retanning

Retanning is a stage in leather processing in which a tanned hide or skin is treated again with additional tanning agents after the primary tanning operation. The goal is to further stabilize the leather and to modify its physical and aesthetic properties to meet the requirements of different leather types and end uses. Retannage is commonly employed in chrome-tanned hides and, in some cases, in vegetable-tanned or synthetically tanned leathers, to adjust shrinkage temperature, fullness, softness, and dye uptake.

A wide range of retanning agents is used, including vegetable tannins (such as quebracho, chestnut, and tara), synthetic tanning agents and aldehyde- or polymer-based resins. The retannage can be carried out in successive baths with different agents, often in combination with fatliquors, lubricants, and surfactants to aid penetration and soften the leather. The choice and sequence of agents determine properties such as grain tightness, resistance to salt and water, color fastness, and overall feel.

Retannage is typically performed after the initial tanning step and before finishing stages such as fatliquoring, dyeing, and surface finishing. It can also be used to alter the appearance of the leather, e.g., to create more uniform color or to achieve a particular character. Environmental and safety considerations include management of chemical residues in effluent, handling of potentially hazardous tanning agents, and adherence to industry regulations.