The mechanical properties of wood and the design of Neolithic stone axes

Despite the importance of wooden tools for early man, and the development of woodworking in the Mesolithic and Neolithic culture, there has been surprisingly little research on how wood can be worked by stone tools or how wooden handles for composite tools were designed. This paper outlines an approach based on an understanding of the structure and mechanical properties of wood. The cell arrangement in wood makes it far less stiff, strong and tough across the grain, especially tangentially. This makes it hard to harvest wood or break it into lengths because it splits down its centre rather than breaking right across. Fortunately, this also makes wood easy to split along the grain, especially radially through its centre into sections and planks. A model of the splitting process predicted that wood is best split using blunt, broad but smooth wedges, as these would use less energy and would be less likely to get stuck in the wood. The predictions were verified in tests in which hazel coppice poles were split using wedges of contrasting angle, width and surface texture. The results help explain the change from the flaked flint Mesolithic tranchet axes to the broader polished stone Neolithic axe and adze heads. However, further experiments are also needed cutting wood obliquely to test this hypothesis. The splitting model also helps to understand the design of socketed axe hafts. Failure usually occurs when the handles split at the distal and proximal ends of the socket. To prevent this, handles are best designed with the growth rings parallel to the socket, and with an expanded head, especially with flanges on the distal and proximal ends of the socket. These designs are seen in some of the Neolithic axe handles that have been found in Britain, including the Etton, Ehenside and Shulishader axes. More experimental research is needed to understand the optimal way of hafting axe heads.