INFLUENCE OF MOISTURE ON EFFECTIVENESS 

 OF FIRE RETARDANTS 



Richard C. Rothermel and Charles E. Hardy 



INTRODUCTION 

 The Problem 



Application of fire-retardant chemicals is fast becoming an integral part of control pro- 

 cedures on forest, range, and brushland fires. Each year new fire-retardant chemicals or 

 combinations of chemicals are introduced, and existing retardants are improved. Fire-control 

 personnel urgently need more information about the effectiveness of present types of retardants 

 as influenced by the environment. 



A complete series of tests for each new or modified product is not economically feasible. 

 Instead, procedures are being developed to classify fire-retardant chemicals so that a minimum 

 number of tests will make it possible to list each one within a group having similar 

 characteristics. 



All research reported in this publication was performed at the Northern Forest Fire 

 Laboratory, Missoula, Montana. It explored the relation between effectiveness of each fire 

 retardant and fuel moisture content as influenced by such environmental conditions as wind 

 velocity, humidity, and temperature . 



Fire retardants may be classified generally as short-term or long-term retardants. 

 Short-term retardants rely entirely upon the water they contain to prevent combustion. Long- 

 term retardants contain, in addition to water, a chemical that effectively prevents flaming 

 combustion even after the water has evaporated. 



Much conjecture surrounds the question of comparative evaporation rates among the 

 various retardant materials. If differences do exist, the slowest drying material is the most 

 desirable one to use in fire control. Determination of evaporation rates in this study is impor- 

 tant not only as a prerequisite to the burning tests, but also as an aid in selecting the most 

 effective retardant thickening or water -holding material. 



Results from earlier research at the Northern Forest Fire Laboratory 1 indicated that 

 long-term retardants --those containing fire -retarding salts --are markedly more effective than 

 short-term retardants. The present study was more concerned with how wind and relative 

 humidity affect the rapidity of moisture loss from retardant -treated fuels than on how moisture 

 affects the burning characteristics of retardant -treated fuels. 



Both experience and previous research show the superiority of long-term over short-term 

 retardants. However, the short-term retardants continue to be used because they cost less and 

 some of them are easier to mix and use. Results of this study should help the fire-control 



Hardy, C. E., R. C. Rothermel, and J. B. Davis. Evaluation of forest fire retardants-- 

 a test of chemicals on laboratory fires . U.S. Forest Serv., Intermountain Forest and Range 

 Expt. Sta. Res. Paper 64, 33 pp., illus. 1962. 



