Textiles 71
damage to the polymer. As with other forms of attack, microbial damage will
preferentially occur where there is improved access to the polymer chain, i.e.
in the amorphous regions and in areas of pre-existing damage, such as pho-
todegradation or swelling. Hemicelluloses and pectins are also the targets of
biological degradation, but lignin appears to exhibit some resistance.
3.4 Acidity Measurements on the VictorySail
In the absence of active biological attack, the two dominant processes leading
to natural deterioration of cellulose are acid-promoted hydrolysis and oxida-
tion. The latter is quite slow, and we would expect cellulosic artefacts that are
above neutrality and kept in a stable environment to remain in relatively good
condition for hundreds of years. However, those that are somewhat acidic can
show significant signs of deterioration within a few decades.
As we have indicated above, the ageing of fibres like flax often results in the
formation of acidic degradation products. These, together with adsorbed acidic
atmospheric pollutants, can continue to promote further damage through the
hydrolytic depolymerisation of cellulose. So, in relation to its stability, it seemed
important to estimate the acid content of the Victory sailcloth. There are two
approaches that can be taken: surface measurements on the fabric and acid
extractions.
We recorded the surface pH by pressing a moistened non-bleeding indica-
tor paper to the fabric for a short period, after which the resultant colour
change was interpreted. These revealed pH values between 3.3 and 4.0 in the
various locations tested, whereas for new linen sailcloth the value was around
6.0. While it is difficult to deduce the true acid content from this type of
assessment, nevertheless values below pH 4 are generally regarded as a cause
for some concern over the longer term, leading to recommendations for reme-
dial action.
More detailed total acidity assessments were performed on short sections
of the isolated yarns by extracting mobile and fibre-bound acid with degassed
saline, before measuring the acidity with a microelectrode and pH meter.
These saline extractions allow a better understanding of acid distribution, with
the presence of salt ensuring the equilibration of immobile acid within the
solution. Such acids may be strong (e.g.those arising from atmospheric pol-
lutants) or weak (such as the carboxylic acids that are natural components of
plant fibres or generated through oxidative deterioration). The total acid con-
tent deduced in this manner will represent a lower estimate since the weak acids
may not be fully ionised, and this underestimate will be most marked for the
more acidic samples.
Yarns from the modern, unaged sailcloth had a small acid content, prob-
ably just reflecting residual weak acid constituents of the plant fibres, such as