T W E N T Y F I R S T E D I T I O N
T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 4
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SERVICES & TREATMENT :
PROTEC TION & REMEDIAL TREATMENT
4.1
confused or substituted with standard
hot-water pressure washers or other steam
machines, which can cause severe damage.
More severe staining and soiling may need
to be removed with laponite*, sepiolite* or
paper-pulp poultices. Over-painting, often the
result of sloppy decorating of an adjacent wall
surface, will need to be removed using solvents
or paint strippers. Strongly alkaline products
should be avoided.
Chelating agents, such as EDTA* and tri-
ammonium citrate, affect ionic bonding and are
useful on some types of soiling. However, they
are difficult to control on marble, and many
conservators consider them too unpredictable
for use on historic surfaces.
Laser cleaning of marble and other
stones has been regularly carried out in the
conservation studio for the last 20 years
and Q-switched Nd:YAG laser systems
are occasionally used on-site for specific
targeted cleaning (for example on the
Cosmati Pavement in Westminster Abbey).
These operate by emitting rapid short pulses
of light energy which are readily absorbed
by a dirt layer causing it to expand and lift
away from the surface of the marble. Highly
effective at removing dark staining from light
surfaces, the use of lasers involves a good
deal of organisation, an appropriate level of
training and strict adherence to stringent
health and safety procedures. Concern
has been expressed in the past about the
discolouration of some types of decorative
finish and the marble substrate itself, but
recent developments have been made which
aim to avoid this potential drawback.
The many proprietary marble cleaning
products now available have mostly been
developed for the maintenance of modern
architectural surfaces and are not suitable
for use on marble monuments. Often highly
alkaline or acidic in nature, they can be
very effective at cleaning marble but do not
allow the degree of control necessary for the
careful cleaning of monuments. They can also
leave damaging residues on the surface and
introduce harmful salts.
The removal of most types of metallic
staining, however, can only be achieved by
the use of chemical processes. Recipes are
available for self-mixing but a number of
suitable proprietary products are also available.
Tests should always be carried out and the
manufacturer’s instructions strictly adhered to.
AFTER CLEANING
Following cleaning, a further decision must
often be made in relation to the application of a
protective surface coating. Again, proprietary
products, such as lacquers, polishes, water-
proof coatings and hard waxes, should
not be used for monuments or sculptures.
A standard procedure is to apply a thin layer
of microcrystalline wax which can provide a
degree of protection to marble surfaces which
are prone to condensation (such as monuments
in semi-exposed locations like cloisters or
porches), are vulnerable to bat excreta, or are
located in areas of heavy human traffic and are
liable to frequent touching. The wax can also
be lightly buffed and/or tinted with pigments to
enhance the appearance of dulled surfaces.
After cleaning, exterior marble
monuments can be treated with a lime
shelter-coat or limewashes which will
adhere well to weathered surfaces and
provide a degree of protection to friable
marble. They can also be used to ‘even
up’ an irregularly weathered surface.
After a thorough conservation clean,
only simple routine maintenance should be
required. Any maintenance plan should avoid
the use of harsh and potentially damaging
materials and should focus on providing simple
guidelines for custodians where, for example,
regular dusting is required.
Maintenance guidelines should also
include provisions for dealing with potential
sources of damage such as graffiti or accidental
spillage. If a conservator is not immediately
available, the problem should not be tackled
by recourse to potentially damaging ‘quick fix’
solutions.
Marble monuments convey an important
record of events from the early 17th century
through to the modern day and form
a unique facet of British sculpture and
design. Marble will continue to be used
as a sculptural and decorative material
but never in quite the same way and it
is important that this rich inheritance
is conserved and looked after properly.
Cleaning of these important pieces should
therefore always be a carefully planned
and executed conservation procedure.
GLOSSARY
EDTA
ethylene-diamine tetra-acetic acid,
which is used in a range of cleaning and de-
scaling processes
laponite
a synthetic silicate related to the clay
mineral hectorite
sepiolite
a clay mineral, magnesium silicate
sulphation
chemical reaction between
calcium carbonate (the main component
of limestone and marble) and sulphur
dioxide (a common air pollutant which,
when dissolved in rainwater, forms an
acidic solution) resulting in the formation
of a sulphate skin on limestone and marble
surfaces.
Recommended Reading
English Heritage, Stone (Practical Building
Conservation), Ashgate 2012
G Lott and D Smith, ‘Shining Stones:
Britain’s native marbles’, Building
Conservation Directory 2001 (online at
buildingconservation.com)
P Rogers, The Beauty of Stone: The
Westminster Cathedral Marbles,
Westminster Cathedral, 2008
ANGUS LAWRENCE
BA Dip Cons ACR has
over 20 years’ experience working on a wide
range of conservation projects including
major works at Westminster Abbey, the
Tower of London, Westminster Cathedral and
the City of London. He is a project manager
for Taylor Pearce Ltd
) and is an
assessor for the PACR (Icon) accreditation
scheme.
NOTES
1 Westminster Cathedral (John Francis Bentley,
1895–1903) contains over 100 different
varieties of marble from 25 countries.
2 It has been noted that certain types of Apuan
marble are especially durable and have
survived well in an external environment
in Britain. As a general conservation
measure, however, vulnerable marble
sculpture and statuary are often protected
with covers during the winter months.
3 There are cases of colour tinting of marble
statues dating to the mid 19th century produced
by John Gibson and others working in the
neo-classical style. It is not a finish likely to
be found in church or other monuments.
Applying a sepiolite poultice to draw out and reduce copper staining caused by
the bronze elements of a marble wall monument
Applying microcrystalline wax to a marble figure after cleaning
