Weathering of radial and tangential wood surfaces of pine and spruce

The development of cracks and changes in appearance have been investigated on radial and tangential surfaces of pine (Pinus silvestris L.) and spruce (Picea abies Karst.) which have been exposed outdoors for 33 months. The de gradation of the surfaces has also been studied at the micro-level. Untreat ed samples, samples impregnated with a CCA-agent and samples surface treate d with linseed oil have been tested. The annual ring orientation is the most important factor for crack developm ent on weathering. The type of wood, impregnation treatment and surface tre atment with linseed oil have only a marginal effect on the crack developmen t. No relation has been found between the density of the samples and the cr ack development After 33 months of outdoor exposure, tangential surfaces of pine have 13 ti mes more total crack length per unit area than the corresponding radial sur faces. In spruce, the total crack length on the tangential surfaces is 6 ti mes greater than on the radial surfaces. Tangential surfaces of both pine a nd spruce have a greater number of cracks per unit area and wider cracks th an the corresponding radial surfaces. Tangential and radial surfaces show t he same colour change in the surface as a result of weathering. On the micr o-level, tangential surfaces have more and deeper cracks than radial surfac es. The cracks on the tangential surfaces occur frequently in both earlywoo d and latewood. On radial surfaces, cracks occur primarily at the annual ri ng borders, but to a certain extent also in the earlywood. The radial cell wall of the earlywood has a large number of pits which are degraded at an early stage. Decomposition of the cell wall takes place on b oth radial and tangential surfaces. Cracks arise which follow the S2 fibril orientation in the cell-wall. Delamination in the middle lamella is especi ally noticeable in the latewood on tangential surfaces. No differences have been observed regarding linseed oil treatment, impregnation or type of woo d.