Environmental Engineering Reference
In-Depth Information
Figure 8.26.
On this sculpture of Grégoire de Tours (Cour Napoléon, Louvre palace, Paris),
biological colonization has developed on the upper part of the head and shoulders, which are
often moistened by rain water, while the areas protected against water are covered with a
black gypsum crust
Micro-organisms participate in stone degradation in diverse ways. Moulds
produce organic acids (oxalic acid is the most common) that are able to dissolve
calcareous substrates. Lichens are symbiotic living beings (algae + mould), the
fungal component of which may be very aggressive, particularly in endolithic
lichens [LAL 92]. Lichens have a slow growth rate and once they have colonized a
surface entirely their thallus participates in some way to protect the surface.
The most common algae families on stone buildings are
Cyanophycea
(blue
algae), which develop in wet areas that are not exposed to direct light [BRU 92].
Some algae produce polysaccharides, which contribute to stone degradation, and in
all cases are an organic substrate enabling the growth of harmful bacteria.
Indeed, some bacteria may colonize the stone. Some are heterotrophic, which
means that they need an organic substrate to be able to live and grow. This category
of bacteria is able to metabolize energy sources, such as sugars or methylic
components, into organic acids. The so-called “autotrophic” bacteria can grow
without any organic substrate. Their energy is derived from the atmosphere (NO
x
,
SO
2
). The bacteria involved in nitrogen and sulfur cycles respectively favor nitrate
and sulfate formation in the stones.
