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8.3.3.3.2 Black and hard gypsum deposits (“black crusts”)
Gray powdery deposits may become harder and transform into black crusts
through gypsum crystallization between particles if water enters the system by
condensation, infiltration or hygroscopy [CAM 82]. Gypsum, which behaves as a
binder, originates partly from the atmosphere, where it exists as particles or in
solution in mist droplets, and partly as a result of the interaction between
atmospheric SO 2 and calcareous substrates. Black crusts generally form in areas
protected from water run-off (Figure 8.23). Their thickness may range from some
micrometers to several centimeters, according to the level of sulfur dioxide
pollution, the time they have been able to grow, and the location: the closer they are
to the interface with an area exposed to run-off, the thicker they are. Black crusts
also contain organic components from aerosols, such as aliphatic and aromatic
polycyclic hydrocarbons, tricyclic terpens, steranes, diterpenoids, etc. [SAI 93].
These components have a brownish color and are soluble in water. They may be
mobilized toward the subsurface of the stone during moistening events such as
condensation.
Figure 8.23. Gypsum-rich black crusts develop in areas protected from water run-off
Rouen cathedral, sculpture on the western façade
8.3.3.3.3. Epigenic gypsum layers and other forms of sulfatation
On substrates rich in calcite, such as marble and limestone, a yellowish to
brownish layer is often observed just beneath the black crust [BRO 96, VER 94a].
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