Heat-treated wood

Article By:

Rowell, Roger M. Department of Biological Systems Engineering, University of Wisconsin, Madison, Wisconsin.

Last reviewed:2009

DOI:https://doi.org/10.1036/1097-8542.YB090002

It has long been known that heating wood will change its properties. In ancient Africa, natives hardened wood spears by placing a sharpened straight wood stick in the bottom of glowing coals and then pounded the burned end with a rock, repeating this process many times until the end was sharp and hard. The Vikings burned the outside of their ships to make them water-resistant and flame-retardant. In the early part of the twentieth century, it was found that drying wood at high temperature increased dimensional stability (resistance to swelling and shrinkage) and reduced hygroscopicity (ability to take up and retain moisture). Later, it was found that high-temperature drying also increased resistance to microbiological attack. At the same time, the increase in stability and durability increased brittleness and loss in some strength properties, including impact toughness (resistance against breaking after a sharp blow is applied), modulus of rupture (maximum surface stress in a bent beam at the instant of failure), and work to failure (the energy needed to cause fracture). The treatments usually caused a darkening of the wood, and the wood had a tendency to crack and split.

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