officer determine what environmental conditions justify use of short-term retardants, and what 

 conditions force him to apply the more costly, but more effective, long-term type. 



Objectives 



The major objective of this study was to learn the extent to which environment affects the 

 ability of retardants to slow or stop an advancing fire (1) when the degree of retardant dryness 

 changes, (2) when different amounts of retardant material are applied to the fuel bed, and (3) 

 when ammonium salts should be included in the retardant formulation. 



The experiments were of two sorts: 



Drying test. - -Determination of drying rates of several fire -retardant formulations. 



Burning test. --Determination of effectiveness of a particular retardant in a controlled 

 fire situation. 



The study was designed to yield information that could ultimately be used in developing 

 operational guidelines . These guidelines would assist fire-control officers in choosing the 

 proper type of retardant, and in determining how much to use, according to the fuel and environ- 

 mental situation at hand. 



INITIAL CONSIDERATIONS 

 Three major elements received primary consideration in developing the study plan: 



1. Retardant chemicals- -selection of types, amounts, and methods of application. 



2. The composition of fuel beds upon which each retardant would be applied. 



3. Environmental regimes within which the drying and burning tests would be conducted. 



Chemicals 



Retardants now being used against wildfire consist of water thickened by either a natural 

 organic gum, a synthetic organic gum, or a swelling clay. Use of the thickened material 

 assures that a large percentage of the original volume will reach the ground instead of breaking 

 up into a mist and drifting off, that it will cling to all parts of the fuel surface, that it will build 

 up a thick layer of moisture that will be a barrier between the fuel and the flame , and that this 

 moisture barrier will evaporate more slowly than would a thin film of plain water. 



The ability of the thickened material to retard fire is increased substantially by adding a 

 salt- -usually an ammonium salt such as ammonium sulfate or diammonium phosphate. The 

 ammonium salt alters the combustion characteristics of the fuel, causing it to char rather than 

 flame; this reduces heat transfer which, in turn, inhibits spread of the fire. The inhibiting action 

 persists even though the retardant's moisture has evaporated from the surface of the fuel. 



Representative retardants selected for this study are classified by effective life and thick- 

 ening agents (table 1). 



2 



