Brown stains running out of newly installed oak are caused by Quertannic acid, also known as tannic acid or tannins. This blog will cover everything you need to know about them, using our 25 years+ experience working with oak at Timberpride.
Does Oak Wood Have Tannins?
Yes, both British and European oak trees naturally contain tannins, which act as a natural defence against pests and fungi, enhance durability, and contribute to its rich colour.
What are Tannins?
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 wine barrels, and the headache you get from Australian reds.
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.
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.
How To Get Rid of Oak Tannins?
Given time, tannin 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 can also be applied to the stained area to clean it off if the customer doesn’t want to wait for it to wash away naturally. Oxalic acid can also be used to reduce tannin staining, but it’s important to do some research beforehand to ensure safe and effective use.
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.
How to Block Tannins in Oak Wood
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 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.
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Frequently Asked Questions
Are oak leaves high in tannins?
Yes, oak leaves are high in tannins. Found abundantly in all kinds of oak, tannins play a vital role in the tree’s natural defence system.
The tannin content in oak leaves can vary depending on factors such as the environment and season. During autumn, for instance, the release of tannins enriches the surrounding soil, showcasing the ecological benefits of this remarkable tree.
What is tannin leaching?
Tannin leaching refers to the process where tannins in oak are released. This is commonly seen in fresh sawn timber or newly installed dried oak. When water or metal, like screws, comes into contact with tannins in the wood, a brownish residue or staining may appear. It’s often noticeable during the initial weathering phase of oak products.
Are tannins corrosive?
No, tannins are not corrosive. While some tannins may cause staining, they do not damage the wood or any surrounding concrete or metal itself.
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