Journal of Human Evolution 70 (2014) 49e60

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Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol

Experimental heat treatment of silcrete implies analogical reasoning in the Middle Stone Age Lyn Wadley a, *, Linda C. Prinsloo b a b

Evolutionary Studies Institute, University of the Witwatersrand, PO Wits, 2050, South Africa Department of Physics, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 August 2013 Accepted 6 November 2013 Available online 5 April 2014

Siliceous rocks that were not heated to high temperatures during their geological formation display improved knapping qualities when they are subjected to controlled heating. Experimental heat treatment of South African silcrete, using open fires of the kind used during the Middle Stone Age, shows that the process needed careful management, notwithstanding recent arguments to the contrary. Silcrete blocks fractured when heated on the surface of open fires or on coal beds, but were heated without mishap when buried in sand below a fire. Three silcrete samples, a control, a block heated underground with maximum temperature between 400 and 500  C and a block heated in an open fire with maximum temperature between 700 and 800  C, were analysed with X-ray powder diffraction (XRD), X-ray fluorescence (XRF), optical microscopy, and both Fourier transform infrared (FTIR) and Raman spectroscopy. The results show that the volume expansion during the thermally induced a- to b-quartz phase transformation and the volume contraction during cooling play a major role in the heat treatment of silcrete. Rapid heating or cooling through the phase transformation at 573  C will cause fracture of the silcrete. Successful heat treatment requires controlling surface fire temperatures in order to obtain the appropriate underground temperatures to stay below the quartz inversion temperature. Heat treatment of rocks is a transformative technology that requires skilled use of fire. This process involves analogical reasoning, which is an attribute of complex cognition. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Silcrete a- to b-quartz phase transformation Controlled heating Transformative technology Complex cognition Homo sapiens South Africa

Introduction Living members of Homo sapiens have complex cognition that includes attributes such as fluid intelligence and innovative thought (Wynn and Coolidge, 2011), abstract thought (Barnard, 2010), the ability to multi-task (Amati and Shallice, 2007), and the use of recursion and concepts of past and future (Haidle, 2010; Reuland, 2010). Executive functions of the brain that are dependent on frontal lobe-linked abilities enable these cognitive attributes and facilitate goal-directed actions, anticipation of problems, response inhibition, planning over long distances or time, and analogical reasoning (Coolidge and Wynn, 2001, 2005; Wynn and Coolidge, 2003, 2011). The development of these cognitive attributes appears to have been gradual and incremental and it seems possible to trace their presence through time using archaeological evidence for various technologies. Advances are evident at the

* Corresponding author. E-mail addresses: [email protected] (L. Wadley), [email protected] (L.C. Prinsloo). http://dx.doi.org/10.1016/j.jhevol.2013.11.003 0047-2484/Ó 2014 Elsevier Ltd. All rights reserved.

beginning of the Middle Stone Age (MSA) about 300 000 years (ka) ago, probably coinciding with the advent of H. sapiens. Backed tools appeared in Zambia at about this time (Barham, 2002), and Ambrose (2001, 2010) proposes that composite-tool making began with this invention. He hypothesizes that the procedure involves planning and coordination of numerous, separate tasks and that the new behaviours evolved alongside frontal lobe development in the human brain. Early composite-tool making is endorsed by the East African discovery, at a similar age, of tiny stone points that would have needed hafting for their effective use (Brooks et al., 2006). Other attributes of complex cognition become apparent later in the archaeological record, though future research is likely to shift the chronological boundaries further back in time. As previously suggested (Wadley et al., 2009; Wadley, 2010a, 2013), the making of compound adhesives involves not only careful planning, but also multi-tasking, which is another attribute of complex cognition. The manufacturing process requires combining diverse raw materials, often from remote sources, and transforming them, usually irreversibly, through the use of pyrotechnology. The artisan needs to simultaneously mix ingredients, control fire temperature, and mentally rotate stone tools to create the desired composite product.

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Compound adhesive manufacture took place in Sibudu and Rose Cottage, South Africa, by at least 70 ka (Wadley et al., 2004; Lombard, 2006). An ochre-rich compound described as paint was found in Blombos Cave in a layer dated to about 100 ka (Henshilwood et al., 2011). The manufacture of this paint would have required the same mental processes needed for compound adhesive production. Equipment designed to function, not immediately, but at a future time, implies the cognitive ability to integrate action across space and through time (Wynn and Coolidge, 2003), and it provides another of the attributes denoting living H. sapiens cognition. Snares are a good example of equipment used for remote capture of prey. Being able to delay gratification and envisage action removed from supervision in both space and time engages modern executive functions of the brain (Wynn and Coolidge, 2003). There is circumstantial evidence for the use of snares at Sibudu by 65 ka (Wadley, 2010b), but future research may disclose an earlier origin. The use of bow and arrow at a similar period (Lombard and Phillipson, 2010) seems further evidence for thought processes that were akin to our own. Lombard and Haidle (2012) argue that such complementary tool sets imply cognitive flexibility that would enable the conceptualization of technological symbiosis. Transformative technologies generate discernible links between complex cognition and items of material culture. Recent examples of transformative technologies that imply enhanced executive functions include alloying metals and firing ceramics (Wynn and Coolidge, 2007). The innovative processes transform natural products through combining raw materials prior to a sophisticated use of pyrotechnology. Comparable transformative technologies used earlier than metal working and ceramics include the creation of compound adhesives and other compound products, which were previously discussed, and also the deliberate heat treatment of rock or ochre. This technology involves planning, patience and considerable expertise and understanding of the properties of all the natural products involved. Here we argue that the introduction of heat treatment also implies analogical reasoning and the ability to envisage action removed from view. This attribute of complex cognition has not previously been demonstrated through archaeological evidence. We explain the connection between heat treatment and analogical reasoning in more detail later. The term heat treatment applies when heating is deliberate, especially when fire and rocks are simultaneously manipulated, but arbitrating between deliberate and accidental heating of rocks can be difficult. When hearths are built above buried items such as rocks, stone tools or botanical remains, these can burn and be altered post-depositionally without this being the intention of the artisans (Pierce et al., 1998; Sievers and Wadley, 2008; Asmussen, 2009). However, heat-treated siliceous rocks exhibit changes in lustre (gloss) (Rowney and White, 1997) and sometimes in colour, too. The flaked surfaces of heat-treated rocks commonly have a greasy lustre that is distinguishable from the dull outer surface of the rock prior to it being struck (Purdy and Brooks, 1971; Rowney and White, 1997). Gloss on heated stone tools is present in the scars knapped after, and not before, heat treatment (Brown et al., 2009). Making a distinction between accidental and deliberate burning is therefore possible in these instances. A digital imaging method that limits intra-analyst bias can distinguish colour of unburned and burned silcrete (Oestmo, 2013). Controlled heating of siliceous rocks makes them better for knapping than untreated rocks. For example, Brown et al. (2009) found that thin bifacial points could be knapped with a higher  ski success rate on heated than unheated silcrete. Flints that Doman et al. (2009) buried in sand, then subjected to slow heating and cooling, exhibited substantial reduction in fracture toughness (fracture toughness is measured in joules/mm2 and represents the

difficulty of propagating a crack through a substance). Flakes could be knapped from these rocks using less energy than would have been required without heating. Stone tool knappers who used heattreated rocks thousands of years ago may not have understood the structural changes that took place, but they clearly knew that correct heating processes made siliceous rocks relatively easy to flake with effective end results. Changes made to siliceous rocks during controlled heat treatment can facilitate not only flake removal, but also fine pressure flaking (Cotterell and Kamminga, 1992;  ski and Webb, 1992; Webb and Doman  ski, 2009; Mourre Doman et al., 2010). This retouching technique for shaping stone tools involves using a retoucher, such as a piece of bone, to exert pressure close to a tool’s edge to produce distinctive, invasive scars (Mourre et al., 2010). Pressure flaking was once thought to be an Upper Palaeolithic innovation that appeared about 20 ka ago, but recent knapping experiments uphold the interpretation of pressure flaking on bifacial points excavated from the 75 ka Middle Stone Age levels at Blombos Cave, South Africa (Mourre et al., 2010). The Blombos silcrete points are said to have been heat treated after initial flaking, but prior to pressure flaking (Mourre et al., 2010). In other words, point preforms were heated; they would have been smaller than silcrete cores, but larger than the finished points. Heat treatment at Blombos is thus thought to have taken place during, rather than before, the stone tool manufacturing process. At another South African archaeological site, Pinnacle Point, all twelve of the tested silcrete stone tools from PP5-6, and the Still Bay biface from Blombos Sands, had been heated (Brown et al., 2009). Some of the heat treated pieces are from 164 ka occupation layers. The thermomagnetic history of these silcrete tools determines that they were heated once only to maximum temperatures between 300  C and 400  C (Brown et al., 2009). This is precisely the range of temperatures that Purdy and Brooks (1971) found most suitable for heating chert. Previous heat treatment research The method of heating siliceous rocks affects their eventual  ski et al. (2009) heated flint pieces in a laborausefulness. Doman tory oven to a peak temperature of 400  C that was held for 2 h. Some experimental flints were buried 30 mm deep in a sand bath in the oven; others were exposed. When expansion and contraction are rapid, fractures can be explosive (Purdy and Brooks, 1971), and a high frequency of uncovered specimens exploded in the oven  ski et al. (2009). This is said by these reduring heating by Doman searchers to be the result of the high coefficient of thermal expansion of the quartz in siliceous rocks. They also conclude that siliceous rocks, such as chert and flint, remain undamaged when heated to 300  C and 400  C, but rapid cooling or heating, or temperatures above 400  C, produce brittleness and/or heat fracturing. The term heat fracturing describes the physical stresses (that form fractures like crenation, potlidding, and surface crazing), regardless of whether the heating was intentional or accidental (Mercieca, 2000). While slow and steady heat treatment is one solution, Mercieca and Hiscock (2008) conclude that this may not be a necessity; heat treatment can sometimes be carried out in aerobic conditions if the rock pieces to be heated are small enough. This is because large items fracture at lower temperatures than small ones, which have higher cracking and fracture thresholds. Increasing specimen size reduces the resistance of the rock to heat fracturing when suddenly exposed to heat (Mercieca, 2000). Lithic blanks or preforms can be controlled for size more accurately than can temperature during the heating event (Mercieca and Hiscock, 2008), thus knappers can exercise some control over the outcomes of the heating process through standardizing rock specimen size. Mercieca and Hiscock (2008) used differently sized

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silcrete pieces to demonstrate their point: they cut experimental blocks into 10  10  10 mm (1 cm3) for the smallest cubes and 40  40  40 mm (64 cm3) for the largest cubes. The 64 cm3 cubes were more susceptible than the small cubes to undesirable fracture during increases of temperature. The temperature above which unwanted fracturing occurred in the small silcrete cubes varied from 700  C to 900  C, but it was lower for the larger blocks. While it seems that knappers might be able to use simple, open fires to heat treat the tiniest pieces of rock, it is clear that doing this with larger blanks would be risky, notwithstanding the suggestion by Schmidt et al. (2013) that South African silcrete can be treated in a simple way on an open fire or in coals scraped from such a fire. An added complication when heating a relatively large block of silcrete, even if it does not explode, is that water is not easily evacuated from its centre and heat treatment can be unsuccessful as a result (Schmidt et al., 2013). Not all siliceous rocks behave identically to the same thermal conditions. Flint is of marine origin, while silcrete is a terrestrial sedimentary rock and these geological distinctions produce structural differences that affect the behaviour of the rocks (Schmidt et al., 2012a, 2013). Most heat treatment studies have been carried out on flint. Flint does not occur in South Africa and, instead, silcrete was heated and used by people who lived in the Southern and Western Cape during the Middle Stone Age. This means that it is really important to use South African rocks for experiments, rather than relying on European observations on flint. Purdy and Brooks (1971) maintain that, during heating, the quartz crystals in siliceous rocks recrystallize. Schmidt et al. (2012a,b) argue against this and claim that heating leads to a loss of silanol (SiOH) and the creation of new SieOeSi bonds, a reaction that begins between 200  C and 300  C, resulting in increased hardness in heated siliceous rocks. They suggest that ‘pore-closing’ and pore-water loss help to improve flaking properties of silcrete, most likely by making the rock more fine-grained. When heating relatively large pieces of flint, pore water is not easily evacuated from their centres (Schmidt et al., 2012a) and heat treatment may be unsuccessful as a result. Most experiments (save those by Brown et al., 2009) to heat treat rocks have been conducted in laboratory furnaces. Electrical furnaces reduce variables and enable consistent outcomes during heating experiments, so the method has much to recommend it.

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However, artisans that heat treated rocks in the Middle Stone Age had to do it in the field. They had to cope with variables inherent in firewood, soil surfaces and rocks. The recent laboratory experiments with South African silcrete (Schmidt et al., 2013) contradict the results of earlier field-based experiments by Brown et al. (2009). This suggests that laboratory and field-based heat treatment experiments may have inherent inequalities. We conducted a new field-based experiment to examine the issue. Experimental heat treatment methods Silcrete preparation A single large silcrete block was obtained from the Pinnacle Point area. This was first sliced with a diamond saw into five portions (numbers 1 to 5 in the experiments) and small blocks were then cut from each slice (numbered for example 1ae1d from slice 1). Three sizes of blocks were cut to maximize the rock surface (100  65  20 mm; 80  50  20 mm and 50  30  20 mm; Table 1). The morphology of the original silcrete block meant that slices and smaller blocks could not be cut accurately. The masses and measurements of the small blocks are therefore not even (Table 1). Before and after heating, the blocks were photographed, weighed on a digital balance, and length, breadth and thickness were measured with digital callipers. Silcrete is not a homogeneous rock and two different phases can be visually detected in some of the blocks (Fig. 1). The bulk of the sample consists of a very hard greyish rock, which is difficult to cut even using a diamond saw. This hard phase is interspersed with a light yellow to red powdery phase, which is easy to cut and even to scrape off. When cutting the rocks in rectangles it was inevitable that some of the powdery areas were included in the samples. Fires 1 and 2 and Coal Beds 1 and 2 Fires were lit in the open on a sandy substrate. Soil moisture readings were obtained with a Theta Probe soil moisture sensor (Type ML2x) (Table 2). Acacia erioloba (an indigenous southern African taxon) firewood was weighed and wood moisture readings were taken using a MC-7825S moisture meter (Table 2).

Table 1 Silcrete heat treatment experiment: Mass of each silcrete block before and after heating, size of the blocks, temperatures administered, and the end result (the piece either remained whole or was fractured). Before heating

Length mm Breadth mm Thickness mm After heating Heat loss Heating position Max. temp.  C Hours 300e400  C Hours >400  C

Sample Mass g 1a 5a 1b 1d 2e 3a 2c 3b 4b 5b 3c 4a 5c 1c 2a 4c 2b 2d 3d

72.5 83.8 64.3 289.5 359.5 56.9 87.8 60.2 301.9 313.2 169.7 88.0 280.0 180.1 78.8 340.4 82.3 66.5 264.0

50.8 53.0 52.0 100.5 100.5 51.6 55.6 50.9 100.0 102.1 82.9 50.0 103.1 82.4 53.5 52.9 51.4 53.3 102.6

32.1 32.4 32.1 65.8 65.5 32.0 31.6 32.1 62.7 60.8 50.8 31.3 61.3 52.1 34.0 31.9 31.3 31.3 65.0

17.4 20.7 17.7 17.4 22.1 15.0 20.8 15.5 20.2 19.7 16.2 23.3 17.9 17.4 19.6 20.1 20.0 20.3 15.7

Mass g

g

N/A N/A 64.3 285.0 354.0 56.9 87.7 60.1 296.0 308.0 169.5 87.8 276.8 179.9 78.8 339.4 82.0 66.4 260.5

N/A N/A 0.0 4.5 5.5 0.0 0.1 0.1 5.9 5.2 0.2 0.2 3.2 0.2 0.0 1.0 0.3 0.1 3.5

Control, no heat Control, no heat Buried, Fire 1 Buried, Fire 1 Buried, Fire 1 Buried, Fire 1 Buried, Fire 2 Buried, Fire 2 Buried, Fire 2 Buried, Fire 2 Coal Bed 1 Coal Bed 1 Coal Bed 1 Coal Bed 2 Coal Bed 2 Coal Bed 2 In Fire 1 In Fire 1 In Fire 1

N/A N/A 406.5 406.5 406.5 406.5 419.4 419.4 419.4 419.4 572.7 572.7 572.7 520.8 520.8 520.8 761.8 761.8 761.8

N/A N/A 3.75 3.75 3.75 3.75 4.00 4.00 4.00 4.00 3.00 3.00 3.00 0.25 0.25 0.25 0.00 0.00 0.00

N/A N/A 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 1.75 1.75 1.75 2.50 2.50 2.50 4.25 4.25 4.25

Result

N/A N/A Whole Whole Whole Whole Whole Whole Whole Whole Fractured Whole Fractured Whole Whole Fractured Fractured Fractured Fractured

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allowed to cool underground before being excavated. All blocks were weighed after cooling. Optical microscopy Thin sections of unheated and heated blocks were cut and these were examined under plane and cross polarized light using an optical microscope. FTIR and Raman spectroscopy

Figure 1. Silcrete sample 2c after heating, showing the two phases that create an inhomogeneous rock. The reddish powdery phase is at the top right and the remainder of the sample is the hard, grey phase. The scale bar is 1 cm.

Small excavations were made into the sandy substrate and silcrete blocks were laid flat on the level surfaces under both Fire 1 (blocks 1b, 1d, 2e and 3a) and Fire 2 (blocks 2c, 3b, 4b and 5b) (Fig. 2). Thermoprobes were placed on the exposed surfaces of the blocks, then blocks and probe tips were covered with 25 mm of sand (Fig. 2). In the case of Fire 1, blocks 2b, 2d and 3d were placed on the sand surface directly under the firewood (Fig. 2B). The fire was set using a fire starter, twigs for tinder, and A. erioloba logs (see Table 2 for mass of wood used). A thermoprobe was placed in the middle of each tepee-shaped fire and also near the fire perimeter. After lighting the fires, the first temperature readings were taken at 5 min and at 15 min thereafter, using Extech temperature data loggers (HD200) with dual K-type thermoprobes, each 50 cm in length. In the case of Fire 1, coals were removed after 2 h to create separate Coal Bed 1. Blocks 3c, 4a and 5c were laid on the coal bed and a thermoprobe was placed in its centre. The temperatures at the top and base of the silcrete blocks were not equal even after 2 h, so coals were then heaped on top of the blocks on the coal bed for a further 2 h. The blocks were then removed from the coal bed and also from the fire. Coal Bed 2 was created from a fire lit especially for the purpose. Blocks 1c, 2a and 4c were laid on the coal bed without being covered with additional coals. Buried blocks were Table 2 Experimental fires, mass of firewood, wood and soil moisture, temperatures at 5 min and maximum temperatures reached.

Fourier Transform infrared (FTIR) transmission/absorbance spectra of powdered samples were recorded using a Bruker Golden Gate Attenuated Total Reflection (ATR) cell, which fits into the macro sample compartment of a Vertex 70v (Bruker Optics) spectrometer. The sample compartment was evacuated during acquisitions and the contact area between the sample and the diamond ATR crystal is 2 mm diameter. Spectra were recorded with 64 acquisitions at 4 cm1 resolution over a spectral range of 8000e 600 cm1. Infrared reflectance spectra were recorded using a Hyperion microscope attached to the same Vertex 70v (Bruker Optics) spectrometer. The samples were placed directly under the 10 microscope objective and a spot selected for analysis. The spectra were then recorded using a 15 IR objective after optimizing the focus to obtain the maximum signal. Spectra were recorded with 128 acquisitions at 4 cm1 resolution over a spectral range of 8000e600 cm1. Micro-Raman spectroscopy was performed with a T64000 Raman spectrometer from HORIBA Scientific, Jobin Yvon Technology (Villeneuve d’Ascq, France). The 514.6 nm laser line of a mixed gas KryptoneArgon laser (Coherent) was used as excitation source. The 10, 50 or 100 objectives of an Olympus microscope were used depending on the spot size of the area of interest. The spectra were acquired using two acquisitions of 120 s with a spectral resolution of w2 cm1. XRD and XRF measurements In order to determine the elemental composition and mineralogical phases, the samples were powdered and analysed using Xray fluorescence (XRF) and X-ray powder diffraction (XRD). The samples were milled in a tungsten carbide vessel and prepared for XRD analysis using a back loading preparation method and then analyzed using a PANalytical X’Pert Pro powder diffractometer with X’Celerator detector and variable divergence- and receiving slits with Fe filtered Co-Ka radiation. The phases were identified using X’Pert Highscore Plus software. The relative phase amounts (weight%) were estimated using the Rietveld method (Autoquan Program) (Table 3). Amorphous phases, if present, were not taken into consideration in the quantification. The samples were dried at 100  C and roasted at 1000  C to determine Loss On Ignition (LOI) values. For each sample, 1 g Sample was mixed with 6 g Lithium teraborate flux and fused at 1050  C to make a stable fused glass bead. The Thermo Fisher ARL9400 XPþ Sequential XRF with WinXRF software was used for analyses. Blank and certified reference materials are analysed with each batch of samples and are included in the results.

Initial mass wood kg

Total mass wood kg

Wood moisture %

Soil moisture 3 m m3

Fire 1 Coal Bed 1

5.0 N/A

16e39

0.008e0.014

460 N/A

775.0 572.7

Fires 1 and 2 and Coal Beds 1 and 2

Fire 2 Coal Bed 2

6.5 4.5

8.0 Coals from Fire 1 9.5 7.5

0.008

530 123

765.0 619.0

The maximum below-ground temperature in Fire 1 was 406.5  C and the maximum above-ground temperature was 761.8  C

Experimental fire

Max. Temp.  C at temp.  C 5 min

Results

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Figure 2. A. Silcrete blocks prior to burial in sand under Fire 1. The position of the probes on samples 1d and 2e enabled the recording of their temperatures throughout the experiment. B. The setting of Fire 1 prior to lighting it. Sample 3d is visible under the wood. C. Fire 1 (top left) and Coal Bed 1 (bottom right) with silcrete samples 3c, 4a and 5c.

(Table 1). The maximum temperature reached by Coal Bed 1 was 572.7  C. The fire ramp rate was steep: after 5 min the fire temperature had reached 460  C and by 15 min the temperature was 540.9  C. The underground ramp rate was considerably slower with temperatures above 300  C being reached only after 1.5 h. Underground temperatures over 300  C were held for 3.7 h (Table 1). The temperature in the centre of Fire 2 ramped to 530  C in 5 min and the maximum temperature reached was 765  C. The perimeter of the fire remained cooler (Fig. 3), reaching a maximum temperature of 671  C after 2 h. Notwithstanding the variation in temperatures at the perimeter and in the centre of the fire, the two underground temperatures developed similarly. All of the underground temperatures rose slowly and steadily and decreased in the same way. Coal Bed 2 was maintained for only 2.5 h, which is the period that Schmidt et al. (2013) suggest is necessary for heat treatment of silcrete. The maximum temperature reached was 520.8  C, achieved when the coals were first scraped from the fire.

The type and amount of firewood influence the above ground temperatures and their duration. The A. erioloba used for the experiments maintained above ground temperatures of between 700  C and 600  C for several hours, produced good coals and successfully heated the subsurface to between 400  C and 300  C. Fire 1 burned 8.0 kg of wood and Fire 2 burned more wood (9.5 kg). Notwithstanding the increase in the use of fuel in Fire 2, the fire performance was unaffected and neither maximum above nor below ground temperatures were substantially different from those achieved by Fire 1. The visual effect of heating on the silcrete blocks Blocks 1b, 1d, 2e and 3a buried in sand under Fire 1 remained whole at the end of the process (Table 1). All other blocks directly under the fire fractured and would not have been usable for knapping (Fig. 4). Blocks 3c and 5c on Coal Bed 1 fractured, but the small, thick 4a remained whole. Blocks 2c, 3b, 4b and 5b buried

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Table 3 XRF results for the silcrete control sample, sample 2b (heated in Fire 1) and 3b (buried under Fire 2). % SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Cr2O3 NiO V2O5 ZrO2 CuO LOI TOTAL

Control

2b

3b

95.84 1.54 0.01 0.23 0.01

Experimental heat treatment of silcrete implies analogical reasoning in the Middle Stone Age.

Siliceous rocks that were not heated to high temperatures during their geological formation display improved knapping qualities when they are subjecte...
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