Fire investigators are trained to work backwards to fill in the story. On this page, you’ll obtain information to understand how investigators look for evidence and communicate their claims.
In these side by side burns conducted by the Fire Safety Research Institute we can observe the effect when only one variable is changed – ventilation.
Pay close attention to how ventilation affects the fire, its development and behavior. Then you can look for these patterns in the burn scene.
As you watch the video, look carefully for the evidence a fire leaves behind. Listen for how investigators use this evidence to build a claim about the cause of the fire and where it started.
This will prepare you to conduct your own investigation in the next section.
Investigators look for observable changes that result from a fire, its development, and behavior. Once the observations are made, they are analyzed to determine a pattern.
Let’s take a close look at the vocabulary that investigators use to communicate about their observations in a fire scene.
Nearly all materials expand when heated. This expansion can result in observable changes to the structure after a fire. Bending, buckling, or distortion of shape can all indicate how hot the material got.
Patterns of deformed materials can indicate levels of heat transfer in an area. This helps investigators identify what materials were closest to the fire.
Smoke and soot can settle on surfaces. The hot soot and smoke can only condense on surfaces that are cooler than themselves. Where smoke and soot settle there is a noticeable color change to the surface.
Patterns of discoloration help investigators identify where hot gases flowed during a fire and work backwards to find the origin of the fire.
When the temperature of a material increases it will either change phase (physical change) or go through a chemical change. Objects melt and ignite at different temperatures. Chemical changes often result in an observable change of color to the affected surface. Often this is called char.
Patterns of char can help identify what materials were on fire. Patterns of melting can identify what materials experienced heat transferred from the fire.
Matter cannot be created or destroyed. Materials in a fire room undergo physical and chemical changes to become fuel gases that can burn. This observable mass loss, it is evidence that the object was converted to fuel gases.
Patterns of mass loss are visible and identify which objects were consumed as fuel during a fire. When more mass loss is visible it can indicate the length of time the object was on fire.