retannering

Retannering, also known as retanning, is a stage in leather processing where additional tanning agents are applied to leather after the primary tanning step. The purpose is to modify and optimize the material’s properties, including fullness, softness, strength, texture, and dye uptake, as well as to improve resistance to water and heat. Retannage is used across different tanning systems, including chromium-tanned and vegetable-tanned leathers, and is often tailored to the intended end product, such as footwear, upholstery, or automotive leather.

In practice, retanning employs a mixture of agents chosen to influence the leather’s performance. These can include synthetic tannins (syntans), fat tenderizers or fatliquors, mineral salts, resins, and additional vegetable tannins. The exact formulation and dosage depend on the leather type, the desired balance of properties, and the processing sequence. The retanning bath is typically circulated in a drum or circulated bath, with controlled temperature, pH, and time to ensure even penetration and distribution of agents.

After retannage, the leather is usually washed, braked or neutralized to stabilize the bath, and often fatliquored to restore suppleness. Retanning can improve color development and dye uniformity, help stabilize the leather against shrinking or cracking, and refine the hand and surface feel. See also tanning, fatliquoring, and dyeing.