A deflagration is a very fast moving and hot fire that moves as heated materials ignite cold ones. This distinguishes it from combustion, a slower rate of spreading, and detonation, an even faster-moving fire that can also create a shock wave and cause additional damage. Fire safety professionals differentiate between these kinds of fires because they need to be handled differently. Likewise, scientists and researchers who work with flammable materials need to be aware of the risks of combustion, deflagration, and detonation.
An example of deflagration can be seen when a panicked cook throws water on a grease fire. Instead of dying down, the fire actually spreads explosively because the water superheats, creating steam that carries heated oil particles to nearby materials. These particles set the formerly cold materials on fire and cause it to spread. The more water the cook adds, the worse the problem will become.
Deflagrations can be challenging for firefighters to manage because they move very quickly and can also be unpredictable. It is possible for a fire to restart abruptly after crews think the fire is out because heated material smoldering underneath can burst into flame again and may also cause neighboring materials to catch fire from the extreme heat. This is one reason why firefighters douse fires very thoroughly, even after they appear to be out, and will stay at the site of a fire to confirm that all the hot spots are eradicated.
It is possible to create an explosion through a deflagration. The rapid increase in temperature can create a pressure differential by heating gases and other materials in the area, and this may erupt explosively without adequate ventilation. The firing mechanism for a gun is an example; the gunpowder ignites through deflagration to explode and force the bullet out with a sudden spike in pressure inside the gun barrel. If the bullet jams, the barrel itself can explode to relieve the pressure.
Consumers need to be aware of the risk of deflagration when they work with very hot materials. Even without an open flame, it is possible for a fire to start through the transfer of heat, and it may be hard to control. This can be especially important while camping and using fires outdoors, as smoldering coals may ignite and cause a fire even after people think their fires are out. Likewise, researchers in lab environments have to control temperatures and materials carefully to limit the chance of explosions, or to create them, depending on what they intend to do.