Tannin
A naturally occurring organic compound in bark, wood, and leaves of many tree species that contributes to wood color, decay resistance, and can cause staining with iron fasteners.
In-Depth Explanation
Tannins are polyphenolic compounds that serve as a natural defense mechanism in trees, deterring insects and resisting fungal attack. Species high in tannins include oak, walnut, cedar, and redwood, which are among the most naturally durable lumber options. When tannin-rich wood contacts iron or steel fasteners, a chemical reaction produces dark blue-black stains that can be difficult to remove. This reaction also occurs with some water-based finishes containing iron compounds. Woodworkers working with high-tannin species should use stainless steel or coated fasteners and test finishes on scrap pieces first.
Usage Example
The white oak deck boards developed black stains around every nail head because the tannins reacted with the steel fasteners.