Conservation Science

100 Chapter 5

the skins. Thus traditionally alum-tawed materials cannot be classed as true
leathers. However, if the aluminium complexes are modified by adding ligands,
such as formate or citrate, and the pH is increased, more stable products with true
tanning power can be produced. Commercial aluminium tanning materials of this
type are available. Aluminium tanned leathers have the advantage of being white
but they lack the stability and handling qualities of chrome-tanned products.
Coordination complexes of titanium(III), titanium(IV), zirconium(IV), iron(III)
and the lanthanides all have tanning properties, particularly if they are stabilised
by the addition of appropriate ligands. None of these, however, give leathers
with the properties of chrome-tanned materials.

Vegetable Tannages. Aqueous extracts of barks, leaves, wood, roots and
twigs of a large number of vegetable species have been employed to produce

Cr

H 2 O OH 2

H 2 O

H 2 O OH 2

SO 4

Cr

H 2 O OH

H 2 O

H 2 O OH 2

SO 4

OH 2

Cr

H 2 O

H 2 O

H 2 O

OH

OH OH 2

O

OH 2

Cr

O

O O

S

Cr

H 2 O

H 2 O

H 2 O

H 2 O

OH

OH

Cr

OH

OH

Cr

OH

OH

Cr

OH 2

OH 2

OH 2

H 2 O

H 2 O

OH 2

OH 2

OH 2

+

+ H+

4+

33% basic chromium(III) complex

50% basic chromium(III) complex

2+

hydrolysis

olation

basification

Figure 2Formation of polynuclear chromium(III) complexes