Concept of Drying Wood

A standing tree contains much water, and the amount of water will vary due to variety of wood and season of the year. A tree felled in May when its leaves are out and a higher demand for water is required by the leaves will be much heavier than a tree felled in December when it is dormant.

Timber holds two types of water; Free water and Bound water. This is water held in the cell cavities and water held in the cell walls. Looking under a microscope the fibres of the tree will be made of cells and in cross section they look like tubes. These run from the base of the tree to the top, and they take moisture to the required parts of the tree mainly by capillary action.

When a tree is felled it is the water inside the tubes (free water) which is the first to be lost. Once this moisture has been lost the timber has reached what is known as the fibre saturation point. The moisture content at this stage will generally be somewhere in the region of 28% to 30%. After this stage the bound water can now be lost. It is from this stage that a degree of shrinkage will be noticed as the cell walls lose moisture.

Logs For Wood Burners

Wood burner “user manuals” generally advise people to burn wood with a moisture content no higher than 20%. Unless you buy kiln dried timber this is a fairly unrealistic instruction.

To get beyond the fibre saturation point the wood will find its equilibrium moisture content. Moisture content in timber is heavily reliant on temperature and relative humidity. The higher the temperature and the lower the relative humidity the quicker and more wood will dry.

Also, wood is hygroscopic. If a moisture reading is taken in the middle of August of 20% with the temperature being 20 degrees Centigrade and a relative humidity of about 60%, that same piece of timber will obtain a moisture content reading of about 28% in January with the temperature at 4 degrees centigrade and a relative humidity of 95%.

In short, wood will absorb moisture as well as lose it. With the climate we have here in England it is very difficult to keep wood below 20%, unless you have rather a large humidor!

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