The effect of drying and storage conditions on case hardening of scots pine and norway spruce timber (parte a)
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Universidad del Bío Bío
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Case hardening is a feature of dried wood that causes the wood to deform (cup) after re-sawing andequalising the moisture content. VTT has analysed case hardening with the aid of the simulation modelPEO and with experimental drying, conditioning and storage tests. Case hardening cannot be predictedby the moisture content gradient alone. However, the case hardening gap after drying to a high finalmoisture content of 18% is nearly identical to the calculated gap caused by cupping when the moisturegradients of the two halves of the test piece are equalised.24 hours (proposed in ENV 14464) is too little time to show the total cupping when keeping thesliced specimens in a plastic bag. Thus the test according the standard doesn t show the whole cuppingtendency of, for example, panels when the moisture content is equalised after re-sawing the timber andplaning the billets.Increasing the kiln drying rate increases the resulting case hardening tendency. With effectiveconditioning at the drying temperature, or with steaming after cooling, it is possible to reduce or removethe case hardening. But at normal temperatures in end-use or storage of timber the case hardeningdiminishes very slowly, despite the equalising of the moisture content in the cross-section.
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