In nature, tannins perform vital functions for plants, including defence against herbivores and antimicrobial protection. They are also antioxidants. Historically, tannins were used to tan leather and are present in many plants and fruits, contributing to astringency and bitterness.
Most tannins react with proteins and with metallic salts such as iron. Chemically, they are polyphenols divided into two groups.
Exploring tannins
Tannins can be explained to children through the protective bark of a tree: like our skin, it protects the inside. The tree releases protective substances that darken the bark — tannins.
When humans discovered that plant juices could preserve animal skins, tanning was born. Combining tannin-rich plant extracts with iron can produce black.
From tannin to black
Oak galls are among the richest natural sources of tannins and may contain around 45%. Tannins are also found in many barks. When boiled and combined with rust, they can produce a durable black. Depending on the source and iron concentration, the colours may range from beige to dark brown and black.
The resulting black reveals deep, subtle tones when fabric or paper is exposed to air. Its depth comes from the complex interaction between natural compounds in the plant and the fibres.