Basics of Fire Investigation: Fire and Combustion

What is Fire?

Fire is a chemical reaction in which a material undergoes rapid oxidation, releasing heat and light. Most fires involve the combustion of organic materials. Organic materials are substances that contain carbon, such as wood and plastics. Each material has different physical properties. For example, cotton and gasoline are both organic, but their chemical structures and complexities are different. Therefore, their combustion behaviors also differ.

The Combustion Process

For a material to burn, it must first be heated. Heating causes the material to chemically decompose, releasing gases. In the case of organic solids, this is called pyrolysis. When the released gases mix with oxygen in the air in the right amounts and ignite, combustion begins. Visible flames appear, and the burning process releases heat and light as byproducts.

For combustion to continue, the heat generated must further decompose the fuel, producing more flammable gases. This process requires a sufficient supply of oxygen. If oxygen is limited, combustion weakens, and only smoldering may occur. Smoldering is a slow, flameless form of combustion.

The minimum temperature required to sustain combustion is called the ignition temperature. Ignition means the establishment of a self-sustaining combustion reaction with a proper mix of fuel and oxidizer. Sometimes the term “vapor” is used, but vapor is not a basic state of matter. It refers to a gas formed from a material that is typically solid or liquid at room temperature.

Fire Tetrahedron and Elements of Combustion

The fire tetrahedron illustrates the four elements necessary for combustion: fuel, oxygen, heat, and a chain reaction. Without all four, fire cannot occur.

A simple example is lighting a piece of paper with a candle. The candle provides heat, the paper serves as fuel, and the surrounding air supplies oxygen.
As the candle’s flame heats the paper, it begins pyrolysis, releasing flammable gases. When these gases mix properly with oxygen, ignition occurs.
Once the paper ignites, it can continue burning even if the candle is removed. The burning paper generates enough heat to keep decomposing the unburned paper, sustaining the fire.
During combustion, the paper changes color, chars, and eventually decomposes into ash.

Fire is an exothermic reaction. Once flames are established, they will spread as long as the combustion conditions are maintained. Different fuels can produce smoke, soot, toxic gases, and irritating gases during burning — for example, carbon dioxide (CO₂) and carbon monoxide (CO).

Summary

Fire is a chemical reaction caused by the rapid oxidation of materials. For fire to occur, there must be fuel, oxygen, heat, and a chain reaction. If any of these are missing, fire cannot start.
Fire investigators must thoroughly understand this basic mechanism to determine the cause of fires at the scene.

Reference page: “Key fire characteristics: combustion elements, smoke risks, toxic gases, and safe firefighting techniques.”

References　National Fire Protection Association (NFPA)