Big Fire, Small Fire: The Effects of Burning on Flaked Stone Artifacts

By Sharon A. Waechter
Senior Staff Archaeologist
Far Western Anthropological Research Group, Inc.
Daniel G. Foster
Senior State Archaeologist
California Department of Forestry and Fire Protection
July, 2003



The California Department of Forestry and Fire Protection wears many hats. The Department's primary mission is to protect the people of California from fires, respond to emergencies, and safeguard the state's natural and cultural resources, as well as providing social, economic, and environmental benefits to the public.

"Cultural resources" are the archaeological and historical sites that occur virtually everywhere in California: Native American camps and settlements, pioneer homesteads, old mines and mills, and many others. These are the fragile and irreplaceable remains of California's past.

Because of their huge task (CDF has responsibility for nearly one-third of the state), the Department must evaluate the possible impacts from all of the different activities under their jurisdiction, and concentrate most of their efforts on those activities with the greatest potential to do harm. For example, a timber harvest where whole logs are dragged across the ground by a cable obviously has more of an impact to the environment than one where the logs are partially lifted ("one-end suspension") before dragging. And large, raging wildfires cause more harm to archaeological sites than smaller, cooler fires.

One way that CDF protects against wildland fires is to reduce the amount of flammable vegetation on the landscape, so that any fires that do start will have less to feed on. The most effective method is prescribed burning - burning-off areas of dense brush and dry grass under controlled conditions. Prescribed burning has many other benefits, as well: it prepares the ground for seeding and planting, improves wildlife habitat, perpetuates fire-dependent species, controls disease, and returns important nutrients to the soil. By doing these prescribed burns over small areas, when the weather is not too dry or the wind too strong, the firefighters can control the size and movement of the fire, and protect surrounding trees, wildlife, and structures.

A few years ago, a landowner near Jackson requested that CDF conduct a prescribed burn of a planned golf course and residential development. The landowner had done the required environmental work for the development, including a survey of the land by professional archaeologists. These archaeologists found and recorded several prehistoric or historic-era sites, which would need to be protected during the prescribed burn. Dan Foster, the Department's lead archaeologist, first recommended that firelines be constructed around the sites, so that the fires would not sweep across them. This is in keeping with CDF's policy to avoid impacts to cultural sites whenever possible.
At one of the sites, however, this would have created an "island" of vegetation in a sea of clear ground - an island that most likely would attract attention and possibly lead to vandalism and looting of the site. Foster was convinced by CDF Forester Jim Smith to let the fire burn across the site, but only if he could conduct an experiment at the same time. The burn would be a test case to see just how much impact a small, controlled fire really could have on an archaeological resource.

Earlier Studies

This would not be the first time archaeologists had experimented with fire. Scientists have known for more than 20 years that burning has an effect on obsidian - the dark, glossy volcanic glass that Native Americans often used to make knives, arrow points, and other flaked stone tools.

What we are still learning, however, is exactly how and to what extent burning changes obsidian. Several studies done over the last decade indicate that fires can alter or even destroy the hydration band that lets archaeologists determine the age of an obsidian tool. This can create obvious problems, because obsidian hydration can be critical for dating a site, especially in the absence of other lines of evidence.

Research shows that very hot fires can destroy hydration bands or make them unreadable. In 2000, CDF sponsored a study of the effects on obsidian hydration from prescribed fires. The goal of the study was to develop guidelines to help CDF managers write prescriptions for these fires. The study showed that the degree of damage depended on several factors: whether the obsidian was on the surface or buried underground; how high the temperature rose during the fire; and how much moisture was present in the soil and in the air. Moist soils and temperatures below 100 degrees Celcius seemed to have little effect on obsidian hydration. These results led many to assume that controlled burning caused minimal impacts to some types of archaeological sites.

But what about other prehistoric artifacts, other kinds of flaked stone? While studies abound of fire's impact on obsidian hydration, little or nothing has been done to test its effects to another common toolstone, chert. Many Native Californians used chert even more than obsidian - perhaps because natural chert formations are found in many more places around the state. They often subjected it to mild heat-treatment, to improve its texture and make it easier to work. If the stone gets heated too much or too quickly, it can get surface cracks and "pot-lids" (tiny, round pieces that pop off) which make it less useful for stone tools. CDF's prescribed burn experiment would document some of the other ways heat and fire can affect this important material.

The Experiment

The prescribed burn experiment took place at an archaeological site with both prehistoric and historic-era remains: bedrock mortars, waste flakes from the manufacture of stone tools, an old road, a structure pad with rock wall, and the remnants of an orchard with apple and pear trees (by the time of the prescribed burn, much of the historic component was gone). The site lay in a grassy area beside a small creek, surrounded by oak trees.

The first step was to survey the area and locate surface artifacts. Bright orange pin-flags marked the locations of these artifacts, to make them easier to find again after the burn. Two tools made from caramel-colored chert were photographed before (and again after) the fire, for comparison.

Now it was time to burn.

To protect the native oaks on the site, the CDF fire crew began by setting very small, controlled fires in the grass around them, to keep the larger fire from coming too close. These small burns were done early in the morning when the grass was still moist and the wind had not yet come up - safe conditions for keeping the flames under control.

Next, the crew laid down a perimeter of non-flammable foam at the down-wind edge of the burn area, to keep the fire from spreading beyond that line. Once the foam was down, trained fire-fighters set the grass alight and stood back to monitor its progress. The fire spread through the dry grass, with flames sometimes reaching 15 feet in length. It moved quickly toward the center of the archaeological site, and within a few hours had done its job.

Now it was up to CDF archaeologist Dan Foster to evaluate the results. He began by searching for the artifacts he had marked before the burn - indicated now by blackened and melted pin-flags. At the base of the pin-flags, he found the two flake tools he'd photographed earlier. Surprisingly, one of the tools had been burned almost beyond recognition, while the other was visibly unchanged.


The Results

Why were the effects on these two artifacts so dramatically different? Clearly it was not a simple matter of whether prescribed burns did, or did not, impact archaeological sites - there had to be other forces at work here. As it turned out, the undamaged artifact lay in an area where the fire burned very hot and passed through very quickly, while at the location of the damaged artifact, the fire burned more slowly and thus heated the specimen for a longer time.

Suggestions for Further Reading

Deal, Krista
2002 Effects of Prescribed Fire on Obsidian and Implications for Reconstructing Past Landscape Conditions. In The Effects of Fire and Heat on Obsidian,
J. Loyd, T.Origer, and D. Fredrickson, eds. U.S. Bureau of Land Management Cultural Resources Publication.
California State Office, Sacramento.

Halford, Kirk
2002 The Trench Canyon Prescribed Burn: An Analysis of Fire Effects on Archaeological Resources within the Sagebrush Steppe Community Type. In The Effects of Fire and Heat on Obsidian, J. Loyd, T. Origer, and D. Fredrickson, eds. U.S. Bureau of Land Management Cultural Resources Publication. California State Office, Sacramento.

Hull, Kathleen
1991 Archeological Survey and Post-Fire Surface Evaluation of the Foresta, Big Meadow, and McCauley Meadow Areas, Yosemite National Park, California. Yosemite Research Center Publications in Anthropology No. 11. National Park Service.

Smith, Jim
2002 Protecting Archeological Sites with Prescribed Fire. In The Effects of Fire and Heat on Obsidian, J. Loyd, T. Origer, and D. Fredrickson, eds. U.S. Bureau of Land Management Cultural Resources Publication. California State Office, Sacramento.

Solomon, Madeline
2000 An Assessment of the Potential Effects to Obsidian Hydration Bands Caused by Prescribed Fires. CDF Archaeological Reports Number 26. California Department of Forestry and Fire Protection, Sacramento.