What is 'crazing' in relation to glass in fire investigations?

Crazing in the context of glass during fire investigations refers to fine, often shallow cracks that form on the surface of the glass due to rapid cooling or thermal shock. This phenomenon typically occurs when glass is heated and then suddenly exposed to cooler temperatures, which causes the surface to contract more quickly than the internal layers.

When glass is subjected to high heat during a fire, it can expand. If the cooling happens too quickly, the differential temperature change leads to stress on the glass, resulting in these characteristic cracking patterns. Crazing is significant for investigators as it can provide insights into the thermal history of the glass and help establish fire dynamics.

The other options describe different glass behaviors: total fragmentation refers to complete shattering, expanding cracks due to heat suggest a continuous or widening fracture from thermal stress, and surface melting indicates the glass has reached high enough temperatures to lose its structural integrity. While they are related to glass behavior in fires, they do not define crazing specifically.