Conservation Science

76 Chapter 4

a high degree of chemical resistivity. The amorphous and semi-amorphous
regions that comprise the remainder of the fibre contain a high proportion of
residues possessing bulky side-groups, which dictates the low degree of order-
ing here.
As with other natural fibres, silk has a hierarchical microstructure – about
five anti-parallel -sheets, each with around 12 chains, aggregate to form par-
allel,crystalline microfibrils (approximately 10 nm in diameter), bundles of
which make up fibrillar elements (roughly 1m across), which in turn asso-
ciate to comprise the individual fibroin filaments (7–12m); at each level of
organisation, the ordered elements are embedded within amorphous matrices
derived from the non-crystalline components. Once again, then, the behaviour
of the structural composite can be understood in terms of the semi-crystalline
array of its component parts.

5 .7 Å

3. 5 Å

5 .7 Å a

b

c

9.4 Å

N

N

N

N

N

H

H

O

O

R

R

N

R

H

H

H

O

O

O

H

H

O

N

N

H N

N

N

H

H

H

O

O

O

O

R

R

R

R

R

R

O

O

O

O

H

H

H

N

N

H

N

N

N

O

H

H

O

O

O

H

H

H

R

N

R

R

O

O

H

H

N

N

N

N

N

O

O

N

N

N

N

N

H

H

O

O

R

R

N

R

H

H

H

O

O

O

H

N

N

N

N

N

H

H

O

O

N

H

H

H

O

O

O

H

R

R

N

N

N

N

N

H

H

O

O

N

H

H

H

O

O

O

H

R

R

Figure 15Structure of silk -crystallites; b lies along the fibre axis