This document provides an overview of the wet blue production process for leather. It involves 15 main steps: 1) selection of raw hides, 2) trimming, 3) weighing, 4) soaking, 5) liming, 6) fleshing, 7) selection, 8) weighing, 9) chemical wash, 10) de-liming, 11) bating, 12) pickling, 13) chrome tanning, 14) basification, and 15) draining and fixing. Key goals of each step are described, along with chemicals used, parameters to monitor quality like pH and swelling, and best practices. The end result is wet blue leather that is fixed and ready for further processing.

1. Production of Wet Blue
Cow Hides
Leather technology
Dayalbagh Education Institute
(Dayalbagh , Agra)

2. Flow Chart of Wet Blue Production
1. Selected raw stock
2. Trimming
3. Weighing
4. Soaking
5. Liming
6. Fleshing
7. Selection(if required)
8. Weighing
9. Chemical Wash
10. De liming
11. Bating
12. Pickling
13. Chrome tanning(with or without pre tanning)
14. Basification
15. Drain out the chrome liquor and pile up for 4-7 days for completion of
fixation and polymerization reactions
Leather: Leather is a durable and flexible material created via tanning from putrescible raw hides
and skins, primarily from cattle hide.
Tanning is the process of controlled denaturation of protein of the raw hides and skins.

3. Selection,Trimming and Weighing
1. At first select raw hides/skins and put aside the putrefied stock to treat
separately
2. Next we trim the unnecessary part of the hides or skins
3. After that measure weight, because we use chemical till fleshing based on
this weight.

4. Soaking
It is a process of rehydration of preserved raw hides or skins
Quality Control
pH Check-9.0 to 9.5(if pH is 10,no problem)
Cross Section Check-Take cross section, if the entire part feel wet
uniformly then soaking has done.
Duration-22to24 hours in industry. Agitate the drum for 10 minutes interval
of every hour
Different checks-1.Two hour after soaking, touch the hides with hand,if it
feels softy then its ok.
2.Drop the soaked hide from high place,if it falls down in a place
uniformly then its ok.
3.If flesh side become slippery then soaking has done

5. We use NaCl to remove Hyaloglunic acid
Baume check- Conc.check of salt. It's measured during soaking and
pickling(sometime in liming).For soaking the value is 3-4(in several tannery
they keep 2-3)
If the pH goes over 11,it is called immunization of soaking. In this case hair
gets fixed in its root and cause problem in liming process.

6. Soaking

7. • Soda Ash or Na2CO3- It increases the pH & works as bleaching agent. It helps to keep pH
within 6.5 to 7.0 of raw h/s
• Caustic Soda or NaOH- It works as Soda ash
• Wetting agent-It wets back wet or dry salted leather during soaking. Some wetting agents are:-
1.LD600(buckman) 2.Lissapol EF(ACI) 3.Eusapon FB(BASF) 4.Sinteral BD(Alpha)
5.Metapol WA 6.7794
• Bactericide- It prevents bacteria's growth in raw h/s
• Soaking Enzyme- It works as catalyst to return the wet condition completely in h/s just like
green condition and also remove loose plunk which grows during mechanical agitation of
soaking.It also assists to control the pH in optimum level.
Chemicals

8. Liming-Liming is the treatment of soaked h/s with milk of lime(CaO) with or
without addition of Na2S to remove the hair from the grain surface of the h/s.
Objectives
 Remove the hair from h/s
 Swelling & plumping the collagen fiber
 Split up the fiber structure at the fibril bundle layer
 Remove the non collagenous substances
 Remove inter fibrillary cementing materials
 Emulsify fats and greases
 Opening up more reactive site for tanning
 Hydrolysis the peptide bonds, amide side chains, guanine side chains
 Removal of dermatan sulfate

9. pH-12 to12.5
Cross section test-Drip phenolphthalein in the cross section of limed pelt, if deep pink
color appears, liming is confirmed.
Layer- If double layer appears in the cross section of limed pelt, then liming has done.
Swelling & Plumping- If the pelt uniformly distended then that’s plumping, it needs
8000lb pressure to bring the water from inside. If the pelt inflate in different place that’s
called swelling, it requires 48000lb pressure to bring out the water.
Duration- 24 hours in industries, but it varies in laboratory.
Hair Slip test-Before soaking we should test hair slipness, if hair slips then the raw stock
is putrefied.
Controls

10. Chemicals
1. Sodium Sulfide (Na2S)
2. Calcium Oxide (CaO)
Conditions
 If the hide has great thickness and area
Sodium Sulfide (Na2S)-3.5%
Calcium Oxide (CaO) -3.25%
 If the hide has mediocre thickness and area
Sodium Sulfide (Na2S)-3.5%
Calcium Oxide (CaO)-3.5%
 If the hide is thin and small in size
Sodium Sulfide (Na2S)-3.5% to 3.75%
Calcium Oxide (CaO)-4.0%
 In case of goat skin(Any size and thickness)
Sodium Sulfide (Na2S)-5.0%
Calcium Oxide (CaO)-7.0% to 7.5%
Chemicals & Use condition

11. Different facts of liming
 CaO inflates the hides and skins
 Normally after 3 months the h/s got Wrinkle, after that h/s got Jam in belly
portion
 In liming we have to add water in afternoon
 If the stock got Wrinkle we have to take water less than the stock equal to two
joint segment of finger, if the stock got Jam then we have to add water more than
three joint segments of finger.
 We should add reactive chemicals in three steps in 30 minutes interval or
applicable time interval

12. Piled up limed pelt

13. Objects
Lowering the pH from 12-12.5 to 8.5-9.0
Remove the lime
Remove swelling and plumping
Depleting the pelt
Deliming: It is an intermediate process between liming and
pickling

14. Quality Control
 pH- 8.2 to 8.8
 Cross section test-Drip phenolphthalein in the cross-section,
if light pink color appears or the pelt become colorless
deliming has done
 After deliming we press the pelt by hand, if it feels soft, then
it does mean the protein inside pelt has prepared for fixation
reaction

15. Deliming Chemicals
We can use anything which has ability to reduce pH, but the
popular chemical is NH4Cl
Water
Hydrochloric acid
Sulfuric acid
Boric acid
Lactic acid
Formic acid
Acetic acid
Ammonium sulfate
Carbon di oxide

16. If the pelt is thick and large
(NH4)2SO4- 2.75% to 3.0% (duration-75 mins)
Medium thick and medium size pelt
(NH4)2SO4- 2.5% (duration-75 mins)
Less thick and small pelt
(NH4)2SO4- 2.0% to 2.25% (duration-50 min-60min)
For all types and sizes goat and sheep skin
(NH4)2SO4- 1.5% (duration- 45 min)
If bath falls down, upload the pelts in the drum, add 80% water and 0.25%
(NH4)2SO4 then run the drum for 10 minutes
Condition

17. Purpose: Peptizing the fiber structure and eliminate the alkali swelling with the aid
of specific enzyme.
Tests
pH check- 7.5 to 8.0
Cross section test- Drip phenolphthalein in the cross section of the pelt and if it
turns into completely colorless, then bating has finished.
Thumb test- Press the bated pelt with thumb, if the left a print their that means
bating has done
Bubble test- It basically does for goat & sheep skins as those have more compact
weaved structure so it requires both acid and basic bate. We’ll wrap the bated skin if
bubble comes out then it’s ok
Bating : Depending on the use of leather we treat h/s with enzymes to soften
them, this process is done after deliming is called bating.

18. Bating-things to keep in mind
Hides bating depends on the final stage of the deliming
If light pink color exist after deliming and the pelt remain light
soft and thick duration of bating 60 minutes
If the pelt turn into colorless and become too soft then the
duration of bating is 45-50 minutes
In case of sheep and goat skin, alkali bating is done prior to acid
bate. Add 0.2% HCOOH in the alkali bate and revolve 10
minutes then start the acid bate and the duration will be 90
minutes

19. Enzymes
Theoretically there are three types bating enzymes. They are:-
1. Weak bating enzymes- 500 to 1000 LVU
2. Mid strong bating enzymes- 1000 to 1500 LVU
3. Strong bating enzymes- 1500 to 2000 LVU
 Practically the available enzymes are mixture of the upper classified enzymes.
There are two types available commercially, they are:-
1. Alkali bate
2. Acid bate
 Available enzymes are-
1.Oropon K (TFL, Alkali bate)
2.Buzyme 146 (Buckman, Alkali bate)
3.Nissabate EF (Bantage, Alkali bate)
4.Tripsol D (Alkali bate)
5.Oropon BRS (TFL, Acid bate)
** LVU- Lohlein Volhard Unit

20. Pickling
The treatment of delimed or bated pelt with a solution of acid and
salt is known as pickling. Recently it’s been strongly supported that
by proper adjusting of salt & acid in this operation a large amount
of hydrolysable tannin can be easily reduced.
Objects:
Preserve the pelt for further treatment
Acidify the pelt, the pH bring down from 7.8 to 2.8
Oxidize hair root and remove if hair remains after liming
Remove all swelling and plumping
Produce softer, thinner white pelt
Reduce astringency of chrome tanned leather

21. • pH- 2.6 to 2.9
• Cross section test- CS area+BCG =yellow color (if yellow color
appear pickling done)
• Beume Check- In pickling the range for beume will be 7-8
Tests and Controls

22. • If light green color shows in the pelt we have to add Sulfuric
acid from 0.1M to 0.6M and run 30 minutes
• If cow hides, the volume of H2SO4- 1-1.2%
• If goat skins, the volume of H2SO4- 0.8%
• Run 3-4 hours after adding H2SO4 in cow hides and run 1
hour for sheep or goat skins
• If pickling bath falls down, load the stock in drum+ 80%
water+ 7.5% salt+ 0.5% HCOOH and run additional 10
minutes
Emergencies in pickling

23. Salts & Acids
Salts
1. Common salt – NaCl
2. Sodium Sulfate – NaSO4
3. Sodium Formate - HCOONa
4. Sodium Acetate – CH3COONa
Acids
1. Sulfuric acid- H2SO4
2. Formic acid- HCOOH

24. Pickled pelt

25. Pretannage
Which pelts are normally white those are chrome tanned and pre
tanned simultaneously after pickling. The pre tanning chemicals are
prepared from replacement synthetic tans. In this step the pH value
is 3.4-3.6

26. Chrome Tanning
Control
pH check- 3.7 to 4.0
CS test- CS area+BCG= If Reddish yellow turns into green, then tanning has
ended
Penetration check- Take CS of wet blue leather if color is uniform through all
the parts then the Cr(OH)SO4 has penetrated into the pelt.
Surface tanning is known as case hardening, it’s a defect. If pickling is
defective then it happens

27. Chrome tanning process
These are added to pickle bath
1. 4.0% Cr(OH)SO4 (run for 30 min)
2. 4.0% Cr(OH)SO4
3. 1.0% HCOONa
4. 0.5% Carbonyl sulfide (OCS) as chrome stable fat

28. Piled up pelts for fixation reaction

29. Wet blue leather

30. It’s done to fix up the Cr salt in the hide protein
In industry MgO is used as basification agent as it reacts slowly than NaHCO3
thus basicity of the bath get increased gradually. That’s why chromium
molecules get fixed up inside hide protein properly and uniformly
Basification

31. Advantage of Basification
• Increase strength properties
1. Tensile strength- 200 kg/cm2
2. Stitch tear strength- 120-180 kg/cm2
3. Ball burst strength- 20 kg/cm2
• Increase soften property

32. Miscellaneous
• Soaking & liming is done in same drum
• Deliming and bating is done in same bath
• Chrome tan is accomplished in other drum
• One day prior to soaking Cao & Na2S keep mixed
• Enzyme prevents hides from forming loose plunk and reduce the spots sculptured
during mechanical action
• Water level measurement for industry is
1. Equal to hides level= 100% water
2. 1 segment of finger joint less= 80% water
3. 1 segment of finger joint up = 200% water
4. 2 segment of finger joint up = 300% water
• Cross section test is the major and confirmatory test for every steps except goat or
sheep skin require to perform bubble test in bating
• Soda Ash
• NaHCO3

36. Mr. G.P Sharma
A VERY SPECIAL THANKS TO

37. Thanks to All
Govind Mishra