Methods of determining the moisture content of wood

Introduction

Seasoning enhances many properties of wood, and minimises dimensional changes of wood products in service. It is a requirement of the Australian Standard Specifications for a wide range of timber products. The need for seasoned timber has long been recognised by many specifiers and users of wood products, but there is still some confusion about the precise meaning of the term 'seasoning'.

Wood consists of a large number of small cells, which are generally tubular in shape. The cells in a living tree always contain water. It is in the form of 'free' water in the cell cavities, and 'bound' water in the cell walls, which are fully saturated. The cell cavities are usually not completely filled with free water, but contain both water and air.  The free water in the living parts of the tree - the sapwood and inner bark - carries the dissolved minerals and foods that are essential to the life of the tree. The heartwood cells also contain free water, but the amount can vary. In some species it can be as much as is in the sapwood, but in other species, e.g. pines, it may be much less.

When a tree is felled and sawn into timber the wood starts to dry.  The free water evaporates first. The stage where all the free water has gone, but the cell walls are still fully saturated, is known as the fibre saturation point. The cell walls then lose water until the wood has dried to a condition where the amount of bound water is in equilibrium with the moisture in the air around thewood. This moisture content is known as the equilibrium moisture content (EMC), and it depends mainly on the relative humidity of the surrounding air and, to a lesser extent, on its temperature.  In most applications wood products are not used in constant environments and the amount of bound water varies, following changes in the relative humidity and temperature. The range of EMC depends on the size of the variations in the conditions, and the rate of response of the wood.