The influence of drying method on the warp behavior of Norway spruce (Picea abies (L.) Karst.) sawn timber

The guarantee of a consistent wood moisture content of 15 +/- 3 percent is an important prerequisite of timber for construction purposes, according to international standards. It was the aim of this investigation to identify and quantify the effect of two different drying methods (air-drying and kiln-drying) on the warp behavior of timber. In this study, 60 Norway spruce trees from a 111-year-old, even-aged, homogeneous stand were processed into beams of practical dimensions. After the measurement of the moisture content when green, the total group was divided into two subgroups according to the later method of drying. The beams of group A were kiln-dried using a conventional drying schedule, those of group B were air-dried and technically post-dried. For both groups, the defined average target moisture content was 15 +/- 3 percent. To derive the distortions of each beam, the surfaces were scanned twice, before and after drying, using Freiburg's Improved Timber Scan (FRITS) measuring device. The warp variables according to international standards (E DIN 4074-1:2001-01), indicated as bow, crook, twist, as well as the angle of twist, were calculated based on mathematical algorithms. Both drying methods led to low warp with only slight differences. A t-test revealed a statistically significant difference in the twist of the sample beams dried by the two methods. Bow, crook, and the angle of twist did not significantly differ between the two groups.