Conservation Science

66 Chapter 4

samples fibre fracture and slippage of the ultimate cells (which are around
7 cm in length) are important. Slippage of the fibres themselves needs to be
borne in mind as well, once the length of the yarn sample approaches that of the
staple, i.e.the length of the fibres, which is generally a few tens of centimetres.
We were now in a position to calculate the load-bearing capacity of the
Victorysail, and, taking account of the holes and losses at the head of the sail,
could estimate, conservatively, that the sail would still be able to take its own
weight and as much again, despite its aged condition. However, besides the dan-
ger of ripping the yarns and fabric, we were also concerned with the perman-
ent stretching that could result from free-hanging display. In addition to the
rôle of slippage in catastrophic failure, it can also affect such deformation; this
occurs when individual fibres in a yarn slip past each other, when the ultimates
within a single fibre slide over each other, and when the secondary bonds that
bind polymer chains together break, allowing the chains to creep past each
other before new bonds are formed. From further direct investigations we deter-
mined that the sail would undergo a permanent deformation long before it fails.
We were then able to offer advice concerning the handling of the sail during
its conservation to avoid undue and prolonged stress, and suggestions for its

head

foot

17

m

4.6 6.4

5. 3

3 .7

5 .8

6. 3

5 .2

4.8

4. 5

24 m

Figure 8Average yarn tenacities (cN tex^1 ) at various point across the sail