Chemical Fire Control

Water has the capacity to absorb and carry away heat, even as it is converted into steam. However, water’s strong surface tension causes it to bead up and roll off most fuels and away from heat too quickly to be able to absorb its full heat capacity, especially if quickly converted to steam by a hot fire. Retardants and foams make water more efficient as a suppressant by spreading water out over a surface and penetrating more deeply into porous fuels, much like a detergent extends water’s usefulness for cleaning purposes. Some retardants also lower ignition temperatures, thus quickening fuel conversion while discouraging flaming and hastening extinction.   Fire engines on the ground control operations make use of water and water enhancers such as fire retardants and foams.

The retardant dropped by airtankers and helicopters (sometimes called slurry) consists of water (85%), diammonium phosphate or sulfates (10%), and minor ingredients (5%). The mixture weighs about 9 lb per gallon, or approximately 1 kg per liter. Diammonium phosphate is the active ingredient that inhibits combustion and fertilizes the soil after the fire is out. Minor ingredients include iron oxide (a red dye, allowing the pilot to check the accuracy of the drop by providing visual evidence of spray pattern), and gums and clay thickeners. Use of aerial retardants may be restricted near creeks and streams due to possible toxicity effects on water fauna.