Abstract: A heated wood sample (46) having a water content exceeding 20 percent is compressed axially in the direction of the grain or fibres of the sample in a compression mould (10). Oppositely directed compressive forces are applied not only to the end surfaces of the sample, for example by means of a hydraulic cylinder (18), but also to side surface parts of the sample as frictional forces. The sample may be arranged within a compression chamber which is at least partly defined by longitudinally overlapping side wall parts (30, 37, 47, 48), which are mutually displaceable in the longitudinal direction of the chamber, and these side wall parts may be pressed into frictional engagement with the sample (46) for example by means of an inflatable bag or an inflatable hose section (32, 34), and means may be provided for mutually displacing the side wall parts so as to apply frictional compressive forces to the wood sample.