Oak logs dry very slowly in the round (complete log). Logs left for 15 years can still have a moisture content of 50%. We like to mill them quickly to get the drying process started.
The speed of drying is critical. If oak dries too quickly at first it will shake (cracks will open upon the face of the board). This is caused by the outside of the board drying and shrinking relative to the wet centre. Boards must, therefore, be put in stick (with small batons to separate the planks and allow air flow).
In the first year of drying, we use 5 -10mm sticks to slow the initial drying down. After a year, larger 20mm sticks can be used to allow the oak to dry a bit faster.
We are looking to achieve a moisture content of 14 to 18% after the board has been left to dry for 1 year per 25mm of thickness. During this time in stick, the timber moisture content will fluctuate with the weather and the seasons, each expansion and contraction allowing some of the natural tension in the wood to relax.
If this prolonged air drying is skipped in favour of modern kiln drying from green, these tensions will still be present and the timber will not be fit for quality joinery, though it will be cheap.
Allowing for shrinkage
During the drying process, the timber will shrink as it loses moisture. A tree with 80% moisture content drying to planks at 15% moisture content has lost 65% of its weight. However, this shrinkage is not linear.
Little shrinkage will be measured in the length, relatively little on the radius of the tree, but quite a lot on the tangent. This means that the crown boards (top and bottom thirds of the tree) will shrink more in width than they do in thickness.
The quarter sawed, heart boards will shrink in thickness and not width, so heart boards must be milled to a greater thickness than crown boards to achieve the same finished thickness.
At about 30% moisture content the timber will be at fibre saturation point. This is not a defined level as each tree has a different density. The theory is that above fibre saturation point only the free water (not held in the cell structure) will be lost, below this level water held in the cell structure will be lost.
Most shrinkage occurs below fibre saturation point. As timber dries below this level it gets progressively harder and stronger. This is partly because the resins held in the water (sap) dry out and set in the fibres.
Once oak has reached equilibrium moisture content, it is in balance with its environment, it is stable and ready to use for external joinery.
If it is to be used internally it must now be dried to a lower moisture content. As equilibrium moisture level in a house is likely to be closer to 10%, this cannot be achieved naturally. A kiln is simply a vessel which pumps warm air over the boards so that moisture will evaporate.
It is critical that the moisture gradient, the difference between the centre of the board and the surface, is no more than 5%. Higher gradients will lead to splitting. This can be avoided by raising the temperature in the kiln, but the risk of collapse/case hardening is too great for reputable mills. Collapse is where cavities open up inside the board, they are often invisible from the surface and will cause your joinery to fall apart.
By cutting carefully, and allowing our oak plenty of time in stick, we avoid the dangers of splitting and collapse.