Drying Oak for Use in Oak Worktops
Most people have seen old oak beams in buildings where the beams are all twisted, split and warped. Whilst this is fine for a building, the same cannot be said for a wooden kitchen worktop. Once it's in, you want it to say stable, flat, and not start cracking and splitting. Of course as any timber adapts to its environment, and any changes in that environment, it will move. Consider the change in humidity from a warm, dry, heated kitchen in winter, to a hot, much more humid unheated kitchen in summer. Correct treatment, installation and maintenance of an oak worktop will significantly reduce these changes to the point where they are unnoticeable, however it still requires that the oak was dried correctly in the first place.
As soon as an oak tree is felled, it needs to be planked and laid out 'on sticks'. Here, the oak planks are laid on a flat surface with small wooden bearers in between layers. The bearers should be about 12 - 18 inches apart. This allows the air to flow freely around the oak which aids the drying process, however this must be carefully regulated to prevent the oak from drying unevenly or too fast, which introduces exactly the problems we are trying to avoid.
One of the first steps to stop the oak from drying too fast is to paint the ends with a wax based paint. Where the end grain is cut, it exposes the capillaries which carry water and nutrients up and down the tree, so clearly significant moisture loss will occur at this point. Painting with wax minimises this, and forces the moisture to flow out through the sides of the timber evenly.
The next step is cover the top of the stack of oak, which is usually provided by a fixed roof in a shed called a 'T' shed. This is simply a roof to keep the sun and rain off, and also to provide a fixing for 'curtains'. The curtains are in fact just like heavy duty curtains made from a tight mesh that allows air flow, but crucially stops wind from blowing over the oak. Wind is a big enemy when drying oak, as it takes the moisture out too quickly, and leads to splitting and checking.
Finally, it's a good idea to place a few sheets of thick ply on the top, and then pile on some railway track to weigh it down and hold it flat. Once all that's in place, the oak should be left for a minimum of six months, but preferably 12 - 15 months.
After this, the next stage is to kiln dry the oak. The longer it has been sat on sticks the lower the moisture content will be as it goes in to the kilns, although of course the cost of the timber is tied up for longer.
The kiln drying process is where most of the damage occurs from trying to dry the oak too quickly. Running a kiln costs around $ 12,000 dollars a month for a 120 cubic metre kiln, so it's easy to see why most companies rush it. It's not just a matter of putting it in the kiln, turning the heat on and leaving it for three months either. The temperature needs to be built up very carefully so that the balance between the core moisture content of the oak and the surface reading are never too far out. Obviously there has to be a gradient here, but it's the relative steepness that matters. Too much, and the moisture is being forced out too quickly, and will split and damage the fibres of the oak, which will then be in there forever. Modern kilns use an array of moisture sensors both inside the oak, the surface and all in between, across a complete cross section of the kiln to ensure evenness of drying.
Once the oak is down to around 6 percent moisture, it is then conditioned with heated steam back to up to 7.5 percent, which makes the oak stable. Obviously each step adds complexity and considerable cost, but without these elements, production of quality dry oak is all but impossible.
Oak which is force dried too quickly will eventually equalise, however during this equalisation it will almost certainly warp, split and crack. If the overall moisture content it too high, then it will shrink once installed, which will lead to glue line failure in the majority of cases.