Reasons to Dry Timber: An Introduction to Timber Drying

Introduction

Generally, at least in temperate countries, it is desirable to get timber dried as quickly as possible after extraction from the forest, for financial reasons and to avoid degradation by fungi and insects (but not so fast as to incur unacceptable drying defects).

There are 2 main methods used to dry timber: air seasoning and kiln drying, although usually a combination of both these methods is used.

Reasons for Drying Timber

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 utilized:

• 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 doesn't 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 timber 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.

Clearly there are a number of important reasons for drying timber. The next page explains important fundamental concepts and definitions needed to understand the drying of wood.