plaster. However, neither this damage, nor destruction by earthquakes, col-
lapse, demolitionor war will be considered in this chapter.
3 Damaging Processes
Deterioration processes, especially caused by soluble salts interacting with
moisture and microbial colonisation, will be presented in this chapter, in rela-
tion to restoration/consolidationtreatments and their effects. Damage will also
be considered thathas occurred as a later consequence of extended ingress
of moisture through damaged parts of the building or the use of unsuitable
building materials.
3.1 Deterioration by Soluble Salts
We have to consider that unless the causes of deterioration due to soluble
salts have been intensively investigated, the methodology of intervention in situ
will not be clear. The majority of the wall paintings contain salts like carbon-
ates, chlorides and nitrates of sodium, potassium, calcium, magnesium and
ammonia dispersed within the porous material, or concentrated in efflores-
cences of different aggregates of crystals on the surface or in form of subflo-
rescences below the paint layer. Subflorescences, in particular, cause damage
to the wall paintings during repeated cycles of crystallisation and hydration
(reversible phase transitions) of salts (from an aqueous solution into the crys-
talline stage) as a result of climatic changes.
The ions originate from the building materials themselves. Bricks and mor-
tars from coastal areas, may from the beginning, contain the salts present in
seawater. Monuments reinforced or isolated with incompatible materials like
concrete or Portland cement may suffer from serious problems due to salt
deterioration. Air pollution emitted from natural or artificial sources may
deposit on the buildings and may then react with the materials. The deposition
of sulfur dioxide from combustion, resulting in the formation of black gypsum
crusts, is the best-known example. Nitrogen compounds from traffic as well as
agricultural sources, aerosols from the sea and chlorides from de-icing salts
may contribute to the ionic loading of the aqueous solutions in the porous sys-
tems of brick, stone, plaster and the paint layer itself. Furthermore, acids and
alkaline solutions applied for cleaning may interact, and result in the forma-
tion of additional salts, when reacting with the materials or existing salts.
In addition, several salts are of biogenic origin. In the surroundings of
human settlements, chlorides and nitrates accumulate. Microorganisms, like
nitrifying bacteria, produce nitrites and nitrates from excrements, waste and
even corpses (graveyards), which penetrate building materials by infiltration
in rising damp. In addition to this external production of biogenic nitric acid
Wall Paintings: Aspects of Deterioration and Restoration 243