INTRODUCTION
Tanning is an important industrial chemical process in which raw animal hides and skins are converted into
leather. Raw hides contain collagen fibers which can easily decompose due to microbial action. Tanning
stabilizes these collagen fibers by forming cross-links, making leather resistant to decay, heat, water, and
mechanical stress.
OBJECTIVES OF TANNING
• To prevent putrefaction of hides
• To improve strength and durability
• To increase resistance to water and heat
• To provide flexibility and softness
• To make leather suitable for industrial use
CLASSIFICATION OF TANNING METHODS
The tanning methods are classified into:
1. Vegetable tanning
2. Mineral tanning
3. Oil tanning
4. Aldehyde tanning
5. Synthetic tanning
The detail of these are given below.
1 Vegetable tanning:-
We read in previous lecture in detail.
2 Mineral tanning:-
Mineral tanning uses inorganic salts, mainly chromium and aluminum compounds.
• Chrome tanning:
We read about this topic in previous lecture in detail.
• Alum tanning:
Alum tanning is one of the oldest and most traditional methods of tanning leather. It is primarily used
to produce soft, white, and pale-colored leather, often referred to as kid leather or morocco leather.
1. Definition
, Alum tanning is the process of converting raw hides or skins into leather using aluminum salts, mainly
potassium alum (KAl(SO₄)₂·12H₂O) in combination with sodium or ammonium salts. It produces
leather that is soft, flexible, and water-insoluble.
2. Chemicals Used
• Potassium alum (KAl(SO₄)₂·12H₂O) – primary tanning agent.
• Sodium chloride (NaCl) – sometimes used as a preservative in soaking or liming.
• Alum and cream of tartar (KHC₄H₄O₆) – for improving leather softness.
• Other salts – for pH control and enhancement of tanning effect.
3. Principle
• Alum tanning works on the principle of coagulation of collagen fibers.
• The Al³⁺ ions from alum react with the carboxyl and hydroxyl groups of collagen in the skin.
• This reaction cross-links the protein fibers, preventing putrefaction and making the skin durable
but less resistant to heat and microorganisms than vegetable-tanned leather.
4. Process of Alum Tanning
The process can be divided into several steps:
Step 1: Preparation
• Soaking: Dried hides are soaked in water to remove dirt, blood, and salt.
• Liming: Hides are treated with lime to remove hair and epidermis.
• Fleshing: Mechanical removal of excess flesh and fat.
• Deliming and Bating: Acidic solutions (like ammonium salts) are used to neutralize lime, and
enzymes are used to make hides soft.
Step 2: Pickling
Hides are treated with acidified salt solution (like NaCl + sulfuric acid) to bring the pH to 3–4. This is
necessary for aluminum salts to penetrate the hide effectively.
Step 3: Tanning with Alum
• Hides are placed in tanning vats containing aqueous solution of potassium alum.
• Gradual addition of alum ensures uniform penetration.
• The tanning time may vary from 3–10 days, depending on hide thickness.
Step 4: Softening
• After tanning, the leather is washed to remove excess alum.
• Leather is then treated with oils or fats to improve softness and pliability.
Step 5: Drying and Finishing
• The tanned leather is dried carefully to avoid shrinkage.
• It may be buffed, dyed, or polished depending on the intended use.
Tanning is an important industrial chemical process in which raw animal hides and skins are converted into
leather. Raw hides contain collagen fibers which can easily decompose due to microbial action. Tanning
stabilizes these collagen fibers by forming cross-links, making leather resistant to decay, heat, water, and
mechanical stress.
OBJECTIVES OF TANNING
• To prevent putrefaction of hides
• To improve strength and durability
• To increase resistance to water and heat
• To provide flexibility and softness
• To make leather suitable for industrial use
CLASSIFICATION OF TANNING METHODS
The tanning methods are classified into:
1. Vegetable tanning
2. Mineral tanning
3. Oil tanning
4. Aldehyde tanning
5. Synthetic tanning
The detail of these are given below.
1 Vegetable tanning:-
We read in previous lecture in detail.
2 Mineral tanning:-
Mineral tanning uses inorganic salts, mainly chromium and aluminum compounds.
• Chrome tanning:
We read about this topic in previous lecture in detail.
• Alum tanning:
Alum tanning is one of the oldest and most traditional methods of tanning leather. It is primarily used
to produce soft, white, and pale-colored leather, often referred to as kid leather or morocco leather.
1. Definition
, Alum tanning is the process of converting raw hides or skins into leather using aluminum salts, mainly
potassium alum (KAl(SO₄)₂·12H₂O) in combination with sodium or ammonium salts. It produces
leather that is soft, flexible, and water-insoluble.
2. Chemicals Used
• Potassium alum (KAl(SO₄)₂·12H₂O) – primary tanning agent.
• Sodium chloride (NaCl) – sometimes used as a preservative in soaking or liming.
• Alum and cream of tartar (KHC₄H₄O₆) – for improving leather softness.
• Other salts – for pH control and enhancement of tanning effect.
3. Principle
• Alum tanning works on the principle of coagulation of collagen fibers.
• The Al³⁺ ions from alum react with the carboxyl and hydroxyl groups of collagen in the skin.
• This reaction cross-links the protein fibers, preventing putrefaction and making the skin durable
but less resistant to heat and microorganisms than vegetable-tanned leather.
4. Process of Alum Tanning
The process can be divided into several steps:
Step 1: Preparation
• Soaking: Dried hides are soaked in water to remove dirt, blood, and salt.
• Liming: Hides are treated with lime to remove hair and epidermis.
• Fleshing: Mechanical removal of excess flesh and fat.
• Deliming and Bating: Acidic solutions (like ammonium salts) are used to neutralize lime, and
enzymes are used to make hides soft.
Step 2: Pickling
Hides are treated with acidified salt solution (like NaCl + sulfuric acid) to bring the pH to 3–4. This is
necessary for aluminum salts to penetrate the hide effectively.
Step 3: Tanning with Alum
• Hides are placed in tanning vats containing aqueous solution of potassium alum.
• Gradual addition of alum ensures uniform penetration.
• The tanning time may vary from 3–10 days, depending on hide thickness.
Step 4: Softening
• After tanning, the leather is washed to remove excess alum.
• Leather is then treated with oils or fats to improve softness and pliability.
Step 5: Drying and Finishing
• The tanned leather is dried carefully to avoid shrinkage.
• It may be buffed, dyed, or polished depending on the intended use.
