# Drying Behaviour of Western Hemlock with Schedules Developed for Norway Spruce and Scots Pine
###### Abstract  
Determining moisture content (MC) distribution during the drying of porous materials such as wood is crucial for developing drying schedules and assessing their suitability to achieve optimised processes. This study aimed to determine the causes of the unique drying behaviour and the well-known unusual longer drying time of western hemlock compared to other similar softwoods. In situ _X_-ray computed tomography (CT) was used to study the evolution of MC in timber during the drying process. The drying behaviour of western hemlock (_Tsuga heterophyllla_ (Raf.) Sarg.) was compared with Norway spruce (_Pica abies_ (L.) [PERSON]) and Scots pine (_Pinus sylvestris_ L.) from green to oven-dried condition with industry-proposed drying schedules used for steering a custom-made experimental kiln combined with a CT scanner. CT scanning was performed at 30 min intervals during the complete drying period of 30 h, and the CT images were processed to calculate the MC evolution within the specimen. Western hemlock showed a considerably slower capillary-phase drying and did not go into the transition and diffusion phases when a schedule adapted to pine and spruce drying was applied for its drying. CT images and MC gradient calculations showed a lower drying rate and severe non-uniformity in MC distribution, which could be due to the effect of higher green MC and the presence of wet pockets. Furthermore, the evaporation front at the first 5 h of drying receded faster into the hemlock specimen, and as drying proceeded, it slowed down compared to other specimens. CT scan; experimental research; image processing; moisture content; porous material; timber drying; wet pockets Article  
[PERSON]; [PERSON]; [PERSON]; [PERSON] Drying Behaviour of Western Hemlock with Schedules Developed for Norway Spruce and Scots Pine  
these parameters with the aim of reaching a final target MC of the timber without causing distortion, cracking, or other quality lowering changes of the sawn timber. Green wood contains liquid water in the cell lumen (free water) and water chemically bonded to the cell wall (bound water). At the initial stages of the drying process, the so-called capillary regime, liquid water is transported from the inner parts of the wood to the surface, where it evaporates. As the moisture level is lowered and the FSP is reached, the moisture transport transitions into being diffusion-driven when the bound water is removed [3]. The anatomical structure of a tree can be divided into the sapwood, which is the outermost part that transports water from the roots to the leaves, and into heartwood, which is the central part of the stem that does not transport free water. Nevertheless, the structure and properties of wood vary largely between species and within a single tree, making it complicated to extrapolate drying methods, as different woods may absorb or desorb water differently before reaching the equilibrium moisture content (EMC). Western hemlock, growing in coastal regions of North America [4], and Norway spruce and Scots pine, growing in Scandinavia, are common softwood species used in sawmilling, with very extended use in timber engineering. However, western hemlock has been considered more difficult to dry than the Scandinavian-grown Norway spruce and Scots pine, and the reasons for that have been suggested to be a higher initial MC in green conditions, a higher extractive content, and a lower permeability [5; 6]. A motive for this study is that when drying technologies from Scandinavia were transferred to North America, the drying times for hemlock were reported to be approximately twice those for spruce and pine. This unexpected behaviour was surprising, given that these three species are softwoods with a close range of density values. Literature records show that hemlock is prone to a phenomenon known as wet pockets (also known as wet wood or sinker wood), formed mainly due to bacterial infection in the living tree, and occurs mostly in the heartwood and in the transition zone between heartwood and sapwood [5; 7].