CHIEF PROCESSES IN LEATHER MANUFACTURE
1) Pre-tannage (beam house operations)
a) Flaying- removing the skin from the animal
b) Curing- preserving during transport or storage
c) Washing (soaking)- restoring to raw condition
d) Liming- loosening hair follicle, fat, etc. and “plump” up the skin for tanning, deterioration of epidermis
e) Unhairing- removing hair
f) Fleshing- cutting away unwanted fat and flesh splitting and smoothening
g) Deliming- neutralizing the alkali (from d)
h) Bating- enzymatic loosening of hide fibers, cleaning the skin(softens)
i) Pickling- drenching or souring- adjusting pH for tannage
The purpose of these operations is to increase the amount of water in the hide, to the amount close to that of the “living” hide, remove foreign bodies and loosen structure. This loosening makes it easier for the tanning agents, fats, dyestuffs and other substances, to penetrate into the hide. In the beam house the non collagenous proteins are removed from the hide, so is its epidermis, hair and globular proteins, melamins, components of cell walls, while collagen fiber skeleton remains practically untouched.
2) Tannage (tannery operations)
Tanning by appropriate method
3) After tannage (finishing operations)
a) Shaving or splitting- to achieve uniform thickness
b) Washing- discarding surplus chrome salts
c) Neutralizing- adjusting pH
d) Dyeing- to get required color
e) Setting out- removal of wrinkles and flatten
f) Stuffing- impregnating with oil and fat (ie. waterproofing)
g) Oiling- making flexible and of good color
h) Drying
i) Rolling- compressing for firming and flattening
There are many variants on this simple outline. All these processes, their choice and control which determine the quality of the leather made form the basis of Leather Technology. In almost all stages, substances are moving either into or out of the skin or hide. Process of wetting back, conditioning and drying involve mainly the movement of water. During liming, deliming, pickling and neutralizing salts, acids and alkali are involved. In tanning, dyeing and fatliquoring, various chemicals move into the skin, while in liming and bating, unwanted materials migrate to the surface and pass into the surrounding liquor. There is a large variation in the pH .
| Processes | pH |
| Pickling | 1,0 – 2,0 |
| Cr tanning | 2,0 – 4,0 |
| Vegetable tanning | 3,0 – 5,0 |
| Neutralizing and Fatliquoring | 4,0 – 6,0 |
| Deliming | 5,0 – 9,0 |
| Bating | 7,5 – 9,0 |
| Oil tannage | 6,5 – 10,0 |
| Liming | 12,0 – 13,0 |
| Dyeing acid dyestuffs | 3,0 – 4,0 |
| Direct dyes | 4,0 – 6,0 |
| Special types | 6,0 – 8.5 |
In a wet rawhide, the peptide groups and particularly acid and basic groups hydrate. Water molecules are attached to and bound to these groups. The more water attracted to the protein molecule, the more it becomes separated from the adjacent molecule, so that the molecules are pushed apart and the skin is said to swell. By increasing the ionization of either the acid or basic groups by the addition of alkali or acid respectively, the attraction for water is increased and the skin swells or “plumps” more.
In a wet rawhide containing 70% water, the bulk of water is mechanically held as free water and its loss doesn’t create a hardening effect, however when the drying has proceeded so that the hide contains ~25% water, the bulk of this 25% is chemically bound (hydrated) to the peptide and aminoacids of the skin and as this is removed by drying, hardening and stiffening will occur.
