Liming (leather processing)

In Liming

Liming

Liming is the application of calcium and magnesium rich materials to soil in various forms, including marl, chalk, limestone, or hydrated lime. This neutralises soil acidity and increases activity of soil bacteria. However, oversupply may result in harm to plant life...

for parchment

Parchment

Parchment is a thin material made from calfskin, sheepskin or goatskin, often split. Its most common use was as a material for writing on, for documents, notes, or the pages of a book, codex or manuscript. It is distinct from leather in that parchment is limed but not tanned, therefore it is very...

 or leather

Leather

Leather is a material created through the tanning of hides and skins of animals, primarily cattlehide. The tanning process converts the putrescible skin into a durable and versatile material....

 processing, the hides are soaked in an alkali solution by a drum/paddle or pit based operation where four main objectives are met. The obectives are :

• Removal of interfibrillary proteins .
• Removal of Keratin proteins.
• Collagen swelling due to the alkaline pH.
• Collagen fibre bundle splitting.

Liming operations of cattle hides usually last 18 hours and are generally associated with the alkaline phase of beamhouse operations.

In Liming

Liming

Liming is the application of calcium and magnesium rich materials to soil in various forms, including marl, chalk, limestone, or hydrated lime. This neutralises soil acidity and increases activity of soil bacteria. However, oversupply may result in harm to plant life...

for parchment

Parchment

Parchment is a thin material made from calfskin, sheepskin or goatskin, often split. Its most common use was as a material for writing on, for documents, notes, or the pages of a book, codex or manuscript. It is distinct from leather in that parchment is limed but not tanned, therefore it is very...

 or leather

Leather

Leather is a material created through the tanning of hides and skins of animals, primarily cattlehide. The tanning process converts the putrescible skin into a durable and versatile material....

 processing, the hides are soaked in an alkali solution by a drum/paddle or pit based operation where four main objectives are met. The obectives are :

• Removal of interfibrillary proteins .
• Removal of Keratin proteins.
• Collagen swelling due to the alkaline pH.
• Collagen fibre bundle splitting.

Liming operations of cattle hides usually last 18 hours and are generally associated with the alkaline phase of beamhouse operations. Historically, the liming pits were used and were one the longest operations.

Removal of interfibrillary proteins

• The interfibrillary proteins are denatured
  Denaturation (biochemistry)
  Denaturation is a process in which proteins or nucleic acids lose their tertiary and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat. If proteins in a living cell are denatured, this...
  
   by the presence of alkali (particularly sodium sulfide), rendered soluble, facilitating their removal from the leather. Removal is done by the mechanical action of liming or reliming, but more prominently when the pelt is deswelled (during deliming). Failure to remove these proteins results in a hard, tinny leather (due to fibre glueing upon drying) that is brittle and inflexible.

Keratin removal

• Keratin
  Keratin
  Keratins are a family of fibrous structural proteins; tough and insoluble, they form the hard but un-mineralized structures found in reptiles, birds, amphibians, and mammals...
  
   that is present in the hair, scales and in the epidermis of the skin is hydrolyzed
  Hydrolysis
  Hydrolysis is a chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions, which may go on to participate in further reactions. It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization...
  
   in the presence of alkali
  Alkali
  In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Alkalis are best known for being bases that dissolve in water. Bases are compounds with a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base, especially for...
  
   (at pH values greater than 11.5). The disulfide
  Disulfide
  In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. Disulfide usually refer to a chemical compound that contains a disulfide bond, such as diphenyl disulfide, C₆H₅S-SC₆H₅.-Inorganic vs. organic...
  
   bridges found in keratin protein is cleaved but can reform. Long periods of liming will result in hair removal. The main removal of keratin is performed using the unhairing operation. In traditional processing liming/unhairing
  Unhairing
  In the tanning industry the unhairing stage concerns the removal of animal hair from the skin by chemical burning of the hair root, or by chemical degradation of the hair shaft. The type of hair removal depends on the type and length of the hair itself. Two main groupings of hair removal exist:...
  
   was indivisible and took place at the same time. Modern liming methods, and in particular, the processing of sheepskins the hair is removed first and then limed in a liming drum. In hair-save technology, the hides are unhaired first and then limed for a further 12-18 hours.

Alkaline collagen swelling

• The presence of calcium hydroxide
  Calcium hydroxide
  Calcium hydroxide, traditionally called slaked lime, hydrated lime, slack lime, or pickling lime, is a chemical compound with the chemical formula Ca₂. It is a colourless crystal or white powder, and is obtained when calcium oxide is mixed, or "slaked" with water...
  
   results in the alkaline swelling of skin. The result is the influx of water into the hide/skin and a marked increase in fibre diameter and fibre shortening. The thickness of the skin increases, but the surface area of the pelt decreases. The weight increase, due to the uptake of water results in a doubling of the hide/skin weight. But, this weight also needs to take into consideration that proteins (especially the hair) have been removed and the fleshing operation is often performed after liming.

Collagen fibre bundle splitting

• The action of liming, in particular the swelling of the skin results in the splitting of the fibre bundle sheath. Due to the fibre diameter increasing, the bundle sheath cannot contain the thicker fibres and it bursts open. This allows increased access to the fibres which allows better tanning, retanning, dyeing and fatliquoring.