English Heritage Practical Buildings Conservation: Glass & Glazing. London, 2011.
- Clare, S. Stained Glass: Art, Craft, and Conservation. London, 2013.