CaCO 3 in the form of calcite. Although biomineralisation has been observed
for many years, the potential for its use in stone consolidation has only
been explored relatively recently. Biocalcifying bacteria have been used to
successfully promote carbonatogenesis on the surface of limestone buildings,
statuary and monuments. This newly-formed carbonate is often resistant to
mechanical stress, possibly due to the incorporation into crystals of organic
molecules, produced as a result of bacterial metabolism.
Bacillus cereushas been shown to protect exposed mineral surfaces by the
formation of sacrificial layers of calcite, vaterite or aragonite crystals, which
may be dissolved in a polluted environments but can be renewed when nec-
essary. Other non-sporing bacteria such as Micrococcus xanthushave also
been shown to produce calcite ot vaterite crystals which strongly adhere to
the original stone and production can be controlled by changing the environ-
mental conditions. Such bacteria precipitate calcium carbonate in their
immediate environment (Figure 10) and encrust cells in the process of car-
bonatogenesis (Figure 11).
It is now understood that application of consolidants to stone surfaces
may hinder the movement of salts that then accumulate, leading first to
unsightly discolouration but ultimately physical damage. While some reser-
vations exist regarding the application of bacteria to stone, calcium carbonate
generated by bacteria could offer a solution to this problem because the pro-
duction of a layer of calcium carbonate on calcareous stone that does not
block the natural pore structure, allowing free passage of soluble salts through
the stone.
236 Chapter 9
Figure 10Calcinogenic bacteria in laboratory culture, showing calcite crystals developing
within colonies