How, if at all, is depth of char related to how long the surface of a material was exposed to fire?

• A. The shallower the char, the higher the intensity
• B. Char depth does not relate to exposure time
• C. The deeper the char, the higher the intensity of the fire
• D. Char depth decreases with longer exposure

Answer: (C)

When a surface is heated, it undergoes pyrolysis and forms char. As heating continues, more of the surface carbonizes and the char layer thickens. The depth of that char reflects how much heat energy has been delivered to the surface over time—the longer and/or hotter the exposure, the deeper the char can become. So, deeper char indicates the surface has experienced stronger or longer heat exposure, i.e., higher fire intensity at that location.

That’s why deeper char is the best choice: it aligns with the idea that more heat has penetrated the surface, showing a more intense exposure. A shallower char would suggest less exposure, and claims that char depth is unrelated or that it decreases with longer exposure contradict how char forms under heat.