How did mordants help natural dyes remain attached to cotton fabric?

Natural dyes generally lack a direct affinity for cotton, meaning they cannot bond to the fabric on their own. To overcome this, craftsmen use mordants—typically inorganic metal salts such as alum, iron, copper, or tin—as a pretreatment.12

The process works through the following mechanism:

• Adsorption: The mordant is first adsorbed onto the cotton fibre.1
• Chemical Reaction: Once on the fibre, the mordant reacts with the dye to form a complex.31
• Fixation: This reaction produces a less soluble form of the dye that is effectively held to the fabric, ensuring the color remains attached.1

Beyond aiding retention, mordants also serve to vary the final color produced by a single dye, as different metal salts react to create distinct hues. While ancient methods relied on various metal salts, chromium salts became common around 1850 due to their superior dye-retention properties.1