Wood is a natural material containing various compounds, including tannins. These substances are present in many wood types and significantly influence its characteristics and how it interacts with its environment. Understanding wood tannins and their effects is helpful when working with wood.
What Exactly Are Wood Tannins?
Wood tannins are complex chemical compounds known as polyphenols. These large molecules are found in plants, including trees, and are characterized by their ability to bind readily with proteins, cellulose, starches, and minerals, forming insoluble substances. While different types exist, they are all organic compounds with complex molecular structures that contribute to their roles within wood.
Where Do Tannins Come From in Wood?
Tannins are naturally produced by trees as part of their biological processes. They are found in various parts of the tree, including bark, leaves, and wood, with concentrations often higher in the heartwood. Certain wood species are known for high tannin content, such as oak, redwood, cedar, walnut, chestnut, and mahogany.
Trees produce tannins primarily as a natural defense mechanism. Tannins also play a role in the natural coloration of wood, influencing its distinctive hues.
How Tannins Affect Wood
Tannins significantly impact the appearance and behavior of wood. They contribute to its natural color, giving many hardwoods their characteristic darker tones. When wood is exposed to moisture, these water-soluble tannins can dissolve and migrate to the surface, a process known as “tannin bleed.” This often results in yellow or brownish stains, particularly noticeable on lighter paints or finishes.
Tannins also react with metals, especially iron, leading to dark blue-black “iron staining.” This occurs when tannic acid in wood comes into contact with ferrous metals. While this reaction typically does not damage the wood structurally, it can mar its appearance. Furthermore, tannins can sometimes affect the adhesion of certain glues or finishes, leading to uneven staining or blotchiness, especially with water-based products. On the positive side, tannins enhance the natural durability of some wood species by acting as natural preservatives against decay and insect infestation.
Managing Tannin-Related Issues
Managing tannin-related issues often involves preventative measures and appropriate product selection. To prevent tannin bleed, especially when painting or applying clear finishes, using a tannin-blocking primer or sealer is effective. Shellac-based primers are widely recognized for their ability to seal in tannins, though oil-based and some acrylic latex primers also offer this protection. Applying two coats of primer can be beneficial for heavy staining or porous woods.
Proper wood drying techniques are important to minimize moisture-induced bleeding, as tannins are more likely to leach from wet wood. When working with high-tannin woods, selecting non-reactive fasteners, such as stainless steel or brass, can prevent dark staining caused by the interaction of tannins with iron. For existing stains, oxalic acid or bicarbonate of soda can sometimes be used for removal. In some applications, particularly with outdoor structures or furniture, the natural color changes and reactive staining caused by tannins are embraced as part of the wood’s aesthetic aging process.