Natural weathering of soft- and hardwoods modified by contact and flame charring methods

Exterior cladding boards benefit from a known service life that makes planning of maintenance and replacement procedures easier. Among the different wood modification methods, surface charring of wood is expected to increase the lifespan of wooden elements in building façades. This paper reports the properties of surface charred Norway spruce, Scots pine and Silver birch in Southern Finnish climate over a natural weathering period of one year. Several modifications were examined, namely variants of contact and flame charring. These also included oiled and brushed surfaces. The flame charred samples of spruce and birch withstood the weathering well, with some minor flaking and cracking. The thick pine samples cracked extensively regardless of modification, raising questions on suitable density and thickness of wood destined for a charring modification. Contact charring did not seem suitable for outside uses at least in direct sunlight, as the colors faded, and surfaces cracked within all examined groups. The spectroscopical methods employed also revealed degradation of contact charred wood lignin, whereas the flame charred surface consisted mostly of recalcitrant carbon structures rather inert towards weathering. This highlights the importance of sufficient structural degradation of wood components in creating a weathering resistant surface, and also shows that a thicker thermally modified layer does not necessarily improve the weatherability in contact charred wood.