Relative Humidity, EMC, and Collapse Shrinkage in Wood

Small red oak (Quercus sp) specimens conditioned at very high humidities showed equilibrium moisture contents (EMC) between 60 percent and 30 percent and very substantial collapse type shrinkages at temperatures of both 40?C (104?F) and 50?C (122?F) for both sapwood and heartwood. The collapse was almost completely recovered by adsorption steps to higher humidities and also, to a lesser extent, by continued desorption at lower humidities. Both collapse and recovery occurred almost entirely above 90 percent humidity and 20 percent moisture content (MC). This behavior was attributed to highly imperforate parenchyma cells. Prefreezing had a moderate effect but precompression had no effect on the oak except when compression was extreme. Limited high humidity data on red gum (Liquidambar styraciflua L.) and yellow-poplar (Liriodendron tulipifera L.) indicated excessive MCs but to a much lesser degree than in Southern pine (probably Pinus taeda L.) had normal MCs and shrinkages. Redwood sapwood (Sequoia sempervirens (D. Don) Endl.) showed very slight collapse and nearly normal MCs, but redwood heartwood exceeded 40 percent MC for both desorption and adsorption to 99 percent humidity. Initial swelling of redwood heartwood as the free water was removed indicated that extractives enter the cell wall as the wood dries. Freezing also produced this swelling, thus indicating why prefreezing reduces collapse.

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