Forest Products Notes on...
DRYING TIMBER
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-
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 <7%.
• 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.
• A commercially important reason to dry timber is that wood is lighter after drying and so transport costs are reduced. Clearly these savings must be weighed against the cost of drying. Also the chances of infection during transportation (insects, fungi) are reduced.
• Wood burns better when it is dry. It must be remembered that although in the so-
• Dry wood is a better thermal insulating material. The (excellent) thermal properties of timber are only recently being rediscovered.
• The electrical resistance of dry wood is much greater than wet wood.
• A very important reason for drying timber is that when the moisture content is reduced to <20% biodegrade by insects and fungi is prevented, although it is very important that the moisture content must then be maintained at < about 20% as drying alone does not confer immunity.
The NEXT PAGE explains important fundamental concepts and definitions needed to understand the drying of wood ...
