Tannic Acid in European Oak
I regularly receive questions, comments or “complaints” about brown spots that occur in European oak, so I have written an article about the cause:
European oak contains a certain amount of tannic acid, also called tannin, depending on the place of cultivation/substrate. This is the tree's natural protective substance to ward off fungi and insects.
In high concentrations this is quite a substance. For example, the “bark pile” from our sawmill used to be collected with a lot of love and joy from local tanneries because the bark contains a lot of tannic acid.
In low concentrations it is completely harmless and anyone who regularly drinks a glass of red wine or good whiskey also absorbs this substance. The reason wine, port and whiskey are aged in oak barrels is because they impart their typical taste.
Oak floors and stairs can be “smoked” with ammonia, which reacts with the tannic acid that is still present after drying, causing the wood to initially become grayish. But if you then go over it with an oil or varnish, it will become a deep, warm brown color. The longer the wood is exposed to ammonia and the higher the tannic acid content, the darker the oak becomes. Please note that the sapwood does not contain tannic acid and therefore does not discolor and even becomes lighter during this process. This ensures a great contrast after completion.
The tannic acid content of European oak depends largely on the place of cultivation and substrate. The “chalk oak” grown in the chalk region of the Jura line in France, which extends beyond the Caucasus, has, on average, a higher proportion of tannic acid. This is why oak is so sought after by wine barrel producers. “Sand oaks” from sandy soils in northern Germany, for example, contain significantly less tannic acid. American “white” oak also contains hardly any tannin, which is why winemakers in California, for example, buy many European barrels.
The tannic acid in wood also has disadvantages:
With freshly sawn oak, the typical brown spots appear on the surface during the drying process due to the escape of moisture. This is completely normal and not a “defect” in the material. Companies with little knowledge of oak sometimes think they must reject the wood on this basis, but this is always wrong. In the case of exterior constructions made of oak, it sometimes happens that after a rain shower the tannic acid is washed out and stains tiles, footings and walls with brown spots. This can be removed with “drainage water”
Another problem can be that when fresh wood comes into contact with metal, oxidation immediately occurs by reacting with tannic acid, resulting in blue and/or black spots. The jaws of the saw, the forks of the forklift and metal straps can leave marks on the undried material. These blue spots are also difficult to prevent when planing fresh oak wood. However, plastic feed rollers as well as a stainless steel feed tray and cutting tools can significantly limit this. This can also be removed with drainage water.
After air or forced drying, the reaction with the tannic acid has almost disappeared and it can be planed and processed without any problems. However, be sure to always use stainless steel or other non-corrosive fasteners. You probably know facade claddings with black spots where this was forgotten. Even after drying, smoking is still OK.
Why aren't oak beams dried for construction purposes?
This is a very common question. The answer to this is that oak is a very difficult type of wood to dry and is not suitable for forced (artificial) drying of large formats. Beams can break completely, crack, twist, or even fall apart. A collapse can also occur, in which the structure of the oak literally implodes and a rectangular beam ends up warped and twisted after drying. Oak can be dried artificially up to a maximum thickness of 100 mm - very slowly and after prolonged air drying. This mainly occurs when used in high-quality interior areas, but not for structural timber. Air drying is very slow and only dries one centimeter per year, so take that into account for your 25x25cm oak beam.
Other types of wood with a lot of tannic acid are: chestnut, Sequoia and merbau.
For the flooring industry and the parquet installers among us: Did you know that Merbau is ideal for smoking?
Hopefully I was able to clear up some misunderstandings on this topic.
You can contact me at any time if you have any questions.
Maarten van de Voort
info@maartenvdvoorthout.nl
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