Last Updated: Nov 22, 2016 Views: 25
From Dr. Robert Brill, Research Scientist Emeritus:
Deformed or sagged pieces of window glass or glass artifacts are often found in the remains of burned buildings. The temperatures reached in a burning building are certainly sufficient to cause pieces of glass to become deformed. The extent of deformation depends upon the maximum temperature the glass is exposed to and the length of exposure. Minimum temperatures of about 700 degrees C (~1300 degrees F) are required to soften most glasses quickly, and they can be attained in a large, intense bonfire. Any debris that comes in contact with the softened glass is likely to become attached to it when the glass sets up. We have frequently seen fire-sagged and deformed glass from ancient archeological sites, for example from Pompeii.
Probably the best way to tell if sagged glass comes from a building fire is just by examining it and examining the context in which it was found. (For instance, were there pieces of dishes or other ceramic wares or other artifacts found nearby?)