- 1. Introduction
- Research outputs
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You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Silcrete heat treatment, along with a suit of other innovations, have been used to argue for an early onset of modern or complex behaviours in Middle Stone Age hominins. However, the exact moment that this technological advancement occurred still remains unclear.
This is partly due to the scarcity of silcrete assemblages dating to the first half of the Middle Stone Age. This period corresponds to the time that Homo sapiens became the major hominin species in the southern African subcontinent and it is roughly the time that silcrete use became widespread in the second half of the Cape-coastal Middle Stone Age. This opens interesting new questions on the relation between silcrete use and heat treatment and on why early modern humans spontaneously invented heat treatment when they began using silcrete in the Cape region.
Silcrete heat treatment is commonly understood as a technical process that aims at improving the quality of raw materials for knapping. It has in the past decade become one of the arguments for an early onset of modern or complex behaviours in the Middle Stone Age MSA see for example: Sealy, ; Wadley, This is because it was argued to proxy for several archaeological and anthropological traits like abstract thinking Wadley and Prinsloo, or high investments in resources Brown and Marean, Although other authors Schmidt et al.
Knowing the exact moment of its first invention is therefore a crucial factor for our understanding of human evolution. When heat treatment was first documented at Pinnacle Point Brown et al. This is even more interesting, as this date falls into a period where silcrete use was generally rare in southern Africa Will and Mackay, This situation is uncomfortable for MSA archaeologists. Thus, at least from the SB onwards, heat treatment seems to have been an important step in the reduction sequences associated with silcrete.
The possibility to artificially improve its knapping quality might even have governed the choice of using silcrete as a raw material. At least, there seems to be a correlation between silcrete use and heat treatment that needs to be explained. There are, however, two arguments that might change our view on MSA heat treatment.
1. Introduction
It has been argued that heat treatment might not have been practised to improve knapping quality but rather for heat-fracturing raw material blocks, to reduce nodule size before knapping even began Schmidt et al. This argument was proposed because at some sites, many silcrete blocks broke from the action of fire before knapping see for example: Schmidt et al. Improved knapping quality would in this case only be a by-product. The other argument is that natural fires might have caused what archaeologists recognise as heat treatment.
It could be imagined that bushfires or fire-based site maintenance Goldberg et al. If this were the case, the entire MSA heat treatment signal might not reflect any human activity at all. Based on these considerations a few important questions can be posed: do all silcrete assemblages in the Cape coastal region show signs of heat treatment? If heating proxies, as they have been used to identify heat treatment in MSA assemblages so far, can be identified on all silcrete assemblages regardless of their age, it might be worthwhile to investigate the bushfire hypothesis or other natural causes.
If on the other hand, we can identify silcrete assemblages without heat treatment, the bushfire hypothesis becomes unlikely. In the latter case, if intentional heat treatment were real, the time of its invention becomes important. For example, can we identify a gradual onset of heat treatment during the MSA or did it appear with the earliest silcrete use in the MSA? Was there a period in the MSA where unheated silcrete was used? If there was, can we determine at least approximately when heat treatment was invented?
If there was not, we may conclude that at least in early assemblages there was an intricate, perhaps causal, relationship between silcrete use and heat treatment. One way to approach these questions is by investigating the earliest silcrete-bearing MSA assemblages and comparing them with silcrete assemblages from before the MSA. Some of the oldest known silcrete assemblages that have yielded sufficient artefacts for such a study come from sites located on the south western coast of South Africa Will and Mackay, As the Ysterfontein 1 assemblage appears problematic the dates were rejected, see: Avery et al.
We included DFT2 in our study investigating whether heating proxies are associated with all coastal silcrete assemblages i. These artefacts were randomly chosen by picking bags, one after the other, each time inspecting all silcrete artefacts from within the bags. It is therefore suitable as a pre-MSA point in this study. Seventy-two of these silcrete pieces underwent a quantitative surface roughness analysis using the replica tape method Schmidt, In parallel, we inspected artefacts from HDP for macroscopic indicators of heat treatment.
Forty-one of these came from the HDP1 site Will et al. There is currently a research project attempting to obtain an absolute age for the HDP3 deposit.
While results have not been published yet, one of their observations relevant to our study is that the HDP3 sediments were likely deposited during the last interglacial, as revealed by paleoclimatic arguments Hare, , pers. Contemporaneity of HDP1 and 3 is, therefore, highly likely, based on stratigraphy and paleoclimate. Fifty-two of these silcrete artefacts from HDP underwent quantitative surface roughness analysis using the replica tape method. We chose not to integrate artefacts made from one silcrete type in our analysis.
It can be difficult to distinguish heat-treated from unheated silcrete with similarly large clasts, based on fracture pattern Schmidt et al. In total, there were 20 artefacts of this silcrete type in the HDP assemblage that we excluded from our analysis. Photos of analysed Lithic pieces from Hoedjiespunt a — d and g and Duinefontein 2 e and f. Note the scalar features indicted by the black arrow.
In parallel, an experimental references collection was produced from 30 South African west coast silcrete types. Samples were chosen to represent a large variety in terms of grain-size and texture. The roughness data measured on this piece references collection is published in tabular form elsewhere Schmidt, , Table 2 but they are used here as comparison with our DFT2 and HDP archaeological data.
As initially proposed by Schmidt et al. HINC fracture surfaces can be recognised due to their strong surface roughness, the presence of scalar features on the surface Schmidt et al. Fracture surfaces were only identified as HINC fractures when they are cross-cut by a post-heating removal.
This technological relationship indicates that the failure occurred during heat treatment, i. In the opposite case, when such a fracture surface is not cross-cut by a flake removal, it may result from fracturing at any stage, e.
Research outputs
In some previous work Delagnes et al. Such pieces have consistently been used to identify heat treatment in assemblages since the beginning of archaeological research on heat treatment see for example: Bordes, ; Inizan et al. In light of these considerations and the acceptance of diagnostic pieces in the archaeological community, it can be concluded that they unambiguously result from heat treatment and, consequently, that they can be used as comparative reference to identify pre- and post-heating fracture scars on undiagnostic samples provided that these are made from the same silcrete types.
Practically, this meant that a set of diagnostic artefacts was laid out on a large table and all other undiagnostic artefacts were compared with the pre- and post-heating scars on these diagnostic pieces. Artefacts that could not be clearly identified as belonging to one of the frequently occurring silcrete types for which no diagnostic comparisons could be identified were left indeterminate in this study. HINC surfaces were identified through the presence of concave, sometimes angular, structures and scalar features Schmidt et al.
To estimate the quality of this visual classification of heat treatment proxies, quantitative fracture surface analysis was conducted using replica tape for three-dimensional 3D surface mapping. The replica tape method is explained in detail in Schmidt and only the details absolutely necessary are repeated here.
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A layer of compressible foam is applied with force to the measured surface the method it is entirely non-destructive. The foam replicates the surface irreversibly by creating a negative of it. The so-produced surface negative contains thicker and thinner parts that correspond to the valleys and peaks of the original surface, respectively.
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These thicker and thinner areas on the replica tape, when scanned by light transmission, appear more or less transparent. Transparency values measured in this way can then be converted to a 3D map of the surface. Measurements made on DFT2 artefacts ventral surface was measured where possible were compared with roughness data of the west coast reference collection as in: Schmidt, , Table 2.
Ten pre-heating removal scars, large enough for replica tape measurements, and 13 suitable post-heating removal scars were identified on HDP diagnostic artefacts. The advantage of such an internal calibration is that, instead of using our external reference collection containing a random number of silcrete types from the greater west coast region, with this method only the silcrete types actually used at HDP are taken into account. The 3. As proposed by Schmidt , we transformed Ra values to their natural logarithm, so that the data can be fitted with a linear function in a scatter plot of S over Ln Ra.
As both values are tightly correlated, the variance between samples in such a plot is one-dimensional and lies on the fitted function the best fit of the scatter plot. Data quality can be visually estimated by evaluating the straying of data plots around the fitted function. It can be quantified by calculating the mean distance of the plots from this function. The fracture patterns on DFT2 silcrete artefacts are rather rough looking.
None of the artefacts showed recognisable roughness contrast between adjacent fracture negatives or between different artefacts. Although silcrete types from both sites are fairly similar macroscopically, only surfaces on the HDP assemblage could be assigned to distinct groups using the visual identification protocol.
These groups are summarised in Table 1.
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Most artefacts were knapped after heat treatment. Nine percent of the heat-treated artefacts show signs of heat-induced fracturing during heat treatment HINC fractures after which knapping continued. None of the artefacts show black tempering residue. Figure 2a is a plot of Ln Ra and S values measured on unheated and experimentally heated reference samples from the West Coast region as taken from: Schmidt, Values measured on artefacts are summarised in Table 2.
The reference scatter plot Fig. Only the zone in the lower left of the plot, where no unheated reference samples plot, is of importance here. Comparing Fig. The results for all DFT2 samples are therefore listed as either Indet. In other words, all surfaces on DFT2 artefacts yielded roughness values that can be comfortably explained by unheated west coast silcretes.
None of the DFT2 artefacts would have yielded roughness values only explicable by heat-treated west coast silcrete. Thus, there is no reason to suggest that any of the DFT2 artefacts were heat-treated. The black line marks the threshold, left of which no unheated samples plot, i. The line cuts the best fit at Ln Ra : 2. Note that none of Duinefontein 2 test samples show values that could not be explained by the variability of unheated Western Cape silcrete.