Firefighters douse a smoldering ridge southwest of Denver with slurry. The blaze has already destroyed 16 homes in the area and continues to spread. (via)
What is slurry?
In general, a slurry is a “thick suspension of solids in a liquid”. But, in particular, the slurry used as a fire retardant in this particular photo, according to the US Air Force, is composed of 80-85% water and 10-15% ammonium sulfate. The ammonium sulfate acts as both a gelling agent and red dye, which helps pilots determine areas that have not already been canvased by previous loads.
These long-term fire retardants are more efficient than plain water, as it works in two phases. First the water extinguishes its portion of the fire, but once the water is completely evaporated, the chemical residue that remains prevents vegetation and other materials from igniting again by binding to cellulose, until it is eroded or washed away with rain.
The residue left over has no ill health affects unless it seeps its way into the water supply, so pilots are careful not to spray near waterways. It also causes no damage to buildings and is relatively easy to clean off, due to its dispersion as a mist. Along with its extinguishing properties, this concoction makes a decent fertilizer.
Read more about slurry here.