Charring of Norway spruce wood surface – an alternative surface modification technique?

IRG/WP 19-40870

M Petric, M Pavlic, J Zigon

Charring is an old wood surface treatment technique, mainly performed on wooden façade elements with the aesthetical aims, but also with the purpose of protection. This flame treatment method can be grouped into heat treatment processes. Below the top charred layer of damaged wood there is a layer which is during flame treatment exposed to high temperatures in anoxic or semi-anoxic conditions. The conditions of forming this layer are similar to those wood is exposed during a conventional thermal modification process to. As such, it might have similar properties than surfaces of conventionally thermally modified wood. Therefore, the aim of our preliminary study was to compare some selected surface characteristics of charred, thermally modified and unmodified Norway spruce wood in order to indicate possible similarities between thermally modified and charred wood surfaces. The charred surfaces that were in the focus of our investigation became substantially darker than surfaces of thermally modified spruce wood, presumably due to substantially higher treatment temperatures. The charred wood became hydrophobic, in wettability similar to that of thermally modified wood. ATR FT-IR measurements indicated that both during charring and thermal modification similar chemical reactions might occurred. The pull-off adhesion strength test of a polyurethane coating that was applied to the substrates exhibited low tensile strength of thermally modified wood and of the charred but undamaged second layer below the surface of flame treated wood. There are only limited scientific data available on properties of charred wood surfaces and so, it is believed that this topic deserves more attention in future, so from scientific as well as from applicative points of view, respectively.

Keywords: Norway spruce wood, charring, scorching, thermal modification in vacuum, colour, hydrophobicity, ATR FT-IR

Conference: 19-05-12/16 Quebec City, Canada

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