The effect of pretreatment with either boron or glycerol followed by thermal modification on the durability of Douglas-fir heartwood was evaluated in an American Wood Protection Association ground proximity test in Hilo, Hawaii. Non–thermally modified samples were generally more heavily decayed than any of the modified woods, but there was no consistent effect of different thermal modification conditions on decay resistance. Thermally modified woods tended to perform better than untreated timbers but not as well as copper azole–treated Douglas-fir heartwood lumber in test at the same site. The results are discussed in relation to how the extreme site conditions might have made it difficult for thermally modified materials to perform.

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