Crazing

The formation of fine, irregular cracks on the surface of glass or ceramics due to rapid heating and cooling. Crazing patterns can be analyzed to understand the heat conditions during a fire.

Crazing most commonly appears on tempered glass, annealed glass, and ceramic surfaces exposed to fire conditions. Tempered glass tends to shatter into small, relatively uniform pieces rather than developing classic crazing patterns, while annealed (standard) glass is more likely to show the fine network of surface cracks investigators look for. The type of glass and its thickness both affect how and when crazing develops.For investigators, crazing has real value as an indicator of heat exposure, but it has also been widely misunderstood. For decades, some training materials taught that tight, fine crazing patterns indicated rapid heating (and therefore an accelerant), while larger patterns pointed to slow, natural fire growth. Research and testing, including work referenced in NFPA 921, has shown this is not reliable. Crazing patterns are influenced by too many variables, including glass composition, thickness, frame material, ventilation, and water application from suppression, to draw simple conclusions about fire speed.That said, crazing still provides useful information when properly documented. The location and extent of crazing on windows throughout a structure can help establish which areas experienced the most heat exposure and for how long. Investigators should photograph crazing patterns in place before any glass is moved, note the window orientation and room of origin relationship, and record whether suppression water contacted the glass.The key takeaway is that crazing is one piece of the puzzle, not a standalone indicator. It should always be evaluated alongside other physical evidence like char depth, smoke deposits, and witness statements.For more on how to interpret crazing at a fire scene, see our full guide: Understanding Crazing: What It Is, Why It Matters, and How to Interpret (https://www.blazestack.com/blog/understanding-crazing-what-it-is-why-it-matters-and-how-to-interpret).