We’ve all seen the recent news videos: fixed-wing aircraft swooping down to drop a load of fire retardant alongside the smokey edge of a wildland fire. But did you ever wonder what those fire retardants are made of and how they work?
First, let’s start the composition of fire retardants. Fires are classified by the fuels they consume. Wildland fires consume Class A fuel (solid combustibles materials that are not metals), so the wildland fire retardants are known as Class A foam. One commonly used Class A foam, known as Phos-chek , is made by mixing “ammonium phosphate and diammonium sulfate..into a liquid concentrate that includes thickeners, flow conditioners, corrosion inhibitors and water. The end result is about 88% water and 12% retardant.”
Next, you add air to this concoction to make a foam. Much like shaving cream can be runny or dry, depending on how much you shake the can, the structure of Class A foam depends upon on the amount of air you incorporate. Generally, there are four types of foam, described as solution, wet, fluid or dry.
The type of foam sprayed from aircraft is known as “fluid” foam, which has the consistancy of runny shaving cream. It is often given a red color, so that the air crews can see where it has (and has not) been applied.
Now let’s take a look at how they fire retardant foams work. We’ll start with a classic teaching tool - the fire triangle.
According to this National Park Service website , “The three-sided fire triangle shows that oxygen, heat and fuel in the proper proportions are necessary to create a fire. If any one of these three elements is removed, a fire cannot exist.” To understand how fire retardant foam works, we can organize the details found in this article  from the National Wildfire Coordinating Group around the concept of the fire triangle:
a) “The foam separates the surface of the burning fuel from oxygen in the air.”
b) Flammable vapors are trapped and prevented from spreading.
a) The foam works as an insulator, preventing “heat from escaping and preheating other fuel particles.”
b) The foam “acts as a heat sink by absorbing the heat generated by the combustion process and using this energy to evaporate the water
contained in the foam...”
a) “...the fuel component is blanketed with an opaque layer whose reflective qualities intercept radiant energy and inhibit rekindling.”
b) The retardant “contains wetting agents which facilitate the water’s spread over a large area, allow it to adhere to the fuel surface, and
impart greater penetrating ability.”
Simply put, just like shaving cream moisturizes your skin, fire retardant foam moisturize the fuel. And just like shaving cream provides a physical barrier that protects your skin from the blade, fire retardant foam provide a physical barrier that protects the fuel from heat and oxygen.
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Article: SCV Outdoor Report: Fire Retardant “Shaving Cream” 
Source: Santa Clarita News
Author: Wendy Langhans