Discuss the various methods of preservation of hides and skin. Elaborate the different steps involved in conversion of hides and skin into leather.

Methods of Hide and Skin Preservation

Preservation is crucial to prevent bacterial decomposition and maintain the hide's structure. Methods are broadly categorized as biological, chemical, and physical.

Biological Preservation

• Salting: This is the most common and traditional method, especially in India. Hides are layered with dry salt (sodium chloride or rock salt) which draws out moisture and inhibits bacterial growth. Approximately 10-15% salt by weight is needed.
• Chilling: Rapidly cooling hides to near-freezing temperatures (0-4°C) significantly slows down enzymatic activity and bacterial proliferation. This requires refrigerated storage facilities.
• Freezing: Freezing at -18°C or lower effectively halts decomposition, but requires significant energy and specialized equipment.

Chemical Preservation

• Pickling: Involves treating hides with acid (typically formic or acetic acid) and salt. This lowers the pH, preventing putrefaction and preparing the hide for tanning.
• Chrome-based preservatives: These compounds act as short-term preservatives and are often used in conjunction with other methods.

Physical Preservation

• Drying: Exposing hides to sunlight or artificial heat reduces moisture content, inhibiting microbial activity. This method is suitable for arid climates.
• Smoking: Historically used, smoking deposits antimicrobial compounds onto the hide surface.

Method	Advantages	Disadvantages	Suitability
Salting	Simple, inexpensive, widely applicable	Can damage collagen fibers if excessive salt is used	Suitable for most climates
Chilling	Effective in preventing decomposition	Requires refrigerated storage	Large-scale operations
Drying	Low cost	Can lead to stiff and brittle leather	Arid climates

Conversion of Hides and Skin into Leather: Step-by-Step

The process of converting hides/skins into leather is complex, involving multiple stages.

1. Pre-Tanning Processes

• Soaking: Rehydrates dried hides and removes dirt, salt, and other impurities.
• Liming: Removes hair and epidermis using lime (calcium hydroxide). This process also swells the collagen fibers, making them more receptive to tanning agents.
• Fleshing: Removal of residual flesh and fat from the flesh side of the hide.
• De-hairing: Enzymatic or chemical process to remove hair.

2. Tanning Process

• Vegetable Tanning: Uses tannins extracted from plant sources (bark, leaves, roots). This is a traditional method producing thicker, firmer leather.
• Chrome Tanning: Uses chromium salts. This is the most common method, producing softer, more flexible leather.
• Synthetic Tanning: Uses synthetic tanning agents like phenols.

The tanning process stabilizes the collagen fibers, preventing them from decaying. The type of tanning agent significantly influences the leather’s properties.

3. Post-Tanning Processes

• Neutralization: Adjusts the pH of the leather.
• Retanning: Improves leather characteristics like fullness, tightness, and dyeability.
• Dyeing: Adds color to the leather.
• Fatliquoring: Introduces oils and fats to lubricate the leather fibers, making them softer and more pliable.
• Finishing: Applies surface coatings (pigments, resins, waxes) to enhance appearance, water resistance, and durability.

The entire process can take several weeks, depending on the desired leather characteristics and the tanning method used. The leather industry in India faces challenges related to environmental regulations, particularly concerning wastewater treatment from tanneries. The Central Leather Research Institute (CLRI), Chennai, plays a crucial role in research and development of sustainable leather processing technologies.

Case Study: The Hazaribagh Tannery Cluster, West Bengal: This cluster, one of the largest in Asia, faced severe environmental issues due to untreated effluent discharge. The National Green Tribunal (NGT) intervened, mandating stricter environmental norms and promoting zero liquid discharge (ZLD) systems. While progress has been made, challenges remain in achieving full compliance. Question: What is the difference between hide and skin? Answer: Hides refer to the outer covering of larger animals like cattle, buffalo, and horses. Skins refer to the covering of smaller animals like sheep, goats, and pigs. The processing methods can vary slightly based on the thickness and characteristics of the material. National Mission for Himalayan Tannery Cluster Development: This scheme aims to upgrade infrastructure and promote sustainable practices in Himalayan regions, addressing environmental concerns in the leather industry. Launched in 2016. Tannin: Naturally occurring polyphenols found in plant tissues. They react with collagen in animal hides, converting them into stable, non-decaying leather. India’s Leather Exports: India's leather exports were valued at approximately $5.5 billion in FY 2022-23 (Source: Directorate General of Commercial Intelligence and Statistics). However, this figure fluctuates based on global demand and trade agreements. Collagen: The main structural protein found in animal connective tissues, including hides and skins. Tanning involves crosslinking collagen fibers to prevent decomposition. Water Consumption in Leather Tanning: Traditional leather tanning processes can consume up to 200 liters of water per kilogram of leather (Source: UNEP, knowledge cutoff).