One of the problem areas in the kiln drying of western hemlock lumber is the wide variation in final moisture content of the wood. This variation in moisture content is due to the presence of sinker or wetwood in the heartwood. The features of wetwood which differentiate it from the normal heartwood include higher specific gravity, higher extractives content, and lower permeability. The apparent higher specific gravity can be fully accounted for by the higher extractires content. The principial extractive is a-conidendrin. The wetwood in western hemlock often occurs together with ring shake and under these circumstances the white depcisit on the shake surfaces is also ~-conidendrin and not matairesinol, the substance usually asso4iated with ring shake in western hemlock.
A viewpoint is presented on the origin of wetwood as the endproduct of a reaction by the tree to injury, i.e., ring shake, in which additional extractives are deposited. The ex4ractives result in a greatly lowered permeability, which prevents loss of moisture during heartwood formation and thereby resulting in wetwood. Bacteria usually found in wetwood and responsible for many of the symptoms associated with wetwood are a result of the high moisture content which favors bacterial growth in wood. Presumably, the two primary sources of loss in kiln drying of western hemlock, shake and wetwood, are often intimately associated.