Fire

Back to life in early Spain for a few weeks.

Archaeologists have problems establishing when hominins deliberately used fire. For a while they believed anything that looked like a heated red sediment indicated combustion had occurred. Then, geologists determined the red color at Schöningen came from iron compounds that formed when the nearby lake dried. Similarly, researchers determined the remains that suggested fire and wood ash in China’s Zhoukoudian cave were actually the natural consequences of "water-deposited organic-rich sediment and colluvial reworking of loess."

Archaeologists believe sediments underlying fires were only reddened when they were heated to 390 to 750 degrees F (200 to 400 Celsius). Now they test artifacts with infrared microspectroscopy to determine the highest temperature to which they’ve been exposed and thermoluminescence dating to determine when. Unfortunately, machines can’t distinguish between temperatures caused by man-made fire, wild fire, or volcanic eruptions.

Fire is a chemical reaction that occurs when carbon molecules are heated in an oxygen environment until the two recombine to create carbon dioxide and water. The point of conversion is the explosion we call combustion.

Not all carbon molecules burn easily. Some are embedded in sacs that make ignition difficult. All grass and wood cells contain cellulose fibers suspended in lignin. Both are organic compounds that contain carbon.

Some tars and resins may alter fire behavior slightly by species. Moisture in the sapwood can slow the process. Relative amounts of lignin and cellulose may differ by plant. But, the

chemistry is absolute. When the temperature of wood reaches the boiling point of water, 212 degrees (100C), chemical bonds begin breaking.

By the time temperatures reach 392 (200C), wood begins losing moisture and releases water vapor.

Between 392 and 572 degrees (300C), lignin loses its liquid chemicals and turns to char. Cellulose does not. Technically it has entered the pyrolysis phase. Charcoal is made by this type of controlled heating.

The temperature at which any organic material can burn depends on the chemistry of the material. The temperature at which it can maintain the combustion without contact with a heated surface is absolute.

Cellulose begins to disintegrate around 660 degrees (350C). Carbon bonds begin breaking around 700 to 750 degrees (370 to 400C).

The ignition point is reached around 750 degrees F (400C). After that, a fire is self-sustaining. The heat from the explosion of molecules warms the air so others can ignite. The more explosions, the greater the generated heat and the greater the number of subsequent explosions.

Before that magic temperature, you can only keep something burning by maintaining contact with something hotter. You use flaming kindling to ignite wood in a camp fire. When people use flame throwers to burn weeds, the tools provide a constant heat source to organic matter too widely dispersed to spread heat. One company advertises the propane-fueled temperature of its thrower is 2050 degrees F.

At 842 degrees (450C) wood cells no longer emit the volatile chemicals that color smoke and poison the air. What remains is char that is converted to carbon dioxide and water, until only ashes remain.

Bones burn differently because their carbon is held in different molecular structures. Several students have tried to reproduce paleolithic fires. A girl in Wisconsin found long bones from white-tailed deer would crack to expose the marrow, which dripped into the flames. The fire temperature varied between 275 (135C) and 200 degrees (150C). She used braided grass for her fuel.

She tried again with ribs and long bones from a deer and long bones from a North American elk. This time the temperature reached a thousand degrees (540C) and produced "calcined (decomposed carbon) bone." She’d used a different local grass with a smaller stem diameter.

A group in Finland tried igniting fresh, unbroken bones of elk, bear and grey seal with "dried pine and birch wood." They found a fire composed of 25% bone and the rest wood had a stable temperature, while one that was 75% bone had one that fluctuated. The latter also did not produce enough heat to burn bones thoroughly.

The four also found ribs burned quickly at a high temperature, while long bones burned "slower but at a more stable temperature for a longer time." When they finished the bear bone experiment, they stacked the bones aside. They found after an hour their temperature was 750 degrees (400C), even though they were cold to the touch.

Archaeologists believe the earliest fire users didn’t yet ignite fires with flint and pyrite. Instead, they perpetuated chance fires by keeping them burning and by carrying miniature fires when they migrated. Terrence Twomey thought they might have carried glowing logs from place to place. The Finnish students implied bones kept at the ignition temperature of 400C would have worked better.

Notes:
Berna, Francesco, et alia. "Microstratigraphic Evidence of in situ Fire in the Acheulean Strata of Wonderwerk Cave, Northern Cape Province, South Africa," National Academy of Sciences, Proceedings 109:E1215-E1220:2012, quote on Zhoukoudian cave.

_____, Mareike Stahlschmidt, et alia. "The Schöningen Hearths: Perspectives on Multidisciplinary Geoarchaeological Research and Middle Pleistocene Fire Use in Northern Europe," Developing International Geoarchaeology Conference, 2013.

Glazewski, Megan. "Experiments in Bone Burning," Oshkosh Scholar 1:17-25:2006.

Twomey, Terrence Matthew. Keeping Fire: the Cognitive Implications of Controlled Fire Use by Middle Pleistocene Humans, 2011.

Vaneeckhout, Samuel, Juho-Antti Junno, Anna-Kaisa Puputti, and Tiina Äikäs. "Prehistoric Burned Bone: Use or Refuse - Results of a Bone Combustion Experiment," International Council for Archaeozoology, international conference, 2010.

White, Robert H. and Mark A. Dietenberger. "Fire Safety of Wood Construction" in USDA, Forest Products Laboratory, Wood Handbook, 2010; defines the stages of wood burning.