Oak is rich in tannic acid (commonly known as tannin), an important natural preservative and pesticide which is prevalent in the heartwood.
When oak is felled and dried, the tannin remains when the water evaporates. Once the wood is first introduced to moisture the water provides a solution for the tannin and as it is rinsed out from the oak it can mark it on some of the vertical surfaces, like table legs, and will sometimes create marks on the environment around the furniture, such as pale paving. Left to itself, the furniture will weather and silver, and any marking will even out and fade, and the sun and rain will eventually wash away marks on paving too. If unsightly, furniture markings can be removed with a wash and a light sanding, and there are cleaning products that can remove markings from stone too, please get in touch for details if this is of interest. Due to the closed cell structure of oak, the tannin will only leach from the outer few millimetres of the sawn ends of the timber, and after the first few heavy rainfalls the tannins close to the surface will have washed through, while the rest is safely locked away to do its job preserving the wood.
We can lessen the effects of tannin marking by slowly seasoning the boards once the timber has been cut. Drying the wood properly means it is less penetrable by water, which as we know is the catalyst to releasing tannin. Our furniture also goes through a final process before it leaves our workshop to start its life outside, whereby it is ‘fumed’ in a chamber of ammonia, an age-old technique which darkens the oak, it was originally used to make oak look older. Introducing the alkaline ammonia to the tannic acid causes it to react and neutralise, this results in less surface tannin, which means reduced marking and a more even colour for the finished outdoor furniture.
Tannin is corrosive to ferrous metals, when oak comes into contact with any material that has iron content the tannin will cause a reaction. This won’t do any permanent damage to the wood, but it will cause blue-black staining – place a penny on a piece of oak and you will find it leaves an imprint of the coin! For this reason we recommend using a paving stone that has no iron content for a patio that will house oak furniture.
Tannin is an important part of why oak is such a suitable material for making outdoor furniture. It preserves the wood and without it the furniture wouldn’t be so durable and last anywhere near as long as it does. Although, it is not just the outdoor furniture making business that finds tannin so useful, the name tannin actually derives from the leather tanning industry which has used oak tannins for thousands of years to tan animal hides into leather. Tannin is also very important to wine making, as the wine is aged in oak barrels, tannin escapes and helps to colour and flavour the wine. It is also used to clarify wine, the tannin reacts with proteins in the wine and creates sediment which can be filtered out. In fact, so important is oak to the wine industry, in France, forests are specifically managed to provide prime oak for the production of wine barrels.
So in conclusion, while tannin can be an annoyance and cause staining, this is for a relatively short period and with time and patience the benefits of tannin are huge and quite frankly we’d be lost without it!