Summary:    I dug a little playa dirt from the 1999 Burning Man site, made mud pies, took them to Ocean Beach in San Francisco, and set a fire on top of them. Here's what happened with different burn scar prevention methods:

Table 1 - Changes Due to Fire in Four Samples of Playa Soil (March, 2000)

1) Control Sample	This sample was dried and left unbaked. Very pale yellow throughout sample, with whitish glaze of mineral salts on surface. No difficulty breaking or scratching with pocket knife.
2) No Protection - Surface Fully Exposed to Fire	Changes occurred throughout sample. Difficult to break. From surface to a depth of 0.5 inches, cannot be scratched with pocket knife; difficult to scratch below this. Surface discolored orange-red, dark yellow, and light to dark gray; below surface, discolored dark to light gray throughout sample. Surface has fine cracking, with 0.25-inch spacing. Aluminum pan partly melted at edge.
3) One Layer of Corrugated Steel	Changes occurred throughout sample. Moderately difficult to break. From surface to a depth of 0.75 inches, resistant to scratching with pocket knife. Surface discolored light yellow to light red, extending to 0.25 inches; below this, discolored dark yellow throughout sample. Surface has little fine cracking.
4) One Layer of Corrugated Steel over Gypsum Board	Changes occurred throughout sample. No difficulty breaking sample or scratching with pocket knife. Slightly harder in upper 0.5 inches. Gypsum board paper turned to black ash; gypsum fragmented into 0.5-inch to 3.0-inch pieces. From surface to a depth of 0.0625 inches, discolored dark gray to black, due to paper ash; medium to light gray extending throughout sample.
5) Two Inches Sand over Fire Blanket	Very little change in sample. No difficulty breaking sample or scratching with pocket knife. Surface color changed slightly to pale yellow; very slight color change throughout sample.
6) Two Layers of Corrugated Steel	Changes occurred throughout sample. No difficulty breaking sample or scratching with pocket knife. Orange-yellow discoloration extending throughout sample.
7) Two Layers of Corrugated Steel w/ Sand	One inch of sand was placed under, between, and over two layers of corrugated steel. Very little change in sample. No difficulty breaking sample or scratching with pocket knife. Surface color changed slightly to pale yellow; very slight color change throughout sample.
8) Bare Fire Blanket	This test was done on the playa, July 4th. The 3' x 3' fire blanket was purchased at REI; they're designed for low-impact wilderness camping. Note that the manufacturer recommends that a couple of inches of sand be placed on top of the material before building a fire. Here, we wanted to test what would happen without the insulating layer of soil, as a worst-case scenario. A very small fire consisting of an armload of wood, burning for about an hour, melted and shattered the silica fire blanket, and charred the playa underneath. Cleanup required removing the top 1/4" of impacted playa soil. The damaged cloth is shown in the second picture. Damage appeared to be a combination of melting and fracturing due to too-high heat. According to knowledgable sources, this type of cloth is rated to about 1100 degrees F. The average wilderness campfire is about 750 degrees F. The lesson is -- use a few inches of sand or sandy soil over fireproof cloth, as the manufacturer recommends!

Conclusions

• The fire blanket covered with two inches of sand appeared to prevent burn scars, although there was slight discoloration. The fire blanket cannot be used without soil on top.

• Two layers of corrugated steel, with sand between, above, and below the layers, appeared to prevent burn scars.

• One or two layers of corrugated steel alone did not prevent burn scars or discoloration.

• Gypsum board caused intense black discoloration due to charred paper. Fragmentation of the gypsum occurred due to the loss of the paper coating.

• Full-scale test burns should be done at the Nevada site.

Follow-up: trial burn platforms on the playa this spring have worked very well! Setup and cleanup were only about an hour each for an 8-foot-square platform.

Additional Information

Mitigation of the deleterious effects of burning wood fires on the clay soil of the Black Rock Desert dry lakebed, or "playa", in northwestern Nevada was investigated by obtaining a representative surficial sample of the soil at the site of the 1999 "Burning Man" event; performing ASTM standard laboratory tests to characterize the soil type; subjecting samples to a typical wood bonfire, using four different types of "burn scar" protection methods; observing the effects of fire on each sample; and making preliminary conclusions on the most appropriate methods.

The Burning Man event has been held annually in the Black Rock Desert dry lakebed in northwestern Nevada since 1990. The event has involved many wood bonfires set directly on the soil surface of the playa. Burning Man staff, participants, and regulatory personnel with the Bureau of Land Management (BLM), who permit the event, have noted the difficulty of eliminating burn scars associated with the bonfires.

Typical burn scars are characterized by reddish to dark gray discoloration and irreversible hardening of the soil surface, extending to a depth of 0.5 to 2.0 inches. Interest has developed in mitigating the effects of the bonfires, as well as in speeding the cleanup after the event.

On March 19, 2000, a representative surficial sample of playa soil was obtained by the author using hand digging equipment, from the uppermost four inches of the soil from an approximately two-foot by five-foot area, enough to loosely fill two five-gallon buckets. The latitude/longitude coordinates of the sample location were 40 degrees 41 minutes 18.8 seconds north by 119 degrees 18 minutes 37.9 seconds west using a Global Positioning System device accurate to approximately 100 lateral feet, corresponding to the approximate location of the site manager's shop ("DPW") for the 1999 Burning Man event.

The sample was transported to the author's geotechnical laboratory in San Francisco, California, where a portion was reserved for soil index testing. Soil testing consisted of sieving the soil over a 200-mesh sieve, to estimate the amount of fine soil particles in the sample in accordance with ASTM D422; testing in accordance with ASTM D4318 to determine the liquid limit, plastic limit, and plasticity index of the soil; and visual-manual index testing of the soil type in accordance with ASTM D2488. The fines content (clay and silt) of the sample was 99 percent (passing the 200-mesh sieve); and the sample had a liquid limit of 44 percent moisture, with a plasticity index of 18. This indicated a borderline clay to silt soil of medium plasticity (USCS classification CL to ML).

Portions of the soil were wetted to a pliable consistency, molded into two-inch-thick cakes, and placed in nine-inch-diameter metal pie pans. The cakes were dried eight hours in a 90-degree-Celsius oven one day before burn scar testing was performed.

Burn scar testing consisted of subjecting the soil cakes to the heat of an approximately three-hour fire of fir and hemlock wood, burned in a three foot by three foot space on Ocean Beach in San Francisco, California. After the fire died, the author removed ash and debris from the burn locations, and disposed of the material elsewhere. Ocean Beach sand is a poorly graded, fine- to medium-grained, brownish gray sand (USCS classification SP).

Samples were reserved for observation. See Table 1 for test results.

A special thank-you to the GGNRA Ranger who facilitated my first burn experiment. He pointed out that although there is no ordinance specifically prohibiting fires on the beach, the GGNRA enforces the general restriction that fires on Ocean Beach should be confined to the area south of Stairways 26 and 28 and no closer to the sea wall than the seasonal high tide line; fires should be no bigger than three feet by three feet, not more than a couple of feet high, and wood with nails should not be used. My bright red conflagration, though immodestly larger than usually permitted, was allowed to burn unmolested, to a happy conclusion.

Cheers,

© Bob Stahl | Home