As an archaeologist with primary research and training experience in North American arid lands, I have always found the European Stone Age remote and impenetrable. I did not know much, but I knew there were better things I could be doing on a Saturday night.
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Pieces in the middle row were assigned to the group Gloss contrast. Pieces in the bottom row were assigned to the Test group. A set of reference samples of the same Jurassic chert, that had clearly never been heated, was collected in the surroundings of Helga-Abri. No permissions were required for collecting the rock samples used for this study as chert is not a rare or precious resource in Germany. The land owners of all sampling locations gave permission to collect samples on their land. No other permissions were required for conduction the study and the study did not involve endangered or protected species.
As no detailed studies on the raw materials used in Helga-Abri are available, we collected samples in secondary position from ploughed fields; sampling locations were chosen as a function of their proximity to Helga-Abri and their accessibility. At both these locations individual nodules vary in size, ranging from 1 to 10 cm.
Most chert is broken there, due to intensive ploughing, but some nodules can still be found intact showing white cortex all round. We collected three of these larger intact nodules with cortex as reference material. All three samples are whitish and show a dull surface aspect on fresh fracture surfaces. Sample numbers and experimental heating temperatures are summarised in Table 2. The theoretical background and detailed experimental setup of the analyses are explained in Schmidt et al. The analyses rely on the measurement of the transmission of near IR radiation, directly through lithic artefacts zones of remaining cortex and patination should be avoided.
The non-destructive measurements result in an IR absorption spectrum between and cm -1 that contains an absorption band caused by SiOH. The mechanism behind this is the chemical interaction of this pore-water with surface SiOH hydrogen bonding. More pore-water causes a shift to lower frequencies, less pore-water causes a relatively larger band-component at higher wavenumbers [ 33 ].
When chert is heat-treated, it gradually loses such open pore-space [ 25 , 34 , 35 ]. Schmidt et al. Not-heated reference samples ideally come from the same find layer in the analysed site internal reference because they were subjected to identical taphonomic processes. This is important because some taphonomic processes were found to alter the hydroxylation of chert [ 36 ], consequently also influencing the measured IR signal.
If such an internal reference is not available, or if it cannot be established with certainty that the internal reference is unheated, an external reference made from the same material may be used see for example [ 37 ]. Both samples compared in this way, the one tested for past heating and the reference, must undergo an identical protocol, allowing for total filling of their open pore-space with deionized or distilled H 2 O.
Heating temperature can be estimated by combining these measurements with measurements of experimentally heat-treated reference samples of the same rock. The comparison between the ratio values of archaeological samples and the ratio of the reference allows to estimate the temperature range the archaeological sample was heated to. Geological reference samples were treated along with the 46 archaeological samples, applying the identical protocol. One of the 3 reference samples underwent experimental heat treatment to estimate the heating temperature of the archaeological samples. After each temperature step, the samples were cooled to room temperature overnight to avoid fracturing induced by excessively fast cooling and then rehydrated in deionized H 2 O for 48h at room temperature and ambient pressure to saturate their open pore space with water.
No fracturing of the sample was observed with this protocol.
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In a second step, the surface roughness of 42 of the Helga-Abri artefacts was measured with a Laser Scanning Microscope LSM four of the artefacts only showed surfaces that were either too concave, not large enough or that contained topographic features that made analysis with a LSM impossible.
LSM analyses were performed because several works see for example [ 27 , 28 , 39 , 40 ] found that heat treatment modifies the fracture pattern of silica rocks, allowing smoother surface removals after the treatment.
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Samples previously assigned to the group Gloss contrast were measured on both types of removal scars pre-HT and post-HT where this was possible. On four of these Gloss contrast artefacts it was not possible to measure both types of removal scars same reasons as explained above.
The surface roughness values obtained in this way were then compared with the IR spectroscopic analysis. Spectra were acquired between and cm -1 with a resolution of 8 cm The diameter of the IR beam was cut to 5mm by a circular diaphragm. No other sample preparation was necessary and the analyses of all archaeological samples remained non-destructive. These IR analyses were performed in collaboration with K.
This error is due to sample heterogeneities and reflects the inter- and intra-sample variability of Jurassic chert from the Swabian Alb.
This error takes into account the sample heterogeneity of Jurassic chert but also reflects the range of different values produced from samples collected in the Helga-Abri archaeological deposits these may also be influenced by taphonomic agents. We chose this larger error for the experimental series to increase the significance of the temperature determination for all analysed artefacts.
The measured spectral range contains a SiOH combination band. The four spectra in the lower part correspond to the experimental heating series of geological reference sample SJc. Note the gradual shift of the band to higher wavenumbers with rising heating temperature use the vertical line as visual guideline. The four spectra in the top were recorded from archaeological samples. Spectra are vertically offset for readability.
To obtain 3D surface models of the removal scars, four tiles were stitched together, producing 1. No additional filtering was applied and the produced Ra values must be considered a mixture between surface waviness and roughness.
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These analyses were conducted in collaboration with C. Fig 3 shows the spectra of the geological reference sample experimentally heat-treated to successive temperatures and four spectra of archaeological samples for comparison a more detailed deconvolution of these SiOH bands can be seen in S1 Fig and S2 Fig. The SiOH band of the experimentally heated sample shifts progressively to higher wavenumbers with rising temperature. These ratio values are listed in Table 2 for geological and experimental sample and in Table 3 for artefacts.
Fig 4A is a plot of the ratio values of the 46 archaeological samples. Archaeological samples previously assigned to the group Gloss contrast i. The range of these values are most likely caused by different temperatures used for heating [ 20 , 37 ]. Samples previously assigned to the Test group produced ratio values between 1.
Samples from the Test group that plot within the blue bar of Fig 4A may be considered as not-heated based on these results.
This is the case of eight pieces. However, comparing the archaeological groups Gloss contrast and Test with geological reference samples Fig 4B , a slightly different pattern can be observed. The grey bar in Fig 4B marks the scattering ranges of values obtained from geological reference samples the upper part of the blue bar from Fig 4A is maintained here for comparability.
Here, only two samples appear as clearly not-heated. However, it must be stressed that the difference between taphonomic processes acting upon archaeological Not-heated and geological samples is unknown and the significance of the comparison in Fig 4B cannot be evaluated.
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The only secure observation stemming from these results is that two of the test samples were most likely not-heated and six Test pieces remain indeterminate with respect to them being heated or not. Archaeological samples are named using the sample numbers shown in Table 1. The range of values produced by samples of the group Not-heated is marked by a blue bar.
The range of geological reference values is marked by a grey bar and the range of values obtained from artefacts of the group Not-heated is still marked by a blue bar. Note that Not-heated artefacts produced slightly higher values than the geological reference. In this way, the resulting temperature estimations takes into account the uncertainty of our assignment but also the possibility that inter-sample heterogeneity of Jurassic chert used in Helga-Abri was larger than the one found in our three geological reference samples. Fig 5 is a plot comparing the archaeological groups Gloss contrast and Test with experimentally heat-treated reference sample.
Ratio values of the experimental series are gradually increasing after each temperature step. The experimental series allows however to estimate the heating temperatures of all artefacts in the group Gloss contrast and 19 samples from the Test group. Ratio values of the progressively heated geological reference samples are displayed on the left of the graph. Temperatures, as calibrated by this experimental series, are shown as horizontal lines.
The blue bar is maintained from Fig 4A and marks the range of values produced by Not-heated archaeological samples. Ra values measured on the surfaces of samples previously assigned to the group Not-heated plot between 1. The range of Ra values obtained on pre-heating removal scars of the group Gloss contrast roughly fall within this range.
In all four cases where it was possible to measure pre- and post-heating scars on single Gloss contrast artefacts, post-heating scars have lower Ra values than pre-heating scars as indicated by the broken arrows in Fig 6A. The range of Ra values measured on samples of the Test group is reasonably close 1. Because, at least, 19 of the Test samples were estimated to be heat-treated by IR analysis, surface roughness appears to be not significant for identifying heat treatment on Jurassic chert from the Swabian Alb.
This observation is further strengthened by the correlation graph in Fig 6B. Note that the surface roughness values of Not-heated artefacts plot in the same range as Test samples. Two values are plotted for some samples of the Gloss contrast group. In this case, square dots are pre-heating removal scars and round dots are post-heating scars.
Note that for all cases, where these double measurements were possible, post-heating scars are smoother than pre-heating scars. Note the absence of correlation between surface roughness and heated vs. Our IR analyses confirm that all archaeological samples with gloss contrast i. In order to produce, control and maintain such temperatures, a specific heating environment or oven-like structure must be built.
However, temperatures recorded on Jurassic chert from Helga-Abri are significantly higher than the ones recorded from Chassey and Solutrean artefacts. How can these differences be explained? The degree of standardisation allowed by the technique used at Helga-Abri also seems to be significantly lower than in other periods. Standardised heating techniques, such as sand-baths or earth-ovens, are unlikely to produce such great scattering ranges of heating temperatures, precluding the hypothesis of their use in the Mesolithic of southwestern Germany.