How We Learned to Stop Worrying and Love the Tannin.
There aren’t many certainties in this life but the presence of Tannin in Oak and Chestnut is definitely one of them.
Both of these popular timber species have high levels of Tannic acid (it’s actually Quertannic acid but let’s call it Tannin for short). In fact, most tree and plant species contain tannin in varying levels and strengths. We’re mostly concerned with Oak and Chestnut because these two beloved timber species have high levels of tannin and we use them for just about everything.
But, why talk about tannin?
Well. Tannin’s worth talking about for two reasons and we want you to know all about them so you don’t get scared off of using these species. If you’re new to specifying Chestnut cladding or Oak timber frame structures you’d be forgiven for being alarmed at the tannin reactions that can happen.
In reality, Tannin is something we manage, we work with, we work around and in the end we’re thankful for.
We want you to feel confident about your specification of Oak and Chestnut so a little choice Tannin knowledge can go a long way.
Probably, the first thing to understand is that moisture plays a big part in the life of tannic acid. When wood is dry the reactions we talk about below very rarely take place. It’s all about the moisture i.e. water. The water provides a solution for the soluble tannin. This means that kiln dried timber (low moisture content) rarely reacts, whereas fresh sawn timber (high moisture content) often reacts.
These two pictures show the movement of the moisture carrying tannin on the surface of this waney edge Oak cladding.
But now let’s take a look at those two scary reasons for talking tannin; corrosion… eeeeeek! and leaching… aaaaaargh!
Tannin in wood and tannin escaping out of wood.
Tannin in wood i.e. corrosion
The Tannic acid in wood is corrosive to ferrous metals (irons) or metals with a ferrous (iron) content.
The corrosion doesn’t do any damage to the wood (the same can’t be said for the metal) but it can result in a stain on the wood, a bluey-black stain. The wetter the wood, the more soluble the tannin, the more it will stain.
So when you’re working on wood with strong tannin content it means considering tools, fixings, blades… anything metal that touches the wood whilst you work on it and anything metal that touches it once it’s in place. It’s why we recommend using austenitic stainless steel fixings for external timber use.
Here’s an example of some light blue-black staining on a stack of fresh sawn oak beams
What to do about it your tannin?
If it’s the corrosion you’re concerned about, then take care over choosing fixings. If you’re concerned about stains then you have to think about not only fixings but blades, machine beds and storage racks, you even have to take care of the mineral content of soil in the mud on the soles of your shoes if you’re going to walk on your beams!
If you find you have stains then, in our humble opinion, you have two options.
Option number 1: leave it
If the stains are on external Oak or Chestnut and you intend to leave it to weather naturally to a silvery grey then, as it weathers the stain will mellow and finally disappear so love it and leave it and in a couple of years you’ll have forgotten all about it.
Option number 2: treatment
The blue black stain can be treated with oxalic acid to try to reduce the discolouration. There are pros and cons of using oxalic acid and we’d recommend lots of research before going ahead. We know some people that have had some success with it and others not so much. We don’t want you to be disappointed.
Sadly, sanding it, planing it or washing it won’t make any difference. It does help to let wet timber dry out, the staining fades with the drying.
Tannin leaching out of wood
Tannin is soluble and leaches out of wood in water and where the water evaporates it leaves the tannin sediment behind… as a stain.
Yup, another one!
Like the corrosion, the leaching doesn’t have any detrimental effect on the mechanical properties of the wood. The tannin washes out of the timber in a solution and gets left behind as very fine, rust coloured particles. The particles are like a fine sediment, dust or powder. You can wipe them away from a smooth surface with your finger but on a porous surface… you guessed it… the pores absorb the rusty coloured dust.
Here’s an example of the rust coloured tannin leaching. It sits as a fine powder on the surface of the concrete ring beam. Clever detailing has meant the water has run down behind this Finger jointed Chestnut cladding rather than down the face.
What to do about Tannin leaching?
If you have tannin staining as a rusty coloured tea stain on the floor under your structural oak posts, on the wall under your new chestnut cladding or as watermarks on the surface of your new garden furniture then, in truth, there’s not a lot to be done.
The answer to tannin leaching is time. The seasoning of the timber over time means that the timber dries and hardens and becomes less penetrable by water and less likely to release tannin.
If it’s a stain underneath your timber you can rinse regularly to help the sediment dissipate.
If it’s a tannin stain on the surface of your wood then any washing or rinsing will probably create new stains as the the water dissolves the tannin and then evaporates again (leaving you a nice new pattern).
Tannin that has leached and stained will disappear over time and washes away naturally. If the stains are on porous material e.g. portland stone, then logic says it will take longer to dissipate. If the stain is on a non-porous material e.g. zinc, it will wash off very quickly.
Whilst tannin content in timber sounds like a bad thing ironically (no pun intended) the presence of tannin makes a timber species more durable. Swings and roundabouts we say.
We really can’t stress how easy it is to work with Oak and Chestnut and how understanding the basics of working with tannin content means you get the benefit of choosing from two of the toughest, strongest, most durable, most beautiful, most sustainable of timber species.
For all you tree scholars & woodgeeks out there… scholarly articles on ‘tree species with tannic acid‘
Here’s a straightforward video about the use of Oxalic acid on wood… it’s not rhubarb 🙂