07 Aug 2015

Brown stains running out of newly installed oak are caused by tannins, this is what you need to know about them.

Tannins are the collective name given to the acidic chemicals held in solution in the liquid sap, they are still present in dry oak as they are left behind when the water evaporates.  They get their name from the leather tanning industry which used to use oak, and oak bark to tan leather. It is also responsible for the oak flavour wines pick up from oak barrels, and the headache you get from Australian reds.

When oak is first cut the tannins will be washed out of the newly cut surface by rain, they will run onto surrounding timber stone etc and can leave stains. Given time these stains will wash away as they are water soluble, but at first, it can look like creosote coming out of the wood. Sugar soap or brick acid will clean it off if the customer is impatient. Oiling the oak will seal it and slow the loss of tannins to the point where it will wash away without staining the stone too much, but oil doesn’t soak into wet oak very far as the water will stop it. Diluting it with white spirit will help it soak in further.

Tannins are also the reason you will usually see dark tide lines on the underside of properly seasoned oak boards, rain washes it off the top face then it runs to the underside and evaporates.

Tannins will only wash out of the outer few mm of the oak, so if you cut or plane it the fresh surface will release tannins all over again.

Tannins is one of the few things which can stain oak during seasoning, if oak boards are air dried using oak sticks, the double dose of tannins can stain the board 5-10mm deep under the sticks, these stick marks may not be visible until the timber is planed and they usually go too deep to plane out. This is why it is vital to use softwood or poplar sticks when drying oak.

Further Reading – Click to Read:

The Great British Oak Tree is Under Threat

Why Building with Oak and Stone is Carbon Negative