Depilation or opening up the skin(liming)

the type concentration and duration of immersion of the depilation methods as well as of the liming and sulphiding chemicals influence the future properties of the leather with regard to softness and strength.
Control of liming process

operational:

1. temperature measurments, especialy when working with higher temperature (not >28 degree c ) and for enzymatic liming or sweating treatments.
2. test of the hair-slippiness.
3. check of the degree of swelling and plumping by touch test press your thumb or finger if it is pressed how much time it taken to resotre or not to restore.

Analytical
1. Determination of total alkalinity.
2. Determination of sulphide sulphur.
3. Determination of efficient alkalinity.
4. Determination of ammonia content, particularly recommended when using a recirculation liming process. If the content is too high, the liquor should be changed.
Test of the liming chemicals
Analytical
1. Determination of purity and concentration should always be performed as an incoming inspection, in particular if suppliers change, in order to ensure a uniform process.
2. Determination of the iron content of sulphides and hydrosulphides. The cheapest products tend to contain high quantities, which cause blue-black iron sulphide liming stains.
Lime painting methods
Painting on the flesh side
Good draining of the soaked raw stock and a lime paint consistency that is not too thin are necessary to obtain an impeccable wool quality without defects. Liming paste is usually prepared by making solution of sodium sulphide and lime in additionally china clay is added to viscous the paste.
Grain lime painting (drum painting, Darmstadt through feed method)
Especially suitable for sensitive hides with prominent growth marks and or very flat substance gives smooth pelts without wrinkled or loosed grain.
Liming methods
Pure hydrated lime pit depending on the raw stock hair loosening occurs in 6 10 days with good opening up of the skin. If using slaked lime check for insoluble components which can damage the grain. Old lime liquors tend to contain uncontrollable decomposition products which may result in increased microbial activity and thus cause loose grain, greater loss of skin substance or spongy leathers .however; in these old lime liquors the hair loosening effect is more intensive then in freshly prepared limes.
If the lime pit is used for reliming, the slipperiness of the pelts caused by sulphide lime liquors is reduced and thus a drifting of the skin in the splitting machine is avoided. Furthermore, it is used to improve opening up of the skin.

Pure sulphide lime pit
Mainly used for the goat skins to produce fine-pored, crack-resistant chevreaux leathers. Also used for veal skins or small cattle skins to obtain nubuk and suede leathers with a tight fibers texture. This method is also chosen for rapid hair loosening, poor opening up of the skin and to ensure the yield.
Sulphide/hydrated lime pit
Still the most commonly used liming method for leather production. Lime liquor concentration with over 0.2% Na2 S
Cause damage to the hair, more than 0.5 % Na2 S will destroy the hair (however, the amount of liquor, temperature and the ratio of hair coat to skin substance are influencing factors). If the pH value is below 11.0 the hair is not attacked or loosened up.
Any desired liming effect can be achieved by adjusting the parameters: proportioning and concentration of chemicals, amount of liquor, temperature, drumming intensity, duration and also the preliminary soaking method.
Sulphydrate/hydrated lime pit
Pure NaSH solution does not result in depilation. Only the addition of Ca (OH) 2 or NaOH achieves a similar effect to that of sulphide lime. This liming process causes a slight swelling of the pelts, reduces the accentuation of growth marks, grooves and wrinkleness. The fact that the leather are flatter and firmer has to be taken into account in the subsequent processing steps.
Sulphide/sulphydryl/compounds
A method which is gradually being used more often. Excessive lime swelling is reduced by replacing sulphide with Sulphydrate, however, only if the content of NaSH prevails. An initial addition of NaSH is also advantage.
Organic sulphydryl compounds
The most commonly used compounds are hydroxymercaptans or thioalcohols, especially 2-mercapto ethanol as alkali salt. They have a very high further oxidation speed and therefore do not contaminate the waste water and the waste water treatment plants. If they are used alone, rapid mixing in the lime pit is necessary. Due to their higher price they are mostly used with a low content of sulphide. These lime liquors lead to even less swelling than the use of Sulphydrate and thus to a good area yield.

Enzymatic liming
The chemical used are specially isolated bacterial or fungal proteinases which are also used in combination. Preliminary alkaline soaking or preliminary treatment and/or addition of activating salts such as sodium hydrogen carbonate, sodium bisulphate or sodium sulphite promote loosening of the hair. Fat residue or defective sections of skin and scars have an inhibiting effect. Reliming is necessary in most cases.
Enzymatic painting
Sprinkling with enzymatic preparation on the flesh side has proven a good method for sheep skin. Hair loosening occurs in about 18-24 hours. Reliming is necessary.