processes by interacting with salts in stone. When limestone has been subjected
to both microbial and salt weathering, under different temperature/wet/dry
cycling regimes, weight loss was higher with microbes alone (7.7%) than
Na 2 SO 4 alone (4.9%) but the two agents together more than doubled the addi-
tive effect and caused extensive exfoliation and fissure formation (Figure 9).
Thus, by interacting with the effects of the salt, microbial biofilm growth can
enhance water content and enhance physical, mechanical pressures on stone
during wet/dry cycling.
4 Conservation Practices
A number of definitions of conservation have been developed in relation to
stone degradation, but most have their origin in the Venice Charterof 1964 that
tried to establish guiding principles for the preservation and restoration of
ancient buildings. The Charter highlights the importance of contextualizing
the building to be conserved and emphasises the need for recourse to scientific
methods for conservation and preservation practices. It also provides guide-
lines for practices to be employed, stating these should not be out of scale
with the setting of the monument and that restoration should be:
to preserve and reveal the aesthetic and historic value of the
monument and is based on a respect for original material and
authentic documents
By 1994, the Nara Conference on Authenticity built upon theVenice Charter
to define conservation as:
All operations designed to understand a property, know its history
and meaning, ensure its material safeguard and, if required, its
restoration and enhancement.228 Chapter 9
Figure 9Stone discs showing exfoliation after treatment with salts and mixed microbial
populations