Why did the oak plank turn black? Why did brown stains appear? Explanations and solutions

As it happened, within two days I saw two people posting photos of blackened oak, independently of each other, asking what had happened. At the same time, a third person asked me what could have happened to the oak in the workshop which has turned a dark blue, almost black colour and is sure it can't be mould. That's why I decided to return to the subject of tannin, even though not long ago I wrote about the pluses and minuses of tannin present in wood and its uses. Oak is a wood that contains a lot of tannin which influences its behaviour during processing, finishing and use.

The amount and composition of tannin in oak depends on many factors

What is tannin or tannic acid as it is also called, I have written extensively here. It is found in varying amounts in most species, the rule being that the lighter the wood, the lower the tannin content. Oak is one of the species with a high tannin content. The content is not the same, however, and differs depending on the type of oak, growing area, soil composition, age and other such criteria. Therefore, not everyone faces the same problems under the same working conditions, and the reactions are not always equally intense.

As a species, European oak is richer in tannin than American oak. In Europe, oak growing in the north is more tannin-rich than oak growing in the south, and oak growing in the west is more tannin-rich than oak growing in the east. The most tannin-rich oak in France is used in manufacture of barrels for ageing wines and for obtaining chips for putting in wine, and the one in the UK where colouring methods based on tannin content have been used for hundreds of years.

Reaction with iron and ferrous metals

Why did the oak plank turn black? Because in that area the wood came into contact with iron or iron-containing metals. The reaction between iron and tannin, in the presence of water, forms a bluish-black substance that colours the wood. The substance is very resistant and sometimes penetrates deep into the wood. Some sources give the use oxalic acid for bleachingbut the method is not always successful or needs to be repeated several times to see a change. If the stained wood is used outside, the colour will open up over time. At the same time, unprotected wood takes on a natural bluish grey colour (in 12-18 months) and so the colour becomes more uniform.

The reaction of the iron with the tannin in the oak requires careful choice of both the tools and the way the elements are fixed. The recommendation is to use stainless steel. The problem is that in addition to the staining of the wood, corrosion of the iron fasteners occurs. These can fail over time and so the design can be affected.

The reaction with tannin is very sensitive and can happen even if it appears that the iron has not come into direct contact with the wood. In one of the cases that caught my attention there had been some iron work nearby, something had been polished. The fine iron particles settled on the oak plank and with the help of moisture in the air and the wood the reaction was triggered. The surface of the plank filled with bluish-black stains.

Care must also be taken when storing. If oak items are stored on unprotected iron rails, the wood will certainly blacken in the contact area.

Tannin generally reacts with iron salts. Depending on their type, the colour changes. In reaction with iron sulphate the colour obtained is dark blue, and with iron chloride a green colour is obtained. It is a method of obtaining inks.

The reaction of tannin with iron can be used as a staining method

But tannin's reaction with iron doesn't just have negative effects. It can be used to stain wood a bluish-grey colour similar to unprotected, weathered wood. This is done by treating the wood with a solution made from iron and vinegar. For a deep colour, the wood shavings should be as fine as possible and kept in vinegar for several days. Find the method described in detail here.

The higher the tannin content, the more intense the reaction. To increase the tannin content on the wood surface, green tea (solution) can be applied beforehand. Tea is very rich in tannin. This method can also be used to stain wood with a low tannin content, such as pin or spruce.

Brown spots occur due to the solubilisation of tannin in water

Why do unevenly coloured brown stains appear on oak? Because tannin is soluble in water. When water comes into contact with the wood it extracts the tannin and a solution is formed which runs off the wood. The colour is given by that of the tannin, which in its purified state is yellow. The solution is brown because other substances are also extracted. After the water evaporates, a fine yellow-brown powder - tannin in the form of a solid powder - remains on the wood.

As with staining on contact with iron, tannin release does not affect the strength of the wood. It is an aesthetic problem that can be removed by mechanical woodworking. However, if it comes into contact with water again, the stains will reappear because the tannin is throughout the wood. To remove it, the wood should be boiled several times in water, as is done with wood used in aquariums. However, it may take weeks before the aquarium water no longer stains at all.

A problem can arise with natural stack drying of oak. The battens used to space the planks and create the airways must not be oak because the wood will stain in the right place. Because the time spent in the stack is long, the stain goes deep and compromises that part of the plank.

Smoke staining with ammonia - another reaction of oak tannin

One method by which the tannin reaction is used to stain wood is ammonia smoking. It is a traditional method discovered by chance a long time ago in England. Oak planks were stored in a stable where horses were kept. After a while the wood took on a grey-brown colour. Analysis revealed that it was ammonia fumes from the horse urine that caused the wood to change colour.

The intensity of the colour depends on the tannin content and therefore each piece of wood has a unique colour. Exposure to direct light leads to slight discolouration and a shift towards amber. This is a natural phenomenon, and the natural appearance thus obtained is very special. See here a full description of the method of smoking oak wood with ammonia.

I hope you find the above information interesting. If you have had any problems due to tannin in wood share them with us. Complements are as usual very welcome. And if you have any questions or queries, leave them below in the space provided. I will certainly answer them.