Leather Manufacturing Series – Part 4: Tanning to Crust
Estimated Reading time: ~16-18 minutes
Introduction:
In Leather Manufacturing Series – Part 3: Transforming Hides into Leather, we explored how raw hides become stable, tanned “wet blue” or vegetable-tanned leather.
Now, in Leather Manufacturing Series 4: Tanning to Crust, we enter the decisive wet-end phase where leather acquires its final softness, strength, color, and performance characteristics. These mechanical and chemical operations — setting, samming, drying, neutralization, retanning, dyeing, fatliquoring, and fixation — turn a stiff, wet, pale material into supple, vibrant crust leather ready for finishing.
For tanning basics, revisit Leather Manufacturing Series – Part 3. For sustainability standards, see the Leather Working Group.
Setting
Purpose
Setting is the first critical mechanical step after tanning. Freshly tanned leather is heavily swollen with water and tanning agents, wrinkled, and uneven in thickness. Setting mechanically stretches, flattens, and smooths the grain surface while expelling excess interstitial water. This operation dramatically improves area yield (up to 5–7%), eliminates creases that would otherwise become permanent cracks after drying, creates a uniform starting surface for subsequent wet operations, and prepares the leather for even moisture removal in samming and drying. Without proper setting, leather would dry with deep wrinkles, reduced usable area, and inconsistent dye uptake later.
Process
Leather passes through a setting-out machine with helical blades and felt-covered cylinders under controlled pressure.
Machinery
Setting-Out Machine – Aletti Giovanni & Figli, Mercier Turner
Professional Tips
- Keep roller pressure 20–30 psi for bovine hides; reduce to 12–18 psi for delicate calf.
- Clean blades every 2 hours to avoid scoring marks.
- Aim for 50–60% moisture after setting.
Sammying
Purpose
Sammying mechanically removes up to 20–25% of free water trapped between collagen fibers after tanning and setting. This rapid dewatering step prevents the formation of hard, glassy surfaces during drying (case-hardening), reduces total drying time by 40–60%, saves energy, minimizes fiber sticking, and allows better control of the final moisture content (usually 40–48%). Proper sammying is essential for achieving uniform drying and preventing cracks or brittleness in crust leather.
Process
Leather passes between high-pressure felt or rubber rollers that literally wring water out.
Machinery
Samming Machine – Tannery Trade. cmspa machines
Professional Tips
- Use automated moisture sensors to adjust pressure in real time.
- Recycle expressed water through filtration for sustainability.
- Transfer to drying within 5 minutes to avoid re-absorption.
Drying
Purpose
Drying removes the remaining bound and free water to bring leather to 10–14% moisture — the stable level required for crust storage and subsequent dry operations. Controlled drying locks the three-dimensional collagen fiber network created during tanning, prevents microbial growth, stops further chemical reactions, and determines final hardness/softness balance. Different drying methods allow tanners to influence grain tightness, stretch, and break characteristics. Poorly controlled drying causes irreversible defects: cracking, looseness, or boardy feel.
Methods
Toggle | Vacuum | Hanging | Paste | Tunnel
Machinery
Drying Machines – Vacuum Dryer – Mecman Toggle Dryer – Fratelli Carlessi
Professional Tips
- Never exceed 50°C to prevent fiber damage.
- Use vacuum drying for splits and corrected grain (2–3 times faster).
- Condition dried leather to 12–14% moisture before staking.
Crusting (Wet-End Finishing)
Purpose
Crusting is the final wet chemical treatment that defines the leather’s commercial character before any surface finishing. It neutralizes residual acids, re-tans for desired tightness or softness, imparts permanent color through dyeing, lubricates fibers with fatliquors (preventing brittleness and improving tear strength), and fixes everything in place. This stage can increase softness by 200–300%, improve water resistance, and determine whether the leather will be suitable for upholstery, footwear, or garments. Crusting turns a pale, stiff dried leather back into a colorful, supple, high-value material ready for mechanical finishing.
Key Operations
Neutralization → Retanning → Dyeing → Fatliquoring → Fixation
Machinery
Rotating Dye Drums – Unuo Instruments, Liangangtech
Professional Tips
- Neutralize to pH 5.0–5.5 before dyeing for maximum color yield.
- Use 2–4% synthetic fatliquor for garment leather; 6–8% for upholstery.
- Run drums at 10–15 RPM and 35–40°C for even penetration.
Conclusion
Leather Manufacturing Series 4 has taken you from freshly tanned wet leather through every mechanical and chemical step that creates supple, colored crust. These often-overlooked post-tanning operations are where craftsmanship and chemistry combine to produce the leather we love.
Next week — Leather Manufacturing Series 5: Crust to Finishing — we’ll cover staking, buffing, embossing, coating, and final polishing that transform crust into the stunning finished leather you see in luxury goods. Stay tuned!
