Leather 113
The results showed that while none of the books stored in the clean Welsh
environment had deteriorated to any significant extent, nearly all the volumes
subjected to London’s acidic pollution exhibited evidence of decay, some
within less than 10 years. In addition, the superior ageing properties of leathers
prepared with hydrolysable tannins were confirmed. Of the various chemical
analytical determinations undertaken, the only results to show any correla-
tion with the degree of deterioration observed were those for the number of
N-terminal amino acid groups on the protein. This figure reflects the amount
by which the collagen polypeptide chain had been broken and was consid-
ered to be evidence for hydrolytic deterioration.
Further work over a long period confirmed these general conclusions. It
also showed that retanning vegetable-tanned leather with mineral-tanning
materials, particularly aluminium salts, gave increased resistance to acid deteri-
oration and this led to the formulation of a British Standard for Archival
Quality Bookbinding Leathers. Unfortunately, imparting the desired chemical
resistance to acidic deterioration was associated with a loss of the specific
physical handling characteristics required by bookbinders.
From the beginning of the twentieth century it has been suggested that
chemical deterioration of leather is due to a combination of oxidation and
hydrolysis. Research undertaken by a group of conservation scientists funded
under the European Commission STEP and ENVIRONMENT programmes has
confirmed and refined this hypothesis. These projects examined the changes
that occur to the amino acids and tannin polyphenols as vegetable-tanned
leathers are aged naturally and artificially. From the many results published
the following appear to be the main conclusions.
In less polluted environments, deterioration is associated mainly with oxida-
tive change. This can be activated by the effects of heat, light or the action of
free radicals. The latter can arise from the autoxidation of lipids or the break-
down of tannin polyphenols. The main effect on the collagen macromolecule
is to convert the basic side groups of lysine, hydroxylysine and arginine to acidic
groups, such as glutamic and -amino adipic acids (Figure 10). In a similar
manner, proline and hyroxyproline can be oxidatively degraded to give glu-
tamic and aspartic acids (Figure 11). Leathers naturally aged under relatively
unpolluted conditions were therefore found to contain reduced quantities of
basic amino acids and higher amounts of acidic ones compared with new
leather. They were also found to contain -amino adipic acid, which was
absent in new leather. In heavily polluted atmospheres, on the other hand, the
main effect appears to be the direct acid hydrolysis of the bonds between
amino acid residues in the main protein chain (Figure 12).
Interestingly, it was found that sulfur dioxide was absorbed twice as rap-
idly from polluted atmospheres by leather produced with condensed tanning
materials than those processed with hydrolysable tannins. It was also noted that