Timber Drying Fundamentals: Wood Moisture and Key Concepts

Reading time: approximately 8 minutes.

Understanding how moisture exists within wood is essential before studying timber drying. This page explains why timber is dried, where water is found inside wood, and the key concepts—including free water, bound water, fibre saturation point (FSP) and equilibrium moisture content (EMC)—that determine how wood behaves during drying.

Why Timber is Dried

Note that all values of moisture content given refer to the moisture content expressed as a percentage of the (oven) dry wood content. This is the preferred, unambiguous way of expressing moisture content and it is to be hoped that this is the method employed world-wide.

There are a number of reasons for drying timber before it is used:

• The main reason timber is dried is to render it as stable as possible in service so that large, unacceptable shrinkage or swelling of the wood does not occur once it is in service.

  It is therefore very important that a component or product must be produced, or more accurately installed, at the same moisture content it will be at once it is in service (taking into account the fact that, for example, a house will have the heating on subsequent to completion). For this reason the drying of timber (to maybe 12% moisture content) is more important nowadays because more people have central heating in their houses.

• Suggested moisture contents for a large number of products are given in British Standard. 1186: 1971. Example recommendations are: Coffin boards:18%, Garden furniture: 16%, Furniture (slightly heated room: 14%, centrally heated room 9%), Mantel pieces: less than 7%.
• A very important reason for drying timber is that when the moisture content is reduced to less than 20% biodegrade by insects and fungi is prevented, although it is very important that the moisture content must then be maintained at less than, about 20% as drying alone does not confer immunity.
• Dry wood is easier to polish, paint, glue, nail, screw and machine.
• Below the “Fibre Saturation Point” (or FSP, i.e. when there is no liquid water in the cell lumen, but the cell walls are full of water – see later) several mechanical properties (e.g. strength) are improved (but very dry wood is brittle).
• Some treatments (e.g. preservation, pulping) require the wood to be well below FSP so that there is space for the relevant chemicals in the cell wall.

So the aim of drying is the removal of the water (sap) as economically and with as little damage as possible.

How much water must be removed during drying? Well how much is present in timber when it is removed from the forest?

Green Moisture Content of Freshly Felled Timber

Freshly felled timber contains widely differing amounts of water depending on the species and whether heartwood or sapwood is being considered.

Species	Heartwood	Sapwood
Yellow Birch	64	68
American Beech	58	79
American Elm	92	84
Douglas Fir	40	116
Western Hemlock	93	167
Sitka Spruce	50	131

In the above table note the variations in moisture content between species and between heartwood and sapwood. There will also be a variation with the time of year when felled. The drying “load” on a timber kiln may also vary depending on where and for how long a stack of sawn timber has been stored.

Key Wood Moisture Concepts

To understand and optimally control timber drying it is necessary to be familiar with a number of concepts and definitions:

Free water

This is the liquid water in the cavities of wood (the “lumens”). The removal or replacement of this water does not cause shrinkage and swelling or the material.

Bound water

This is the water contained intimately within the structure of the cell walls of the wood. It is the removal or replacement of this water which causes shrinkage and swelling of the material.

Fibre Saturation Point (FSP)

This is defined as the point at which the cell walls are full of water, but there is no water in the lumen. In practice it is very difficult to determine the Fibre Saturation Point accurately. Importantly within a piece of wood it is possible that some regions may be above FSP (and so not inclined to shrink) at the same time as others are well below FSP (and so may be shrinking or attempting to shrink).

Equilibrium Moisture Content (EMC)

Wood will always adjust its moisture content so as to be in equilibrium with its surroundings (but not at the same moisture content as it’s surroundings). In fact the Equilibrium Moisture Content of wood depends on the “drying potential” of the surrounding air – this is determined by both the “relative humidity” of the air and its temperature. Problems commonly occur when timber adjusts its EMC to new conditions! Hence the need to produce products at a moisture content appropriate to the service conditions.

Note:

The percentage Relative Humidity = 100 multiplied by the amount of water vapour in a certain volume of air, divided by the maximum amount of water vapour the air can hold at that temperature.

Charts exist which enable the EMC to be determined for combinations of relative humidity and temperature.

An understanding of the above fundamental concepts is critical in understanding how and why timber is optimally dried to maximise financial profit.

---