  The reduction of wood response to variations of ambient humidity, described
as kind of ageing, has been studied from both points of view of moisture uptake
and dimensional changes. Clearwood specimens from 3 gymnosperms (Pinus
sylvestris L., Pinus pinaster Ait., Pinus insignis Dougl.) and 4 angiosperms
(Populus spp., Quercus pyrenaica Willd., Entandrophragma cylindricum Sprague,
Chlorophora excelsa Benth. & Hook f.) were subjected to five wet-dry cycles and
their moisture content and dimensional changes in radial and tangential
direction at three increasing levels of relative humidity (57.6, 84.2 and
90.2%) were measured before and after the treatment. For a given increase of
relative humidity, a coefficient of hygroscopic ageing was defined as the
relative decrease of moisture uptake, and the coefficient of dimensional ageing
as the difference of swelling strain before and after ageing. All species
exhibited a clear ageing effect, much stronger in the wet range than in the dry
range. Calculation of swelling coefficients suggested that in some cases the
relation between swelling strain and moisture content is no longer linear in
aged wood.
