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A hearth is an archaeological feature that represents the remnants of a purposeful fire. Hearths can be extremely valuable elements of an archaeological site, as they are indicators of a whole range of human behaviors and provide an opportunity for obtaining radiocarbon dates for the period that people used them.
Hearths are typically used to cook food, but may also have been used to heat-treat lithics, burn pottery and/or a variety of social reasons such a beacon to let others know where you are, a way to keep predators away, or simply provide a warm and inviting gathering place. The purposes of a hearth are often discernible within the remnants: and those purposes are key to understanding the human behaviors of the people who used it.
Types of Hearths
Over the millennia of human history, there have been a wide variety of intentionally-built fires: some were simply piles of wood stacked on the ground, some were excavated into the ground and covered to provide steam heat, some were built up with adobe brick for use as earth ovens, and some were stacked upwards with a mix of fired brick and potsherds to act as ad hoc pottery kilns. A typical archaeological hearth falls in the middle range of this continuum, a bowl-shaped soil discoloration, within which is evidence that the contents have been exposed to temperatures between 300-800 degrees centigrade.
How do archaeologists identify a hearth with this range of shapes and sizes? There are three crucial elements to a hearth: inorganic material used to shape the feature; organic material burned in the feature; and evidence of that combustion.
Shaping the Feature: Fire-Cracked Rock
In places in the world where rock is readily available, the defining characteristic of a hearth is often plenty of fire-cracked rock, or FCR, the technical term for rock that's been cracked by exposure to high temperatures. FCR is differentiated from other broken rock because it has been discolored and thermally altered, and although often the pieces can be refit together, there is no evidence of impact damage or deliberate stone working.
However, not all FCR is discolored and cracked. Experiments recreating the processes that make fire-cracked rock have revealed that the presence of discoloration (reddening and/or blackening) and spalling of larger specimens depends both on the kind of rock being used (quartzite, sandstone, granite, etc.) and the kind of fuel (wood, peat, animal dung) used in the fire. Both of those drive the temperatures of a fire, as does the length of time the fire is lit. Well-fed campfires can easily create temperatures up to 400-500 degrees centigrade; long-sustained fires can get to 800 degrees or more.
When hearths have been exposed to the weather or agricultural processes, disturbed by animals or humans, they can still be identified as scatters of fire-cracked rock.
Burned Bone and Plant Parts
If a hearth was used to cook dinner, the leftovers of what was processed in the hearth may include animal bone and plant matter, which can be preserved if turned to charcoal. Bone which was buried under fire becomes carbonized and black, but bones on the surface of a fire are often calcined and white. Both types of carbonized bone can be radiocarbon-dated; if the bone is large enough, it can be identified to species, and if it is well-preserved, often cut-marks resulting from butchery practices can be found. Cut-marks themselves can be very useful keys to understanding human behaviors.
Plant parts can also be found in hearth contexts. Burned seeds are often preserved in hearth conditions, and microscopic plant residues such as starch grains, opal phytoliths and pollen may also be preserved if conditions are right. Some fires are too hot and will damage the shapes of plant parts; but on occasion, these will survive and in an identifiable form.
The presence of burned sediments, burnt patches of earth identified by discoloration and exposure to heat, is not always macroscopically apparent, but can be identified by micromorphological analysis, when microscopically thin slices of earth are examined to identify tiny fragments of ashed plant material and burnt bone fragments.
Finally, non-structured hearths--hearths that either were placed on the surface and were weathered by long-term wind exposure and rain/frost weathering, made without large stones or the stones were deliberately removed later and are not marked by burned soils--have still been identified at sites, based on the presence of concentrations of large quantities of burnt stone (or heat-treated) artifacts.
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