Dimensional instability in timber

Abstract

Dimensional instability is one of the major impediments in the processing and use of lumber. Three separate facets need to be distinguished and considered: shrinkage on drying, movement in service, and the responsiveness of lumber to a fluctuating environment. The issues demand the attention of workers and management at every stage of manufacture. There are more complaints about the instability of lumber than any other matter and rectifying problems is expensive. Figure 4.1 provides unhappy examples of the consequences of movement in service. Some typical moisture content values for green wood are noted in Table 3.1. These values are considerably greater than the fibre saturation point. Absorbed water at the surface will evaporate and the lumber will dry provided the surrounding atmosphere is not totally humid. Indeed the absorbed water in the lumens cannot remain there in equilibrium with the atmosphere unless the relative humidity of the air is in excess of 99% (Table 3.3). If the wood is left under cover - keeping the rain off - it will eventually dry to a moisture content that will vary according to the temperature and humidity of the air (Figure 3.2). This moisture content will be below the fibre saturation point so all the absorbed water and some of the adsorbed water will have evaporated. If an even lower moisture content is required it is necessary to use a kiln to lower the relative humidity and raise the temperature (Figure 3.2).