Combined Wood Modification Using Bark Extractive Impregnation and Ammonia Fuming

The present study investigated, for the first time, the synergistic effects of combining bark extractive impregnation with ammonia fuming, revealing promising enhancements in both the aesthetic and functional attributes of wood. This combined approach uses the bioactive and protective properties of natural extractives while also making ammonia fuming effective for wood species with low extractive content. Bark extracts of pedunculate oak (Quercus robur L.) and European beech (Fagus sylvatica L.) were investigated on wood specimens of European beech (with and without red heartwood), hornbeam (Carpinus betulus L.), and gray poplar (Populus × canescens). Oak bark extract contained substantially higher total extractives and polyphenols than beech bark extract. Treatments caused only minor changes in density and equilibrium moisture content (generally within ±8%), while some shrinkage parameters changed more noticeably. The largest improvement occurred in beech false heartwood, where volumetric shrinkage decreased by up to 16%. Both treatments darkened the wood surface, with oak bark extract producing significantly larger color changes; ammonia fuming further intensified these effects, especially in poplar. Durability improved markedly: mass loss caused by Coniophora puteana (Schumach.) P. Karst. and Trametes versicolor (L.) Lloyd decreased from 27%–38% in untreated samples to 0.58%–7.73% after treatment, improving durability classification from Durability Class 4 to classes 1–2. Bark extract treatments slightly reduced hardness but generally maintained or slightly increased compression strength.