Kiln Drying for Wood

What is the kiln drying process for wood?


Kiln Drying timber consists of lowering the quantity of water kept inside the wood.


To do so, timber, lumber, panels, cladding ... are stacked in a sealed warehouse (the Dryer) in which temperature, air flow and humidity are monitored to drag the water out of the wood by evaporation. That being said, there are two "kinds of water" to remove: free water and bound water.

Clearance of free water is the easiest to do as it is mobile and free of movement (hence the name). The bound water is more tricky to remove as it is chemically bound to the inner wood fibers (hence the name); if bound water is forced to go out of the wood, it might result in a distorsion of the wood structure itself and a modification of the initial dimensions of the piece. The analogy can be made with a sponge soaked in water: the free water is easy to expell with a handpress yet the sponge remains wet because there is still bound water. After a while under decent temperature and air flow, one can notice that sponge is dry (bound water has evaporated) but has also slightly strained. A well-controlled drying will lower the chances of distorsion.


When all the free water is gone, the wood has reached the "saturation point of the fibers" which is the humidity rate before removing bound water. All of the bound water is not cleared though, and there will always remain some water in the fibers which cannot be remove, unless the fiber itself disappears with the water (e.g. during a combustion).


The required humidity rate depends on the species, the final use of the wood and its surroundings. The humidity rate for indoor timber shall generally be around 10-12% as the outdoor timber shall stand around 16-18%.


Are there various processes for drying wood?




Natural drying. Timber is stacked oudoors under a shed while wind and sun do the job. It may take years to drop the wood humidity to around 15%. Sometimes it is impossible to go below, that is why artificial kiln drying has been developped.


There is a third kind of drying, quite similar to the traditional artificial drying except that the air pressure is lowered on top of extra heat and air-flow. This allows the water to evaporate at a lower temperature than under the normal atmospheric pressure.



What time does it take to kiln dry timber?


It depends on many factors among which the initial rate of humidity, thickness, species and of course the required rate of humidity at the end. The duration of kiln drying may vary between a few days for thin softwoods and up to 4 months for thick hardwoods.





Why kiln drying after fire retardant treatment?


Fire retardant treatment by impregnation soaks timber with water. Construction standards requires timber between 10% (wood flooring, wood walls, wood ceiling...) and 18% (carpentry, structures, wood cladding..) depending on the use. At the end of a cycle fire retardant timber stands around 25% humidity rate, so Woodenha includes the kiln drying process on a consistent basis when providing autoclave fire retardant treatment. Kiln drying also improve the bound and stability of the fire retardant agents inside the wood.




Why is kiln drying a tricky process?


If conducted to fast or ill-monitored, kiln drying may turn lumbers not suitable for use. Issues may be numerous among which color dying, cracking, breaking, distorsions...  Woodenha has been well-known as premium specialist of kiln drying since they started and has grown skills and expertise that allow them to lower the hazard chances close to zero during their kiln drying operations.





Is kiln drying an enhancement of wood features?




Without Kiln Drying, wood would contain so much water that fungi would settle and destroy quickly the fibers.It is considered as the first preservative step for wood. KD timber is way more resilient to structural and mechanical stranins.


Wood is an hygroscopic material i.e. it has the ability to give and take water to/from its environment. During that water exchange, its dimensions change, either shrinking when drying or swelling when getting wet. KD allows timber to reach its point of dimentional stability depending on its surrounding when mounted. From this balance comes the sustainability of the wood work.


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