and, as soon as micro-fissures or other lesions occur, water and salts from the
atmosphere start penetrating behind the coating and will subsequently lead to
efflorescences. Therefore an artificially-induced oxalate layer should permit
some porosity. First attempts to apply microorganisms at site to induce excre-
tion of oxalic acid, and then transformation, were restricted, because it would
have been difficult to restrict microbial activity to a particular level – a prob-
lem that has been discussed before.
Calcification by bacteria. Lime-water application seems not to guarantee
sufficient consolidation. Precipitation of calcite by microorganisms is known
to occur in freshwater and marine environments, as well as in soil, and this led
to trials of bioremediation on heritage surfaces. Very recently, approaches
using calcification due to bacterial activity have even been carried out partic-
ularly on stone and architecturalsurfaces (see page 236). Remediation by
several bacteria was shown to result in the formation of newly developed cal-
cite precipitations. For this, chemoorganotrophic bacteria, as well as nutrients,
were supplied to the surfaces. Up to now the role of bacteria is not completely
understood, however, studies on marine bacteria revealed three main mecha-
nisms: (i) calcium-bindingmembrane proteins, (ii) ionophores, and (iii) extra-
cellular materials on cell surface.
These bioremediation treatments are based on the assumption that bacteria
will not cause biodeterioration at some point in the future after the applied
nutrients have been exhausted. If the new precipitated calcite completely
encloses the cells, this might be true. If this is not true, there is no reason why
they should not live on other nutrients available from the environment.
Laboratory investigations resulted in bio-induced calcification from both
deadcell material and organic matrix macromolecules from the shells of mol-
luscs. Different results were obtained by accident after lime impregnation
was done on site in Denmark. Calcite precipitation on fungal hyphae was
demonstrated after impregnation, which resulted in at least a temporary con-
solidation. However, fungi have to be considered as potential detrimental
microorganisms, especially if calcification does not result in embedding of
the cells, so inhibiting exchange with the environment and resulting in cell
death. Interestingly enough, carbondioxide release by cell respiration may
also contribute to calcification but this has not been considered.
To summarise, bioremediated calcite precipitation as a consolidation treat-
ment for wall paintings has not yet reached the stage of development to be rec-
ommendedwithout more intensive tests.
It is clear that a consolidation treatment for wall paintings suitable for all
situations cannot be expected in the near future. The treatment will be deter-
mined by the amount of consolidation necessary, the material and the form of
application possible at the specific site. It is also necessary to take climate into
Wall Paintings: Aspects of Deterioration and Restoration 259