How to Dry Wood to Produce Good Quality Timber

Introduction to How to Dry Wood

The notes begin by addressing the reasons for producing dry wood: one very important reason for reducing the moisture content in wood is to ensure that when the wood is put into service a change in the moisture content brought about by environment a product is installed in does not cause the wooden product to shrink or swell. Another good reason to dry wood is to avoid fungal degradation of the wood. A number of other reasons to dry wood are explained.

After giving some important fundamental concepts and definitions which need to be understood if dry wood is to be produced efficiently (for example the concepts of “Fibre Saturation Point” and “Equilibrium Moisture Content”) the notes then describe the factors which control the drying of wood. Some of the controllable factors relate to the environment where the wood is dried (such as the temperature and humidity of the air) but to dry wood efficiently and so produce timber of optimum quality the internal structure of the wood must be appreciated. Consequently the notes also include an introduction to the structures of hardwoods and softwoods and their effects on the drying process.

After explaining these factors a number of pages of the notes describe the best practices to produce dry wood by air seasoning in timber yards and in drying kilns. An introduction to air seasoning of timber is given, where the expected rates of drying and the limitations of air seasoning in producing dry wood are described, and the optimal layout of a timber drying yard is described - this being critical to dry wood efficiently, as are the design and arrangement of the stacks in the drying yard.

There follows a description of the theory of the kiln drying of wood, where the need to provide a sequence of optimal temperatures, relative humidities and air circulation rates is described and the concept of a “Kiln Schedule” is explained. Different types of kiln drying equipment are described. There follows a comparison of the advantages and disadvantages of kiln drying timber compared to air seasoning of timber.

The use dehumidifiers to dry wood is also described as are high temperature drying methods and the use of solar kilns to dry wood.

The notes end with descriptions of the various defects in dried sawn timber which can arise and how these defects can be avoided. The various defects arising in the anisotropic nature of wood are described (spring, twist and checking, for example) as is the serious defect of case hardening of timber. The final page of these notes explains how case hardening of timber can be avoided and mention is made of a number of other defects which can arise when producing dry wood.