Conservation Science

Textiles 81

Acid and Alkaline Hydrolysis. Silk is quite resistant to chemical attack, as
a result of its highly crystalline nature. However, some reagents, e.g.those
used in the original processing of the cloth and atmospheric pollutants, will
affect deterioration, notably over the longer term.
Acids, particularly in higher concentrations, attack the amorphous regions
relatively rapidly and at random points along the chain, leading to the hydroly-
sis of peptide bonds (Figure 19). Microstructural changes will also ensue as
the hydrogen bonding and salt linkages that dictate the secondary and tertiary
structure of the polymer are disrupted. This results in brittleness and the loss
of mechanical strength. Additionally, the presence of acids can induce a
rearrangement of peptides to esters and ethers. The action of some acids is
used commercially to induce sought-after properties: weak acid (particularly
acetic and tartaric) treatment imparts ‘scroop’, a characteristic rustling believed
to be the result of microcrystalline deposits, while concentrated sulfuric acid,
which leads to the contraction of the fibres and the modification of side-chains,
alters lustre and softness to yield fibres for the production of silk crêpe.
Alkalis hydrolyse silk more slowly than acids, the reaction proceeding
most rapidly from the ends of the chains. However, as with acids, the pres-
ence of alkaline agents can additionally break hydrogen bonding and salt
linkages, leading to a similar disordering of the structure. Concentrated alka-
line solutions, especially if hot, then cause the complete dissolution of silk.
Although generally effecting chain shortening (Figure 20), alkaline conditions
also have the potential to cause cross-linking, e.g.via the formation of lysine
to serine bridges (giving lysinoalanine), leading to some increased rigidity.
Fibre-included weighting agents can promote hydrolysis of the fibroin –
e.g.over time tin salts are themselves hydrolysed to amphoteric species that
can affect both acid and alkaline attack.

N

HO

( )RH

n O^2

·OOH

( )n

N

HO

R·

Figure 18Free radical thermal oxidation of silk

N

O

R H

R' O

R

OH

R'

H 2 N

H+

H+

H 2 O N

R H

R'

OH 2

OH

+

Figure 19Acid hydrolysis of silk