Procedure of producing Chrome tanned leather is given here

1. By Samina Tannery,Hazaribag,Dhaka,Bangladesh

2. 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.
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. ChemicalWash
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
16.

3. Selection,Trimming and Weighing
 At first select raw hides/skins and put aside the
putrefied stock to treat separately
 Next we trim the unnecessary part of the hides or skins
 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
 Beume 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. Chemicals
 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.

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. Controls
 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.

10. Chemicals & Use condition
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%

11. Different facts of liming
CaO inflates the hides and skins
Normally after 3 months of Qurbani, 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
Durrani Tannery

13. Objects
Lowering the pH from 12-12.5 to 8.5-9.0
Remove the lime
Remove swelling and plumping
Depleting the pelt

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
 Ammoniumsulfate
 Carbon di oxide

16. Condition
 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

17. 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.
 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

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. Tests and Controls
 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

22. Emergencies in pickling
 If light green color shows in the pelt we have to add
Sulfuric acid from 0.1M to 0.6Mand 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

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. Basification
 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

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 is known as গরম স োডো in industries
 NaHCO3 is known as ঠোণ্ডোস োডোin industries

36. References:
1. Syed Rubel Hossain
2. Tanning chemistry: The science of leather

37. A VERY SPECIAL THANKS TO
ABDULLAH AL MAHMUD SOHEL SIR

38. Thanks to All