Last Updated: Aug 08, 2017     Views: 26

Julie Sloan is a Consultant in Stained Glass Conservation & Project Manager --  she has experience in dealing with glass that has been in a fire. She has additional resources on her website, such as links to useful information for stained glass owners and an article list

From her book, Conservation of Stained Glass in America: A Manual for Studios and Caretakers. Art in Architecture Press, 1993, p. 87-88:

Glass that has been exposed to a fire will also develop annealing cracks. While many of these will appear within a day of the fire, the glass may continue to fracture for many years. Alternatively, a piece of glass exposed to fire may show no signs of breakage immediately following the event, but may develop fire cracks years later.
Some internal stress in glass can be seen using a polarizing filter. this is actually a pair of filters between which a piece of glass is placed. Viewing the glass through this sandwich, various stresses appear in bands of light and dark, or as differently colored stripes.

She discusses "fire-damaged stained glass" on pages 111-114:

"Heat causes the most serious damage to stained glass windows. When glass is heated very quickly to a very high temperature and then rapidly cooled, which happens in a fire when the flames are extinguished, erratically curved cracks can result. The most difficult and common type of cracking seen in fire damaged windows, though, is the internal crazing which develops as time passes. For years after a fire, internal cracks may develop which cannot be felt on the surface of the glass. This is due to the complex physical nature of glass...."

Sloan's book provides additional explanation of this process. She also describes the effects of heat on historic glass in the article: Mavis Bimson and A. E. Werner, "The Danger of Heating Glass Objects." Journal of Glass Studies, 1964, pp. 148-150.

Additionally, members of the Stained Glass Association of America (SGAA) may have advice for you. The AATA Online Database provides abstracts of international conservation literature. Once you register to use the site, searching "fire stained glass" or "fire glass" in conservation reports, etc.