What is seasoning and how long does it take green oak to season? Seasoning is the term used to describe the loss of moisture from cut timber to give a drier, lighter and stronger end product. This will naturally occur in all oak timber but can be hastened by air drying or kiln drying. Green oak is not green in colour, in this context ‘green’ describes the freshness and rawness of the timber. For this reason, it is also sometimes referred to as ‘fresh sawn’ oak. Seasoning begins as soon as a tree is felled but is a slow process. If left as logs, say for firewood, the moisture content can still be as high as 50% after 15 years.
All green oak will have been cut within 18 months of felling and has a moisture content of 60 – 80%. This makes the wood relatively soft and flexible and therefore easier to cut and shape than dried timber. This makes it perfect for oak framed buildings, furniture making and decorative work. Additionally, green oak is available in the large lengths and thicknesses required for beams, rafters and roof trusses. It is possible to dry timber in these dimensions, but the time taken and the low demand make it expensive and inconvenient. As a hardwood, oak is dense and non porous and water escapes slowly. Once this does happen, oak is very resistant to reabsorption of water, making it ideal for outdoor structures.
Methods of Seasoning Green Oak
1. Unassisted/ Natural Drying
Left to its own devices, oak will dry naturally at a rate of 1 year for each inch (25 mm) of thickness. This will result in a shrinkage of about 4.5% radially (across the grain of the wood) and about 0.15% along its length. Any reputable designer and manufacturer will factor this shrinkage in and it should be of no concern. In fact, this will increase the structural integrity of the build as the shrinkage tightens joints between timbers. However, this does make it unsuitable for glazing surrounds or lintels as the shrinkage will put stresses on the brittle adjoining materials.
2. Air Drying
During air drying, sawn timber is placed in stacks separated by sticks or batons. This allows air to circulate around the timber, drawing moisture away and speeding the seasoning process up. This should be done in a cool, dry shaded area, protected from the elements. The flow of air should be continuous and uniform to ensure a constant rate of drying. After the first year of air drying oak, the baton size is usually increased to allow faster drying. As air dried oak has a lower moisture content than green oak, it is more rigid and suitable for glazed structures and supporting lintels. Air dried oak will never be completely dry as its moisture content will eventually become the same as the surrounding air. When this equilibrium is reached, desorption will stop.
3. Kiln Drying
This involves placing stacks of timber in a sealed container and applying heat using gas, electricity or solar heat. Vacuum kilns also exist that use the lower boiling temperature of water in a vacuum to extract moisture from the timber. Heated kilns allow a constant ambient temperature to be applied, guaranteeing a consistent drying rate. This can speed drying times up considerably, but fast drying can lead to shaking (splitting of the wood). While this does not weaken the timber as the splits occur along its length, it makes the timber unsuitable for quality joinery.
Mechanics of Oak Drying
Once oak is milled, the interior of the wood is exposed, allowing water trapped within to escape. To understand how this happens, we need to look at the types of water contained within the wood.
While water in the form of vapour exists within wood cells and vessels, the amount is negligible. It will diffuse readily out of the timber and is of no concern to the overall drying process.
2. Free water
Free water is held in a liquid state within the vessels that would transport it around a living plant. It is not chemically bound to the wood itself, but is held in place by capillary forces. As cutting the wood also cuts through these vessels, the free water is exposed to the surrounding air. In low humidity, the air flowing over the surface of the timbers encourages the free water to evaporate (desorption) and carries it away. As this is lost it causes capillary forces to draw the free water through the vessels to replace it. This is in turn lost and the process continues until all the free water is gone. In a kiln, the heat of the surrounding air also encourages desorption, further speeding up the process.
3. Bound or Hygroscopic Water
This term describes water bound to the wood by hydrogen bonds. Once the free water has completely evaporated, the outer surface becomes dry. The hygroscopic water then diffuses through small channels between cell walls to equalise the moisture content. Once this reaches the outer surfaces, It too diffuses into the surrounding air. The rate of diffusion is again decided by the humidity and temperature of the external environment. This diffusion is mainly a lateral movement, that is it happens across the width of the timber rather than the length.
How to Stop Green Oak Splitting
As noted earlier, speeding up the drying process can cause shaking in oak as splits appear along its length. While this is a natural feature that many accept as a desirable side effect, sometimes users wish to minimise this. It is almost impossible to avoid in kiln drying as the rapid drying of outer surfaces causes them to contract around a larger core. The stresses caused to the timber by this result in the splitting itself. Lowering the heat within the kiln to adequately slow the seasoning is not financially viable. It can however be minimised by slow air drying, using small batons and slowly increasing their size. This controls the rate of air flow between timbers and consequently the rate of seasoning. By regularly weighing the timbers, the rate of drying can be accurately measured through weight loss. Adjustments can then be made to the process to keep the drying out sufficiently slow. Shaking can never be completely eliminated, but it can be minimised in this way.
At Hardwoods Group, we are industry leading suppliers of European oak. We can supply green oak, air dried oak and kiln dried oak, cut to size and rough sawn or planed. All our oak is responsibly sourced from sustainable and managed forests to guarantee an ethical yet high quality product. Contact us to discuss your requirements, we will be happy to help.
What Is Green Oak?
Working With Green Oak
Air Dried vs Kiln Dried Oak