Tannins In Timber
Most tree and plant species contain tannins, in varying levels and strengths.
Tannins in hardwood timber occur naturally and are stored within the internal structure of the timber. Tannins are water soluble. When moisture or water soaks into the timber it will bring these tannins to the surface, causing brownish stains when washed onto walls, concrete or pavers.
When the tannins are brought to the surface of the timber it is called “leaching”
It’s important to note that when staining like this occurs, the stains are due to tannin, not from an applied oil coating “washing off”.
Timber is a natural product and this process will occur at some stage.
The stains will naturally wash away as the tannins are water soluble
Some timber species are more likely to leach than others. Sawn timber does not regenerate tannins, once they have washed out then the timber will stop leaching.
When tannins leach out of timber and react with ferrous metals it can leave a blue-black stain, the wetter the wood the more soluble the tannins, the more it will stain.
Leaching doesn’t have any detrimental effect on the mechanical properties of the wood.
Tannin that has leached and stained will disappear over time and washes away naturally. If the stains are on porous material then it will take longer to dissipate. If the stain is on a non porous material then it will wash off very quickly.
The presence of tannin typically makes a timber specie more durable.
Should it be required to accelerate the removal of tannins, from any surface it has leached onto,there are two options.
Oxalic Acid can be used in accordance with instructions for the surface being cleaned.
Oil Away (Optainable from Builders Warehouse) can be used for porous tiles or cement surfaces.
Oxalic Acid can be used to clean any blue / black marks formed on Timber as a result of ferrous (Iron Fillings etc.) reaction with Tannins.