Leather Manufacturing Series – Part 2: Preparing Hides for Tanning
Estimated Reading time: ~15-18 minutes
Introduction: The Chemical Bridge to Perfect Leather
Welcome to Part 2 of the Leather Manufacturing Series. In Part 1, we covered the physical and chemical groundwork — from raw hide selection to soaking, liming, unhairing, fleshing, and splitting. These steps opened the fiber structure and removed unwanted material, but the hide remains swollen, alkaline, and structurally unstable.
Now begins the true art of leather tanning preparation: deliming, bating, and pickling. These three sequential processes are the make-or-break stage in leather quality. They control pH, remove non-collagen proteins, soften the grain, and preserve the hide — ensuring uniform tanning agent penetration, consistent dye uptake, and long-term durability.
For tanneries supplying global brands (Nike, Gucci, BMW), failure here means rejected batches, uneven color, or leather that cracks under stress. But when executed with precision, these steps deliver a supple, open, and chemically balanced pelt — the ideal canvas for tanning.
Let’s explore each process in detail.
Deliming – From Alkaline Swell to Neutral Ground
After liming, hides are saturated with calcium hydroxide (pH 12–13), causing inter-fibrillary swelling that stiffens the structure and risks permanent damage if not reversed. Deliming uses mild acid salts to gently collapse this swelling, neutralize excess alkali, and create a stable environment for enzymatic action. This step is critical — too fast, and the hide burns; too slow, and production lags.
Key Parameters:
- Chemicals: Ammonium sulfate or chloride (dilute, 1.5–2.5% w/w); low-ammonia variants preferred for cleaner effluent
- Equipment:
- PPH Drum by Shibiao Machinery — polypropylene construction resists corrosion, integrated pH sensors
- Target pH: 7.8 – 8.2
- Time: 60–90 mins
- Temp: 28–32°C
- Float: 100–150% water
Best Practice: Adopt CO₂-assisted deliming — inject food-grade CO₂ during the final 20 minutes. This accelerates pH drop, reduces ammonia release by 60%, and improves LWG environmental scores. Widely used in Italy and South America.
Bating – Enzymatic Art of Softness
With alkalinity neutralized, the hide is ready for bating — a biotech-driven process that uses proteolytic enzymes to selectively degrade non-structural proteins (elastin, globulins, mucoids) trapped between collagen fibers. This loosens the grain, enhances softness, improves dye and fatliquor penetration, and reduces surface defects like scud or vein marks.
Key Parameters:
- Enzymes:
- Traditional: Pancreatic trypsin (animal-derived)
- Modern: Microbial protease (e.g., Novozymes Oropon, Buckman Bating Enzyme) — consistent activity, vegan-compliant
- Equipment:
- UI-FT800 Drum by Unuo Instruments — stainless steel, ±0.5°C temperature control, ideal for enzyme-sensitive processes
- Dosage: 0.8–1.5% enzyme on pelt weight
- Duration: 45–75 mins
- Temp: 35–38°C (optimal enzyme activity)
- Float: 80–120% warm water
Pro Tip: Pre-dissolve enzyme in 37°C water and add after 10 minutes of drum rotation. This prevents cold shock, ensures uniform dispersion, and increases enzyme efficiency by 25%. Standard in European automotive leather production.
Pickling – Preservation & Tanning Readiness
The final step in leather tanning preparation, pickling sharply lowers pH to 2.8–3.2, contracts collagen fibers, and creates a preservative brine that inhibits microbial growth. This allows safe short-term storage (up to 6 months) and ensures deep, even tanning agent fixation — especially critical for chrome tanning.
Key Parameters:
- Chemicals:
- Sodium chloride (80–100 g/L) — creates osmotic pressure
- Formic acid (0.8–1.2%) + sulfuric acid (0.5–0.8%) — split dosing
- Equipment:
- GHR Series Drum by Shibiao Machinery — acid-resistant stainless steel, auto-circulation, interlayer heating
- Target pH: 2.9 – 3.1
- Time: 90–150 mins
- Temp: 22–26°C
- Float: 60–80%
Pro Tip: Add acid in 4 stages (25% each, 15 mins apart). Use formic acid first (70%), then sulfuric. This prevents “acid burn” on the grain, reduces salt usage by 15%, and ensures pH stability — standard in LWG Gold tanneries.
Sustainability in Global Leather Production
These chemical-intensive steps generate high BOD, COD, and salt loads. Leading tanneries worldwide adopt:
| Innovation | Global Impact |
|---|---|
| Closed-loop systems | 80–90% water reuse via ultrafiltration + RO |
| Enzyme + CO₂ deliming | ↓60% ammonia, ↓40% BOD |
| Solar-powered drums | ↓30% energy, ↑LWG score |
| Automated dosing | ±0.1 pH accuracy, zero waste |
Pro Tip: Install condensation recovery units on drum exhausts. Capture 1.5–2 L of distilled water per hide — reduces freshwater demand by 12–18% in any climate. Used in Brazil, Italy, and China.
Conclusion: The Hide Is Now a Canvas
Deliming → Bating → Pickling = Leather Tanning Preparation Complete.
The Pelt is now:
- pH-balanced (2.9–3.1)
- Enzymatically cleaned (no scud, open grain)
- Soft & pliable (ideal for finishing)
- Preserved & stable (ready for tanning or storage)
This chemical trilogy is the foundation of all premium leather — from Italian upholstery to Japanese automotive seats.
Next in Series 🔗 Leather Manufacturing Series – Part 3: Various Leather Tanning Chrome, vegetable, aldehyde, wet-white, and hybrid systems — how the world creates durable, beautiful leather.
Read Part 3 →
