Annex 2: Leather Processing

Leather tanning is essentially the conversion of raw animal hides (cows, sheep, goats, buffalo) into leather by a series of chemical reactions that alters the protein structure to preserve the hide. It involves three main stages: the first to produce “wet blue” leather, the second to produce “crust leather,” and the third to produce finished leather.

1. “Wet blue” stage

Hides are first soaked for one or two days in water, wetting agents and bactericides, to remove the salt. They are then treated in pits or drums with lime and sodium sulfide to remove hair and excess flesh, in processes called liming and unhairing. These two stages are particularly polluting, causing the release of hydrogen sulfide and sulfur dioxide gases (which can cause sulfuric acid in the atmosphere) while the effluent contains calcium hydroxide as well as toxic sulfides and large amounts of suspended solids.

After liming, the hides undergo fleshing, either manually or in a fleshing machine, a process which strips the remaining flesh and fat from the hide. De-liming then removes the lime from the hides, often by ammonium sulfate or ammonium chloride as well as sodium metabisulfite. Bating, to soften the leather, uses a protein-digesting enzyme. Pickling then prepares the hide for tanning, often using salt, sulfuric acid, and formic acid.

Tanning can involve chrome tanning, synthetic tanning or vegetable tanning. Vegetable tanning uses tannins that occur naturally in the leaves and bark of certain plants. Chrome tanning, which is common in Hazaribagh, involves treating the hides with chromium sulfate then sodium carbonate or sodium bicarbonate. Some 60 percent of the chromium is normally discharged as solid or liquid waste.[274]

The hide then undergoes pre-crusting operations. Sammying, pressing the hides through heavy rollers, removes water from the hide. The hide may be split horizontally to adjust the thickness of the leather, the upper part being the most valued leather. In shaving, rotating blades of a machine smooth the rough part of the leather, generating a fine dust of leather particles.

Some tanneries will sell the resulting “wet blue” hides to other tanneries, while others continue further processing (crusting, then finishing) themselves.

2. “Crust leather”

The leather can be rechromed in order to increase its density and quality. Chromium sulfate is again used along with sodium carbonate or sodium bicarbonate. Re-tanning spreads tanning agents evenly through the leather. This can involve tanning agents, resin, vegetable tannins and other chemicals. The hide is then ready for dying with acid dyes, alkaline dyes, or various others. Ammonium hydroxide, ammonium chloride, labeling agents, synthetic tanning agents, vegetable extracts, formic acid, and acetic acid are also used.

The hides then undergo fat liquoring, which treats the hides with natural or synthetic oils. The hides then go through a setting machine to remove the wrinkles in the hide, and then drying (by hanging, in a vacuum machine, or toggling in the sun) before being trimmed and plated (or smoothed out under high pressure and heat).

Some tanneries will sell the resulting “crust leather” to other tanneries, while others continue the finishing process themselves.

3. Finishing

Leather can be finished in a variety of ways. In general, the finishing process gives a decorative and protective surface coating to the leather. Buffing in a machine smoothes the leather, often creating a fine dust. A finishing solution is applied by a spray machine or by padding. Dyes, binders, adhesives, fillers, waxes, resins, polymers, modifiers, fixatives, thinners, oils and preservative may be used in this part of the process.