Cleaning Marble Monuments
Angus Lawrence
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Eroded and soiled garden statuary marble, half of which has been cleaned using a controlled nebulous spray system |
Marble has long been an important
material for sculpture and
monumental compositions, notably
in religious settings. Whether related to a
significant event or as a mark of remembrance,
a monument will often become an important
focal element in an architectural space.
As with all natural materials, marble
is prone to varying degrees of soiling and
deterioration which in turn can be detrimental
to the appearance and appreciation of a
piece, detracting from its significance as
a work of art. Therefore cleaning aimed at
refreshing or restoring a marble surface is
often considered appropriate. As soiling
may also hide evidence of deterioration,
cleaning is often carried out as a precursor
to conservation and repair work, in order to
fully understand the condition of the piece.
Cleaning, then, often forms a necessary
and desirable part of a well-designed
conservation specification. This should
involve the simple removal of damaging
or disfiguring deposits from the surface.
However, cleaning is an irreversible
process and so the choice and application
of the right materials and techniques are
vital. Well-intentioned but damaging
cleaning has sometimes been carried out
in the past with the use of inappropriate
processes, tools and materials.
Faced with an ever-expanding group
of specialist marble cleaning products
(in the form of powders, liquids, gels,
pastes, etc) it is important to approach
the cleaning of marble monuments with sound conservation principles and an
awareness that a historic marble surface
should be properly examined and assessed
before any programme of cleaning.
THE MATERIAL
Marble (from the Greek marmaros meaning
a white, shining stone) is a metamorphic
rock formed from limestone (CaCO₃) which
has been broken down under pressure and
heat to recrystallise and produce a granular
mosaic of calcite crystals of roughly equal size.
During this process, the original sedimentary
elements of a limestone are lost and a pure
marble is therefore monomineralic, free from
fossils and white in colour. The vast range of
coloured marbles is a consequence of small
amounts of impurities being incorporated
with the calcite during this metamorphism.
Marbles are found across the globe but
the best known and most desirable come from
Italy, Greece and Turkey. There are a handful
of true British and Irish marbles, but they are
relatively rare and most are only of geological
interest. Most notable are the rocks from the
Scottish islands of Iona, Skye and Tiree and
Connemara Marble from the west coast of
Ireland. Other stones that take a polish, such
as Purbeck, are often called marbles but are
in fact largely fossiliferous limestones, shale
stones and other varieties.
HISTORY AND USE
Since indigenous marbles are both difficult
to quarry and largely unsuitable for carving,
marbles from abroad have been an important
decorative and sculptural material in Britain
since the Roman occupation. Although the
architecture of ancient Rome is equated in
the popular mind with the use of marble,
the Roman buildings of Britain consisted
mainly of timber, local building stones,
and fired terracotta bricks and tiles, all
cemented together with lime mortars and renders. Marble was, however, imported around the Roman Empire in the form of
statuary, sculpture and monumental pieces to
furnish temples, public buildings and private
dwellings. Fine marble remains associated with
the 3rd-century AD Temple of Mithras, which
was excavated in 1954 in the City of London,
are on display in the Museum of London.
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A finely decorated marble cartouche on a monument from the 1780s, before and after cleaning to remove dust, decorators paint and surface soiling. The monument had
been divided in two by a stud wall, hence the vertical paint marks. The solvents used included acetone, saliva and deionised water. |
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The use of marble tesserae in mosaic
flooring is also well recorded and in the
Middle Ages Italian craftsmen imported
material from archaeological remains in Italy
to reuse with indigenous stones in order to
achieve the decorative patterns and colour
combinations of the Cosmati pavements
at Westminster Abbey and Canterbury
Cathedral (both 13th century).
Alabaster was the main stone used
for religious sculptures and monuments
in Britain from the mid 14th century.
A vibrant and profitable industry grew up
in London, Nottingham, York and Burton
upon Trent and mass-produced sculptures
based on stock themes and imagery were
exported in large numbers to mainland
Europe. Sometimes mistakenly identified
and referred to as marble, alabaster is
geologically very different, being a form of
gypsum – hydrated calcium sulphate.
The 17th century saw the eventual
replacement of alabaster with marble as
the main material for church and other
monuments. Sculptural and architectural
styles based on European examples were
studied, copied and then developed in Britain
and the demand for marbles increased as
monuments became more exuberant and
ambitious in breaking free of the staid
conventions of the alabaster workshops.
A typical example of this development can be
seen at St Mary’s, Bottesford in Leicestershire,
where the use of alabaster for the fine
memorials to the Dukes of Rutland was
phased out in the middle of the 17th century in
favour of statuary marble.
Realistic portraiture became a
key component of many monumental
compositions with the finest statuary marble
being sought out to produce sculpture of
the very highest quality. Marble funerary
monuments were important commissions
for the top sculptors and carvers during
the late 17th century through to the late
19th century, with sculptors such as Gibbons,
Rysbrack, Westmacott, Flaxman and others
producing a range of monuments often
comprising large and complex architectural
arrangements. At the same time, fairly simple
marble wall tablets with incised and gilded or
painted inscriptions became commonplace
throughout the British Isles.
The start of the 17th century also saw the
beginning of the use of marble as a decorative
feature for internal floors and walls, again
influenced by the architecture of Europe.
One of the earliest examples in Britain is at
the Queen’s House, Greenwich, where Inigo
Jones, who had returned from Italy in 1614,
used marble for the flooring in 1635. Dark and
brightly coloured marbles, which were deemed
unsuitable for sculpture, were often used to
decorate and enrich grand interiors.[1]
DURABILITY AND DEFECTS
Statuary marble has been used externally for
garden statuary and sculpture, churchyard
monuments and mausolea but its relative
vulnerability to frost and other atmospheric
conditions meant that limestones, sandstones
and granites became established as the
preferred materials for external stone pieces.[2]
Even in an internal location, marble
monuments are subject to a range of processes
that can cause deterioration. Moisture
is usually the key agent of decay, with
condensation or penetrating and rising damp
(often the result of a poorly maintained roof
or rainwater goods) leading to direct surface
erosion, soluble salt action and corrosion of
iron fixings.
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Dust and soiling on a church monument caused by atmospheric pollutants, waxes and
other applied surface materials |
It should be emphasised that the most
immediate consideration with any monument
is the structural integrity of the piece which,
if compromised, may constitute a significant
health and safety risk to users of the building
or surrounding area.
Of major concern is
the corrosion and sometimes disintegration
of iron fixings, which can occasionally lead
to catastrophic failure. Larger monuments
may contain dozens of metal fixings, all
playing a role in keeping the various parts in
balance and uniting the whole as a coherent composition. Simpler memorials may rely on
just a handful of fixings. Clear indications
of corrosion can be seen in rust staining,
cracking, spalling and movement.
Surface soiling can be attributed primarily
to airborne dust and pollutants, degraded
coatings, staining through condensation or
water ingress, corrosion of metallic elements
and fixings, and human activity (such as
previous maintenance and restoration,
handling and graffiti). Conservation of monuments may
therefore include partial or total rebuilding,
pinning and repair of breaks and losses,
re-pointing of failed and open joints, and
consolidation of decaying stonework or
decorative finishes.
Surface cleaning may therefore appear
to be low on the priority list for marble
monuments. However, it can enhance a
monument and allow a clearer reading of
both fine detail such as inscriptions and
the sculptural whole. There is also the
understandable desire to maintain a piece in
its best possible condition as a memorial to
an event or person and as a work of art in its
own right.
CLEANING ASSESSMENT
Before any cleaning is undertaken, careful
assessment and recording should be carried out.
As part of this initial process, an
assessment of the surfaces to be cleaned must
be made to get a better understanding of what
might be removed by cleaning and what may be
left. This must also take into consideration the
residues that could be left on the surface from
the cleaning process/material.
A close examination of the surface
should help to identify the material, assess
its condition in terms of structure and
surface, understand that condition and,
if possible, determine what has caused it.
Any surface finishes such as the remains of
painted decoration or gilding should also be
located and identified. Decorative finishes
on marble monuments tend to be restricted
to inscriptions, heraldic devices and other
minor decorative elements (for example,
column fluting and capitals being picked out
in gilding).[3]
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Solvent swabbing a lightly soiled marble surface |
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Analysis of a painted finish through crosssection
examination or identification of paint/
pigment types can be used to distinguish
original decoration, which should always be
retained, from later interventions that may be
removed if appropriate. A close examination
of a monument’s surface should also reveal
any structural defects such as hairline
cracking or joint movement which may need
to be addressed before cleaning.
Following close examination, careful
and objective tests should be specified
and undertaken before continuing with a
programme of cleaning. Experience of similar
projects will inform the conservator in what
materials and methods should be trialled,
but it is always sensible to start with the most
benign procedures.
The key is to develop the tests, preferably
in groups that are similar in nature (such
as various mineral solvents), and to give
them the time to work properly. That
also means, for example, allowing time
for a trial area of marble to dry out fully
so that the stable cleaned surface can
be judged. A wet surface can often give
a false idea of the final appearance.
THE CLEANING PROCESS
The decision to proceed with cleaning may
be left to the conservator alone, but more
often than not a range of people take part in
the process. This often involves managing
people’s expectations. Trials to select the
most appropriate cleaning method and to
establish the degree of cleaning that can
be achieved without risk are essential, and
the conservator should present a clear and
concise explanation and assessment of the
trials to the client and others responsible
for decision-making. It is important that the
process and materials being recommended
are fully understood by all concerned,
rather than just the visual result.
At the most basic level, a light dusting
with a fine, soft cloth (an ‘e-cloth’ for example)
or a fine, soft-bristle brush can remove a
dulling layer of dust and grime. A brush
should be used with a vacuum cleaner with
the edges of the nozzle covered with padding
(such as ethafoam). If there is any danger of
loose sections dislodging, a layer of gauze can
be placed over the end of the nozzle to prevent
any original material being sucked up.
Mechanical cleaning may include
careful removal of poorly executed fills
and repairs which are obscuring areas of
original surface. Extreme care should be
taken using hand tools such as scalpels and
fine chisels, and abrasion of the marble
surface should be avoided at all costs.
Wet cleaning is likely to involve a range
of detergents and/or solvents. These may be
used singly or in combination. For example,
a standard non-commercial formula (known
to conservators as ‘Kill quick’ or the ‘V&A
mix’) combines water and white spirit with
a small amount of non-ionic detergent.
This is applied by swab or brush and can be
lightly agitated with a bristle brush before
removal and careful rinsing with clean water.
Wet cleaning techniques act by dissolving
or mobilising deposits so it is important
that these are lifted from the cleaned
surface using cotton wool or absorbent
paper to prevent re-soiling of the marble.
Steam cleaning can be carried out
using small portable dental cleaners which
have been successfully employed both in
conservation studios and on site. These allow
very controlled targeted cleaning to be carried
out at a pressure of c3-5 bar (40-75 psi).
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Left: using a portable steam cleaner to clean a decorative marble font. Right: a marble figure during steam cleaning. |
A new generation of larger steam
cleaners which superheat the water have been
developed for use in building conservation.
These are controllable enough for use in
cleaning external marble monuments where,
for example, removal of biological growth
or rinsing after poulticing of sulphation*
is carried out. Again, the marble should be
carefully examined first and only sound
surfaces should be cleaned by this method.
However, these machines should not be 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 triammonium
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.
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Above left: applying a sepiolite poultice to draw out and reduce copper staining caused by
the bronze elements of a marble wall monument and, above right: applying microcrystalline wax to a marble figure after cleaning |
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, waterproof
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.
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Glossary
EDTA ethylene-diamine tetra-acetic acid,
which is used in a range of cleaning and descaling
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, Practical Building
Conservation: Stone, Ashgate, Farnham, 2012
G Lott and D Smith, ‘Shining Stones:
Britain’s native marbles’, The Building
Conservation Directory, Cathedral Communications Limited, Tisbury, 2001
P Rogers, The Beauty of Stone: The
Westminster Cathedral Marbles,
Westminster Cathedral, 2008
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.
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