Distinguishing between traces created by different mineral materials can indeed be challenging, especially between those with similar physical properties like hardness, crystal habits, fracturing tendencies, etc.15,41,46,47,48,49. Variability in the appearance of traces produced by the same contact material can complicate their assessment both on experimental and archaeological specimens and may be caused by a number of factors: variability between individual rock/mineral types (size, structure, contact surface morphology, etc.), a contact material behaving differently on different types of flint, variable preservation conditions, or that each archaeological biface was employed in a different series of functions after the mineral use traces were imparted50.

Experimental results

The use traces imparted onto a stone tool appear as one or more of the following types of surface damage, depending on the material being worked and the duration of the task: polish, linear traces (i.e. striations, scratches, grooves), rounding, fractures, surface/edge removals and crushing (i.e. abundant overlapping fractures causing extensive surface removal) (Figs 5–7). Generally speaking, the mineral use-wear traces observed on our experimental and archaeological pieces can be broken down into four main categories: retouching/flintknapping, non-directional percussive, directional percussive and directional frictive traces. Retouching/knapping traces consist of single or clustered linear gouges in the surface of the flint, sometimes overlying (semi-)circular percussion marks46,51, often oriented in a similar direction allowing for the determination of the direction of motion (Fig. 5f,g, Supplementary Figs S47, 48). The non-directional percussive traces seem to indicate some sort of pounding activity where the direction of force is roughly perpendicular to the surface of the tool, creating isolated or grouped circular percussion marks (i.e. incipient Hertzian cones) on flatter surfaces without associated linear gouges, or extensive crushing of salient points and ridges (Fig. 5c,d, Supplementary Figs S38, S40). When the battering is excessive, it may be difficult to distinguish between pounding and flintknapping activities due to fracturing and surface loss. The directional percussive traces are also comprised of single or clustered percussion marks, but instead of being fully circular, they are instead C-shaped, indicating a more oblique blow (Fig. 5a,b, Supplementary Figs S39, S44). Experiments have shown that the Cs open towards the direction the percussor is travelling, and thus can indicate the relative motion of the two elements. Finally, the directional frictive traces created during activities involving grinding, forceful rubbing or, at times, oblique percussion, manifest as polish and/or striations, the latter often indicating the relative directionality of the interacting elements (Figs 6,7).

Figure 5 Images of experimental wear traces at low-magnification. (a) Unidirectional C-shaped percussion marks produced while making fire with pyrite (Exp 3471, Supplementary Fig. S39); (b) unidirectional C-shaped percussion marks clustered along a flake scar ridge while making fire (Exp 3474-Zone D, Supplementary Fig. S44); (c) percussion marks and heavy crushing produced during fire making (Exp 3470, Supplementary Fig. S38); (d) crushing and percussion marks along flake scar ridge produced during fire making (Exp 3472, Supplementary Fig. S40); (e) very small unidirectional C-shaped percussion marks produced while ‘backing’ a flint flake, caused by the sudden change in relief as the flake passed over the step-fracture and dropped onto the lower surface (Exp 3473-Zone B, Supplementary Fig. S41); (f) percussion marks and linear and ovate surficial gouges produced while flintknapping another flint biface (Exp 3476-Zone A, Supplementary Fig. S47); (g) percussion marks and linear surficial gouges produced while retouching the edge of a scraper (Exp 3476-Zone F, Supplementary Fig. S48); (h) iron-oxide mineral residue (after cleaning) deposited while abrading/grinding iron-cemented sandstone (Exp 3477-Zone D, Supplementary Fig. S50). Full size image

Figure 6 Images of experimental pyrite microwear traces at high-magnification. These traces generally occur as zone of matte, rough polish containing densely packed clusters of parallel to sub-parallel striations and scratches. (a) Exp 3470 (Supplementary Fig. S38), (b) Exp 3471 (Supplementary Fig. S39), (c) Exp 3472 (Supplementary Fig. S40), (d) Exp 3473-Zone D (Supplementary Fig. S42), (e) Exp 3475-Zone B (Supplementary Fig. S45), (f) Exp 3474-Zone C (Supplementary Fig. S44), (g) Exp 3476-Zone G (Supplementary Fig. S48), (h) Exp 3477-Zone E (Supplementary Fig. S49). Full size image

Figure 7 Images of experimental microwear traces of other mineral materials at high-magnification. See Supplementary Table S3 for more detailed descriptions of the experimental tools pictured here. (a) Flint, Exp 3473-Zone A (Supplementary Fig. S41), (b) Flint, Exp 3476-Zone F (Supplementary Fig. S48), (c) Quartz, Exp 3474-Zone B (Supplementary Fig. S43), (d) Quartz, Exp 3474-Zone B (Supplementary Fig. S43),(e) Sandstone, Exp 3474-ZoneA (Supplementary Fig. S43), (f) Sandstone, Exp 3474-Zone A (Supplementary Fig. S43), (g) Iron-cemented sandstone, Exp 3474-Zone C (Supplementary Fig. S44), (h) Iron-cemented sandstone, Exp 3474-Zone C (Supplementary Fig. S44), (i) Quartzite, Exp 3476-Zone C (Supplementary Fig. S47), (j) Quartzite, Exp 3476-Zone D (Supplementary Fig. S47), (k) Calcareous cortex of a flint nodule, Exp 3475-Zone D (Supplementary Fig. S46), (l) Calcareous cortex of a flint nodule, Exp 3475-Zone D (Supplementary Fig. S46), (m) Limestone, Exp 3475-Zone A (Supplementary Fig. S45), (n) Limestone, Exp 3475-Zone A (Supplementary Fig. S45), (o) Hematite, Exp 3478-Zone A (Supplementary Fig. S51), (p) Hematite, Exp 3478-Zone B (Supplementary Fig. S51), (q) Goethite, Exp 3479 (Supplementary Fig. S52), (r) Goethite, Exp 3479 (Supplementary Fig. S52), (s) Manganese dioxide, Exp 3480 (Supplementary Fig. S53), (t) Manganese dioxide, Exp 3481 (Supplementary Fig. S54). Full size image

Fire making traces

The traces produced by pyrite on flint during fire making generally conforms to a combination of directional percussive and frictive traces. At the macroscopic level, this activity can produce clusters of unidirectional C-shaped percussion marks, rounding of flake scar ridges and some crushing (Fig. 5a–d). At the microscopic level, these traces generally occur as zone of matte, rough polish containing densely packed clusters of parallel to sub-parallel striations and scratches (Fig. 6). While usually a percussive task, percussion marks are not always present or readily noticeable. This could be due to a number of reasons, including the nature of raw material (percussion marks are sometimes more difficult to observe in coarse-grained stone, e.g. Supplementary Figs S34, S49), the force of the blow (often dependent on the size of the pyrite fragment, with larger fragments yielding larger incipient cones), and/or the surface morphology of the pyrite fragment (salient/convex surfaces are more likely to produce percussion marks than a flatter surface due to the greater concentration of force). Therefore, it is possible to produce what appear to be purely frictive traces while employing oblique percussion. Moreover, it is also possible to create sparks using a purely frictive, forceful rubbing gesture (e.g. Exp 3475-Zone B; see Supplementary Table 3 and Supplementary Fig. S45), though this method was not as effective at producing sparks/fire as using oblique percussion.

While C-shaped percussion marks were common, other macroscopic traces observed in our experiments include crushing and/or heavy rounding of edges, flake scar ridges or other salient surfaces (Fig. 5, Supplementary Figs S38, S40). Microtraces include densely packed clusters of (sub)parallel striations within discrete zones of flat, matte polish, as well as microscopic manifestations of the crushing, rounding, and surface removals mentioned before. Often times these traces are associated with small pits (described also as ‘micro-potlids’45 and ‘craters’ or ‘micro-craters’49,51). Johansen and Stapert45 attribute these to friction heat, much like potlids formed when flint is exposed to fire, but based on our experiments, they may be at times more related to a fragment or salient portion of the pyrite plucking out portions of the flint surface as it carves out a striation, as indicated by the linearity of some of these pits (e.g. Fig. 6, Supplementary Fig. S46), small pyrite fragments tumbling between the two surfaces, or they may sometimes simply be an artefact of the surface topography of the flint.

Non-fire making traces

The experimental traces created by grinding iron oxide (hematite, goethite) and manganese dioxide minerals across flake scar ridges to produce powder52 produces a bright, flat polish lacks pronounced striations (Fig. 7o–t, Supplementary Figs S51–54). Linear groupings of closely spaced C-shaped incipient cones (also referred to as a frictive track or ‘chattersleek’) were common within the goethite and hematite traces (Supplementary Figs S51, 52). These traces differ substantially from those observed on the archaeological bifaces41, with the degree of wear to the ridges also being much too minimal, and can likely be discounted as candidates for explaining the unidentified mineral use traces. Moreover, iron oxide residues (e.g. Fig. 5h, Supplementary Figs S50, 51) were particularly difficult to remove from the experimental pieces during cleaning, even when subjected to harsh acids, suggesting these residues, if ever present on archaeological pieces, would be more likely to preserve than pyrite residues.

Siliceous rocks (flint, quartzite, quartz, sandstone) tend to exhibit a streaky polish, not as flat as pyrite and sometimes having a reticulated appearance (i.e. features perpendicular to the motion direction, somewhat similar to a frictive track) (Fig. 7a–j). Striations are variable in expression, both in number and nature. Quartz striations are generally wider and poorly expressed (Fig. 7c,d, Supplementary Figs S43, S49). Sandstone and quartzite often create packed clusters of shallow striations with occasional wider, deeper, U-shaped cuts into the surface of the flint, likely corresponding to salient individual sand grains (Fig. 7e–j; Supplementary Figs S43, 44, S47, S50). Flint polish appears more domed with only occasional striations with widths and depths intermediate between sandstone/quartzite and pyrite (Fig. 7a,b; Supplementary Figs S47, 48). The surface of the flint often has a ‘cloudy’ appearance due to resistant, additive siliceous residues. Of these, iron-cemented sandstone was the most apt to produce polish and striations somewhat similar to the mystery traces in question. Linear gouge marks generally associated with retouching and flintknapping (Fig. 5f,g, Supplementary Figs S47, 48) are not usually produced during other percussive activities (e.g. fire making), and non-directional circular percussion marks without associated (uni)directional frictive traces are more likely resulting from pounding activities. Grinding, rubbing or abrading activities with these materials result in directional frictive traces, but rarely produce percussion marks. These, if present, are relatively few in number and tend to be found at points where there is a sudden change in relief, where the object moving across the surface of the biface either encounters a step fracture causing the abrading piece to suddenly drop onto a lower surface of the biface, as was the case with Exp 3473-Zone B (Fig. 5e, Supplementary Fig. S41) used to back a flake, or if it encounters a more raised surface like a high flake scar ridge, as seen on Exp 3473-Zone A (Supplementary Fig. S41) used to abrade the edge of another flint biface.

Calcareous stone was found to be neither hard nor abrasive enough to impart the heavy ridge rounding observed on the archaeological pieces without considerable effort. The resultant polish is domed, with wider more shallow (undulating) striations (Fig. 7k–n, Supplementary Figs S45, 46). Sand grain inclusions would occasionally create deeper isolated striations more akin to those created by sandstone (Fig. 7m, Supplementary Fig. S45).

Archaeological results

All the artefacts examined for this study are listed in Supplementary Table 1, which also indicates their interpreted uses and associated figure numbers. Based on the comparisons with experimental material, both the character and distribution of the use traces imparted onto experimental bifaces used to make fire compare well with those encountered on a number of the archaeological specimens: 26 surfaces on 20 bifaces appear to exhibit traces that indicate either probable or possible use of the tool as a strike-a-light (e.g. Figs 1, 2). Ten surfaces on eight of the archaeological pieces exhibit what we consider retouching/flintknapping marks that are not associated with comparable zones of directional frictive traces (e.g. Fig. 1c; Supplementary Figs S7, S27), while eight other surfaces have what appear to be overlapping zones of retouching/flintknapping and directional percussive/frictive traces that are likely unrelated to one another, reinforcing the multi-use nature of these tools (e.g. Supplementary Figs S1, S20). When present, other non–mineral microwear traces (as reported in38,41) are indicated in the Supplementary Information figures and listed in Supplementary Table S1.

Orientation and distribution of probable fire making traces

As was the case in our experiments, adjustments to how a biface is held can result in different spatial distributions of the traces. However, despite the variability observed in the distribution of the traces on the archaeological pieces, when the location of the traces is considered together with the orientation of the striations and percussion marks, the inferred motion is likely to be indicative of the orientation of the biface and finger placement during use, and may even be indicative of the handedness of the user. Moreover, the size of the biface relative to the corresponding piece of pyrite used to make fire likely dictated which was used as the active element. Larger biface specimens were likely held passively while being struck with a smaller piece of pyrite, in some cases with the proximal (prehensile) end of the biface positioned downward, perhaps resting on the ground or some other stable substrate, with the tinder placed at its base. This could, for example, explain the proximal crushing observed on biface BvD 12582 (Supplementary Fig. S31). The location of the traces on other archaeological bifaces (e.g. CPN 99 W9, Supplementary Fig. S2; CPN E15-324, Supplementary Fig. S13) suggest that they may have been held with the distal end pointed downward, the tip of the biface either resting on the substrate, or more likely, held above the tinder material (See Supplementary Video S1). On some of the smaller bifaces (e.g. CPN E13-718, Supplementary Fig. S8), it is possible that they were struck against a passively held block of pyrite (Compare with Exp 3472, Supplementary Fig. S40, Supplementary Table S3). This variability in sizes, plus the variable nature and orientation of the flake scars on each biface, as well as the fact that most of the bifaces were further reduced and reshaped after use, and of course not forgetting personal preferences, can all account for the different locations of use zones between the archaeological pieces. Moreover, all of these methods were found to be effective at producing showers of sparks.

On the archaeological bifaces, use traces are consistently oriented parallel either to the longitudinal axis or to one of the lateral edges of the tool, and often perpendicularly cut across flake scars produced while shaping the biface. This is likely due to the flake scar ridges acting as a rough, abrasive surface that aids in creating sparks when struck with pyrite. However, experiments of longer duration (e.g. Exp 3470, Fig. S38, Supplementary Table 3) have shown that these surfaces become worn and less effective at producing sparks over time, which can have a limiting effect on the amount of time any one surface is used. Some bifaces exhibit particularly heavy mineral use-wear on both sides of the tool (e.g. Fonseigner 77, A2 Base Foyer, Niveau B, Supplementary Fig. S24), or on one side with variable directionality (e.g. BdV 2692, Fig. 2; Meyrals, Fig. 2; CPN F15-55, Supplementary Fig. S20; CPN F15-397, Supplementary Fig. S21). This phenomenon could indicate that the tools were used for more than one fire-making event, or that difficult conditions for making a fire (e.g. inclement weather, poor quality or slightly damp tinder) required a longer period of use that necessitated using a fresh surface after the utilized surface became too worn and less effective at producing sparks. However, the act of reshaping a biface through flintknapping effectively rejuvenates the surface of the biface, though in the case of the archaeological bifaces, it is likely that this would have been an added (though largely unintended) benefit of normal edge resharpening practices geared towards obtaining fresh cutting edges for other tasks like butchery. It is therefore interesting to note that some of the most well-developed directional percussive and frictive mineral use traces occur on bifacial thinning flakes (e.g. CPN E14-243, Fig. 3; CPN E19-318, Fig. 3; F15-397, Supplementary Fig. S21).

Bifacial thinning flakes

Included in our analysis were ten bifacial thinning flakes from CPN exhibiting mineral use traces. Of these, eight possess probable or possible strike-a-light microwear (Fig. 3; Supplementary Figs S4,S10–15, S21), one appears indicative of use for flintknapping/retouching (Supplementary Fig. S6) and another appears to have been used for some other unidentified percussive task (Supplementary Fig. S17). Four other bifacial thinning flakes with mineral use traces are known from CPN that were not included in our analyses (see Supplementary Table 2). Together with the biface evidence, these additional strike-a-light use zones make a total of 34 surfaces out of 49 analysed possessing these traces. That microtraces attributable to pyrite are observed on bifacial thinning flakes has two major implications, which—assuming these traces do indeed correspond to fire making—are consistent with the expedient strike-a-light model: 1) microwear evidence of a biface being used to make fire can potentially be lost as the tool is subsequently resharpened during its use life34,35; however, 2) identifying strike-a-light microtraces on resharpening or bifacial thinning flakes provide evidence that the inhabitants of a site were making fire using bifaces, either on- or off-site, even if the tools themselves were ultimately taken elsewhere.