Drying timber to EMC
Once the tree is harvested and processed into timber, it will begin to dry naturally - quickly at first, and then more and more slowly as the moisture content drops down below fibre saturation point.
This point is around 30% MC in most species.
This is called equilibrium moisture content (EMC).
In most indoor coastal situations, EMC is between 10% and 15%.
In dry inland areas, or inside air conditioned buildings, it may be as low as 6 or 7%.
Some kilns have solid concrete or block walls, while others have plastic membranes stretched over a frame.
Either way, they all use mechanical ventilation systems and heat and humidity controls to allow the timber to dry much faster under controlled conditions.
Advantages of seasoning
Traditionally, building timbers were generally used in an unseasoned state, particularly in the days when house frames were built 'stick-by-stick' on-site.
Even cypress pine floor boards were typically green, because cypress has a very low shrinkage rate when it dries, and the flooring was often covered with carpet or linoleum anyway.
The changing nature of EMC
Once the timber reaches EMC, that doesn't mean its moisture content will remain stable.
As the humidity in the atmosphere continues to rise and fall, the EMC in the timber will change accordingly.
This is why in wet weather you sometimes find that timber doors and drawers tend to stick.
What's happening is that the timber swells as it takes up moisture from the atmosphere and then shrinks back again when the dry weather comes.
between EMC, relative humidity and air temperature.
You can see that as the RH rises, the EMC will also rise. This is why the general definition of seasoned timber under the Australian Standards is timber with a moisture content of between 10% and 15%.
However, the definition goes on to allow for EMC levels outside this range when they've been specified by the client. This caters for situations when the EMC is normally lower than 10% (such as in air conditioned buildings or dry inland areas) or above 15% (such as in very humid areas).
The speed of change will depend on various factors, including the cross-sectional size of the piece, whether there are any chemical treatments in it, and most of all, how exposed the surface of the wood is to the atmosphere.
However, if the humidity stays very high or low, and the surface finish starts to break down and allow direct contact with the wood fibres, then the EMC will change more quickly in response to the humidity levels.
Learning activity
We said earlier that seasoned timber generally has a moisture content of 10 to 15%, unless otherwise specified.
If the air temperature was 30° C, what would this equate to in terms of relative humidity?
Click on the graph at right to see it in a larger window. This will help you to work out an approximate percentage range.