Definition: acidic chemicals held in solution within the liquid sap; are water soluble, so moisture or water permeating through the timber will bring them to the surface causing stains
What are tannins?
Tannins in wood are natural organic compounds found in various species of trees. They are part of the wood’s composition and are responsible for giving wood its characteristic color, particularly the darker hues in many hardwoods.
The word tannin comes from the old German word tanna meaning oak. It refers to the use of wood tannins derived from oak trees that were used to convert animal hides into leather.
Tannins are found commonly in the bark of trees, wood, leaves, buds, stems, fruits, seeds, roots, and plant galls. In all of these plant structures, tannins help to protect the individual plant species. In addition to influencing the wood’s color and taste, tannins also play a role in protecting the tree from pests, fungi, and microbial degradation.
Courtesy of http://www.fs.usda.gov
What does it look like when tannins come or “bleed” through?
This is an example of when tannins cause staining or discoloration on finished surfaces.
If wood is painted with products not specifically designed to trap tannins, the tannins can bleed through to the surface resulting in brown, tan or yellow stains. Discolouration can also occur if wood is not fully dry when it’s painted.
What causes tannins to come to the surface?
Tannins in wood coming to the surface is commonly known as “tannin bleed” or “extractive bleed.” It occurs when the tannins, along with other water-soluble extractives and natural resins present in the wood, migrate towards the wood’s surface and become visible.
This process is typically triggered by exposure to moisture, humidity, or changes in environmental conditions.
Several factors contribute to tannin bleed:
- Moisture Exposure:
- When wood comes into contact with water, whether through spills, rain, or high humidity, the tannins and other extractives can dissolve in the water and move towards the surface of the wood.
- Temperature and humidity changes:
- Fluctuations in temperature and humidity can cause the wood to expand and contract. During these changes, the dissolved tannins can be pushed to the wood’s surface.
- New wood or freshly cut surfaces:
- Tannin bleed is often more noticeable in new wood or freshly cut surfaces, as they contain higher concentrations of extractives that have not yet been leached out through weathering and exposure.
- Finishing Products:
- Certain types of finishes or wood treatments can react with the tannins, causing them to be drawn to the surface. Example, using certain water-based stains or finishes can lead to tannin bleed in some wood species.
What woods have the greatest tannins?
The acidity of a species of wood depends primarily on its tannin content.
Higher tannin woods
- oak
- walnut
- cherry
- mahogany
- cedar
- pine
Lower tannin woods
- maple
- birch
- poplar
Note: Tannin content can vary even within the same species, depending on factors like the tree’s age, growing conditions, and location.
Here are a few related tannin topics- how to remove using Osmo’s Tannin Spot Remover and how to prevent bleedthrough when painting!
References:
https://logic-bespoke.com/timber-leaching-and-tannins
https://www.aquacoat.com/blogs/news/how-to-prevent-tannin-bleed
https://www.fs.usda.gov/wildflowers/ethnobotany/tannins
https://www.woodmagazine.com/wood-supplies/lumber/avoid-acidic-woods-to-protect-a-precious-flag
https://en.wikipedia.org/wiki/Tannin
Chat GPT- search “list the woods with high tannin content”
Chat GPT- search “list the woods with low tannin content”
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