Liming

1.0 Purpose

1.1 Unhairing to remove hair and epidermis
1.2 Liming to act upon the skins' epidermis, so that the skin may be properly unhaired
1.3 Liming to partially saponify skin fat and grease
1.4 To put hides and skins in proper chemical condition for liming
1.5 To swell and rehydrate skin protein to desired extent

2.0 Sweating

2.1 Oldest method
2.2 Skins are kept in dark warm room at 21-27 degrees C. bacteria are allowed to
proliferate, and they attack the region between the papillary layer and the epidermis first.
2.3 Hairs are picked off as possible - normally 20 to 40 hours
2.4 This process always damages the leather somewhat
2.5 Used for dewooling sheepskins when the wool is valuable e.g. Merino type
2.6 Essentially uncontrolled enzyme process

3.0 Enzyme

3.1 A Development of sweating
3.2 Antiseptic (NaClO2) controls unwanted bacteria
3.3 For sheepskins use 2% enzyme sprayed on the flesh
3.4 For hides use 1% in pits
3.5 Enzyme attacks the keratin
3.6 Optimum temperature for enzyme reaction is close that for bacteria in sweating
3.7 Advantage with effluent disposal is the absence of sulphide
3.8 There is no swelling, thus if tanned directly will give a hard leather
3.9 It is difficult to prevent drawn grain in tannage

4.0 Dimethylamine

4.1 Gives smooth grain
4.2 No sulphide in effluent
4.3 Temperature control is important
5.0 Oxidative
5.1 Oxidation of disulphide bond by chlorite
5.2 No sulphide in effluent

6.0 Reducing Agents

6.1 Cyanides - poisonous and disliked
6.2 Arsenic sulphide used to be used. Gives no pH increase and consequent swelling
6.3 Arsenic sulphide gives calcium hydrosulphide which is milder than sodium hydrosulphide
6.4 Sodium sulphide crystal 30% conc. while flake (more common) is 60% conc.
6.5 Sodium hydrogen sulphide
6.6 Na2S + H20 goes to NaOH + NaHS
6.7 As pH increases unhairing becomes faster.
6.8 A high pH needs no sulphide. From a low pH + Na2S gives NaOH creating a high pH.
6.9 The power of the reducing substances to increase the unhairing rate is primarily
dependent upon the OH concentration
6.10 Red arsenic is not soluble in water. It is dissolved in lime.

6As2S3 + 21 Ca(OH)2 goes to 3Ca(SH)2 + 4Ca(AsO3)2 + 2Ca3(AsS3)2 which goes to
6Ca(SH)2 + Ca2(AsO2)2 + 6Ca(OH)

7.0 Painting

7.1 Hair saving process - used with sheep etc.
7.2 Decrease water content after soaking to prevent swelling during painting (60%)
7.3 Paint on flesh side; use fibre brush (or spray)
7.4 Mixture; 1 part sulphide. 4 parts lime, 12 parts water
7.5 Na HS, CaCl2, Kaolin sometime used.
7.6 If trying to obtain wool fold flesh inwards down the backbone and stack - normally overnight
7.7 Used with calf - if hair is removed before liming growth marks are reduced.
7.8 Wool is removed over a beam - by knife or by hand - and graded
7.9 Shorter hairs are often not removed, and the subsequent liming is used for this
7.10 There is less damage to the skins than sweating
7.11 less wool is obtained than with sweating.

8.0 Flat Liming

8.1 Dropping good into pits
8.2 Handling and allowing to drip down (said to give leather better grain in finished leather)
8.3 Plunge liquor and redyke
8.4 See goods are thoroughly wet when 1st dyking
8.5 Can be used to burn hair off bull hide

9.0 Sole Leather Hair Saving Process

9.1 2.5% lime, 0.1% sulphide, hide:water 1:6, seven days. This is a two or three pit process, using old liquors.

10.0 Hair Burning Process

10.1 3% lime, 2% sulphide, 1% hydrosulphide, for about 2 hours

11.0 Methodology Notes

11.1 Tanners prefer to pile hides overnight after liming
11.2 preserving hair uses 0.1% sulphide
11.3 Destroying hair uses 1.0% sulphide or over
11.4 Rockers - prevent pockets of bacteria
11.5 Paddles - not run continuously or grain would be damaged
11.6 Drums - 1-3 rpm maximum
11.7 Paddles and drums allow low float
11.8 An increase from 16 to 27 degrees C will halve the time required for loosening hair: more significantly it will double the rate of the solution of collagen.