Tannins are natural compounds found in many plants, including trees, significantly influencing wood’s characteristics. They impact its appearance, durability, and interaction with finishes and fasteners. Understanding tannins is key for anyone working with wood, as they present both desirable qualities and potential challenges.
What Are Tannins in Wood?
Tannins are complex organic compounds, a class of polyphenolic biomolecules, found throughout various parts of plants, including wood, bark, leaves, fruits, and roots. Within the wood structure, tannins are primarily located in cell vacuoles, particularly concentrated in the heartwood, which is the denser, non-living core of a tree.
The name “tannin” originates from their historical use in tanning animal hides into leather, a process that relies on their ability to bind with proteins. Tannins are known for their astringent properties, which create a dry, puckering sensation, similar to what is experienced when tasting unripe fruit or strong tea. Common high-tannin wood species include oak, redwood, cedar, walnut, and chestnut.
Why Trees Have Tannins
Tannins play an important role in a tree’s natural defense system. These compounds act as deterrents against various threats, including pests, fungi, bacteria, and herbivores. Their ability to bind to proteins allows them to inhibit digestive enzymes in herbivores, reducing the nutritional value of plant material and making it unpalatable or toxic.
Beyond defense, tannins contribute to the tree’s overall resilience by providing protection against ultraviolet (UV) radiation. They also help prevent infections by precipitating enzymes and proteins from bacteria and fungi, hindering their ability to infect the tree. The concentration of tannins can vary within a tree, often being more abundant in vulnerable areas like new leaves and flowers.
How Tannins Affect Wood
Tannins significantly influence wood’s aesthetic and physical properties. They are largely responsible for the natural color of many wood species, contributing to darker hues, especially in hardwoods. Tannins naturally decrease the lightness of wood, making it appear darker and browner, while also increasing red and yellow color values.
A common issue stemming from tannins is staining, often referred to as “tannin bleed” or “extractive bleed.” This occurs when tannins, which are water-soluble, migrate to the wood surface, especially when exposed to moisture or humidity. When tannins react with iron, such as from certain fasteners or steel wool, they can cause a distinct bluish-black or dark stain, known as iron tannate stain.
Tannins also contribute to wood’s natural durability. They help protect wood from decay and insect infestations by making it less susceptible to biological degradation. However, tannins can impact the adhesion and appearance of certain finishes. When water-based finishes are applied to tannin-rich woods, the water can dissolve and activate the tannins, leading to discoloration or staining of the finish.
Dealing with Tannins in Woodwork
Managing tannins in woodworking projects involves several strategies. Proper wood drying prevents moisture from drawing tannins to the surface and causing bleeding. For finishing, use appropriate primers or sealers to prevent tannin bleed-through.
Shellac-based primers effectively block tannins due to their fast drying time. Oil-based primers also block stains well and are suitable for tannin-rich woods. Some water-based primers are formulated to block tannins, though they may require multiple coats and longer drying times.
When working with high-tannin wood, select non-corrosive fasteners to avoid bluish-black staining from iron reactions. Applying a tannin-blocking primer before the topcoat helps ensure a consistent appearance and prevents discoloration, especially for outdoor applications or with water-based finishes. If tannin stains appear, they can be treated with oxalic acid or bicarbonate of soda solutions.