Burning dirt?

Elmer and his son, Elmer Jr., are off on a weekend campout to explore nature and to get in some bonding. As they drive to their campsite, Junior notices several dirt paths snaking through the deep wooded areas. "Are those walking paths?" he asks. "No, son, I don't think so," says Elmer. "They're fire breaks to help contain a blaze should one occur in the forest." After Elmer and Junior have consumed the last of the roasted marshmallows, Elmer instructs his son the smother the fire by covering it with dirt. As he sets to his task, Junior gets a puzzled look on his face. "Dad, does dirt ever burn?" Poor Elmer -- he's done a good job all day of impressing Junior with his knowledge of the outdoors, but this is one question he simply can't answer. While he works to divert Junior's attention to other tasks, let's get an SwRI Whizard to answer Junior's question.

"The fire breaks that Junior noticed are known as fire lines or fire barriers, which serve to delay or prevent fire from propagating from one place to another. To prevent spread across fire lines, it is important to trench down to mineral soil so that fire will not eat through dried ground duff (partly decayed organic matter) and roots and thus pass the intended barrier.

"A fire will start and will continue burning as long as there is an ignition source, oxygen, and a combustible material. Take away one of them and the fire is out. When we cover a fire with dirt, we are cutting off the oxygen supply, thus snuffing the fire. At the typical temperatures encountered in a camp fire, dirt is an inert solid that will not ignite, unless it is mixed with dry leaves and combustible waste, such as paper or plastic.

"Even if a fire appears to be out, it can restart after hours or even days, if the dirt contains a significant amount of organic material. A similar internal heating and self-ignition phenomenon is sometimes observed in compost heaps and piles of contaminated forest litter.

"So, to answer Junior's question: dirt is not going to ignite during a small blaze such as a camp fire. However, in the event that there is a continuous, high-intensity ignition source, some of the components in the dirt could melt, but will not contribute to the growth of the fire. When attempting to put out a fire, Junior must make sure that the dirt is free of dry leaves, roots, and combustible waste -- such as the hot dog and marshmallow packages -- which could ignite and increase the growth of the fire."

Thanks to this month's Whizard, Gladys Miller (Finley), a group leader in the Fire Technology Department of the Chemistry and Chemical Engineering Division. Finley manages the Listing and Labeling Program. Her current responsibilities include monitoring the production quality of products to ensure that their fire performance is maintained.

The Lighter Side SwRI Home

March 25, 2013