Environmental Engineering Reference
In-Depth Information
8.3.5.2.
Clay- and iron-rich patinas
Sandstones, granites, and also some iron containing stones develop with time a
very thin (5 to 100 µm thick) black to brown film on their surface [JEA 79]. The
layer has a clay and ferruginous matrix [JEA 79, SCH 95], embedding many
minerals originating from the substrate (quartz, clay minerals, feldspars), terrigenous
particles (i.e. particles detached from the ground and able to stay in suspension in
the atmosphere until they are trapped by a surface), and particles linked to industrial
activity. In certain instances, the matrix only contains iron oxides [NOR 92]. These
patinas are always difficult to distinguish from black gypsum crusts (Figure 8.28).
Figure 8.28.
Facade of Strasbourg Cathedral, France. The so-called “grès à meule” forms
an iron- and clay-rich patina when in contact with the atmosphere. Such patinas are often
confused with black gypsum crusts
