Wood is a natural product that is hydroscopic, meaning it will continually adjust its moisture content to match the changing relative humidity of the air around it. This happens more quickly at the surface layer of the piece, with the inner layers catching up slowly, eventually moving the moisture from the core to the surface where it will be evaporated until equilibrium is reached throughout all layers of the wood, and the wood with the air.
Wood contracts as the moisture content decreases, and will expand with higher moisture levels. Before reaching moisture equilibrium while drying, the surface layer of the wood will contract around the still expanded inner and core layers, causing an internal pressure imbalance that can cause checking or cracking if the piece is dried too quickly. Faster drying causes a greater pressure imbalance, since the core layer of the wood loses its moisture as a somewhat constant rate, while the wood’s outer region dries much more quickly the drier the air around it.
All this has ramifications for wooden items that have been grown, carved, and stored in uncontrolled environments within tropical rain and cloud forests, where the relative humidity hovers between 90 and 100 percent year-round. To complicate matters even further, the Ifugao season their wood intended for carving without the benefit of modern kilns, but instead accomplish this by simply storing logs in a sunny, open place until deemed “seasoned” enough to carve.
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