Leather 99
Formaldehyde was utilised to produce washable white leathers from the
end of the nineteenth until the last third of the twentieth century when its use
was phased out due to health and safety concerns. Other aldehydes have tan-
ning properties but only glutaraldehyde has been employed successfully on a
commercial scale.
Mineral Tannages. The effects of chromium salts on skins were first inves-
tigated during the mid-nineteenth century and by the 1870s, leather was being
produced commercially with these materials. Today over 90% of leather is
manufactured using trivalent chromium compounds.
The reactions involved in the chrome-tanning process are those of coord-
ination complexes. They involve the interaction between charged carboxyl
groups on the collagen macromolecule and polynuclear chromium(III) coord-
ination compounds. The most widely used chrome-tanning material is 33% basic
chromium(III) sulfate produced industrially by reducing sodium dichromate
with sulfur dioxide.
In aqueous solution, most chromium(III) compounds exist as coordination
complexes. Depending on pH, concentration, temperature and other factors
these react to form a range of stable basic complexes. Under the conditions
usually chosen at the start of chrome tannage (pH 2.5–3.0, temperature 20°C)
the predominant species contain two chromium atoms linked with one or more
hydroxyl ligands forming ol bridges. Bridging sulfato ligands are also present
which further stabilise the structure. At this pH the majority of carboxyl side-
groups on the protein chain are not ionised and generally non-reactive. The
relatively small chromium compounds can therefore penetrate into the skin
structure. As the tannage progresses the pH is gradually raised to 3.5–4.0.
This has a dual effect. The protein’s carboxyl side-groups become increasingly
ionised and the chromium complexes become larger as the number of ol bridges
increases. Boththese factors enhance reactivity and favour the formation of
stable cross-links.These effects are increased by raising the temperature as
tannage proceeds, and by the addition of other ligand-forming compounds to
control the rate and type of reaction. An outline of some of the reactions,
which take place during the tanning process is given in Figure 2.
The use of naturally-occurring potash alum for leather-making dates back
at least to pre-dynastic Egypt. Impregnating the skin with pastes of alum, salt,
flour, egg yolk or olive oil and water yielded a soft, white product.
As with chromium (III) compounds, aluminium (III) complexes react with
collagen’s free carboxyl groups. However, under the conditions prevailing
during tannage, the complexes do not contain stabilising sulfato ligands and
are readily hydrolysed. Nor do the carboxyl groups coordinate so readily into
the aluminium complex. The cross-links between adjacent collagen molecules
are therefore much weaker and the aluminium salts are easily washed out of