Environmental Engineering Reference
In-Depth Information
8.3.5 . Patinas
The superficial aspect of stones changes dramatically over time. In urban
environments, if the atmosphere is rich in sulfur dioxide, black to brownish gypsum-
rich deposits may form. In rural environments, biological colonization prevails. All
these very superficial developments are called “patinas”. Patinas other than the ones
already described do exist. We will hereafter describe the natural ones (oxalate
patinas, iron- and clay-rich patinas, and calcite encrustations). Man-made patinas
(including lime washes, paint layers, past treatments, rust or copper salt
discoloration) are described elsewhere [VER 00]. The characterization of patinas is
very important prior to any cleaning campaign.
8.3.5.1. Oxalate patinas
These orange to brown superficial layers (some micrometers to hundreds of
micrometers thick) have been thoroughly described, both on geological substrates
and on monuments [FAS 89]. They are composed of calcium oxalates, crystallized
as weddellite or whewellite, and are often associated with calcium- and/or silicium-
rich particles [VER 94]. They have two accepted origins: some authors have
described such deposits as the end product of the transformation of a superficial
organic matter by bacteria [LAZ 89]; others interpret their origin as linked to
transformation of the stone surface by the reaction of mineral substrates with oxalic
acid originating from lichen or bacteria metabolism [DEL 87]. The third possibility
suggested in the literature is that patinas are simply the result of deposition of
calcium oxalates present in the atmosphere. Generally, oxalate patinas are found in
parts of buildings protected from the direct or indirect impact of rain water, for
instance on portals arches (see Figure 8.27).
Figure 8.27. Oxalate patina relicts on the surface of a carved bas-relief,
Saint-Trophime portal, Arles, France
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