Sheep Skin
Subject: Chemistry | Topics:


Sheepskin is used to produce sheep skin leather products and soft wool-lined clothing or coverings, including diplomas, gloves, hats, footstools, automotive seat covers, baby and invalid rugs and pelts. In particular, sheepskin is the principal material used to make ugg boots (a type of sheepskin boot), footwear traditionally produced in Australia and New Zealand. Sheep skin numnahs, saddle pads, saddle seat covers, sheepskin horse boots, tack linings and girth tubes are also made and used in equestrianism.

The fleece of sheep skin has excellent insulating properties and it is also resistant to flame and static electricity. Wool is considered by the medical profession to be hypoallergenic.

Testing at the Royal Melbourne Hospital and the CSIRO Textile and Fibre Technology Leather Research Centre confirmed the advantages of medical sheepskin in the prevention and treatment of pressure ulcers.

At the heart of the leather making process is the raw material hides and skins.As the largest organ of the body of mammals, the skin is a complex structure, providing protection against the environment and affording temperature con­trol, but it is also strong enough to retain, for example, the insides of a one tone cow. Skin is primarily composed of the protein collagen and it is the properties and potential for chemical modification of this protein that offer the tanner the opportunity to make a desirable product from an unappealing starting material. It is part of the tanner’s job and skill to simplify or purify this starting material, allowing it to be converted into a product that is both desirable and useful in modern life.

Collagen is a generic name for family of at least 28 distinct collagen types, each serving different functions in animals, importantly as connective tissues. The major component of skin is type I collagen: so. Unless otherwise specified, the term ‘collagen’ will always refer to type I collagen. Other collagens do feature in leather making and their roles are defined later.

Collagens are proteins, i.e. they are made up of amino acids. They can be separated into α-amino acids and (β-amino acids). Each one feature a terminal amino group and a terminal carboxyl group, which become involved in the peptide link (see below), and a side chain attached to the methylene group in the centre of the molecule. When the amino acids are linked together to form proteins, they create an axis or ‘backbone’ to the polymer, from which the side chains extend. It is the content and distribution of the side chains that determine most of the properties of any protein. In the case of collagen, it is the side chains that largely define its reactivity and its ability to be modified by the stabilizing reactions of tanning, when leather is made. In addition, the chem­istry of the backbone, defined by the peptide links, offers different reaction sites that can be exploited in some tanning processes.

All the common amino acids are found in skin or skin components. There are two notable aspects of the amino acid content of collagen. Hydroxyproline (Figure 1.1) is almost uniquely present in collagen compared to other proteins and, therefore, offers the basis of measuring the collagen content in any skin or skin derivative. Tryptophan (Figure 1.2) is absent, therefore making collagen deficient as a foodstuff.

In terms of leather making, some amino acids are more important than others, since they play defined roles . The roles of importance are either in creating the fibrous structure or involvement in the processing reac­tions for protein modification. Other amino acids, not included in Table , are important in defining the properties of the collagen, but play less defined roles in the leather making processes.

Amino acids create macromolecules, proteins such as collagen, by reacting via a condensation process: the amide or peptide link is in bold:


The condensation reaction can be reversed by hydrolysis, by adding the ements of water. Clearly, hydrolysis as set out in this chemical equa

cannot be fast, nor does the equilibrium lie to the left, otherwise the protein would be unstable and useless as the basis of life. In contrast, the hydrolysis reaction is catalysed by general acid and general base – importantly for leather making, it is catalysed by H+ and OH. The impact on processing can be indicated as follows.

In the earliest stage of processing, hair is usually removed and at the same time the skin is given a prolonged alkali treatment, typically conducted over about 18 hours for cattle hides, often in aqueous lime solution, Ca(OH): longer treatment may result in detectable damage to the fibre structure. In saturated lime solution. [OH~]= ^10~~ molar. Conversely, pickling in brine solution with acid is routinely used as a preservation technique for more vulnerable sheep skins, enabling them to be transported across the world between Europe and Australia and New Zealand over a period of several months: here [H~]= – 10~2 molar. Hence, by this practical comparison, in which pickled pelt remains undamaged for months, but limed pelt shows damage after a few hours, hydroxyl ion produces a much faster reaction than hydrogen ion – at least 100 x faster.

An important feature of the peptide link is that it is partially charged. The link can be expressed in two forms. The charged structure makes chemical sense, but nature does not favour charge separation in this way. However, the electro negativity difference between the oxygen and the nitrogen means that the structure can be set out in a slightly different way. shown in Figure .

The two parts of the peptide link each carry only a partial charge, but this still allows the peptide link to piay significant roles in the interaction between the protein and water and in the fixation of reagents in the leather making processes, most important in post tanning, when the leather is dyed and lubricated.


It is a feature of the properties of collagen that it has ‘layers’ of structure, collectively known as the ‘hierarchy’ of collagen structure, which combine to allow the formation of fibres. These can be defined as follows, starting with the most fundamental element of structure.

Amino Acid Sequence:

Collagens are characterised by a repeating triplet of amino acids: -(Gly-X-Y)n-. Therefore, one-third of the amino acid residues in collagen are glycine. Fur­thermore. X is often proline and Y is often hydroxyproline: 12% of the triplets are -Gly-Pro-Hypro-, 44% are -Gly-Pro-Y- or Gly-X-Hypro- and 44% are -Gly-X-Y- where X and Y are not denned. In this way,.,the helical shape of the molecule is determined (see below).

The amino acid compositions of bovine skin proteins are compared . In leather making terms, the amino acid sequence plays only a minor role; indeed, the technologies for making leather are essentially the same for all animal skins: variations in technologies are much more dependent on the macro-structure of the skin and its age rather than the details of the chemistry of the protein. The amino acid content determines the reactivity of the protein towards reagents such as tanning compounds and the sequence influences the formation of electrostatic links, which is important for protein stability but which is also exploited in leather making: the amino acid content determines the isoelectric point (see below), which together with pH controls the charge on the protein. Therefore, these features of the protein influence its affinity for different tanning reagents, but there is little difference between animal species in terms of the outcomes of stabilising reactions. Here, stability or more commonly the hydrothermal stability (resistance to wet heat) is conventionally measured by the temperature at which the protein loses its natural structure: for collagen this is referred to as the ‘shrinkage temperature’, Ts, or sometimes as the melting or denaturation temperature.

The only discernible difference in hydrothermal stabilities of collagens is observed in extreme variations in skin source, where there can be big diffe­rences in shrinkage temperature, depending on the natural environment of the animals concerned . When there are differences in shrinkage temperatures between collagen sources, the difference is also reflected in the

The α-Helix:

The presence of a high content of p-amino acid causes the chain of amino acids to twist, due to the fixed tetrahedral angles, locking the twist in place . Notably, the twist is left-handed, i.e. anti clockwise, because of the natural L-conformation of the molecules.

The Triple Helix:

The notion of the triple helix structure of collagen was first proposed by Ramachandran. In type I collagen, the monomeric molecule, protocollagen, contains three chains, designated αtl(I) and α (2): there are two αl(I) chains and one α (2) chain, which differ only in the details of the amino acid sequence. These three chains twist about each other in a right-handed or clockwise triple helix: this is only possible because of the high glycine content, which has the smallest α-carbon sidechain, a hydrogen atom, so that ‘glycine is always situated in the centre of the triple helix .

Each a-chain is about 1050 amino acids long, so the triple helix takes the form of a rod about 300 nm long, with a diameter of 1.5nm: this is the monomeric unit from which the polymeric fibrous structure is created. In the production of extracellular matrix from the fibroblast cells of the skin, the triple helix is synthesised as the monomer procollagen and then the monomers self assemble into a fibrous form. Before it becomes fibrous, soluble collagen can be isolated as triple helices and it is in this form it can be extracted from immature skin under acid conditions.

In the triple helix there is an inside and an outside of the structure. The minimum size of the glycine sidechain and its occurrence every third residue in the alpha helix allows it to fit into the inside part of the structure. If the side-chain was bigger, the triple helix could not form.

At each end of the triple helices there are regions that are not helical, con­sisting of about 20 amino acids, called the telopeptide regions. If soluble col­lagen is isolated with the aid of pepsin, a proteolytic enzyme, triple helices are obtained without the associated telopeptide regions. In the case of acid extraction of soluble collagen, individual triple helices are obtained with the telopeptide regions intact. In the context of leather making, the importance of the telopeptide regions lies in their role in bonding, to hold the collagen macromolecule together: the covalent bonds holding the triple helices together link the helical region of one triple helix to the non helical region of another triple helix, indicated in . Modelling studies have shown that, because of their flexibility, the telopeptide chains are capable of their own interactions between α-helices and triple helices.In tanning terms, the telopeptide regions probably do not play a significant role in the preparative processes or in the stabilising reactions: in the latter case, the stability arises from the structure created between the triple helices, based on the high degree of structure already in place in the helical region, but which is not present in the more random telopeptide chains.


Importantly, the triple helix is also held together by electrostatic bonds, as follows:

These are the so-called ‘salt links’, formed by electrostatic reaction between acidic and basic sidechains of the protein, which together determine the iso-electric point of collagen. The IEP is an important parameter, because it con­trols the charge on the protein at any given pH: since the reactions in leather making are usually dependent on charge, they are in turn dependent on the IEP and the way the value of the IEP varies during processing. The isoelectric point is defined in several ways, but the most fundamental definition is that it refers to the point on the pH scale at which the net charge on the protein is zero, illu­strated as follows:

Because the charge on the protein can be adjusted with acid or alkali, to make the protein positively or negatively charged, respectively, there must be a point on the pH scale when the net charge passes through zero. Therefore, the IEP depends on the relative availability of groups that can participate in reactions with acid and alkali, the carboxyl and amino groups. So IEP can be defined as follows:

where:  indicates some function of concentration and T indicates total concentration.

The second, expanded form of the equation indicates that it is only the total availability of a species that is important, its form is unimportant, i.e. the IEP does not change with pH.

More strictly, the definition should incorporate the role of the pKa or pAb of these groups, which would require that each specific pH active group would be treated separately, exemplified as follows:

!n this way, all i and j species provide individual contributions to the iso­electric point. Unfortunately, the nature of the functions is not known, so

Table 1 :   Effect on IEP of changing the content of active groups.

 Amino FunctionCarboxyl function
Increase ContentHigherLower
Decrease ContentLowerHigher

 calculation of the precise value of the IEP is not yet possible. However, this is clearly not an intractable problem.

The equations are usefully presented in this form, with the amino function as numerator and the carboxyl function as denominator, because they then pre­dict how varying the functions influences the IEP, in terms of the direction of change .

At the isoelectric point, the following phenomena occur, each of which might be used as the definition, as a whole or in part.

•  net charge is zero

•  content of intramolecular salt links is maximised

•  swelling is at a minimum

•  shrinkage temperature (and other aspects of hydrothermal stability) is


There are two points relating to isoelectric point and its relevance to leather

making that are very important and worth emphasising:

1.  IEP is a point on the pH scale, so it does not change with changing pH of the system. The IEP of collagen is the same whether is in the alkaline, limed state or in the acidic, pickled state.

The importance of this point is that the isoelectric point can only be changed if there is a chemical change that alters the availability of active groups; this can occur in the beamhouse processes, in the tanning pro­cesses and in the post tanning processes.

2.  The charge on collagen is determined by the relative values of the IEP and the pH. If the pH is higher than the IEP, the collagen is negatively charged, and if the pH is lower than the IEP, the collagen is positively charged. Moreover, the further the pH is from the IEP, the greater is the charge, although it is limited by the availability of amino and carboxyl groups.

The importance of this concept relates to the application of charged reagents, particularly post tanning reagents and their interaction with the

charged leather substrate.It can be assumed that at the starting point of processing for collagen, prior to raw pelt going into liming (alkaline treatment for unhairing and modification of the skin’s fibre structure), the IEP is at physiological pH. about 7.4.


The triple helices are bound together in bundles called fibrils . The repeating pattern of charges in the D-period banding becomes apparent in the macro structure of the fibrils: it can be visualised in raw collagen by reacting the charged sidechains with heavy metal salts, to create a corresponding pattern of high electron beam density. This is commonly done with, for example, anionic phosphotungstate to react with positive basic groups and counter staining with, for example, cationic uranyl ion to react with the carboxyl groups of acidic

Fibrils are the smallest units of collagen structure that are visible under the electron microscope. That is, unless the structure is disrupted chemically, e.g. by the effect of acid swelling . The figure shows that there is some sub-structure present in fibrils. However, the nature of the sub-units is not known: it might be thought that the structures are pentafibrils, but the thick­ness of the units visible in the figure is much too large.

Moreover, in figure there appear to be layers of spiral structure so they must represent a discrete layer in the hierarchy of structure. It is possible that what is visible in the electron photomicrograph is unravelled surface. This would be in agreement with Koon’s observations of apparent hollow structure in collagen fibrils recovered from bone . Here the demineralisation conditions are too mild to cause significant damage to protein, so the structure observed by transmission electron microscopy must be a reflection of the actual structure.

This clearly begs the question: what is the structure of fibrils? The question is complicated by the observation that sectioning through fibrils reveals no internal structure .Kronick has reported that the cores of fibrils can be melted at a lower temperature than denaturation of the surrounding sheaths and that the inner material of the fibrils can be degraded by trypsin to leave hollow fibrils, but this reaction is prevented if the fibrils are first stabilized with glutaraldehyde.Other workers have suggested that the core material might be glycosami-noglycansor glycoproteins. Therefore, it is uncertain what lies within fibrils, if anything, but the latter observations are not inconsistent.


Fibrils are arranged in the form referred to here as fibril bundles  in which the fibril bundle are a sub-structure or the constructing units of fibres. This level of the hierarchy of collagen structure is important with respect to opening up of the fibre structure, in preparation for the tanning process. Splitting open the fibre structure at this level, at the junctions between fibril bundles, is important for creating softness and strength in the final leather, particularly in regard to the deposition of lubricating agents.


Fibril bundles come together to create fibres, as clearly seen under low reso­lution light microscopy . The fibres characteristically divide and rejoin with other fibres throughout the corium structure. It is this variation in crosslinking or linking that provides the strength to the material. Some workers have used the term ‘fibre bundle’, but this merely reflects the observation that the fibres have a range of diameters, largely dependent on the position through the skin cross section. Therefore, strictly this does not constitute another level of structure in the hierarchy.

Amino Acid:

The amino acids are the constituents of proteins. They contain the amino group –NH2. The carboxyl group-COOH and the radical –R. the general formula is H2N-CH(R)-COOH The radical -R characterizes the amino acids and classifies them into the following groups

–     Non-polar, non-reactive                                      = Hydrogen, aliphatic, aromatics compounds,

–     Polar, reactive                                                     = -OH, -SH, -COOH, -COO”,

-CO-NH-, -CO-NH2, -COOR, NH2, NH3+

There are 20 different amino acids in the structure of collagen and 21 -22 in that of keratin. Typical of collagen is the presence of hydroxy-proline (HYP) and glycine (GLY); characteristic of keratin is the presence of the sulfur-containing amino acid cystine (CYS).

Bridge linkage in protein:

Cross-linking bridges decisively influence the structure, stability, reaction capacity and overall behaviour of proteins. They may occur within the peptide chain or between two or several adjacent peptide chains like Disulfide bridges of cystine, Ester linkage bridges between carblxyl and hydroxy groups in side chains, side chain peptide bridge linkages between the acid and the basic amino acids, and secondary valence linkage(non-covalent) Hydrogen bridge linkage between peptide groups or side chains and peptide groups.

 Table 2  : Amino acids of importance to leather making.

NameSymbolTypeSidechain (R)   ‘     –Importance in leather making
GlycineGlyα, neutral-HCollagen sturcture
AlanineAlaα, neutral-CH3Hydrophobic bonding
ValineValα, neutral-CH(CH3)2Hydrophobic bonding
LeucineLeuα, neutral-CH2CH(CH3)2Hydrophobic bonding
Isoleucinelieuα, neutralCH3CH2CH(CH3)Hydrophobic bonding
Phenyl-AlaninePheα, neutral-CH2C6H5Hydrophobic bonding
SerinsSerα, neutral-CH2OH      ,Unhairing
CysteineCySHα, neutral, S Containing-CH2SHUnhairing
CystineCyS-SCyα, neutral, S Containing-CH2SSCH2– ‘Unhairing
Aspartic AcidAspα, acidic-CH2C02HIsoelectric point (IEP), mineral tanning
AsparagineAsnα, neutral-CH2CONH2 IEP
Glutamic AcidGluα, acidic-(CH2)2CO2HIEP, mineral Tanning
GlutamineGinα, neutral-(CH2)2CONH2IEP
ArginineArgα, basic-(CH;)3NHC(NH)NH,IEP
LysineLysα, basic-<CH2)4NH2IEP, aldehydic tanning , dyeing, lubrication
Aldehydi tan- ning, dyeing, lubrication
HistidineHisα, basic
ProlineProβ, neutralSee Figure 1Collagen Structure
Hydroxy- prolineHyproβ, neutralSee Figure 1.1Collagen struc- ture, hydrogen bonding


 The principle fibrous protein in the corium or derma layer of a hide or skin. Defined by ALCA glossary.

Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendon, ligament and skin, and is also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral disc.


Collagen occurs in many places throughout the body. So far, only 29 types of collagen have been identified and described. Over 90% of the collagen in the body, however, is of type I

  • Collagen One: skin, tendon, vascular, ligature, organs, bone (main component of bone)
  • Collagen Two: cartilage (main component of cartilage)
  • Collagen Three: reticulate (main component of reticular fibers), commonly found alongside type I.
  • Collagen Four: forms bases of cell basement membrane
  • Collagen Five: cells surfaces, hair and placenta

Fibrous structure of true skin (collagen):

Fiber bundles composed of fibers (20-200 micro meter in diameter) which in turn consist of elementary fibers (about 5 micro meter in diameter) and these of fibrils (10-100 nm in diameter), and these of micro fibrils (about 5 nm in diameter), and these of macromolecules. The collagen molecules (tropocollagen) are about 280 nm long, about 1.5 nm in diameter and have a molecular weight of about 300000. they are composed of three polypeptide chains which are twisted together in form of a helix (triple helix) and which consist of amino acids that are linked together by peptide bonds.1 kg raw skin has a reactive inner fiber surface area of 1000-2500 meter square.

Physical and Chemical properties of Collagen:

  Whitish, hard and brittle in the dry state.

  Insoluble in cold water and organic solvents.

  Water absorption up to 70% on the tissue weight; partly deposited in form of water of hydration or capillary water.

  Water vapour absorption up to 50% on the collagen weight. Decisive advantages over synthetic replacement materials.

  Preservation by dehydration is possible.

  With continuous heating in the presence of water, the fibres shrink to one third of their original length and begin to cement together irreversibly.

  Collagen shows minimum swelling at the isoelectric point.

  Dilute acids and alkalis cause swelling due to the charge, i.e. volume and weight increase owing to higher water uptake (reversible, almost no change in structure of collagen).Increase in temperature and concentration and extension of time result in swelling due to hydrolysis (only partially reversible).

  Hydrotropic substances enhance swelling and lower the cementing temperature, the ones with strong polarity render collagen soluble.

  They are usually white in appearance;

  They stain easily with weakly acid stains, e.g. picric acid/fuchsin (van Gieson’s stain);

  They do not take up silver stains.

  They swell markedly when immersed in acid or alkali;

  They are mostly insoluble in neutral solvents;

  They are more resistant than most fibrous proteins to degradation by proteinases, but are rapidly attacked by collagenase;

  They are generally inelastic;

  They shrink to about one third of their original length at a certain temperature (the shrinkage temperature);

  They are converted in large part into soluble gelatin by prolonged treatment at temperatures above the shrinkage temperature;

  They react with tanning (cross-linking) agents.

Amino acids composition of  collagen:

  1. High content of glycyl-residues (about one-third of the total);
  2. High content of amino acid residues, prolyl and hydroxyprolyl (about two-ninths of the total);
  3. High content ot hydroxyprolyl residues (about one-tenth of the total) so that the ratio of praline to hydroxyproline is about unity;
  4. High content of alanyl residues (about one-ninth of the total);
  5. No cysteyl, cystyl or tryptophanyl residues;
  6. Very few methionyl, valyl, histidyl and hydroxylysyl residues;
  7. Very few phenylalanyl and tyrosyl residues, so that vertebrate collagen is usually low in aromatic amino acids.

When considering whether a protein is collagenous, it should be shown to have this general distribution, i.e. all the amino acids must be taken into account. It may be unsound to use the content of any one amino acid as a guide, since some non-collagenous proteins, e.g. silk fibroin, also have a high glycine content; others, e.g. tongue tissue proteins and those from certain young plants, have hydroxyl-proline contents similar to collagen, whilst hydroxylysine, once thought to be unique to collagen, is present in certain muscle proteins.

It is seen that the composition of vertebrate collagen corresponds roughly to the pattern:

One-third glycyl residues

One-third imino acid residues

And One-third alanyl and other residues

Which suggests a major repeating unit or sequence of the type:

 Glycine, amino acid,any other residue, especially alanine.

Chapter- Two

Some Tanning Process &

The Future Leather Processing

Chrome Tanning:

When the Leather is tanned by chrome powder then the process is called chrome tanning & material called chrome tanning.

Basic chrome powder:

 The use of chromium (III) salts is currently the commonest method of tanning: perhaps 90% of the world’s output of leather in tanned in this way. Up to the end of the nineteenth century, virtually all leather was made by ‘vegetable’ tanning, i.e. using extracts of plant materials ,. The development of chrome tanning can be traced back to knapp’s treatise on tanning of 1858, in which he described the use of chrome alum: this is referred to as the single bath process, because the steps of infusion and fixing of the chromium (III) species are conducted as consecutive procedures in the same vessel. It is usually accepted that chrome tanning started commercially in 1884, with the new process patented by Sshultz: this was the ‘two bath’ process, in which chromic acid was the chemical infused through the hides or skins, conducted in one bath, the pelt was then removed to allow equilibration (but no fixation), then the chrome was simultaneously reduced and fixed in the second bath.

Brief Review of the development of chrome tanning:

The development of modern chrome tanning went through three distinct phases:

I)       Single bath process: The original process used chrome alum, Cr2(SO4)3. K2SO4.24H2O. applied as the acidic salt, typically giving pH ~ 2 in solution. Following penetration at that pH, when the collagen is unreactive, the system is basified to pH ~ 4, with alkalis such as sodium hydroxide of sodium carbonate to fix the chrome to the collagen.

II)    Two bath process: The first commercial application was an alternative approach to the single bath process: it was recognized that a more astringent, and consequently more efficient, tannage could be achieved if the technology of making chromium (III) tanning salts was conducted in situ. This means that the process was conducted in two steps. The pelt is saturated by chromic acid in the first bath, then it is removed, usually to stand overnight. At this time there is no reaction, because Cr(vi) salts do not complex with protein. Next, the pelt is immersed in a second bath containing a solution of reducing agent and enough alkali to ensure the final pH reaches at least 4. At the same time, processes were also devised that combined both valencies of chromium, exemplified by the Och’ process. However, the dangers of using chromium (v) drove change back to the single bath process. Not least of these consideration was the incidence of damage workers by chromium (vi) compounds:  the highly oxidizing nature of reagents typically caused unceration to the nasal septum. Notices warning of the dangers of chromium (vi) to health can still be found in UK tanneries, even though these compounds have long since ceased to be used.

III) Single bath process: With the development of masking (see below) to modify the reactivity of the chromium (III) salt and hence its reactivity in tanning, the global industry universally reverted to versions of the single bath process.

There have been attempts to reintroduce versions of the two bath process. Notably, the Gf process was developed towards the end of the twentieth century in Copenhagen. However, European tanners were reluctant to handle Chromium (vi), regardless of the advised measures to ensure complete reduction. For this reason, the method was never generally adopted.

Vegetable tanning:

When the leather is tanned by various types of natural organic material which obtained from the plant kingdom then the process is known as vegetable tanning and the materials are called vegetable tanning.

Some important vegetable tanning materials are Minosa, Oak, Hemlock, Scnec, Iaebracho, Mangrone, Myrobalar etc.


The vegetable tanning materials one obtained from different ports of plant which is given below.

Vegetable tanning materialsSource
1. Minoba/Wattle, OaK, Henlock, Mongroue, FuebrachoBark
2. Sarac, Mangrone, GambierLeaned
3. Oak, Iaebracho, Chestrat, MinobaWoods
4. Myrobalan, ValoheaFruits.

 Practical vegetable tanning

The history of vegetable tanning concerns the development of both the tanning agents employed and the equipment used for the process. In the former, changes involved the nature of the agent, gradually changing from the use of the plant material itself, to the availability of extracts. In the latter, pits have been the preferred vessel, but the way in which they were used changed. His-trically., tanning was conducted in pits in a process referred to as ‘layering’. A layer of the appropriate plant material. E.g. oak bark, was placed in the bottom of the pit, followed by a layer of hides or skins. Another layer of plant material was placed on the hides, followed by another layer of plant material: the alternate layering continued until the pit was full . Then the pit was filled with water.

The water leached the polyphenols out of the plant material and the dilute tannin diffused into the hides, converting them into leather. The dilute nature of the solution limits the reactivity of the tannins, allowing it to penetrate through the pelt cross section. The reaction is slow, in part due to the static nature of the process, so an early from of quality assurance applied more than half a

Millennium ago in England was the requirement for hides to stay in the pit for a year and a day.

Definition(Cobination tanning): 

The tanning process which is carried out by the use of two or more chemically different tanning agent in the some tanning process is know as combination tanning.

 Combination tannage:

When the leather is produced pure chrome tanningor pure vegetable tanning method. This leather can not show desirable properties to or user. So for providing maximum number of desirable properties of a leather for a particular uses the combination method should be followed.

When the leather is tanned combindly by both of chrome & vegetable tanning the process is known as combination tannage.

For example most of the chrome appear leathers are retanning by vegetable tannins. These leathers are known as chrome retanned leather.


The environmental impact of chromium (III) is low: as a reagent, basic chromium salts are safe to use industrially and can be managed efficiently, to the extent that discharges from tanneries can routinely be as low as a few parts per million. Therefore, clearly, the industry should be able to meet all the future requirements of environmental impact. Consequently, we can reasonably assume that the future of tanning will include a major role for chrome. Nevertheless, the technology can be improved: efficiency of use can be improved, as can the outcome of the reaction, in terms of the performance of the leather (shrinkage temperature) and the effectiveness of the reaciion (shrinkage temperature rise per unit bound chrome).

The role of masking in chrome tanning is an important feature, which can be technologically exploited. The use of specific masking agents, which gradually increase the astringeney of the chrome species, by increasing their tendency to transfer from solution to substrate, is an aspect of the reaction that has not received scientific attention. Here, the requirement is to match the rate of diminishing concentration of chrome in solution with the rate of masking complexation, to maintain or increase the rate of chrome uptake. This type of reaction is already technologically exploited by the use of disodium phthalate. although the degree of hydrophobicity conferred by even a very low masking ratio can cause an undesirably fast surface reaction. Other hydrophobic masking agents could be developed, to give a more controllable increase in astringency.

It is already recognised that the chrome tanning reaction is controlled by the effect of pH on the reacthity of the collagen substrate: a second-order effect is the increasing of the hydrophobicity of the chrome species by-polymerisation. In addition, the rate of reaction is controlled by temperature. However, the efficiency of the process is limited by the role of the solvent, in retaining the reactant in solution by solvation: this is typically only countered by applying extreme conditions of pH, which is likely to create problems of surface fixation, causing staining and resistance to dyeing 1.

The role of the counterion in chrome tanning offers potential for change. The chrome tanning reaction is controlled by the particular counterion present. It was fortunate for the leather industry thai chrome alum [potassium chromium (III) sulfale hjdrate] was the most readily available salt for the original trials of tanning ability: sulfate ion is highly effective in creating a stable supnimolecular matrix, because it- is a structure maker in water. The effect is very different if other salts are used. e.g. chloride or perchlorate, when the outcome is only moderate hydrothermal stability. However, even if these salts are used, the high hydrothermal stability can be acquired by treating the leather with another counterion. Since the effect is independent of a complexing reaction, the process of modifying the moderate tanning effect is fast. This opens up the chrome tanning reaction to modifications that exploit the separation of the link and lock reactions;

•  The environmentally damaging sulfate ion might be replaced by other less damaging counterions. such as nitrate.

•  Reactive counterions can then be applied: options include using the stoichiometric quantity of sulfate or organic anions.

•  The counterion might be replaced  with  polymeric agents,  including polyacrylates with the right steric properties.


It has been suggested’ that the environmental impact of chromium can be alleviated by substituting all or part of the offer by other metal tanning salts; the following options are the likeliest candidates:

Al (III). Ti(III)/(iv), Fe(II)/(III), Zr(Iv), lanthanide(III)

The list of available options is limited to the few presented above by considerations of cost, availability, toxicity and reactivity towards carboxyl groups. All metals salts are mixable in all proportions in this context. However, the following general truism should he noted:

The dumiiging effet-i of any ( allegedj pollutant is nor eliminated and is barely significantly miligaied by reducing rhe degree to wich it is used..

The effects of substituting other metals into the chrome tanning process or even completely substituting it have been discussed, and are briefly reviewed below. There are circumstances when other mineral tanning systems might be appropriate, but it should be clear that there is no viable alternative for general applications and particular]) for those applications that require high hydrothermal stability-


In the current climate of emphasis on ecologically friendly processing and products, attention has been directed to the environmental impact of chromium (III) in tanning and leather. Therefore, this situation has fuelled the movement to produce so-called ‘natural’ leathers, perceived to be and marketed as ‘chemical free’ or using other appellations that are designed to indicate their ecological credentials. The perception of the abuse of resources in leather making has been exploited in the marketing of leather that is not mineral tanned. Hence, it is typically assumed that:

chrome free = organic.

Consequently, leathers and leather articles are offered to the market as non-chrome. The use of the term ‘chrome free1 leather implies that there is a problem underlying the inclusion of chromium(iii) salt in the production of leather. This concept has its origins in the (alleged) environmental impact of chromium (III) waste streams, liquid and solid, and the mobilisation and viability of chromium (III) ions from tanned leather waste. However, latterly the accusation that some chrome leathers are contaminated with chromium(vi) or that Cr(vi) can be generated during the use of the leather article has heightened the notion that chrome tanned leather may be undesirable.

The decision to produce leather by means other than chrome tanning depends on elements of technology and elements of economics: some of these aspects will clearly overlap. In each case, there are points for change and points against. If production methods are changed at the tanning stage, it must be accepted that every subsequent process step will be altered; some will benefit, others will not and additional technological problems will have to be overcome.

Technological aspects:

•  environmental impact of waste streams:

•  ‘just in time’ quick response, wet white technologies;

•  colour, may be counterproductive if vegetable tannins are used;

•  changed properties, e.g. HHB (hydrophobic-hydrophilic balance). IER (isoelectric point);

•  process timings may or may not benefit;

•  resource management will change, e.g. the continued programme of conventional retanning, dyeing then fatliquoring.

Economic aspects:              

•  waste  treatment  and  cost of treatments,  adding  value  and  use of byproducts;

•  recyclabilily’ of leather, which traditionally means compostability, i.e. returning the leather into the environment without adverse impact -although this must involve at least partial denaturation of the leather, either by pH or heat treatment or both.

•  thinking beyond ‘cradle 10 grave”, extending to ‘cradle to cradle’;

•  marketing of’natural’ leather:

•  public perception of leaiher making – the use of ‘chemicals’, renewable reagents:

•  eco-labelling, as a marketing tool for developed economies or just access for developing economies to developed markets.

It is important to retain a clear idea of what is required in the alternative leather. Typically what is required is ‘the same, but different’. Here the obvious difference is the absence of chromium (III) salt, but which elements or aspect? of chrome leather character performance, properties should or need to be retained is less simple. Here, the tanner needs to distinguish between the specific properties required in the leather and the more general property of ‘mineral character”: the former may be relatively easy to reproduce, at least with regard to individual properties performance, but the latter is more difficult to reproduce, since mineral character usually means chrome tanned character.

Clearly, no other tanning system will be able to reproduce all the features of chrome leather. Therefore, it is unlikely that a generic leather making process with the versatility of chromium (III) will be developed – any new leather making process must be tailored so that the resulting leather can meet the needs of the required application, to be ‘fit for purpose’.


•  Other metals:

For example. Al(III). Ti(Iv), Zrd(Iv). possibly Fe(III), lanthanides(III). In each case, the leather is more cationic, creating problems with anionic reagents: the leather can be collapsed or over-filled depending on the metal. The HHB of leaiher depends on the bonding between the metal salt and the collagen. In all cases Ts is lower than Cr(ii).

•  Other inorganic reagents:

This category includes complex reagents, e.g. sodium aluminosilicate. silicates, colloidal silica, polyphosphates, sulfur, complex anions such as tungsiates. phosphotungstates. molybdates. phosphomolybdates and uranvl salts, i.e. those that have application in histology.

•  Aldehydic agents:

This group includes elutaraldeh;de and derivatives, polysaccharidc (starch, dextrin, alginate. ere.) derivatives and analogous derivatives of. for example, hyaluronic acid, oxazolidme and phosphonium salts. These reagents are not equivalent: they react in different polymerised states and their affinities are different, even though the chemistry of their reactivities is the same, similar or analogous. Jn each case, the leather is plumped up and made hydrophilic. The 1EP is raised. The colour of the leather depends on the tanning agent – glutaraldehyde and derivatives confer

Colour, others do not, Oxazolidine and phosphonium salts may produce formaldehyde in the leather or there is perception based on odour (Which may be the original reagent itself.)

Ploant polyphenols, vegetable tannis- hydrolysable, condensed:

Reactivity is a problem for a drum process, so the reaction may require syntan assistance. Filling effects and high hydrophilicity characterise these leathers. Colour saddening and light fastness may be a problem, particularly with condensed tannins. Migration of tannin can occur when the leather is wet. Ts is only moderate, not more than 80 C for hydrolysable, not more than 85 C for condensed.

Syntans retans or replacement syntans:

As for vegetable tannnis, although reactivity is more controllable by the choice of structure. Light fastness similarly controllable.

Polymers and resins:

This category includes: melamine-formaldehyde, acrylates, styrene maleric anhydride, urethanes, etc.

Miscellaneous processes, e.g. oil tanning. in situ polymerisation, etc.

Tanning Combinations:

Clearly, for the brief account of the individual tanning options set out above the number of available combinations is very large. Hence, it is important understand the principles of combination tanning. Analysis can commence with the effects of the components on the shrinkage temperature, as an indicator of the combined chemistries:

The components may react individually, i.e. they do not interact, primarily because they react with collagen via diffrent mechanisms and there is no chemical affinity between the reagents.

Here the contributions to the overall shrinkage temperature can be treated additively. Also, the properties of the leather will be a combination of the two tannages, although the first tannage applied will tend to dominate the leather character.

 The components may interact antagonistically, i.e. they interfere with each other. This may be due to competition for reaction sites or one component simply blocks the availlability of reaction sites for the other component. It can be assumed that the two components wither do not interact or that one component will react with the other in preference to reacting with collagen. Under these circumstances, the interaction does not constitute the creation of a useful new tanning species, i.e. the reaction of the second component does not involve crosslinking the first component. The outcome of the combined reaction is likely to be domination of the properties by the reagent that is in excess or has greater affinity for collagen.

Table 3: Indicative/possible interactions in combination tanning.

Second Regent
First regentMetal saltsInorganicAldehydieVeg tan hydrolVeg tan cond.Syntanresin
Metal saltsIIIAIAI
Veg. tan hydrol.SSIAAAA
Veg. tan. cond.IISAAAA

Here, the hydrothermal stability of the combination will be less than the anticipated sum of the effects of the individual tanning reactions. The effect of destabilising the leather is likely to be undesirable.

The components may react synergistically, i.e. they interact to create a new species, which adds more than expected to the overall hydrothermal stability. Since a new tanning chemical species is created, the properties of the leather are likely to reflect the sum of the individual tanning effects. but to differ significantly from a simple additive outcome.

The additional elevation of Ts may constitute a high stability process. as rationalised in the link-lock theory.

In making choices of combinations, it is important to consider how the reagents combine to create the leather properties other than hydrothermal stability: these could include handle ( softness, stiffness, fullness, etc.) hydrophilic-hydrophobic balance, strength ( tensile, tear, extensibility, etc.) and so on. If the components react individually, it might be expected that the properties of each would be conserved in the leather, dependent on the relative amounts fixed. If the components react antagonistically, the overall properties will be determined by the dominant reactant, although they may be modified by the presence of the second component, unless the antagonism means the second reagent is not fixed. Synergistic reaction means the tanning reaction is changed, so it can be assumed that the outcome will not be the same as the sum of the individual properties, although it can also be assumed that the leather will still reflect the individual properties.

The concept of the link-lock mechanism is useful in this context, because it offers a model of the tanning reaction that can be visualised and hence used in the analysis of reaction and outcome. The matrix model is easier to use than the previous model of  specific  interaction with the collagen sidechains.

All meials are mixable in all proportions for tanning and all metals react primarily at carboxvl groups, tjpically electrostatically. It is unlikely that there will be any positive impact on the tanning reaction from mixed speciation. The filling collapsing effects can be controlled by mixing the metal offers. There is no benefit with respect to Ts or cationic character. Any combination in which the components rely on hydrogen bonding for fixation is likely to be antagonistic. Hydrolysable  vegetable tannins and metals: This is established technology.

The product is full, hvdrophilic leather. Covalent fixation of metal by polyphenol complexation modifies the properties of the metal ions, but there is still cationic character.

Condensed tannin with aldehydic reagent: Prodelphinidin and profiseti-nidin tannins or non-tans are preferred. Not all aldehydic reagents work: oxazolidine is preferred. The result can be high TV Syntan with aldehydic reagent: The- industry standard is to use unspecified syntan with glutaraldehyde. There is apparently no attempt to define the syntan in terms of reactivity to create synergy; in general, reliance is probably placed on simple additive effects, so ihe rationale is not easy to discern.

The technology could be improved by analogy with the condensed tannins, by applying the structure-reactivity criteria known in that context. Polymer with aldehydic reagent: Melamine-formaldehyde polymer with phosphonium salt can be a synergistic combination, depending on the structure of the resin, especially the particle size.

There will be other polymeric reagents capable of creating synergistic tannages. Reliance on syntans and, or resins may carry ihe additionai problem of formaldehyde in the leather, which is subject to limits in specifications. It is possible to avoid this problem by scavenging the formaldehyde by the presence of condensed tanning, preferably prodelphinidin or profisetinidin.’


Polyphenol Chemistry

The best known example of plant polyphenol exploitation for high hydro-thermal stability tanning is the semi-metal reaction. Here, the requirement is for

polyphenol and the locking metal ion: in practice, this means using the hydrolysable tannins, but alternatively some condensed tannins can be used  prodelphinidins (e.g. Myrica esculents, pecan and green tea) and prorobinetinidins (e.g. mimosa) each have the required structure in the B-ring. Many metal salts are capable of reacting in this way. so there may be useful applications for the future.

The condensed tannins can confer high hydrothermal stability by acting as the linking agent, with aldehydic crosslinker acting as the locking agent; other covaleni crosslinkers may also have application in this context. The reaction applies to all flavonoid polyphenols, when reaction always occurs at the A ring. In the case of the prodelphinidins and profisetinidins, additional reaction can take place at the B-ring. The effect is to increase the ease of attaining high hydro-thermal stability. It is useful to recall that the combination tannage does not rely-on conventional vegetable tannins, because it can work with the low molecular weight non-tans: here, there is advantage in terms of reducing or eliminating problems of achieving penetration by the primary tanning component.The link-lock mechanism can be exploited in other ways, even using reagents that at first do not appear to be tanning agents e.g naphthalene diols


Shrinkage temperatures (0C) hide powder treatede with dihy-droxynaphthols (DHNs) and oxazolidine.

DihdroxynaphtholDHN aloneDHN+ oxazolidine
1.6 (2.5)6490

The behaviour of naphthalene diols in the tanning process is highly dependent on the structure of the isomer. Clearly, the presence of a hvdroxyl in the 2-poshion activates the naphthalene nucleus: the 1-position does not work, shown by comparing the 1,5 with the 1,6 (2,5) diol. When there are two groups in the 2,6-positions, they act together. When the hydroxjls are in the 2,7-positions they act against each other. The basis is the inductive effect of the hydroxyl on the aromatic ring, activating the ortho positions to electrophllic attack or allowing those positions to engage in nucleophilic attack at the methylene group of the N-methylol group of the oxazolidine . The results in table also illustrate the principle that the linking agent may only exhibit a very weak effect in tanning terms, but successful locking of the linking species, combined with the ability to link the matrix to collagen in the locking reaction, can result in high hydrothermal stability.

Non-chemical polymerisation is less effective. Applying laccase (phenol oxidase enzyme) to hide powder treated with 2,6-dihydroxynaphlhalene produced the highest rise in shrinkage temperature for the range of this type of linking agents tested (by 26 °C, to 85 °C). Clearly, the locking reaction is more easily and effectively accomplished by applying a second reagent, rather than relying on direct reactions between linking molecules.

Polymer and Crosslinker:

Perhaps surprisingly, it is less easy to create high stability tannage with polymers than it is using oligomers or monomers, depending on the polymeric compound. High hydrothermal stability can be achieved using melamine resin crosslinked with tetrakis (hydroxymethyl) phosphonium salt. Several conclusions were drawn from these studies:

•  Not all melamine linking resins work. Therefore, the requirements for matrix formation are likely to be more important than possessing specific chemical reactivity.

•  Not all aldehydic locking agents work. The locking function does depend on creating stable bonding between the linking molecules and forming a rigid species capable of resisting the collapsing triple helices.

•  The linking reaction is dependent on physical parameters; particle size may be critical. It is not sufficient to provide space filling.

•  The ability to form the basis of a supramolecular matrix must depend on the stereochemistry of the linking agent. This requirement is more easily satisfied with lower molecular weight species.

This reaction provided an interesting aspect of the matrix theory of tanning, when an attempt was made to accumulate hydrothermal stability by adding a matrix to a matrix. Here, the sequence of reagent additions was as follows: melamine resin, phosphonium salt, condensed tannin (mimosa), oxazolidine. The observation was the achievement of high hydrothermal stability from the melamine resin and phosphonium salt, added to by the condensed tannin, reaching a shrinkage temperature of 1290C, thereby matching the maximum shrinkage temperature achieved by chromium in the presence of pyr-omellitate (1.2,4,5-tetracar boxy benzene). The melamine and phosphonium salt create a matrix in which the melamine polymer reacts with the collagen via hydrogen bonds, the phosphonium salt crosslinks the polymer, whilst probably linking the matrix to the collagen. The introduction of condensed polyphenol raises the shrinkage temperature by a small additive effect, since it is applied after the matrix formation, it has no affinity for the resin, but has limited affinity for the phosphonium salt. The subsequent addition of oxazolidine is capable of forming a synergistic matrix with the polyphenol, but the reaction causes the shrinkage temperature to drop significantly (A.D. Covington, unpublished results). The clear inference is that the new reaction is antagonistic to the established matrix: by analogy with other antagonistic combination,” there is competition with the mechanism that binds the first matrix to the collagen, effectively loosening the binding belween the matrix and the collagen.

allowing shrinking to occur, with ihe consequence that the hydrothermal stability is lowered. It is difficult to rationalise the observation by any mechanism other than the formation of matrices.


The concept of ‘compact processing’ is the condensing or shortening of processing by combining two or more reactions into a single process step. In this way. lime is saved and consistency is created in the interaction between the process steps involved, because only one reaction takes place, instead of two or more. This is a powerful contribution to innovation in the tannery. The idea of compact processing can be applied at any stage in processing, although it is more commonly applied in the later stages of wet processing. Therefore, thinking about this concept should not be limited in scope. If it is accepted that high stability can only be achieved  by a two-stage tanning action, then there are opportunities to exploit such an approach to tanning in compact processing.

The use of a non-swelling acid incorporates the notion of pretanning. It may be possible to substitute pretanning for relanning. However, it should be recognised that the first tanning agent controls the character of the leather. In the case of prime tanning being conducted with vegetable tannins, the requirement for further tanning is uncommon. An exception would be organic tanning, which might be based on polyphenols, when there would be a requirement for applying the second component of a combination process for high hydrothermal stability. Such an approach is feasible for the combination of gallocatechin-type condensed tannins with oxazolidine, when the reaction is activated by elevated temperature.

The case of chromium (III) tanning is less clear. No technologies have been offered to the industry in which a retanning agent has been combined with conventional chrome tanning salt. The closest to that situation are products referred to ;is ‘chrome symans’. which are well known to the industry. However, these are primarily offered as filling versions of chrome relanning. rather than a new approach to prime tanning. Because chromium (III) has little affinily for phenolic hydroxide as a ligand. chrome synians are either mixtures of syntan and chrome salt or the syntan may be capable of complexation with chrome, by having some carboxvl functionality in its structure. In ihe latter case, the function of retanning might be accomplished with tanning. Similarly, an extended application of masking, to confer additional features, may be useful.

The application of compact processing to post tanning is discussed in Chapter 15. Clearly, there is potential for further development, using creative chemistry


It is conventionally assumed that tanning has to be conducted in aqueous solution. However, there are options available for the future. Wei has demonstrated the possibility of processing waler wet pelt in a tumbling medium of water-immiscible solvent. The technology is feasible, but brings problems of its own, notably the requirement for diffusion across the pelt, unlike the conventional requirement for diffusion through the cross section. However, such difficulties are solvable, if the willingness is there. The alternative is an essentially non-aqueous process, as indicated in the context of compact processing: this approach to processing, including the use of liquid carbon dioxide, has been discussed for some steps, but industrial development has not yet followed. The advantage of such an approach to one-step post tanning is that the substrate is dry (at least to the touch), so the moisture in the leather would have a limited influence on the process. It is less clear that such an approach would be feasible for primary tanning, in whatever way that might be done chemically. If chrome tanning were to be developed in that direction, it is not difficult to imagine the HHB properties of the chrome complex being tailor-made for the solvent by appropriate masking.

It is useful to speculate on the roles that enzymes might play in tanning. Clearly, biotechnology has an increasingly important part to play in the beamhouse, but it is less clear if it can contribute to collagen stabilisation. From the matrix theory of tanning and collagen stabilisation, the inability of transglutaminase to increase the hydrothermal stability is predictable and understandable, even though it is clearly capable of introducing crosslinking in the conventional sense. Similar reactions, which can introduce crosslinks into collagen, are unlikely to function as useful tanning agents, beyond altering the texture of the protein.

At the other end of the processing procedures, the role of drying remains to be exploited to advantage by industry . Recent studies have linked the properties of leather, including area yield, to the programme of drying conditions: “softness does not depend on the rate of drying, only the moisture content. Using conventional plant, it is possible to modify the drying programme into two stages, to obtain advantage from the relationship between the viscoelastic properties of leather and its water content, so that area gain of intact leather does not have to be at the expense of softness.

In considering alternative technologies, it is useful to examine whether there is an alternative mechanism to link-lock, i.e. whether there could be exceptions. If the mechanism of shrinking, as set out here, is right, then the impact of the tanning reaction on hydrothermal shrinking is right. Consequently, the use of conventional, penetrating reactants, in the way also outlined above, is also right. Therefore, the only exception to the link-lock mechanism would be a single step reaction that combines both features. The requirement would be the formation of the stabilising matrix from a single species polymerisation reaction, which would have to include reaction with the collagen, in this regard, polymerisation of cod oil is the closest reaction known in the art. However, in this case, despite the complexity of the chemistry of polymerisation by oxidation, there appears to be little direct interaction between the polymer and the collagen, despite the possibility of creating aldehyde groups. Hence, the leathering reaction does not elevate the shrinkage temperature significantly. Examples of chemistries apparently capable of multiple bonding with collagen do not react in that manner, because ihere is no evidence of an outcome that might be expected from such tanning reactions . Nevertheless, it is no! inconceivable that such a polymerising tannage might be developed, particularly if an extended range of practically useful solvents is made available to the industry.


The range of chemistries available to the tanner is widening. By considering ihe molecular basis of ihe tanning mechanism, especially those requirements to confer high hydrothermal stability, the options open to the tanner are widened. If high stability organic tannages are desired, they can be created just by observing the following rules:

a.  Apply the first reagent, linking to the collagen wilh high stability bonding, preferably covalemly. The reagent must offer the potential for a second reaction, by possessing usefully reactive groups, but need not have high molecular weight.

b.  Apply the second reagent, to react with the first reagent by locking the molecules together: therefore, the second reagent must be multifunctional. Contributing to linking the matrix to the collagen is useful.

The properties of the resulting leather can be controlled by the choices of reagents: this applies to both mineral and organic tanning options. In the absence of a polymerising tannage capable of meeting the stabilising matrix criteria, we are limited to the two-step process. But the two-step process does not have to extend processing times: this approach can contribute to compact processing, moreover it can offer specifically required properties and therefore offers the basis of the production of bio-vulnerable or so-called recyclable leathers. The latter category has not been adequately explored. The definition of tanning refers to the resistance to biodegradation of a previously putrescible protein material: here the resistance refers to proteolytic attack. Therefore, it is feasible to consider tanning processes that incorporate a degree of vulnerability. In this way. high hydro thermally stable leather could be chemically or biochemically destabilised, to allow denaturation of the collagen at moderate temperatures, so that proteolytic degradation can be achieved. Targeting specific groups in the matrix may be sufficient to degrade its effectiveness: options are hydrolases, oxidases, reductases, etc., depending on ihe chemistry of the tannage. Alternately, tannages can be devised in which the stability is temporary, such as semi-metal chemistry that is vulnerable to redox effects . Other approaches are possible and should not be beyond the wit of the leather scientist.


The expression of the link-lock mechanism has made it possible to take quantum steps forward in developments in tanning technology. This new

theory is a simpler and more powerful view of collagen stabilisation than the older model of direct crosslinking between adjacent sidechains; it is a more elegant view. It is no longer worth pursuing the single reagent alternative to chrome tanning, because it does not exist. Indeed, why should we seek an alternative to chrome tanning? It works well, it can be made to work even better and, anyway, by all reasonable judgements it causes little environmental impact. Nevertheless, organic options provide potential for new products from the leather industry. Now we can develop those options using sound logic, based on sound theory. The literature is littered with examples of multi-component tanning systems: the outcome of each is entirely predictable if we apply our new understanding of the theoretical basis of tanning.The continuation of developing tanning and leather technology depends on constant reappraisal of all aspects of the subject. This is the role of leather science. Conventional, received wisdom should not be relied upon without critically reviewing exactly what it means, what it contributes to processing and products and what the wider implications are for the practical tanner. It is important to recognise that the scrutiny of current technology will often identify inconsistencies and misunderstanding of principles: (he technology may work, but the science may not. However, this is not always a bad thing, because it can lead to new thinking, new developments and more profitability in an environmentally sound, sustainable industry.

Chapter- Three

Advantages of the projected tanning method & Solid Waste Management generated from chemically stabilized pelt .


  1. The shaving and trimming are bio-degradable and represent a valuable nitrogenous fertilizer.
  2. The organic tanning material in the shavings, Glutaraldehyde, breaks down relatively easily to form co2 and water and in the quantities present represent no problems.

c. The leather has sufficient thermal stability to allow shaving to the substance as required in the finshed leather. A very important point here is that due to the spreading action of the shaving machine blades with the “leather” in a very malleable state, a significant improvement in area yield and flatness of grain is created, which is then fixed by the tannage, this is particularly noticeable with a chrome tanned leather.

d.The main tannage, retanning, dyeing and fatliquoring may be carried out as one continuous process.

e. when the main tannage is a chrome tannage, the exhaust bath contains very little chrome.

f. A further point is that leathers produced in this way are recycleable even if the finishing of the leather obliges the method to be thermal. The resultant ash would of course be chrome-free.

g.Chemical efficiency is very much greater simply because the weight basis for the processing from the wet-white is the shaved weight of the wet-white is less than the shaved wet-blue weight and from a retanning aspect the total amount materials is therefore correspondingly less . There has been a lot of nonsense propagated by the anti-chrome are heavily retanned the amount of vegetable tanning material required, most chrome-leathers are heavily retanned anyway and furthermore i was looking at renewable resources offering wonderful employment prospects in less developed countries the in southern hemisphere like Bangladesh.

h.The Cross-linkage of the collagen fibres with glutaraldehyde is very stable and irreversible. No free glutataldehyde is detectable in the leather if it is correctly applied. Futhermore, as opposed to formaldehyde tanning, this cross-linkage remaings stable to hydrolysis during extended storage.

i.The wet-white leather has a light colour and a shrinkage temperature of 70-800c,

And it can be mechanically processed in the same manner as wet-blue leather.

j.Treatment of the pretanned hide with synthetic or vegetable tanning agents will produce high-quality, metal-free shoe, clothing automotive  and upholstery leather without any problems as i presumed.

k.Shaving and trimmings from the wet-white process, pretanned with modified dialdehydes contain virtually no tanning agents and furgicides so that they become useful products (e.g. fertilizers, fodder and protein hydrolysates).


The untanned wastes, slurries containing collagen and the residues of leather grades not tanned with chrome , e.g. vegetable-tanned leather, wet-white, etc. can

Be used directly in agriculture, for instance as mulch for fruit growing. The recent modern biofertilizers and leaf-greeners on a purely biological basis always contain some collagen when produced with a proportionate amount of abattoir waste.

The organic combined nitrogen in the shaving is rapidly mineralized to a very high extent in the soil. This results in an excellent fertilizing effect, but also gives need to limit the application rates to adapt the quantities to the given crop.

When compared with mineral fertilizers that are easily dissolved, the fertilizing effect of the nitrogen contained in the shavings was in the order of 50% . Field tests with shavings for cabbage and potatoes revealed an equally good effect.

 Using shavings as feeding (feedstuff for animals)

It would an alternative protein source for poultry feed as it doesn’t content any heavy metal .There may some shortage of essential amino acid in the extracted collagen. So it  would require the additional purchase of certain amino acids.

Production and use of protein hydrolysates from chrome free shaving:

The resulting hydrolysates could be used in leather production (e.g. as retanning agent) or for the production of wetting agents.The heavy metal content (including chrome) is of no consequence for application in industry.

Caustic soda can be used as a simple hydrolysis agent for the hydrolysis of shavings.

 Chapter- Four



 This project paper was focused on producing an intermediate stabilized material subsequently followed by chrome/vegetable or other combination tanning. The raw material first treated as normal process till pickling then the stabilized pelt produced followed by a pretannage using modified gluteraldehyde along with other syntans to facilitate the pretannage. It was then shaved to the required thickness thus reducing the chemical consumption in the following processes as the base weight was on the shaved pelt. Then separately went into two distinctive processes to show the versatility of the stabilized pelt. One was semi-chrome leather processing and the other was vegetable combination leather processing.

Recipe: 3 Pieces wet salted sheep skin taken .

            Trimming: By knife.

            salted weight taken. (All % based n this salted wt.)

Name of operation     %of Chemical Used                  Time   Analytical Checking

Pre-Soaking:    300% water

                                    0.2% Soda Ash

                                    0.2% LD 600                            Run 60 Min.


Main-soaking:300% water

                                    0.4% Soda Ash

                                    0.3% LD 600

                                    0.2% AracitDC                        Run 30 Min.         pH = 9.0 – 9.5

                                 Leave o/n. in the bath.

                                    Next day Drain, wash well.


8.0% Lime

 2.0% Sod, Sulphide.

 0.2% LD 600

Water Requires to make as paste.

Apply the paste on the flesh side of the skin by hand and piled flesh to flesh for 4-6 hours. Or over night. Then unhearing by hand knife.


300% water

 1.0% Vinkol MTV

 4.0% Lime

 1.0% Sod. Sulphide.

0.2% LD 600

Run 30 Min.

Leave in the bath for 2-3 days with regular hauling.

Then scudding by hand very well.


300% water

4.0% Lime

0.5% Soda Ash.

 0.5% LD 600

Run 30 Min.

Leave in the bath for 2 days with regular hauling.

Then fleshing by machine, Wash well.

Chemical wash:

300% Water at NT

0.25% Meta bi Sulfite.            Run 20 Min.



100% Water

2.0% Amm. Sulphate

0.5% Bi-sulphite Drain.

Run 60 Min. Check with phenolphthalein colorless


  100% Water at 370C

 1.5% PBW1

0.5% LD 600

Run 90-100 Min. then check bubble test.

Scudding: By hand, Wash well.


80% Water

8.0% Salt

0.2% ImpropelCO

Run 15 Min.       Check Be 6.5

0.5% Formic acid                    Run 30 Min.

1.0% Sulfuric acid                   Run 2 hrs.           Check pH 2.8

Leave over night.

0.5% Hypo                               Run 30 Min.

Drain half of the pickle bath.


80% Pickle Float

2.0% Derugan 3080 undiluted            Run 180 Min.

pH = 3.2 -3.4

6.0% Syntan PMX

4.0% Syntan PM

1.0% Sodium Formate             Run 180 Min.

pH = 3.5 – 3.7

No wash, Pile up

Summing, splitting, shaving thickness (0.8-0.9)mm.

Same as Above Recipe for semi chrome & vegetable combination tanned leather up to pre tanning. Left Variation in recipe of semi-chrome & vegetable combination tanned leather given below:

 For Vegetable combination from the stabilized pelt

Wet Back:

                        200% Water

                        0.3% Oxalic acid

                        0.5% Silastol E             Run 30 Min.

                        Drain, Wash


                        100% Water

                        2.0% Sellasol NG

                        1.0% Sodium Formate             Run 60 Min.



100% Water

5.0% Megnapol PGN               Run 30 Min.

1% Lipsol EA                            Run 10 Min.

10% Mimosa

                        10% Quebracho

                        4% Tanigan OS

                        5% Relugan D

                        1% Ukatan GM                        Run 120 Min.

                        50% Water                              Run 5 Min.

Keep O/N Next Morning         Run 30 Min.

0.3% Formic Acid                    Run 20 Min.

0.3% Oxalic Acid                     Run 30 Min.



100% Water                600C

4% Lipsol BSFR

2% Lipsol EA

4%  Lipsol LQ                           Run 60 Min.

3% Formic Acid                       Run 40 Min.


100% Water

                        0.4% Vinkol PB                        Run 10 Min.

                        NoWash Pile up

Next Morning Setting, Vacuum 40C, 40 Sec, hand to dry, conditioning, staking, toggling, dry vacuum.

For semi-chrome tanned From stabilized pelt

Wet Back:      

                        200% Water

                        0.3% Oxalic Acid

                        0.3% Silastol E             Run 30 Min.

                        Drain, Wash.


                        100% Water

                        8% Chromitan B

                        4% Syntan CRN

                        1% LipsolSD

                        1% Sodium Fomate                 Run 90 Min.

2% Relugan RF                        Run 30 Min.

50% Water

                        1% Sodium formate

                        0.7% Sodium Bi Carbonate     Run 60 Min.

                        Keep o/n next morning run 30’ drain water.

                        150% Water

                        2% Sellasol NG

                        1.5% Sodium Formate.                        Run 60 Min.



                        100% Water                            350C

                        5.0%, Magnapol PGN                          Run 30 Min.

                        1.0% Perfectol HQ                              Run 10 Min.

3.0% Tanigan R7                                 Run 15 Min.

                        3.0% Mimosa

                        3.0% Quebracho

                        4.0% Basytan AN

                        1.0% Lipsol BSFR                                 Run 90 Min.


                        50% Water                  600C

                        2.0% Lipsol BSFR

                        2.0% Lipsol EA

                        4.0% Ossipol LN undiluted                  Run 60 Min.

3.0% Formic Acid.                               Run  40 Min.

Next Morning Setting, Vacuum 40C, 40 Sec, hand to dry, conditioning,

Staking, toggling, dry vac

 Flow Chart of working:

Table 5: Technical information about Chemicals used for the completion of project:

Name of ChemicalCompositionCompany
 Soda AshSodium carbonate
LD 600Wetting agentBASF
LimeCalcium Hydroxide
Sodium  Sulphide
Vinkol MTVLiming Auxiliary(Mercaptan based)Schill+Seilacher
Soda AshSodium carbonate
Meta Bi Sulfite
Ammonium Sulphate
PBW1Bating agentClariant
ImpropelCOBleaching agentClariant
Formic acid
Sulpharic acid
Chromitan BChrome Tanning AgentBASF
Sodium formate
Remsol OCSChrome Stable FatClariant
Derugan 3080Modified GluteraldehydeSchill+Seilacher
Syntan PMXBleaching syntanHariyana,India
Syntan PMWhite replacement syntanHariyana,India
Sodium formate
Silastol EDesreasing agentSchill+Seilacher
Syntan CRNChrome SyntanSmith & Zoon
LipsolSDChrome stable fatSchill+Seilacher
Relugan RFacrylic resinBASF
Sellasol NGNeutralizing SytanTFL
Magnepol/PGNAcrylic rasinTFL
Perfectol HQHydrophobic fatSchill+Seilacher
MimosaVegetable tanning agent
QuebraohoVegetable tanning agent
Tanigan OSReplacement syntanLANXESS
Relugan DMelamine SyntanBASF
Uktan GMDispersing SyntanSchill+Seilacher
Lipsol BSFRSynthetic FatliquorSchill+Seilacher
Lipsol EAModified Fish oilSchill+Seilacher
Lipsol LQLecithine based fatliquorSchill+Seilacher
Vinkol PBSodium Poly Phosphate based

Sequestering agentSchill+Seilacher

 Reaction Involved:

As the use of formaldehyde declined, the use of glutaraldehyde as a replacement grew.

Consideration of the structure of the monomer might lead to the conclusion that the tanning reaction involves the creation of a four-centre crosslink, since each end can theoretically link two amino groups. Intuitively, this is doubtful, because the coincidence of four adjacent amino groups is unlikely and the entropy penalty of such a bond would be very high. However, it is not necessary to invoke a mechanism that is chemically unsound: the polymer structure based on condensation of the hydrate, is a linking structure between two reacted glutarldehyde molecules.

  1. Within a complex scheme of reactions, glutaraldehyde forms Schiff bases with protein and these are established by other gluraldehyde molecules.
  2. There is no evidence that crosslinks are formed.
  3. Three glutaraldehyde molecules are fixed per lysine amino group : there is no evidence for polymerized matrix.

Chapter- Five



The  leather samples and their finished leather samples were tested for their various physical properties. These properties indicate the quality of the finished leathers produced. Due to limitation of time and the availability of instruments, selected physical tests were accomplished and these tests are briefly discussed below:

The two main areas of tasting are:

1. Physical Testing

2. Chemical testing

Physical testing: The term “Physical testing” has perhaps come from the English word “Physic” means body. Physical force means body forces. Similarly physical testing of leather are those testing on leather which where once carried out with the different parts of the body of the purchasers. The look, the uniformity of colour, gloss of finished surface etc was judged with the eye. Different organizations of different countries no doubt developed different instruments and methods for physical testing of leather, but unfortunately most of them were not perfect for the purpose even through. Some of them have been accepted internationally and are known as “Official methods” of physical testing of leather. In the countries like India, Pakistan, Bangladesh, and similar developing countries, official methods of physical testing are followed in laboratories for exportable leathers.

The shoe upper leather samples and their finished leather samples were tested for their various physical properties. These properties indicate the quality of the finished leathers produced. Due to limitation of time and the availability of instruments, selected physical tests were accomplished and these tests are briefly discussed below:

i) Tensile Strength and Percentage of Elongation At Break Based on SLP-6, IUP/6:

The tensile strength and elongation at break was measured by Testometer. Tensile strength is the force (Kg) per unit area of cross-section (Sq. cm) required to cause a rupture of the test specimen.

Breaking load mainly depends upon the number of collagen fibers acting in the direction of applied load, so it is more or less constant for a piece of leather specimen because the number of fibers in that piece is always constant.

The extent of elongation of the leather specimen at the time of its breaking, while applying the tensile force, expressed as the percentage on the original length of the said specimens the elongation at break. Elongations   at break for these   specimens are calculated from the distance of the jaws after breaking was occurred.


Tensile Strength and Elongation at break.
Perpendicular (Kg/cm)Parallel (Kg/ cm)
Sample NoTensile strength%

Elongation at breakTensile strength%

Elongation at breakVegetable combination tanned336.363 kg/cm2103%236.363 kg/cm2100%Semi-Chrome117.00 kg/cm269%125.00 kg/cm274%Pre -Tanned Leather156.5 kg/cm278%147.52 kg/cm269%

ii) Stitch Tearing Strength (double hole); SLP-8

The double hole stitch tearing strength can be defined as the load (Kg) required to tear the sample of the leather between two holes of 2mm. diameter each and whose centers are 6mm. apart, express of its unit thickness (cm).


Sample NameStitch Tear Strength
Semi-Chrome16 kg/cm
Vegetable combination tanned90 kg/cm
Pretanned Leather23 kg/cm

 iii) Tearing Strength SLP-8

Table no. 8 Data for Tearing Strength:

Sample NameTearing Strength
Semi-Chrome12.5 kg/cm
Vegetable combination tanned36 kg/cm
Pre Tanned Leather14.5 kg/cm


Table: 9 Data for Shrinkage Temperature:

Sample NameShrinkage Temperature
Semi Chrome890 C
Vegetable combination tanned leather770 C
Pretanned Leather720 C

 During Shrinkage Temperature determination

Table: 10 Data for Moisture Content: 

Sample NameMoisture Content
Semi Chrome17.5%
Full Vegetable13%.
Pretanned Leather52%

 During Fat determination

Table 11 Data for Fat content:

Sample NameFat content
Semi Chrome8.7%(including other soluble matters)
Full Vegetable9.1%(including other soluble matters)

 Data for Boil Test:      Semi Chrome didn’t stand


The shrinkage temperature of the stabilized pelt was 72o C and I found no trouble during shavings. And chemical consumption reduced to a greater extent. I carried out the sammying operation from a tannery and an unexpected chrome contamination occurred which appeared prominently after completing the crust. The properties of vegetable combination tanned leather and semi –chrome tanned leather indicated that it is possible to go for versatile tanning option from this stabilized pelt. The tensile strength, tearing strength, stitch, tearing strength, shrinkage temperature of vegetable combination tanned leather respectively are 336.363. Kg/cm2, 36 kg/cm and 90 kg/cm, 770 C and the body, touch, softness is very good except minor loose in flank, belly part. Also the tensile strength, tearing strength, stitch tearing strength, shrinkage temperature of semi- chrome tanned leather are 117 kg/cm2, 12.5 kg/cm  16 kg/cm, 890 C. The thermal stability of semi-chrome tanned leather indicates its poor strength which might be due to improper distribution of Chrome. There are a lot of scopes to work with multi tannage from pretanned pelt.

There is opportunity of producing some value added products in an environment friendly way from the shavings of the stabilized pelt thus contributing to solid waste management.


As i had limitations in raw materials and other facilities, it wasn’t possible for me to produce other probable combination tanned leather for attaining my aim. The contamination by chrome in the sammying could be solved by the deep color like black dyeing in finishing. As I could not carry out the shrinkage temperature test by DSC so, it wasn’t possible to find accurate result of shrinkage temperature of sample leather. Amino acid analysis should be done for using it as a protein supplement for feeding like poultry feed. Finally, I should have used more fat for semi- chrome tanned leather like in vegetable combination tanned leather. More research should do into the matter of solid waste management generated from stabilized pelt.The chromic oxide content of the produced leather should have measured as I couldn’t do it due to instrumental problem.

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