30
BCD Special Report on
Historic Churches
19
th annual edition
only appear under certain climatic conditions,
which tip the balance between dissolution
and crystallisation, and are often found in
distinct zones (often related to capillary rise).
Taking samples of the efflorescence
for chemical analysis will help diagnose
which salts are present. Some salts are
known to be particularly damaging in
terms of their crystallisation and hydration
effects, such as sodium sulphate.
Other symptoms of salt deterioration
include the presence of blistering and
flaking of the outer few millimetres of
stonework. Such loosening and detachment
of surface flakes of stone is consistent
with subsurface salt crystallisation (called
cryptoflorescence or subflorescence).
In some buildings, alveolar weathering,
which creates clusters of depressions in
the stone surface, is observed and can be
ascribed to salt weathering. In this case, salt
crystallisation within near-surface pores
causes detachment of individual grains. For
reasons which are not yet entirely clear, the
process becomes self-organised into a network
of near-circular depressions. However, both
blistering and alveolar weathering can also
be produced by other processes and are not
unambiguous indicators of salt deterioration.
How can salt damage be
prevented or treated?
Preventing and treating deterioration problems
caused by soluble salts are difficult tasks, and
solutions vary widely depending on individual
circumstances. What might be effective in one
building, may be wholly inadequate somewhere
else depending on the sources of moisture,
the nature of the salts and the type of building
materials. Accurate diagnosis of the problem is
vital before any remedial measures are trialled.
Potential approaches to preventing
future salt deterioration and treating
current problems include: controlling the
ingress of salts and water, removing any
damaging salts already present (desalination),
modifying the environmental conditions
to reduce damaging cycles from any
remaining salts, and/or using materials
known to be less vulnerable to salt attack.
A good example comes from the
work of David Watt and Belinda Colston,
who examined the causes of and potential
solutions to salt deterioration at the redundant
church of Walpole St Andrew, Norfolk. The
widespread and dominant presence of sodium
chloride in this church may be a result of
previous stone treatments (some recipes for
limewash include sodium chloride), rather
than current capillary rise or rainwater.
Desalination and environmental
control (to control temperature and relative
humidity and thus reduce the likelihood of
crystallisation and dissolution cycles) should
be effective in this case, because new salt
ingress is unlikely. Methods that are unlikely
to prevent salt deterioration, and indeed
are likely to enhance it, are the application
of water repellents. Several recent studies
have shown that such techniques encourage
subflorescence rather than efflorescence of
salts, thus increasing the likelihood of damage.
Some promising future prevention methods
are being researched. For example, following
research by Professor George Scherer and
colleagues at the University of Princeton which
identified the disjoining pressure between
growing salt crystal and the pore wall as
exerting a key control on salt crystallisation
damage, methods to reduce this pressure may
well be a useful control method in the future.
A number of experiments have already
been carried out on the use of surfactants
to reduce salt crystallisation damage.
Surfactants may help to reduce the contact
angle between salt solutions and pore walls,
but they themselves can also be affected by
salts and thus do not offer a simple solution.
Another possibility is the use of
microorganisms: it has been demonstrated
that some species have the ability to
remove salts from contaminated masonry,
and some of the surfactants under
consideration are of biological origin.
Salt crystallisation impacts on masonry will
undoubtedly be influenced by climate change,
as the recent EU-funded Noah’s Ark research
project has predicted. Any proposed treatment
and preventive strategies for salt-affected
masonry should take these predictions on board.
For example, areas in the north west of
the UK are likely to be affected by wetter
winters over the course of the 21st century.
Thus, deeper ingress of moisture into
stonework is likely, with concomitant impacts
on transport of salts and a potential switch to
more chemical weathering impacts of salts.
In contrast, in more southerly and
eastern parts of the UK, drier summers are
likely to reduce the overall annual number
of salt crystallisation cycles, potentially
reducing damage from this mechanism.
However, increasing evaporation in summer
in southern and eastern UK may also
enhance the capillary uptake of salts.
Recommended Reading
RM Espinosa-Marzal and GW Scherer, ‘Advances
in understanding damage by salt crystallisation’,
Accounts of Chemical Research, Vol 43, 2010
AS Goudie and HA Viles, Salt Weathering
Hazards, John Wiley, Chichester, 1997
CM Grossi et al, ‘Climatology of salt transitions
and implications for stone weathering’, Science of
the Total Environment, Vol 409, No 13, 2011
C Hall et al, ‘Moisture dynamics in walls:
response to micro-environment and climate
change’, Proceedings, Royal Society of London A,
doi: 10.1098/rspa.2010.0131, 2010
C Rodriguez-Navarro and E Doehne, ‘Salt
weathering: influence of evaporation rate,
supersaturation and crystallization pattern’,
Earth Surface Processes and Landforms, Vol 24,
No 3, 1999
DWatt and B Colston, ‘Investigating the
effects of humidity and salt crystallisation on
medieval masonry’, Building and Environment,
Vol 35, 2000
Heather Viles
is Professor of Biogeomorphology
and Heritage Conservation at the University
of Oxford. She carries out research on rock
breakdown in extreme environments (including
hyperarid deserts, rocky coasts and Mars) as
well as on the deterioration and conservation
of stone. Currently, she is carrying out research
with English Heritage on three projects including
‘
Damp Towers’, which studies problems of
moisture ingress and possible solutions.
A one-day ‘Damp Towers’ conference is to be
held by English Heritage in Exeter on 15 January
2013
to present the results of the research. Please
email chris.wood@english-heritage.org.uk or
for details.
Typical salt deterioration phenomena, with flaking surfaces at Byland Abbey, Yorkshire (left) and alveolar weathering at Whitby Abbey (right)
