The effect of drying and storage conditions on case hardening of scots pine and norway spruce timber

Abstract

Case hardening is a feature of dried wood that causes the wood to deform (cup) after re-sawing and equalising the moisture content. VTT has analysed case hardening with the aid of the simulation model PEO and with experimental drying, conditioning and storage tests. Case hardening cannot be predicted by the moisture content gradient alone. However, the case hardening gap after drying to a high final moisture content of 18% is nearly identical to the calculated gap caused by cupping when the moisture gradients 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 the sliced specimens in a plastic bag. Thus the test according the standard doesn’t show the whole cupping tendency of, for example, panels when the moisture content is equalised after re-sawing the timber and planing the billets.

Increasing the kiln drying rate increases the resulting case hardening tendency. With effective conditioning at the drying temperature, or with steaming after cooling, it is possible to reduce or remove the case hardening. But at normal temperatures in end-use or storage of timber the case hardening diminishes very slowly, despite the equalising of the moisture content in the cross-section.