A DELICATE DRYING PROCESS
The very same characteristics that make oak so naturally suitable for outdoor use, also make it quite tricky to dry ready for manufacturing. The Tyloses and closed cell structure means that oak holds its moisture content really well, and if it is dried too quickly the outside will dry quicker than the inside which will cause it to split. We are aiming for the whole board to be dried at the same rate, it’s a delicate process and if it’s not done properly it’s easy to wreck a good piece of oak that has been growing in the forest for over a hundred years.
FIRST STEP - AIR DRY ON STICKS
When an oak tree is felled the moisture content is around 80%, for us to be able to craft into outdoor furniture the moisture content needs to be around 12-14%, which is about as dry as the wood will ever be outside. The process of drying the boards to the required standard is two fold. Firstly, it is stacked and air dried on sticks. To dry out the boards slowly and evenly, they are positioned to sit on carefully dimensioned sticks, the water can then evaporate off through the gaps between the timber. If the sticks are too large the gap will increase and the wood will dry off too quickly and split, too small a gap and it won’t dry out enough. Oak tends to be air dried for 1 year for every inch of thickness plus 1 year, so a board 2 inches thick will be air dried for 3 years. Once the wood is at about 30-35% moisture it is at the fibre saturation point (FSP) where only the water in the cell walls remain and all the other (free) water has evaporated. Once the wood is below the fibre saturation point the fibres start to shrink and the timber gets increasingly harder and stronger.
SECOND STEP - KILN DRY
Air drying will take the timber down to around 18% moisture but to get it to the 12-14% moisture content required, so next we need to kiln dry it in a dehumidifier kiln. Through the carefully regulated process, where the atmosphere inside the kiln is kept moist, the moisture in the centre of the wood is dried off at the same rate at that on the surface. If the outside is allowed to dry too fast case hardening can occur, this happens when the outside of the wood dries quickly and hardens, and then inside dries more slowly and shrinks away leaving hollow parts which can collapse.
In contrast, ash and beech trees are very easy to dry when compared to oak as they don’t have a closed cell structure and the moisture can evaporate much more freely. The process of drying oak is slow and careful but the result is strong stable wood ready to be made into beautiful outdoor furniture pieces.