Skip to main content

The main phases of a fire are four: the start of combustion, the spread of fire, the flashover and the extinction.
The time between the first and the second phase depends on different factors, like the flammability of the fuel, the likelihood of propagation of flames, the decomposition speed of fuels involved in the fire, the geometry and volume of the spaces, the possibility of heat dissipation in fuels, the ventilation of the space, the qualities of the surface of fuels and the distribution of fuels in the area.
During the spread of the fire, all materials close to the centre of the fire reach the ignition point, producing flammable gases and other products of combustion, reducing visibility by doing so. In addition to this, toxic and corrosive gases are also produced, with pockets in which gases reach the flammability and explosion limits. In this phase, because of the spreading of fire, combustion increases its speed and temperature and, at the same time, also the radiant energy increases.
During the flashover phase, the building hit by the fire runs the risk of structural failure while the flames could spread to closer buildings. The flame speed and the temperature increase precipitously. So does the combustion gas emission, especially upward.  Fuels closer to the centre of the fire ignite spontaneously, while the farthest reach the combustion temperature; during this phase, intense shock waves can be observed.
When all fuels have completely ignited, combustion starts to slow down, entering the extinction phase. Also temperature decreases, together with speed and radiant energy.
The temperature reached by a fire may vary according to the type of fuel, its quantity and the place where it develops. Approximately, solid fuels can reach a temperature that goes from 700°C to 1200°C, while the temperature of flames goes from 1700°C to 2500°C.