Reply: A critique of the Ennos and Chan fire-hardening study, concerning wood material representation in archaeological finds and generalized conclusions

Reply to David A. Agar

As a broadly based biomechanics, fascinated by how our relationships with the materials around us have shaped human evolution and history [1], I am delighted that our article has interested the people who should know most about the effects of fire on the properties of wood–wood scientists.

The comment criticizes our work in three ways. First, that the heat treatment we applied to our wooden rods was crude. Second, that we investigated the effect of fire on a hardwood, not a softwood such as was used to manufacture the Clacton and Schöningen spears. Third, that our rods were composed of sapwood, not the heartwood used for the Clacton and Schöningen spears.

Each of these criticisms seems to me to be due to a misapprehension of the aims of our project. Our main aim was to make a preliminary investigation of the much-vaunted effects of ‘fire-hardening’ on the tools of early humans and modern hunter–gatherers. These are usually assumed by anthropologists to be wholly beneficial, but without any experimental evidence that this is the case.

Our basic treatment, performed as a short undergraduate project, in which we heated wooden rods over a barbecue, was justified because it would have replicated an early human holding the tip of his spear into the smouldering ashes of a wood fire. We agree that the treatment is not as controllable as placing wood in an electric kiln for 36 h, but this is of course not an option for hunter–gatherers. We are confident that our results are valid as they tie in very well with the effects you would expect to get if you heated wood in such a kiln [2]: it increases stiffness and hardness, but reduces toughness. This is also precisely what would be expected theoretically whatever type of wood was used, since heat should start to crystallize the hemicelluloses within wood cell walls without affecting the cellulose fibrils.

We used slender hardwood rods composed of sapwood—poles from coppiced hazel—because of their regular growth form and reproducibility. We feel we are justified in using this type of wood because slender hardwood shoots would be the preferred choice of the first human hunters to make spears. These people, the first to use stone tools to carve wood, would have belonged to the species Homo erectus [3] who then lived in the dry savannahs of East Africa, which are dominated by short hardwood trees, not conifers.

Of course, we entirely agree that our rods were not representative of the oldest wooden spears to actually be found: those from Clacton and Schöningen. They survived because they were used during an ice age in a more northerly continent, where conditions were suitable for the formation of the peat in which wood can be preserved. However, we mentioned these tools largely because the Clacton spear was the subject of the only study we found that investigated the effect of heat treatment on ancient tools [4]. This tested the idea that fire-charring could have helped their makers carve the spear tip. If the makers of the Clacton spear had used fire-charring to help carve their spear points, our results showed that this would have reduced its toughness and might have made it more susceptible to being broken as we pointed out.

We would be delighted if wood scientists were to undertake a further experimental study, using a series of tests that better replicate the techniques of modern hunter–gatherers and use their well-equipped laboratories to shed further light on whether fire-hardening tools really does have real rather than simply imagined, benefits, for instance on improving resistance to rotting.

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Competing Interests

I declare I have no competing interests.

Funding

I received no funding for this study.