Lumber Acid Content
A closer look at when lumber or bark might release tannic acid or acetic acid. April 17, 2009
Question
Does anyone know what type of acid that comes out of walnut when you steam it? We have been told it is called tannic acid, but we cannot find any information on it. We are being asked by our concrete supplier.
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
Although I cannot confirm that it is indeed tannic acid, when building with concrete, we typically use river rock and not lime stone.
From contributor M:
Tanic acid is derived from the bark of fruitwood trees - walnut hickory oak and other trees with nuts. It is used in the process of tanning hides.
From Gene Wengert, forum technical advisor:
The early European settlers in North America used the bark of hemlock for obtaining tannic acid for tanning. Often the trees were cut and debarked and then the wood was not hauled to a mill. I have not heard that nut trees or fruitwood trees have large amounts of tannic acid in their bark compared to trees without nuts.
Note that the question here refers to the acid in wood. All trees have a small amount of tannic acid in them (in both wood and bark), but little of it comes out of the wood when processing. Oak has acetic acid in the wood. This acid will attack lime stone concrete. I cannot find anything to indicate this is the same acid that comes out of walnut when steamed.
From contributor D:
We have often had water from dehumidification kilns tested and we have never found any tannic acid but always do find acetic acid. I have been told that tannic acid doesn't evaporate from wood and the only way to get it is to immerse the bark or wood in water. We test a lot of species. We do it so that we can respond if someone needs to know to get a building permit but also to understand what we are contending with in the equipment to prevent corrosion, hence, we avoid anything galvanized in the kiln and coat anything where two different metals are in contact. Be sure that when concrete is ordered that there is no antifreeze in it because that reacts with acetic acid. It is common in the North for concrete companies to add chemicals in the winter to keep the water in the concrete from freezing before the concrete sets. That is not acceptable for dry kilns.
From Gene Wengert, forum technical advisor:
I do agree with contributor D, but I will add that when steaming wood we do get different chemicals released. For example, with walnut, there is an oily residue on the water and the water is black in color.
AI enhanced answer (added Jan 14, 2025):Tannic acid and acetic acid can be released from lumber or bark due to several natural processes:
1.
Tannic Acid: This is a type of polyphenol found in many types of wood, particularly in hardwoods like oak, chestnut, and walnut. When wood is cut or processed, cells are damaged, releasing tannins. These tannins can leach out when the wood comes into contact with water or moisture. Tannins serve as a defense mechanism in plants, deterring herbivores and pathogens, but in woodworking, they can cause discoloration or staining on surfaces or in water used for soaking or steaming wood.
2.
Acetic Acid: This can be produced through the natural decomposition of wood, especially when wood is exposed to moisture and oxygen. The process involves the breakdown of hemicellulose, a component of wood, by bacteria or fungi. This microbial activity can convert sugars and other organic compounds into acetic acid, which is essentially vinegar. This acid can contribute to the sour smell often associated with wet or rotting wood. In some cases, acetic acid might also be released during the thermal treatment of wood, like in kiln drying, where high temperatures can accelerate chemical reactions leading to acid formation.
Both acids can affect the wood's pH, potentially influencing its color, durability, and how it interacts with finishes or adhesives.
