We will look today at various aspects of seasoning wood.
Newly felled timber will lose moisture very rapidly at first. This is due to the free water evaporating. This loss of moisture than slows down as water within the cell walls is surrendered.
Woodworkers should always look for timber which is well-seasoned. This means that the timbers should have been drained at the sawmill under controlled conditions and in two separate stages.
When it has been recently felled, timber has a high water content. At this stage it is termed green.
It’s heavier and more susceptible to decay than seasoned timber. It can, though, be worked quite easily and it’s more flexible. As it dries, the timber will shrink slightly and it can also distort in shape.
This picture of a cherrywood bowl illustrates nicely how changes can be very dramatic…
This shows a section cut from the base of the tree. The heartwood is a much deeper color than the sapwood. In this instance, the disc formed naturally into a bowl shape. Imagine, though, the havoc this would cause if this deformation occurred within a worked piece of furniture.
If it is left outdoors, timber will dry naturally to some extent. It must be properly stacked in order to allow adequate ventilation.
When treating hardwood, a sawmill will air dry all the converted timber for up to two years before further processing takes place.
The stack of wood should be protected from the elements. The layers ought to be separated by small sticks which will encourage an even rate of drying while reducing distortion.
Some pale species of timber (such as sycamore) are highly prone to staining when very green. This happens particularly at the spots where the sticks are laid. One way of avoiding this is to end-rear the timber. This simply means stacking them on end.
Shrinkage describes the behavior of timber as it dries.
Movement refers to the tendency of wood to expand or shrink after it has been seasoned. This depends upon its surroundings.
Wood species with high stability are less likely to deform if the rates of movement, both tangential and radial, are similar.
Certain species of wood behave much better than others. Douglas fir, iroko, teak and yellow pine are great performers. Pay close attention to this when choosing your wood.
Air drying is a very time consuming process. It can also be wildly unpredictable as it is subject to varying weather conditions and seasonal changes.
Kiln drying, by contrast, allows for much swifter production. The quality and performance of the timber can also be improved by careful control of the drying conditions.
Today, almost all commercially available timber is kiln dried. Beams of large section are the exception to this standard.
Softwoods are generally kiln dried directly after conversion. Any necessary preservative treatment is also usually applied at this stage.
After it has been dried, the timber may be cut up into smaller pieces for distribution. It normally reaches the home woodworker in this condition.
Timber will invariably produce disappointing results of it is worked before the seasoning process is complete.
Calculating the moisture content of wood is very straightforward. It’s simply a percentage of its dry weight.
A sample of fresh-sawn softwood weighing 10kg may well contain as much as 5kg of water and the same amount of dry wood. This would be described as having 100% moisture content.
In reality, wood is never dried completely to a moisture content of zero. Eventually, though, it will reach a level consistent with its environment. This is called the equilibrium moisture content (emc). This can range from 18-20% outdoors down to as little as 8-9% in a warm and dry atmosphere.
Kiln-dried timber is normally produced at a level of 10-12% moisture content.
It’s essential to understand how changes in the emc can affect the performance of a finished piece of woodwork since the wood continues to absorb and lose moisture in response to changing levels of temperature and humidity. This is why doors and windows are prone to sticking during the colder months. A cabinet door that fits perfectly in the workshop can shrink or distort when it’s installed in an air-conditioned, heated house.
Consider storing timber where it is to be used to allow it to acclimatize before being worked.
Moisture content can be estimated using a moisture meter. This meter must be suitably calibrated for the species of wood in question. Also, it will only provide local values for moisture content which will vary through the thickness of the wood. It serves as a guide when monitoring drying timber.
As wood dries, it shrinks and distortion can take place in the finished boards. This, in part, is because the shrinkage is uneven. This can be predicted, to a certain extent, depending on how it is converted.
Wood moves more readily in the direction of the growth rings. A plain-sawn board can lose as much as 10% of its original width during the drying process.
Quarter-sawn boards are somewhat more stable. They are often used for wide areas of panelling for this very reason.
Uneven grain and tensions within the timber can result in defects in a board as it dries or when it is worked.
Here you can see the importance of the seasoning process when it comes to wood.
Next week we will investigate different methods of selecting and storing wood.