Spear study misses the point: a critique of the Ennos and Chan fire-hardening study, concerning wood material representation in archaeological finds and generalized conclusions

Ennos and Chan (E&C) investigated the effects of fire hardening on wooden spears [1]. Their fire treatment of representative wood reduced its strength and work of fracture, and the authors concluded that it had no tangible benefits for spear functionality. Several aspects of the study, which casts doubt on the ingenuity of early hominins, invite a response.

The oldest exemplary spears, namely the Clacton spear point [2], the Schöningen spears (nine spears, one lance and a throwing stick) [3] and the Lehringen lance [4,5], were manufactured from gymnosperm trees: yew (Taxus sp.), spruce (Picea sp.) or pine (Pinus sylvestris). Each of the Schöningen spears came from a single spruce stem, with minimum ages of 21–60 years. As the maximum spear diameter range is just 2.4–4.7 cm, it is clear that the source trees were extremely slow-growing, with compact rings of annual growth, and selected for their high density [6]. The spearheads were carved from the basal end of the stems, the oldest part, where the heartwood proportion is greatest. Heartwood only forms in older trees as parenchyma cells die, leaving encapsulated deposits of resin (fats and waxes) [7]. These extractives permeate the heartwood and increase wood density, ranging from 320 to 720 kg m⁻³ (dry mass) across species and radial position [8]. The absence of extractives is the main reason why sapwood, the young outer layers of wood, has poor durability regardless of species [9]. The sapwood of the Schöningen and Clacton spears was completely removed during the manufacture of the spear tip taper, exposing the heartwood over the tapered section (25–60 and 10 cm from the tip in the former and latter, respectively). As a result, the spearheads consist of heartwood and its dense resinous mass gives these spear points their highly polished appearance [2].

The 2–3-year-old hazel (Corylus sp.) used in the E&C study differed from the aforementioned spears in two ways: (1) hazel is an angiosperm (hardwood) and (2) it contained no heartwood owing to its young age. Hardwoods and softwoods respond differently to thermal treatment owing to their dominant hemicellulose type; xylan (15–30%) in hardwoods and mannan (14–20%) in softwoods [7]. As the former is more thermally reactive [10], hardwoods undergo greater chemical modification and mechanical quality changes than softwoods during equivalent treatment [11].

E&C linked fire hardening with modern thermal treatment of timber and their results generally agree with observed properties changes in wood owing to thermal treatment. However, they implied that, because the timber effects can be generalized across species, their fire-hardening results on hazel are also applicable to other wood species. Yet this is questionable given the simplicity of their method. Commercialized thermal treatment, partial pyrolysis of wood, is a multi-phase process under controlled conditions within a kiln. Maintaining thermal equilibrium within the wood during treatment is a key aim. The 36 h process combines drying and pyrolysis under inert atmosphere up to 215°C with saturated steam and includes cooling with moisture conditioning [12]. Uniform treatment and resulting physical properties are realized throughout the timber.

E&C's fire-hardening process (i.e. barbeque grilling) was short (30 min) and severe by comparison. It was carried out in an oxidizing atmosphere (i.e. in air) with unidirectional heating, creating large temperature differentials, greater hydrostatic tension and explosive vaporization in the wood, factors leading to cellular collapse [13]. It is likely that heat-transfer limitations produced the large variations reported in stiffness and failure breakage of the hazel. This highlights the need for better-defined parameters (e.g. temperature) in experiments to avoid half-baked results. The extrapolation of the grilling results to other species is problematic, not only because of the compositional differences already mentioned but also because the extent of thermal modification itself was poorly defined.

It follows that the mechanical tests literally missed the point—the spear point, that is. The results relate to misrepresentations of ancient finds; spear points are composed of gymnosperm heartwood, not angiosperm sapwood. The Clacton and Schöningen spears show no evidence of fire hardening [14]. An analysis of the former explicitly states (three times) that the spear was neither fire treated nor charred [2]. There is no evidence that the spear tip was sharpened using fire. On the contrary, the Clacton spear point ‘was almost certainly worked while the wood was fresh…’ [2]. Why then do E&C suggest that it may have ‘broken off because it had been fire-hardened’ when this is out of step with the facts? It could be that fire use was not even widespread at the time of their manufacture [15]. But if it was, fire need not be destructive; it is a method to dry wood and maximize its natural hardness [16], which is sufficiently lethal for hunting [17,18]. A more inclusive view on spear functionality evaluates designs based on whether they were sufficient for the task. The fact that both untreated and fire-treated spears exist as artefacts, and we are discussing them, suggests that they were.

E&C gave no attention to the anisotropic features of wood, yet there is clear evidence that ancient hunters did. The wood used in the study was not representative of wood used by early hominins in the oldest examples of wooden spears. The authors have stimulated discussion on spear manufacture, but their wide-ranging conclusions are not justified based on their methods. Future studies need to use representative materials and better-characterized methods.

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Competing interests

I declare I have no competing interests.

Funding

I received no funding for this study.