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ire pit and sand both used for heat treatment experiments. Silcrete is placed beneath a layer of sand and then a fire is built over the top. The temperature is gradually increased and then slowly decreased to avoid cracking the stone. Credit: © Science/AAAS
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Fire Used to Make Better Tools 75,000 Years Ago
By Clara Moskowitz, LiveScience Staff Writer
posted: 13 August 2009 02:04 pm ET
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090813-fire-02.jpg
Fire pit and sand both used for heat treatment experiments. Silcrete is placed beneath a layer of sand and then a fire is built over the top. The temperature is gradually increased and then slowly decreased to avoid cracking the stone. Credit: © Science/AAAS
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090813-fire-02.jpg
Fire pit and sand both used for heat treatment experiments. Silcrete is placed beneath a layer of sand and then a fire is built over the top. The temperature is gradually increased and then slowly decreased to avoid cracking the stone. Credit: © Science/AAAS
090813-tools-02.jpg
Experimentally replicated blade tools produced with heat treated silcrete. Some of these tools may have been mounted unto a wood handle to create a tool with disposable blades, as pictured above. Credit: © Science/AAAS
Early humans crossed a threshold around 75,000 years ago, when they started painting symbols, carving patterns and making jewelry. A new study found they also began to use fire to make tools around that time.
Until now, this complex, multi-step process for tool making was only known to occur as recently as 25,000 years ago in Europe. But the new findings show this breakthrough occurred much earlier, and in Africa, not Europe.
By heating up stones in a fire before chipping away at them to make blades, early humans could make tools sharper and produce them more efficiently.
Scientists think this advancement represents a link between the earlier use of fire for cooking and warmth, and the later production of ceramics and metals.
"Around 800,000 years ago we see some of the first evidence for hominid controlled use of fire," said study leader Kyle Brown, a graduate student in archaeology at the University of Cape Town in South Africa and at Arizona State University. "And then at about 10,000 years ago we see evidence for production of ceramics. And at about 5,000 years ago we see metal working."
"The heat treatment of tools is sort of a bridging technology," he said.
The development of this skill may represent a level of complex cognition that was just beginning in humans at this time.
Brown and colleagues discovered the remains of tools that had been made using fire at archaeological sites in South Africa. The tools were made out of a stone called silcrete. Some of the earliest examples could date back to 164,000 years ago, and the researchers found that by 72,000 years ago this technique was seemingly common for silcrete tools.
The heat-treated tools look almost like stone razor blades, and are small enough that they could have been set into a handle.
"It’s a big debate to figure out what people were doing with these things," Brown told LiveScience. "Some people argue that they are the first arrowheads. Other people argue that they were set in a handle and used as knives."
To make the tools, early humans would have had to bury the stone beneath a fire, then slowly heat it up, keep it at a high temperature for hours, and then let it cool. The process was complicated and could take one to two days of continuous heating.
The heat transforms the stone so that it's harder and more brittle, which allows it to be more easily chipped away into a sharper edge. It also gives the stone a special sheen, which helped the archaeologists identify the tools as resulting from fire treatment.
"The most noticeable thing about heat-treated stone is that it has a luster or a gloss to it that’s fairly distinctive," Brown said. "A stone that's heated will only show that gloss if it's been flaked after it's heated."
The researchers then confirmed that the tools had been warmed in a fire with a technique called archaeomagnetics, which measures the realignment of iron particles in stone that results from heating. Another chemical process called thermoluminescence provided further proof that the stones had been heated.
Brown and colleagues report their results in the Aug. 14 issue of the journal Science.
http://www.livescience.com/history/090813-fire-tools.html
Experimentally replicated blade tools produced with heat treated silcrete. Some of these tools may have been mounted unto a wood handle to create a tool with disposable blades, as pictured above. Credit: © Science/AAAS
Heat Treatment of microcrystalline quartz
http://www.sciencenews.org/view/generic/id/46394/description/Fire_engineers_of_the_Stone_Age
Fire engineers of the Stone Age
People in southern Africa 72,000 years ago heated stones in preparation for toolmaking
By Bruce Bower
Web edition : Thursday, August 13th, 2009
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Hot and notAn unheated piece of silcrete stone, left, is contrasted with an ancient stone tool that was shaped from silcrete. The stone changed color and texture after being heated and chipped into an implement. New analyses of such artifacts suggest the technology of using heat to make tools was being used in Africa at least 72,000 years ago.Brown/South African Coast Paleoclimate, Paleoenvironment, Paleoecology, Paleoanthropology Project
Stone Age toolmakers didn’t need motivational speakers to get fired up at work. People living on the southern tip of Africa 72,000 years ago decided on their own to heat stones with carefully controlled fires in order to make the rock more suitable for tool manufacturing, a new study finds.
Coastal residents of southern Africa may even have heated stones as a first step in toolmaking as early as 164,000 years ago, say Kyle Brown, an anthropology graduate student at the University of Cape Town, South Africa, and his colleagues.
Until now, evidence of heat treatment extended back no further than about 25,000 years ago in Europe, Brown’s team notes in the Aug. 14 Science.
“We’ve shown that, at least 72,000 years ago and perhaps much earlier, modern humans had an understanding of how fire and heat could transform the materials around them to suit their needs,” Brown says. This technological development bridged the taming of fire for cooking, warmth and protection at least 750,000 years ago (SN: 5/1/04, p. 276) to the appearance of pottery around 30,000 years ago and metal working 5,500 years ago, in his view.
Brown’s results “push the antiquity of heat treatment back much earlier than previously supposed,” remarks archaeologist John Shea of Stony Brook University in New York. It’s not surprising that ancient Homo sapiens used fire in this way, he adds. In 2003, another team reported that modern humans living in the Middle East 90,000 years ago heated chunks of a yellow mineral to give it a reddish hue before grinding the mineral into pigment.
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Shore firesAt South Africa’s Pinnacle Point cave site, humans heat-treated stones for toolmaking earlier in the Stone Age than previously thought.Science/AAAS
Heat treatment of stones for toolmaking occurred in several steps that required complex thinking abilities, the researchers assert. Toolmakers buried selected pieces of stone beneath a fire at a campsite or workshop, probably for a day or more, they suspect. Stones were then removed and worked into shape as cutting tools. Aborigines in northwestern Australia engaged in much the same practice until a few decades ago.
At South Africa’s Pinnacle Point cave, the researchers first identified magnetic and molecular signatures of intense heating on 26 tools made of a type of stone called silcrete. The tools came from an excavation of sediment layers dating to between 72,000 and 47,000 years ago. The researchers estimate that the artifacts were heated at maximum temperatures of 300° to 400° Celsius.
High levels of surface gloss, based on a measure of the relative amount of light reflected by stone, further indicated that a majority of 129 stone artifacts from the same layers had been heated, probably before being made into tools.
Gloss levels on 24 Pinnacle Point stone tools with an estimated age of 164,000 years suggested that they too had been heated at high temperatures.
Brown’s group also experimented with modern samples of silcrete from the Pinnacle Point area. They found that intense heating makes silcrete less brittle and allows sharpened edges to be created more precisely, the scientists say.
In contrast, Shea argues that heat treatment yields only marginal improvements in stone quality for the amount of time and energy it demands. He speculates that Stone Age heat treatment in southern Africa represented a kind of conspicuous consumption. Its practitioners may have wanted to gain the upper hand in rivalries for mates and social status that intensified within rapidly growing populations.
South African sites have also provided remnants of symbolic behavior by humans at around the same time as the earliest possible appearance of heat treatment. People ground pigments—apparently for decoration—around 164,000 years ago (SN: 10/20/07, p. 243) and carved geometric designs into pigment chunks 100,000 years ago (SN Online: 6/12/09).
Technological and symbolic behaviors typical of modern humans did not first appear 50,000 years ago, as some researchers have argued, contends anthropologist and study coauthor Curtis Marean of Arizona State University in Tempe. Marean suspects that such behaviors spread rapidly after the evolutionary launch of modern humans around 200,000 years ago.
Anthropologist Sally McBrearty of the University of Connecticut in Storrs considers it more likely that modern human behavior developed slowly over several hundred thousand years. Ancient people gradually developed a series of practical applications for controlled fire beyond cooking, she proposes, including making wooden tools in a charring-and-scraping process and melting tree sap for adhesives. “I don’t think the use of heat treatment required a big cognitive leap,” McBrearty says.
Marean also hypothesizes that heat-treatment expertise gave modern humans a leg up on Neandertals, who apparently lacked advanced toolmaking skills. But unlike Pinnacle Point people, Neandertals had access to good-quality flint for tools, comments archaeologist Nira Alperson-Afil of Jerusalem’s Hebrew University, so they may not have needed heat treatment.
Photo of conchoidal fracture in quartz from:
http://dn.redwoods.edu/coursenotes/renner/geo_images/rocks_minerals/minerals/conchoidal-fracture.jpg
Heat treatment of flint very often makes it much easier to work. Longer, thinner flakes can often be struck from the core, and the stone becomes more glassy, with a greasy look to the conchoidal surfaces left after a flake has been struck.
Text below is an excerpt from the paper:
Heat-Treating Experiments With Onondaga Chert:Preliminary Results
by Frank L. Cowan
which may be viewed at:
http://wings.buffalo.edu/anthropology/Lithics/Files/thermal.pdf
Various investigators have used scanning electron microscopy to compare
the fracture surfaces of heated and unheated cherts (Draper and Flenniken 1984; Flenniken and
White 1983; Johnson 1985; Olausson and Larson 1982; Purdy 1974; Purdy and Brooks 1971;
Robins, et al 1978). In comparing fracture surfaces produced before and after heat-treatment, they
find that heat-treated cherts fracture to form much smoother surfaces. In unheat-treated cherts,
fracture fronts appear to pass between the quartz microcrystals, breaking the relatively weak bonds
holding the crystals together. At the high magnification used in SEM, these surfaces appear
topographically rough. In heat-treated cherts, however, the fracture appears to pass through many
of the microcrystals, leaving a much smoother fracture surface. The smoother surface reflects light
more evenly and produces an increased surface luster or "greasy" texture that is macroscopically
visible and has served as an indication of thermal alteration to many investigators (Bordes 1969;
Crabtree and Butler 1964; Rick 1978).
The physical/chemical changes that take place during heat-treatment are poorly understood.
Purdy and Brooks (1971) have suggested that impurities (particularly iron compounds) within the
microcrystalline quartz latticework may reach a eutectic melting point. They suggest that "minute
amounts of impurities (or compounds of the elements making up the impurities) in the
intercrystalline spaces of the chert are probably acting as fluxes (substances promoting fusion) to
fuse a thin surface film on the cryptocrystals. This fusion occurs when the melting point (eutectic
development) of the impurities is reached... Binding of the microcrystals results in a more
homogeneous material with the ability to fracture like glass rather than like a rock aggregate"
(Purdy and Brooks 1971).
3
Flenniken and Garrison (1975) have countered with the observation that the Purdy-Brooks
hypothesis depends upon particular mineral impurities which are not universally present in chert or
other materials which respond positively to heat-treatment. They suggested that interstitial moisture
present in the crystalline structure would expand forcibly as a gas with heating, and water vapor
would escape by micro-fracturing the crystalline latticework. Several observers have noted that
heat-treating results in small but measurable weight loss probably due to the loss of water
(Mandeville 1973; Purdy and Brooks 1971).
More recently, Schindler, et al. (1983) have offered another mechanism whereby heat-treatment
reduces the tensile strength of Bald Mountain jasper, although they do not claim that it as a
mechanism generalizable to a larger class of materials. In heat-treating the central Pennsylvanian
jasper to approximately 275 degrees C., Schindler, et al. noted that the jasper undergoes a
distinctive color change from yellow to deep red. Concurrent with the color shift, they observed
that there is a 50% reduction in point-tensile strength. In determining that the color transformation
was the result of a recrystallization of goethite to hematite, a denser material, they noted that the
volume reduction of the iron compounds left small "channels" permeating the reddened stone.
They reasoned that these small voids were the cause of the reduction in tensile strength, and
hence the thermal alteration mechanism for Bald Mountain jasper.
The last hypothesis, aside from positing a unique mechanism for a unique material, suffers from
a flawed research design. All of the jasper samples heat-treated and tested were very small (1 cm)
cubes (Schindler, et al. 1983:528). It is typical for heat-treated jaspers to undergo a dramatic color change only near the heated surfaces; reddening seldom penetrates more than a few millimeters
into the mass of the rock. Although unreddened, heat-treated jasper within larger stone masses is
nonetheless thermally altered as clearly demonstrated by reduced fracture toughness and
increased luster of fracture surfaces. The direct equation of color change with the alteration of
fracture characteristics forced Schindler, et al. (1983:537) to advance ludicrous technological
interpretations, such that large cores or bifaces would have to be heat-treated more than once
during the reduction process.
In most cherts (and other varieties of microcrystalline quartz and some quartzites), successful
heat-treatment usually involves raising the temperature of the stone to approximately 275 degrees
C., and maintaining that temperature for a period of several hours (Flenniken and White 1983;
Mandeville and Flenniken 1974). The critical temperature at which thermal alteration occurs and
the length of time the material must remain at that temperature is apparently quite variable with
differing kinds of materials. Flenniken and Garrison (1975) have found that Arkansas novaculites
require very high temperature curves, with thermal alteration only successfully occurring with
sustained temperatures of 450-500 degrees C. Purdy and Brooks (1971) reported that color changes
consistently took place in Florida chert heated to between 240-260 degrees C. when iron was
present in concentrations of at least 1,100 parts per million. Critical temperatures for effecting
textural changes were 350-400 degrees C. Rick (1978:60) reported that Burlington chert underwent
color changes (apparent iron oxidation) at approximately 230-290 degrees C., but that textural
changes were not apparent at less than 290-370 degrees C. Personal experimentation has
indicated that Burlington cherts may be successfully heat-treated at much lower temperatures.
Sustained temperatures of 275 degrees C. appears to be sufficient, although higher temperatures
will produce higher, more vitreous lusters. My experiences with heat-treatment suggest that
generally more vitreous materials, and darker colored cherts and flints require less heat than lighter
colored cherts, and, in fact, many high quality materials can not survive the high temperature
curves suggested in some of the literature. This observation is concordant with the experiences of
other knappers experimenting with heat-treatment (Crabtree and Butler 1964; Flenniken, personal
communication, 1984; Rick 1978:61).
The upper temperature limit of successful heat-treating is definitely fixed at 573 degrees C. (at
one atmosphere of pressure), the low quartz/high quartz inversion point (Hurlbut and Klein
1977:411-412). At 573 degrees C., at one atmosphere of pressure, quartz undergoes an
instantaneous and reversible transformation of crystal structure. "This displacive transformation
from low to high quartz involves only minor atomic adjustments without breaking of Si-O bonds.
Upon cooling high quartz, through the inversion point at 573 [degrees]C, Dauphine twinning may
be produced" (Hurlbut and Klein 1977:415). Although Hurlbut and Klein (1977:411) report that the
inversion may be repeated "over and over again without physical disintegration of the crystal", this
temperature does represent the apparent threshold beyond which microcrystalline quartz
aggregates can not cross without disintegrative effects. Whether because of the reversible
alteration of individual crystal symmetry or the morphological transformation to Dauphine twinned
low quartz, the effect is that a rock unit of chert no longer has any structural integrity, and tends to
become friable and sometimes chalky and will crumble rather than break conchoidally with any
applied force. Several investigators have noted this decrepitation (Mandeville 1973; Mandeville
and Flenniken 1974; Purdy 1974, 1975; Schindler, et al 1982), although apparently some have not
been aware of the crystalligraphic basis for it.
The controlled nature of successful heat-treatment becomes apparent with the consideration of
temperature change rates. Rapid heating results in explosive fractures as excess moisture trapped
in voids, fissures, pockets, and in the interstices between microcrystals vaporizes. Differential
expansion of the brittle material results in characteristic "potlid" fractures. Overly rapid cooling
results in reticulated fractures or "crazing". Rapid cooling and uneven mass shrinkage causes
distinctive "reticulated" fractures which occur abruptly across the thickness of a chert blank and
often leave a rippling surface across the fracture oriented to the broader dimensions of stone (Purdy
1975).
Conceptually and observationally, the distinction between heat-treatment and fortuitous burning
is essential. Poorly controlled heat-treatment attempts can produce all of the fractures discussed
above. Nonetheless, the abundant burnt and pot-lidded chert debitage and artifacts recovered
from most archaeological sites pertains to fortuitous spatial congruences of debitage and fires
burning for other purposes, congruences which are not necessarily the result of synchronous
activities or deposition. Post-depositional fires, whether humanly caused or the result of natural
processes, may alter cherts, although the results are most likely to result in characteristc fractures of
thermal shock, 'smoked' surfaces or structural deterioration to blue-black color and chalky textures.
The distinctive textures of controlled thermal alteration for technological purposes occur on
surfaces flaked after heat-treatment. This is not to say that heat-treating attempts will always be
successful, as the following experiments with Onondaga chert will show. Nonetheless, once the
appropriate heat-treating conditions and variables for particular materials are learned and
understood, it would be likely that the frequency of unsuccessful heat-treating attempts would be
low. If the heat-treatment of chert were a highly risky undertaking, with the likely loss of time, labor
and scarce raw materials, it would be unlikely that it would be pursued as a maximizing strategy.
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Photo of conchoidal fracture in quartz from: