LPT 312: Abattoir Practices and Animal Products Technology (1+1)

CHROME TANNING

• This is the modern and quite popular technique since it yields soft, supple and strong leather, which is permeable to air.
• It also takes less time.
• It can be done by any of the two processes.
• In single bath process, basic chromium salt (chromic sulphate or chrome) is directly applied in solution to the skin in gradually increasing strength.
• In double bath process, chromium salt is formed on the fibres by interaction of chemicals.
• Usually sodium bichromate is reacted with a reducing sugar maltose and sulphuric acid to get chromic sulphate.
• It is applied in 1.5 to 3% range initially at a pH 2.8, which is then increased to pH 3.5 to increase the affinity of the collagen for the chrome.
• Chrome tanning takes places due to the formation of cross linkages between chrome ions and free carboxyl groups in the collagen side chains.
• Tanning operation is accomplished in 5-6 hours.
• Post-tanning operations are also necessary to get finished leather.
  • Setting out or wringing is done to remove excess tan liquor or moisture by passing the hide between two large rollers.
  • Splitting and shaving is done to adjust the leather thickness for the desired ultimate use.
  • Dyeing of the leather is done to produce the desired colour.
  • Fat liquoring is done to adjust the firmness or softness of the leather by lubricating the fibres with oil.
    • It also increases the tensile strength.
  • Staking refers to the softening and making the leather more pliable.
    • It decides the final temper of the product in combination with fat liquoring.
  • Buffing is done to smooth the grain surface of leather for the better appearance and to diminish the blemish.
    • Buffed leather is called corrected grain, which is otherwise called full grain.
  • Glazing is done on chromic leather after seasoning an drying.
    • A glass cylinder clamped to the end of a moving arm is made to roll on the leather.
    • Heat generated by friction softens the wax in the finish and produces a continuous plastic coat of high lusture on the grain surface.
• As the leather gets ready, it is graded for temper, uniformity of thickness, colour, etc.
• The graded leather is properly packed for convenience and to fetch a better price.
• With the use of strong tan liquid and revolving drums, the speed of tanning can be improved.
• This type of tanning is good for the manufacture of sole, belts, harness, saddlery and other heavy leathers.