The document summarizes the key steps in the hides and skins tanning process: 1. Beamhouse work includes soaking, liming, unhairing, fleshing, and deliming to prepare the hides. 2. Soaking rehydrates the hides and removes impurities. Liming uses chemicals to separate the hair and epidermis from the collagen. 3. Fleshing removes remaining flesh and fat. Deliming removes residual lime in preparation for tanning.

1. HIDES & SKINS TANNING

2. Beamhouse work
• The operations that take place in the
beamhouse include: soaking, liming &
unhairing, fleshing, deliming & bating
• Beamhouse work requires 18-24hrs

3. SOAKING (WASHING)
• The first process in tanning consists of
soaking the H/S in water
• The aim is to allow the hide to reabsorb
any water that was lost after flaying, curing
& transport
• Soaking also removes blood, dirt, dung,
preservation salts and soluble proteins

4. • Absorbed water rehydrates any dried inter-
fibrillary protein, loosening its cementing
action on the fibres
• Collagen fibres, keratin cells of the hair &
epidermis also take up water & become
more flexible

5. • Wet salted hides may be soaked for 8-20hrs
(10-16oC) while dried hides take several
days
• The amount of water used ranges from 3-5
times the weight of hides & 6-7 times for
dried skins
• Satisfactory soaking is judged by
cleanliness & absence of salt (determined in
juice squeezed out of the skin, using a
pocket refractometer)

6. • Soaking is not a simple process since
putrefying bacteria act as soon as there is
surplus water or curing agent is washed
out
• Common additions to the soak liquor as
disinfectants (bactericides) are:
Sodium hypochlorite & trichlorophenate
(or formic acid & pentachlorophenate)

7. • During soaking water uptake is speeded up
by:
1. Mechanical action (rocking frames, paddles
or rotating drums)
2. Temperature (the soak is warmed up to
38oC); If temperature exceed 38oC bacteria
action increase, protein fibers shrink
&/or gelatinize.

8. 3. Chemical additions
(i) acid addition – when wool or
hair is kept on (1-2 parts of
formic, hydrochloric or sulphuric
acid per 1,000 parts of water at
60oC)

9. (ii) alkali addition – more common as it
loosens hair (1-3 parts of caustic soda,
soda ash, washing soda or borax per
1,000 parts of water)
Sodium sulphide, sodium polysulphide or
ammonia liquor may also be used

10. • If too much acid or alkali is used the skin
surface will rapidly absorb it & swell to the
extent of distorting surface – loose grain
on leather.
(iii) 3% Salt solution dissolve unwanted
inter-fibrillary protein, thus speeding up
soaking.

11. (iv) Wetting agents (Detergents): 1-2
parts per 1000 parts of water particularly if
hides are greasy
(v) Enzyme preparations (proteolytic
action on
interfibre proteins)

12. UNHAIRING & LIMING
• Unhairing may be considered to be an
extension of soaking
• The purpose of unhairing is to separate the
two structural proteins keratin & collagen
• Unhairing removes hair, epidermis & to some
degree the inter-fibrillary proteins, preparing
the skin for the fleshing process

13. • There are 2 methods for unhairing:
1. Methods based on destruction or
modification of the epidermal tissue
surrounding the hair so that it can be
loosened & removed
2. Methods that attack the hair itself to
destroy its structure (use alkali –
Ca(OH)2, NaOH, NaHS or Na2S)

14. • The first group of methods include:
- sweating [soaked skins hung up in a dark
humid room (22-27oC), to allow bacteria to
attack keratin cells of hair & epidermis]
- enzyme unhairing – enzymes that
preferentially attack cells at the base of
hair roots or epidermis are available

15. - paint unhairing – after soaked skins have
drained, ‘paint’ made from hydrated lime &
sodium sulfide is painted or sprayed – it
attacks keratin cells

16. • Liming chemicals either destroy the hair &
epidermis completely or loosen them to
the extent that they can be removed
mechanically without difficulty
• Liming with lime and sodium sulfide takes
12-18hrs
• Depending on Na2S conc. the hairs are
either completely destroyed or only
partially

17. Fleshing:
• After dehairing, any subcutaneous
connective tissue, flesh and fat adhering to
the flesh side is removed mechanically
using a fleshing machine (or hand fleshing
- two handed knife )
• After fleshing the hide/skin is now called a
‘pelt’ (the pelt wt. form the basis for
calculating amounts of tanning materials
needed)

18. Trimming & Splitting:
• After fleshing any loose, raggy ends of skin
are trimmed (or cobbled) by a hand knife
from the pelt
• Then the pelt is split horizontally on the
splitting machine to a grain split of uniform
thickness & one or more flesh splits of
varying thickness
• Splitting may also be done after chrome
tannage

19. Deliming & Bating:
• These are done in the same
vessels(milling drums, mixers etc.) that are
to be used for tanning

20. Deliming:
• After unhairing, the lime in the skin is no
longer needed & if left it has detrimental
effect on subsequent tannage
• With chrome tanning, the lime give a hard
green inflexible leather & prevents
proper tannage.

21. • With vegetable tanning, the lime slows
down or reduces tannage & gives a dark
colour
• Deliming can be done through washing or
by use of chemicals

22. Deliming by washing:
• The skins are put in a paddle or drum & run
with continuous flow of cold clean water
• Undissolved lime on the surface & some
dissolved lime between the fibres is quickly
removed
• The lime that is chemically held by the fibres
(about 0.4% of the skin wt) is removed very
slowly necessitating prolonged washing

23. • Prolonged washing may lead to lime blast
if the water used is hard (soluble Ca or Mg
bicarbonates or carbonic acid react with
lime to precipitate CaCO3)
• Prolonged washing allows further alkali
breakdown of the skin giving loose
leather particularly if the water is warm

24. Chemical deliming:
• The loose lime is removed by quick washing
followed by addition of controlled amounts of
weak acids or acid producing salts to
neutralize the alkali
• Too much acid damages the skin by causing
violent swelling & dissolving protein, therefore
weak acids are used

25. • Weak organic acids used include: Boric,
lactic or acetic acid
• Acid salts that can be used include:
sodium bisulphite, salts of weak alkalis
such as ammonium chloride or sulphate
• The lime is converted into readily soluble
salts that are rinsed out (deliming subsides
the alkali swelling caused by the liming
chemicals)

26. • Deliming is tested by dropping a few drops
of phenophthalein onto a cross-section of
the pelt (if the process is complete –
colourless; if not it becomes red)
• Delimed pelts must be taken to the next
process immediately since putrefying
bacteria can cause a slimy feel & loose
leather with damaged structure will result

27. Enzyme Bating:
• This is additional removal of protein
material loosened by liming using
enzymes
• Enzymatic bating agents are used which
attack non-structured collagen & non-
collagenic proteins in a controlled manner
to make them soluble

28. • Bating opens up the pelt for the ensuing
tannage, improves softness, grain
elasticity & colour levelness of the leather
• Intensive bating does not impair tensile
strength of the leather & may even
improve the tear strength by increasing
elasticity of the fibres

29. • Originally enzymes used for bating were
pepsin & trypsin from dog dung & fowl
droppings (cause soft smooth & silky
grain)
• Today two main types of bates are used:
1. Pancreatic bates – digestive enzymes
from the
pancreas
2. Bacterial bates – digestive enzymes of
bacteria

30. • Enzyme bates are prepared under sterile
conditions & mixed with fine wood flour &
NH4 salts (sulphate or chloride)
• NH4 salts keep the pH at best level for the
enzyme action
• Alkali bating is done at pH 8.0-8.5 at 37oC
with 1-2% powdered enzyme added

31. • Short bating time (i.e. 1hr) lead to slight
flattening of the grain & increase flexibility
of leather
• Long bating time lead to extreme stretch &
suppleness
• Bated pelts could be scudded if it is
essential to produce very clean grain (hair
roots, fat, epidermis & unwanted protein
are squeezed out)

32. • Acid bates work at pH 4-5 & these give
rise to softer leather because at these pH
the fibres are not swollen & allow easier
access of enzyme to the fibre structure

33. Tannage
• Converts the unstable natural fibre network
of the hide collagen into a stable material
(leather)
• The tanning materials react with the
polypeptide chains of the collagen, the
individual tanning material particles each
forming several bonds with the different
reactive groups of the collagen molecules.

34. Tanning Mechanisms

35. • The collagen molecules become cross-linked
and strengthened both individually &
collectively leading to great stability
• Differences of leather from untanned hide:(i)
Increased resistance to hot H2O
(ii) Resistance to microorg. when wet
(iii) Less swelling in H2O
(iv) Better shape retention
(v) Dries leather-like (i.e. not ‘bony’)

36. • Tannage can be effected by means of very
different types of tanning material which
react poly-functionally with the hide
collagen in different ways
• Therefore there is a vast array of tanning
methods and materials

37. Different methods of tanning
1. Vegetable Tanning:
Extracts from certain plants (leaves, barks,
roots, fruits) are used to convert skins into
leather
Vegetable tanned leathers have a
characteristic colour (vary from pale yellow-
brown to intensive red-brown depending on
the type of vegetable tanning material used &
conditions applied); the colour darkens in
daylight

38. • Vegetable tanned leather is used for making
soles, sandals, straps, belts, upholstery,
bags, decorations, shoe lining & book
binding leather etc.
• Characteristics of vegetable tanned leather:
(i) good strength
(ii) shape retention
(iii) low heat conductivity

39. 2. Mineral Tannages
• Basic salts of chromium, zirconium &
aluminium are used
• Very soft leather is produced compared to
vegetable tanning
• Leather colour is white for aluminium &
zirconium but pale green or blue for
chromium tannage
• Little change of colour in daylight
• Chromium tan does not wash out but Al does
wash out
• Alum tans are used for gloves or fur-skins

40. Chrome tanning:
• The limed, delimed, bated & pickled pelt is
converted into leather by treating with
chrome sulphates & basifying agents
• Chrome leather is highly resistant to hot H2O
& boiling
• The most important process for production of
shoe upper leather, furniture upholstery &
clothing leather

41. • Advantages of chrome over vegetable tannage
a) Chrome leather has favourable properties
(high mechanical strength, outstanding
dyeing properties, absolute light fastness)
b) Ideal as semi-finished product (wet blues)
with good storage stability for subsequent
processing into all types of leather
c) Heat resistant (can withstand modern drying
&
steaming processes used in finishing & shoe
manufacture)

42. d) Chrome tannage offers wide scope for
streamlining
e) Less capital is required because of quicker
turn-over of raw materials & more modest
space requirements
f) Production times are shorter & tanning material
costs lower
Note: ‘semi-chrome’ tannage refers to complete
vegetable tanning followed by retannage with
chrome
‘chrome-retan’ is a full chrome tannage

43. 3. Synthetic tans (Syntans):
• Include substances which were manufactured
to replace partially or completely, the natural
vegetable extracts, to accelerate production
& make it cheaper. They may be of various
chemical structures
• Syntans give leather that is paler than veg
tans, may darken in daylight, does not
washout with water
• Uses are white leathers & specialty leathers

44. 4. Aldehyde tannage:
• Uses formaldehyde or gluteraldehyde
• Produces soft white leather that does not
wash out
• The leather is water absorbent
• Used for water washable gloves & clothing

45. 5. Oil tannage:
• Cod liver oil is used
• Produce light, soft, air-permeable leather
that is resistant to washing
• The leather is pale yellow that bleaches in
sunlight
• Used for wash leathers & washable gloves

46. Other tanning processes:
1. Oil & sulphur tannage
2. Isocyanate tannage
3. Sulphochloride tannage
4. Quinone tannage

47. Dyeing
• Leather dyeing is a transition process
between tanning & finishing
• Some leathers require penetration of
dyestuff into collagen network while others
need surface application
• Synthetic dyestuffs are used & any colour
shade can be applied

48. • Leather is dyed discontinuously in rotating
vessels (drums or Y-shaped dyeing units)
or continuously (through-feed dyeing,
dyeing by roll coating or spray dyeing)
• Anionic or cationic dyestuffs are also
available

49. Fatliquoring
• The last operation in the aqueous phase of
tanning before drying
• The process involves the introduction of oil
into the leather so that individual fibres
become uniformly coated
• The oil should be emulsified to ensure
uniform distribution over the leather (soap is
normally used to emulsify)

50. • This is the process that determines the
mechanical & physical properties of the
leather
• Leather dried before or with little
fatliquoring becomes hard & bony; such
leather may crack in use
• Over lubrication result in excessive
softness & raggy leather

51. • Fatliquoring separate the fibres in the wet
state so that they do not stick together too
much during drying
• It gives the leather softness, suppleness
& handle required for a particular
application

52. • Improves tensile strength, water
permeability & wetability of the leather
• Improper fatliquoring can lead to poor
adhesion of the finish coats & also impair
the adhesion of shoe soles

53. Finishing
• This refers to further processing of the
leather after drying
• The aim of finishing is to suit the fashion
demands of the consumer with regard to
colour, surface effect etc.

54. • Involves treatment with dyestuff solutions,
pigment preparations, top coats & lacquers
or by means of mechanical treatment (e.g.
plating, embossing or dry drumming)
• The poorer the raw material the more
complex the finishing treatment required

55. Finishing of different grades leathers

56. Application of finish products:
• Curtain coating
• Padding
• Spraying
• Roll coating

57. Impregnation:
• Could mean waterproofing of leather or
tightening the grain
• If leather is given a relatively heavy finish,
the result is frequently loose grain or a frizzy,
unnatural grain break

58. • Loose grain is corrected by impregnation
with preparations of acrylic or
polyurethane dispersions or with
polycyanate prepolymers
• The impregnating products penetrate deep
into the leather & stick the grain layer to
the reticular layer beneath it.

59. Other special processes/finishing
effects include:
• Glazing
• Polishing
• Coating
• Special effects
• Tipping

60. Waterproofing:
1. Closed waterproofing
2. Open waterproofing