End-grain erosion of Douglas fir wood during natural weathering

Abstract

We hypothesise that end-grain (transverse) wood surfaces will erode less during natural weathering than side-grain (radial and tangential longitudinal) surfaces. Douglas fir samples were exposed to 10 000 hours of weathering, and confocal profilometry was used to measure depth of erosion of the different planar surfaces. End-grain eroded less than side-grain, but the difference in erosion of end-grain and radial surfaces was only statistically significant if voids in end-grain created by checks were excluded from erosion measurements. Tangential surfaces were the most susceptible to erosion, due in part to the presence of lens-shaped voids caused by erosion of ray tissues. Erosion of earlywood was greater than that of latewood, particularly for end-grain. Erosion of earlywood is caused by thinning of cell walls in combination with transverse and longitudinal micro-checking, which creates loosely bonded cell wall fragments at weathered wood surfaces. We conclude that end-grain erodes less than side-grain surfaces when exposed to weathering, but differences in erosion of these planar surfaces is affected by checking, which complicates measurement of the erosion of wood using confocal profilometry. We discuss this issue, and describe the practical implications of our findings for the use of wooden posts as boundary markers for mining leases.