What's good and what's bad about wood tannin

In connection with tanned wood, I have said that wood insulating materials should be used when finishing so that the tannin does not affect the color or the curing process. In another context, I mentioned that a certain staining solution is only applied to tannin wood to achieve a beautiful, intense color. About oak and gorun I said they are good for barrels because of their high tannin content. At first glance the information seems contradictory and it is not really clear whether it is good or not that the wood has tannin. And so the question arises: what is tannin and how is the behavior of wood influenced by its presence?

What is tannin

Tannin is not a single substance but a mixture of complex chemicals derived from phenolic acids. This is why tannins are often referred to in the plural. They are large molecules that bind easily to proteins, cellulose (one of the main components of wood), starch and minerals. They are found in many plant species in all climates and in all parts of the world. Tannic acid, as it is also called, is soluble in water to form a brownish solution, has an astringent taste, and is a yellow powder in a solid, purified state.

The name tannin comes from Latin (tannare) and means oak bark. Tree bark has been used since ancient times to tan animal hides, hence the connection. In the process of tanning with vegetable tannins, due to the reaction of tannin with various iron-based chemical compounds, coloration of the hides can also occur.

Tannins are commonly found in the bark of trees, but also in wood, leaves, buds, stems, fruit or seeds. Their main role is to protect the plant. Those in the bark protect the tree from infestation by bacteria or fungi. The buds and seeds contain tannins, which prevent animals from eating them and thus ensure the continuity of the species. Raw fruits also contain tannin. The astringent taste we get when we eat a raw fruit is due to the presence of tannin. It is the fruit's way of protecting it to reach maturity. By studying the transformation of tannin in the fruit, fruit researchers can tell when is the best time to harvest.

As a general rule, wood high in tannin is darker in color. So oak tree, nuc, cherry have a higher tannin content than birch, maple or poplar. Hardwoods have a higher tannin content, but there is also tannin in softwoods, but the content is very low. It's not just wood that contains tannin. It is found in tea leaves, coffee beans, herbs such as tarragon or thyme, cloves, vanilla or cinnamon.

How tannin behaves

The tannin content of the wood can be a plus or a minus, depending on what we are going to do with the wood. If we use it to make wine barrelsThe tannin content is welcome because it will enrich the taste of the wine. The astringency that we are pleased to discover in red wine comes not only from the grapes but also from the oak barrel in which it has been aged.

If you want to use wood outdoors or finish it with water-based products, tannin can be quite confusing. Being water soluble, it affects the color applied to the wood if the product is water soluble. I had such an experience years ago with some acacia slats of garden furniture. I was going to paint them white, but I didn't think about the tannin content of the acacia and didn't initially apply a coat of sealer. The tannin extraction was so fast that I hadn't even sprayed the fifth slat and the first one had already changed color.

This water extraction of tannin can also be seen in wood used outside without very good protection. When it rains on it, the water that runs off turns brown. If only a portion of the house is tiled, the runoff will stain the wall under the tiled area or the floor. Unsightly stains like colored drips can appear on wood.

The acid character of tannin is manifested in the presence of iron and ferrous metals. Therefore, tanned wood cannot be assembled with iron-containing metal objects, nor can it be decorated with such ironwork. The properties of the wood are not affected by the reaction, but the metal object is corroded and a bluish-grey or dark blue discoloration appears, staining the wood.

However, the reaction of tannin wood with iron can be used to stain wood. To do this, use a solution obtained by soaking iron filings or a metal dish sponge in vinegar for at least 24 hours. The solution is applied to the wood and reacts with the tannins to produce a bluish-gray coloration like that of wood that has been left unprotected outside in the sun. The process is described in detail here.

In order not to stain the wood and to avoid disturbing some finishing processes, the tannin must be blocked out

To lock the tannin inside, a product called insulator is applied to the wood. This is a kind of hardened primer, diluted to penetrate the wood as well as possible. The hardening of the insulator inside the wood blocks the escape routes and the tannin can no longer influence the subsequent finish coats.

The method is mainly used for tanned wood products used outdoors: oak, ash or meranti windows, garden furniture made of acacia or oak, wood for cladding. It is also recommended for products used indoors where a water-based finish is used on tanned wood or where the subsequent finish is acid (carbamidic) hardened. The tannin can affect the chemical reaction between the two components and the varnish may stop drying or remain soft.

Insulators are both water-based and solvent-based. With water-based, the dilution is much lower, so as not to favor the extraction of the tannin from the inside. It anchors very well to the surface of the wood and hardens, and the tannin can no longer escape.

Uses of tannin

The extraction of tannin from tree bark began as early as the 19th century with the industrial revolution. Previously, bark was used as such for tanning hides and the process was very lengthy, up to 6 months, to get a well-tanned hide. In the middle of the last century, the demand for vegetable tannin fell sharply due to the emergence of synthetic tannins in response to the shortage of vegetable tannins caused by the Second World War.

Vegetable tannin is still used for the natural tanning of animal hides. Compared to the past, now only 10-201TPTP3T of the world's leather is made with vegetable tannins, the rest is tanned with inorganic tannins, which are easier to find and much cheaper. However, tannins extracted from the bark of oak, chestnut or exotic trees continue to be used for very high quality leather. Tannin is also used as a mordant (color fixative) in the natural dyeing of cellulose fibers or to produce different colors in reaction with iron compounds.

Tannic acid is used worldwide as a wine and beer fining agent due to its ability to react with and precipitate proteins. It is also used as a flavoring and astringent ingredient in both alcoholic and soft drinks or juices. In medicine it is used as an astringent and for various other conditions.

As you can see, tannins can be very useful. And as far as tanninized wood is concerned, even if their presence sometimes confuses us, they are not dangerous, and their escape or reaction with iron objects does not affect the strength of the wood. The important thing is to use the right materials to block tannins inside.

If you have any questions or queries, you can leave them below in the dedicated space. I will certainly reply. And additions are always a pleasure and I thank you for them.