Brief description

Tanning is the process of treating skins and hides of animals to produce leather. It permanently alters the protein structure of skin, making it more durable and less susceptible to decomposition.

Use / Function

• Primary: Preservation of animal skins to create leather.
• Secondary: Creating waterproof, durable materials for clothing, shelter, armor, and containers.
• Scale: Domestic to Industrial.

Operating principle

It relies on the chemical modification of collagen. The process removes water from the interstices between protein fibers and cements them together. Tanning agents (such as tannins from plant bark, alum, or chromium salts) cross-link with the collagen proteins, preventing them from rotting, shrinking, or hardening upon drying.

How to create it

1. Preparation

• Curing: Fresh hides are salted or dried to prevent immediate putrefaction.
• Soaking: Rehydrating the hide in water to remove dirt and salt.

2. Liming and Dehairing

• Liming: Soaking the hides in a solution of lime (calcium hydroxide) or wood ash (lye) to loosen hair and remove fats.
• Dehairing: Scraping off the hair and flesh remnants using a dull knife or scraper over a beam.

3. Deliming and Bating

• Deliming: Removing the alkali (lime) from the hide using weak acids (vinegar) or water.
• Bating: Historically involved using fermented dung (dog or pigeon) to enzymatically soften the hide.

4. Tanning (The Core Step)

• Vegetable Tanning: Soaking hides in vats containing water and tannin-rich plant matter (bark, leaves, roots) for weeks or months. The concentration is gradually increased.
• Alum Tanning (Tawing): Soaking in a solution of alum and salt. Produces white, soft leather (tawed leather) which is not waterproof.
• Brain/Oil Tanning: Rubbing emulsified fats (like animal brains) into the hide and often smoking it. Produces soft buckskin.

5. Finishing

• Drying: Controlled drying to prevent hardening.
• Lubricating: Applying oil or fat (fatliquoring) to ensure flexibility.

Materials needed

• Essential: Raw animal hides, Water.
• Tanning Agents:
  • Vegetable: Bark (Oak, Chestnut, Hemlock), Sumac leaves.
  • Mineral: Alum, Salt.
  • Oil: Animal brains, Fish oil.
• Chemicals: Lime (from limestone) or Wood Ash (for lye).
• Tools: Fleshing beam (rounded log), Scraper (bone, stone, or metal), Vats (wooden or pits).

Variants and improvements

• Vegetable Tanned: Stiff, durable, breathable. Used for shoe soles, belts, armor. Takes months.
• Alum Tawed: White, very soft. Used for gloves, bookbinding. Not water-resistant (alum washes out).
• Brain Tanned: Extremely soft, washable (if smoked). Used for clothing.
• Chrome Tanned: (Modern) Uses chromium salts. Fast (days), produces soft, stretchable leather. Bluish-grey before dyeing.
• Aldehyde Tanning: Uses glutaraldehyde or oxazolidine (Chamois leather).

Limits and risks

• Foul Odor: Tanneries produce noxious smells from rotting flesh and soaking agents (dung, urine). Historically located outside towns.
• Pollution: Tanning effluent (wastewater) is highly polluting, containing organic matter, salts, and toxic chemicals (especially in chrome tanning).
• Time: Traditional vegetable tanning is a very slow process.
• Health: Anthrax spores can survive on raw hides. Lime and other chemicals can cause skin burns.