Environmental Engineering Reference
In-Depth Information
8.3.4.
Biological colonization
Bacteria, moulds, epilithic and endolithic lichens and algae may be present as a
colored layer (5-3,000 µm) on the surface of the stone. Their color is extremely
diverse, owing to the presence of organic dyes such as black melanins [SAI 95],
green to black chlorophyll, red to blue phycobiliproteins, or red to orange
carotenoids [URZ 91]. Biological layers may have different origins (mosses, lichens,
algae, etc.) and develop in various environments according to their needs in light,
moisture and temperature (see Figure 8.25).
Figure 8.25.
Left: the gray veil on the body of the sculpture is a biofilm of green algae on a
sculpture in Champs-sur-Marne Castle park, France; right: epilithic and endolithic lichens
on the surface of a sculpture in Méry-sur-Oise Castle park, France
In general, biological colorizations are abundant in humid climates, on stone that
is often moistened and able to behave as a kind of reservoir, because they need water
to grow. They are mainly observed on areas exposed to rain or water run-off (see
Figure 8.26), on western facades, on zones exposed to splashing water, in areas
submitted to rising damp if water from the ground is not contaminated with salts or
if the organisms are tolerant to salts. They cannot develop in areas with high stone
loss rates, close to black crusts (too many sulfates), or if the salt load is too high.
Some metallic salts have biocidal properties; as a consequence the zones
contaminated with such salts are not covered with bio-films.
