most important organic tanning agents are the vegetable
tannins present in tanning liquors.They are prepared
from certain parts
of plants by aqueous extraction. Their
tanning power has been appreciated for a long time and
Babylonian texts have recorded their use.
Vegetable tanning materials occur
in nearly all forms of plant life. They are used
commercially where the amount of tan is high and large
quantities can be extracted economically. Other
considerations are color and particular properties of the
Parts of plants used
as sources of tannins.
Vegetable tannins are dervatives
of phenol(with several OH groups). Phenols are more
acidic than alcohols (pKa~10), but are weak acids
therefore form salts only with strong bases. Solubility
of phenol ~7% in cold water. But the sodium salt is
Vegetable tannins react with atmospheric oxygen,
particularly at high pH values to form quinones (for OH
groups that are ortho-para to one another).
Vegetable tanning liquors are very complex and
continually changing physically, chemically and
biologically. They are partly colloidal but easily
aggregate and will then sediment. Yiests moulds and
bacteria can grow in the liquors, the main consequence
being the fermentation of sugars to acids.
Tannins are not the only constituents of vegetable
tanning liquors. The non-tans include, apart from the
sugars, acids and their salts, hemicelluloses, pectin and
lignin, as well as compounds containing nitrogen and
phosphorus. The acids and their salts are the most
important for the tanner. Apart from the nature of the
tannins themselves, the acids and salts are the principal
means of controlling the astringency of liquors and whole
process of vegetable tannage.
Several acids, such as gallic, oxalic, citric, tartaric,
and phosphoric are present in the original tanning
material and sugar fermentation can yield carbonic,
acetic and lactic acid.Gallic and other phenolic acids
can arise from the breakdown of tannins. Polyuronic
acids, whether from hemicellulose or pectin, contribute
to the acidity.
are mainly two classifications for vegetable tannins:
catechol tans (condensed
tans)- they are similar to catechol,
usually give red-brown color and are astringent.
Condensed tannins are not decomposed by acids. They
gradually polymerize becoming phlobaphenes, insoluble
derivatives. On dilution and standing, they deposit a
thick, reddish sludge called reds
(phlobaphanes). In order to prevent
reds the extract is solubilized by heating
under pressure with sodium bisulphite (3-8 % on the
extract) at 98 0C.
||natural pH= 4.8
||natural pH= 4.8
||natural pH= 4.8
||natural pH= 4.9
obtained from the heart-wood of the quebracho tree which
grows in south America chiefly in Argentina and
Paraguay.Ordinary- or warm soluble Quebracho is the
natural extract rich in condensed tannins(phlobaphenes)
and is not easily soluble. Its use is therefore limited
to tannage of sole-leather according to the process known
as hot-pitting.It results in a red-brown
color and excellent water-proofness.
||natural pH= 4.9
Ordinary extract is subjected to
sulphitation process which transforms it into soluble
tannins. The main properties are: rapid penetration into
the pelt, a high tannin and low non-tannin content.The
rather low acid and medium salt content characterize them
as mild tanning agent(can also be deduced from their pH
Sulfiting as applied to the condensed tannins consists in
treating their solutions with a mixture of sodium sulfite
and hydrosulfite. A part of the bonds in tannin is then
split. Initially phlobaphanes dissolve, then not only the
size of the tannin molecule decreases but changes in the
||natural pH= 4.0
||natural pH= 4.5
||natural pH= 3.5
||natural pH= 4.0
Gambier is a
solid extract(cubes) obtained from the leaves and stems
of Uncaria Gambier, a plant which occurs both wild and
cultivated in the Malayan region. Besides the predominent
catechol tannins it also contains sugar,salts, waxes,
oils, and even minral substances. If used alone,
tends to give a rather flabby leather. When used in
retannage, it imparts suppleness and a smooth feel. A
peculiarity of gambier is to give leathers an excellent
capacity of being glazed and very glossy dyeings.
A lot is known about the tannins of spruce bark.
These consist of polyphenols mainly in the form of
glucosides. The principal polyphenol is called
Glucosidic groups are known to be associated with reduced
tanning power and thus spruce bark is a relatively poor
tanning material. Stilbenes and their glucosides have
also been found in wandoo, an extract of eucalypt.
The poor tanning properties of glcosidic groups is
explained by the ease with which aglucon is oxidated.
Piceatannol in an oxygen free medium does not cause a Ts
increase. In presence of oxygen, like in other flavanols,
quinone systems appear which are bound irriversibly to
the pelt. Tanning consists of oxidizing piceatannol to
its quinone and then addition of free collagen amino
groups in the 1,4 position via covalent bonds. Glycosides
of piceatannol do not have tanning capacity.
The great majority of condensed tanning materials contain
compounds derived from flavan. Flavones, such as
quercetin, are fairly stable, almost insoluble substances
are known to be a frequent source of yellow colors in
nature. The closely related but colorless and soluble
catechin was found in gambier leaves. It is not a tannin
but during the commercial preperation of the extract it
is converted into tannins, the liquor darkening at the
same time. (+)-catechin is also widely distributed in
nature and constitutes a minor component of wattle bark
and heartwood, chesnut bark, spruce bark, oak
bark,quebracho bark, tea and cola. Compounds related to
catechins occur in small amounts in some tanning
materials. (+)-gallocatechin, which has an extra hydroxy
group at the 5-position, is present in the barks of
chesnut, wattle, and oak, as well as in tea, and wattle
bark also contains some (-)-robinetinidol, which is
(+)-catechin without the 5-hydroxy group.
Most of the red and blue colors of flowers are due to
other derivatives of flavan, the anthocyanins such as
cyanidin and delphinidin chlorides. In some flowers and
fruits colorless substances occur, which on treatment
with strong acids give red colors due to
anthocyanidines.The simplest represantatives of these
colorless compounds are flavan-3,4-diols.
Thus wattle heartwood contains a small amount of
(+)-mollisacacidin, whereas in quebracho sapwood there is
almost 2 % of the (-) enantiomer. In oak bark 0.5 %
leucodelphinidin is present and 8-10 % of an isomer in
Leucocyanidines have been isolated from some
species of mangrove.
To the tanner it is more important to know the
constitution of the tannins present in condensed liquors
than to know the formula for the relatively simple
compounds since these are present only in small
amount,except in quickly and very carefully prepared
extracts of freshly gathered material. Condensed tannins
are formed by condensation of these simpler compounds and
it is of importance whether they are a result of acid
condensation or of oxidative condensation.
tans) - esters of glucose and gallic
acid (glucoside tannins) and its derivatives which are
Hydrolizible tannins, having a polyester structure,
easily hydrolize to the respective sugar or polyhydric
alcohol and polyhydric phenol with carboxyl group.
Hydrolysis products my be classified into gallotannins,
derivatives of gallic acid, and ellagitannins,
derivatives of ellagic acid.
These are more yellow-brown than catechols. Their sugar
content may lead to acid fermentation during long
tannage, when a deposit of sand colored sludge known as
bloom is also formed. The later is a result
of enzyme action causing hydrolysis of the ester link,
releasing insoluble acids, eg. Ellagic, chebulinic from
the tannins. They are usually less astringent than
||natural pH= 3.2
Myrobalans has been
extensively studied. There up to 12 % chebulinic acid and
2 % chebulagic acid, two tannins which have been isolated
in crystalline form. Chebulagic acid is identical with
chebulinic acid, except for an additional link between
the positions marked.
about 2.5% chebulagic acid and 5% corilagin.
Tannins of algarobilla contain other related structural
units, namely the quinone of
4,4,5,5,6,6- hexahydroxydiphenic acid
and brevifolincarboxylic acid.The quinone is yellow and
similar components may account for the color of some
myrobalans tannins also.
contains a convenient quantity of soluble organic
non-tannins and of organic acids of natural origin and
others which develop during manufacturing proces and is
characterized by considerable astringency. Such
astringency manifests itself in a relatively low speed of
penetration of the tanning matters into the pelts and in
the property in which these tanning substances possess of
fixing themselves irreversibly and in large amount to the
These properties make Chestnut extract especially
suitable for the tannage of heavy hides and of sole
leather in particular, as by its use it is possible to
obtain a firm and compact yet flexible leather of good
color, light resistant with low water absorption.
Sweetened Chestnut is a chestnut extract with varied
astringency therefore different behaviour in the tanning
||natural pH= 3.6
||natural pH= 4.0
It is obtained from the leaves of
Sumac(Rhus Coriaria). It is the purest among vegetables
hydrolizable tannins. Using sumac as the only tannin a
full and even leather of yellow-hazel color is obtained.
In dyeing with both anionic and basic dyestuffs an
excellent color levelness is achieved
The tannin of Caesalpinia Tinctoria(bear the fruits
called pods) has pyrogallic character but small
quantities of catechol derivatives also occur in it. It
contains practically no coloring substances therefore
permits very bright and light-resistant leathers. Tara
gives leather fullness and softness and at the same time
a fine, closed grain. In leathers tanned with tara the
grain resistance to breaking load is higher than that
achieved with any other vegetable tannin.
obtained from Chinese galls contains apart from tannin,
galic, m-digallic and trigallic acids. On hydrolysis
gives D-glucose and gallic acid in a molecular proportin
of roughly 1:10. It is thought that m-digalloyl groups
are attached to each of the five free hydroxy groups of
glucose, but it is possible that some of them are simple
galloyl ones, whereas others are trigalloyl ones.
Tannins of Turkish galls and Sumac are similar, but with
a lower propotion of gallic acid to glucose. Tannins such
as these which yield only gallic acid and sugar on
hydrolysis are called gallotannins.
Ellagitannins give on hydrolysis a precipitate of ellagic
acid. Ellagic acid is not present as such in tannin
molecules, but is derived from hexahydroxydiphenic acid.
Lignins constitute the material that fills out the spaces
between the microfibrils of the cellulose in certain
plant cells and stiffens the cell structure. They are the
charasteristic constituents of wood, but are also present
in bark and straw.
In young tissue of trees, coniferin ie. coniferyl-4-b
-D-glucoside, is present; a b-glucosidase liberates
coniferyl alcohol from it in the wood and bark and lignin
is formed in situ. Deposition of lignin brings about the
physiological death of the cells. Lignins are insoluble
in water, organic solvents and even sulphuric acid.
They contain 59-67% carbon, some methoxy groups, can be
oxidized to give up to about 25 % aromatic aldehydes and
react with sodium bisulphite, sodium hydrosulphide and
thioglycollic acid, HSCH2COOH. It is known
that coniferol alcohol can be polymerized to give a
number of intermediate products, such as
dehydrodiconiferyl alcohol, a-guaiacyl
glycerol b-coniferyl ether and pinoresinol.
In paper production, lignin has to be removed from the
cellulose fibers, and one way of achieving this is by
treatment with bisulphite under pressure, which
solubilizes the lignin, leaving the cellulose unaffected.
In the process, the hydroxy groups of aromatic side
groups can be replaced by sulphonic groups to give RSO3H,
SO2 can add
to double bonds to give RSO3H and
ethers may split to give -OH + H3OS-. Such
sulphite cellulose, lignocellulose or ligninsulphonicacid
liquors have low tanning power but are very cheap. They
are sold alone or as blends with syntans and tanning
extracts. Their main function is to solubilize the less
soluble components, to speed up tannage, and to act as a
filler.Condensed tanning materials are subjected to the
sulphiting process in order to solubilize the less
soluble fractions, when reactions similar to those with
Theory of tannage with vegetable and synthtic tannins:
The first step in tanning is the binding of hydroxyls of
vegetable tannins to the active collagen centers.The next
step- the binding of further tanning molecules continues
until the interfibriller spaces are filled. The collagen
active centers which react with vegetable tannins are
various functional groups of its side chains and peptide
bonds as well. This stage ends when collagen has absorbed
½ of its weight of vegetable tannins. The difference
between Cr and vegetable tanning becomes striking here,
since 3% of Cr tanning agent is sufficient to form stable
bonds between collagen and tanning agent.
Tanning is carried invariably under acid conditions,
although it can be done at pH 1-9. The reason lies in the
fact that liquors containing vegetable or synthetic
tannins are naturally acidic (at higher pH s polyphenols
may oxidize and dark colors may dye the leather).
Below pH 5, limed collagen is + charged, charges on basic
side chains. Vegetable tannins normally carry negative
charges due to dissociation of their carboxy and phenolic
groups. Tannage therefore involves ionic interaction
between collagen and tan.
In pelt, structure can cause difficulty with
accesability. Because of case hardening
(rapid overtannage of outermost layers), pelts are not
readily penetrated. It is important to distinguish
between penetration and fixation.
Apart from ionic interaction that brings the tans close
to collagen fibres, other factors like H-bonds and van
der Waals attractions must be involved since polyamides
such as nylon can bind tannins via their -CO-NH- groups.
The weak character of such binding forces could well
account for the relatively low shrinkage temperature of
Some firmer covalent bonds (crosslinking collagen chains-
and so contributing to tanning) may arise from quinones
and possible aldehydes formed by the oxidation of
Much of the tannin may be physically held within the
leather structure and be independent of specific chemical
bonding to the collagen, since electronmicrographs of
vegetable tanned leather clearly show the presence of
large aggregates of tannin, lying between hide fibers.
Undoubtly this accounts for the well known
filling action of vegetable tannage and the
physical properties of roundness and fullness associated
with this tannage.