Leather Science: Tanning

  TANNING

Tanning is the process to convert putrescible hides and skins into non-putrescible material known as leather. Tanning is the process which converts the protein of the raw hide or skin into a stable material which will be suitable for a wide variety of end applications.

The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material which putrefies when wetted back, while tanned material dries out to a flexible form that does not become putrid when wetted back. Many tanning methods and materials can be used for making leather but the choice is ultimately dependent on the end application of the leather. The most commonly used tanning material is chromium, which gives the leather, once tanned, a light blue color (due to the chromium) and this product is commonly called "wet blue".The hides once they have finished Pickling will typically be between pH 2.8 and 3.2.

At this point, the hides would be loaded in a drum and immersed in a float containing the tanning liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the full substance of the hide.

Regular checks will be made to see the penetration by cutting the cross-section of a hide and observing the degree of penetration. Once an even degree of penetration exists, the pH of the float is slowly raised in a process called basification. This basification process fixes the tanning material to the leather and the more tanning material fixed, the higher the hydro thermal stability and increased shrinkage temperature resistance of the leather.The pH of the leather when chrome tanned would typically finish somewhere between 3.8 and 4.2.

SOME IMPORTANT TANNERY  FACTS

1. Batch size:

This is the quantity of leather produced by a tannery at one time. The batch size depends upon the number & size of pits, drums, paddles, machinery, manpower & raw material available. This can as follows:

· Small tannery: 25-30 hides,

· Medium tannery: 50-400 hides and

· Large tannery: 400 and above hides

Soaking to Pickling/Chrome tanning should be planned in continuous go and no holidays in between.

2. Process Duration: (For Making of Upper/Lining)

a) Up to Wet-Blue : 3 - 4 Days

b) Wet-Blue to Crust : 8 -12 Days

c) Crust to Finish : 3 - 6 Days

d) Idle Time : 3 - 6 Days

Total : 17 - 28 Days

The raw material stock should cover minimum 2 months requirement. The chemical stock should cover 3-4 months requirement.

 3. Yield factors for Upper leathers: (Approximate values)

A) 100 kg. Wet-salted Cow hide : 120 kg. Soaked weight : 90 kg. Pelt weight (unsplit) : 60 kg. Shaved weight

B) 100 kg. Wet-salted Cow hides Yields 200-250sq.ft. Finished leather

C) 100 kg. Wet-salted Cow hides Yields 10 kg. Chrome split

D) 100 kg. Sammed Chrome leather Yields 10 kg. Shavings

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