Outdoor Metal Sculpture
Quadriga, Wellington Arch, London, a bronze sculpture with
an artifical patina which has been retouched and rewaxed (above),
and (right) the patina at the start of work
s public works of art enliven our landscape and stand as testaments
to our rich social, cultural and political history. It is essential
that those bodies with the responsibility to care for our public
sculptures respect their vulnerability and implement sound conservation
and maintenance practice. This article provides a brief introduction
to the materials and processes involved in the manufacture of
external metal sculptures and the problems associated with their
metals have been used to produce sculpture over the centuries,
including bronze, lead, cast iron, zinc and copper electrotype.
Each has its own sculptural qualities, but equally varied and
just as important are the surface finishes and patinas. Their
influence on the aesthetic quality of the work of art is immeasurable,
and they are often far more fragile than the body of the sculpture
has always been expensive, bronze is ideal for exterior use due
to its comparative resistance to corrosion and its ability to
take a fine, aesthetically pleasing finish. After construction
in the foundry, the surface of the bronze is further finely worked
to disguise the joints and to add further detail. It is then given
an artificial patina (under the direction of the sculptor) using
a variety of chemicals and, finally, coatings of wax and sometimes
lacquers to protect it. Traditionally, the artificial patina chosen
would usually have been a dark or midbrown colour, though over
the past century other colours and shades have also been used
by artists on their bronze castings.
damage to bronze sculptures is comparatively rare and is usually
caused by either the failure of patches and joints made when the
piece was first cast, failure of the internal armature and fixings,
or impact damage (from trees falling or road traffic accidents
for example). The aesthetic surface of the bronze is, however,
far more vulnerable to damage: if the protective coatings of wax
are not maintained they are eventually lost, leaving the original
artificial patina unprotected from environmental influence (water,
gases and air-borne pollutants). This leads to slow corrosion
of the bronze and the development of a natural patina, usually
mid to light green in colour.
that causes the natural patina is slow to form and usually stable,
however the lack of protective coatings can lead to areas of active,
aggressive corrosion on the bronze, which can cause permanent damage
to the surface.
The green copper sulphates forming a natural patina
are washed downwards by rainfall and cause permanent staining
to stone plinths. Although the appearance of this staining can
be slightly improved by conservation treatments such as chemical
poultices, it is almost impossible to remove altogether.
sulphate staining on a stone plinth caused by rainwater runs
Lack of maintenance
on bronze sculptures is so common that the naturally formed green
patina is a familiar sight to us, and is often prized for its
own aesthetic value. Generally, when undergoing conservation treatment,
the patina, either natural or artificial, should not be removed,
but respected as an integral part of the sculpture. However, a
natural patina can sometimes include disfiguring marks or black
streaks that can make the sculpture difficult to read, and it
is generally accepted that these may be removed and the surface
manipulated to reinstate a cohesive whole.
If an artificial
patina requires conservation or retouching, it is most important
to use the same methods and chemicals in their treatment as those
used originally. A few modern alternatives are available, but
this author has yet to see any new methods in this field which
compare favourably to the quality of the result achieved by using
the same materials and techniques as were originally used in the
foundry during the production of the piece: methods which have
changed little from antiquity to the present day for good reason.
of lead sculpture reached its height in England during the 18th
century. At that time it became cheaper to produce cast leads
painted to resemble stone than to acquire genuine stone sculpture,
so leads were frequently painted with lead-rich oil-based paints
to give the appearance of limestone or marble. Many were also
painted in naturalistic polychrome. In most cases the original
paint will have weathered away, though traces may have survived
in recessed areas, and it is essential to take small samples for
analysis before proposing conservation treatment, and to record
the results of the analysis in order to retain this vital map
of the object's aesthetic history (see page 159). Original or
historic paint should not be removed and should be left undisturbed
as far as possible, even if a curatorial decision is made to restore
the sculpture to its creator's original intention and repaint
it in its original scheme.
is generally resistant to corrosion and, if left exposed, the
surface will develop a protective, pale to dark grey patina. As
so much lead sculpture has not had its paint surface maintained
over the years, we are now culturally used to seeing lead with
its natural patina, in the same way that we accept naturally formed
green patina on bronzes and lichen on stone. As a result, these
finishes have become part of our aesthetic understanding of historic
sculpture was cast in one piece with a core material of sand, plaster, or a ground refractory
material such as brick. The
iron armature, and usually the core material, were left inside
after the casting process was complete to provide support to
the soft lead. It is the original armature that is the sculpture's
most threatening problem, since it will inevitably corrode. As
it does, the rust expands up to ten times the volume of the original
iron, causing the lead to split, slump and eventually collapse.
As a soft
material, impact damage is often found on lead sculpture. Superficial
dents and scratches to the surface are most common, and the material
is particularly vulnerable to graffiti and other forms of vandalism (shotgun pellet damage is not unusual). There are even cases
where lead sculptures are being gnawed by squirrels.
repair of a lead sculpture with a rusting armature: top left
and right, the sculpture is opened up to remove the sand core
and expose the rusting armature; above, the new stainless
steel armature; and, right, lead burning the patch back into
of lead sculpture therefore often involves extensive structural
repair, including removal of the original armature and core material,
followed by the insertion of a new stainless steel armature. This
will probably require cutting window-like patches to allow access.
These are then lead-burned back into place, and the seams carefully
worked back and chased using fine chisels and tools to match the
original surface. Additionally, if areas of the sculpture have
become crushed or collapsed it is sometimes necessary to cut further
patches in order to gain access to dress out the lead from the
inside. Alternatively, crushed areas may be built up by adding
successive layers of lead (by lead burning), then carving back
to the original form. These treatments, though intrusive, enable
the structural integrity and original sculptural form of the piece
to be restored, and if performed by experienced conservators,
ensure the sculpture will retain the appropriate, aged appearance
and recover its structural stability.
pellet damage on a lead sculpture which still retains some
of its original stone-coloured paint.
analysis of a paint sample taken from 'Fanny's Urn', a small
lead urn in Sir John Soane's Museum
It is generally
considered poor practice to remove a natural patina which, usually,
will have been acquired by the surface of the lead after many
years in the open air. Nevertheless, it may be appropriate to
selectively remove intrusive stains if they interfere with the
reading of the sculptural form, and indeed a curatorial decision
may be made to recreate the original painted surface for other
If left unpainted,
repaired areas on the lead will be visible due to their shinier,
worked surface. One solution (preferred by the author) is to disguise
these areas with a thin wash of casein-based watercolour paint.
As the paint slowly wears off in an outdoor environment, it is
replaced by natural patina. Unpainted lead requires little maintenance: monitoring of condition and an annual wash will usually be all
that is required.
paint surface of this cast iron sculpture has not been maintained,
allowing the iron to rust.
19th century the development of new technologies and the Victorian
enthusiasm for ingenuity of invention in industrial processes
lead to new, cost-effective materials being used to cast sculpture,
in particular cast iron, zinc and copper electrotyping or electroforming.
The majority of this sculpture was intended for exterior display.
is a brittle material, making it susceptible to impact damage
and stresses caused by expansion rusting of construction joints
in the object. Cast
iron can be repaired using a range of techniques such as pinning,
stitching or welding. However, great care is required when attempting
repairs, especially if heat is involved, as in welding, because
localised thermal expansion can induce further cracking and subsequent
As iron rusts
rapidly if unprotected, almost all iron sculptures were originally
painted. As with lead sculpture, or indeed any important painted
object, it is vital that existing paint is analysed so that original
paint schemes can be identified and recorded, as much iron sculpture
has been over-painted many times in its life.
of paint can obscure sculptural detail, so conservation may involve
their careful removal, followed by the application of primer,
mid and top coats of paint to protect the surface from moisture.
(For further information, see Keith Blackney's articles on cleaning and painting ironwork
which are also available on this website.) The colours
chosen must be dictated by the results of analysis and where possible
other historical research.
of large-scale sculptures in zinc was only in fashion for a short
time between 1860 and 1900 and as a result they are relatively
uncommon. The sculptures were cast in sections and assembled using
lead soldered joints, which were additionally pinned onto structural
elements such as iron straps or frames, or zinc plates, for extra
corrosion causes pustules to form which, when cleaned, reveal
deep pits in the surface
a bronze effect, the completed sculptures were frequently electroplated
in copper, then chemically patinated and protected with layers
of varnish or wax. In Britain, the firm of Elkington was instrumental
in the development of electroplating technology and this company
often finished zinc sculpture which had been imported from Germany,
France or America.
sculptures sited outdoors have usually lost their original surface
coatings and copper plating through corrosion, exposing the grey-coloured zinc. The corrosion causes a very pitted surface in the
zinc, and this is usually accompanied by failure of the lead solder
joints. The result compromises the object's structural stability
and leaves it vulnerable to serious fracture as well as ongoing
corrosion processes, further leading to the rarity of sculpture
in this metal. Structural repairs can be effected and the corroded
surface restored, if appropriate, to replicate the original 'bronzed'
of electrotyping, or 'electroforming', was developed in the 1830s.
There were two methods of manufacture, the method chosen depended
on what was being made as the technique was employed for many
different kinds of objects and works of art, from small furniture
trimmings, tabletop sculpture and relief plaques, to life-size
sculptures intended for outdoor display. Usually, smaller objects
were made using a wax or plaster model (that is to say, a positive)
possibly with an iron armature already inserted for support. Large
objects and sculpture were usually made in sections using a number
of moulds (that is to say, negatives). The model or mould was
placed in a copper-plating tank and copper was allowed to deposit
itself by electro-deposition onto the surface of the positive
object or negative mould, until it had built up to an adequate
In the case
of the model option, the copper built up on the outer surface
of the object, so the thicker the copper became, the more surface
detail was lost. It was therefore not a good technique for large
objects because you could not gain sufficient thickness of copper
to retain much strength without losing visual quality. To counteract
the potential weakness of this method, the wax impregnated plaster
model would often be left inside the object to give support.
In the case
of a mould, however, the copper built up from the outer surface
inwards, so a greater thickness could be built up without blurring
the visible surface of the sculpture, thus making it suitable
for large objects where a greater thickness was required for structural
strength. This process produced highly detailed surfaces, reproducing
every mark of the sculptor's hand.
ingress causes electrotype objects to split. Repairs
(above right) were carried out using copper sheet.
When a large
object was made by the mould method, each copper section was then
cleaned and joined together by brazing or soldering. The thickness
of the copper that could be deposited meant that iron armatures
were seldom required. Electrotypes can be patinated in the same
manner as bronzes, but now frequently have a natural, green patina
which has developed on the copper over time. Either way, they
are easily mistaken for bronzes at first glance.
sculptures which have a very thin copper shell are particularly
susceptible to corrosion if not cared for. Where constructed with
a plaster core and possibly an iron armature, any consequent ingress
of moisture results in the expansion of these materials, leading
to splits and eventual collapse. Repairs can be made by reproducing
any large, missing or heavily damaged areas using copper sheet,
and weak areas may be reinforced by introducing fibreglass resin.
Like lead sculpture, such objects usually require the replacement
of any iron armature with new stainless steel armatures and structural
As with bronze,
protective coatings of wax or lacquer need to be applied to protect
and maintain the delicate surface.
A CONSERVATION AND MAINTENANCE PROGRAMME
can well survive the rigours of outdoor life if structural damage
or aggressive corrosion are identified at an early stage and corrected
by a skilled conservator, and if regular and correct attention
is given to the maintenance of its protective coatings. Because
historically most owners or bodies responsible for the sculpture
have largely failed to carry out this comparatively simple requirement,
our sculptures are now in varying states of repair and much public
sculpture in Britain is in need of urgent conservation treatment.
of the structure and surface of a sculpture is a specialist practice.
It is vital that those entrusted with the care of such objects
understand the principles behind good conservation and maintenance
and obtain the best advice from experienced conservators before
embarking on a program of conservation treatments or establishing
a maintenance regime. Work must be specified and carried out by
qualified conservators who are sufficiently experienced to have
gained the skills required. These include an aesthetic sensitivity
to sculptural form, an understanding of the subtlety of surface
finishes and knowledge of art history. Verifiable proof of the
experience and professionalism of the conservator should always
be obtained before commissioning advice or work.
Once a sculpture
has undergone full conservation and is put in good order, it is
equally important that it is maintained regularly as a matter
of course. The goal of a preventive maintenance programme for
sculpture is to preserve as far as possible the artist's original
intention, while being respectful of the historical experience
the object may have undergone (including not only the acquisition
of a sound, natural patina but also, in some cases, the damage
associated with an historical event, such as shrapnel damage for
example). This goal can be achieved by slowing down corrosion,
maintaining structural soundness and maintaining or improving
the aesthetic appearance, while ensuring the unique and visually
varied character of a sculpture's surface is also protected.
years the United Kingdom has seen a marked increase in the commissioning
of new public sculpture: works of both traditional and modern
materials and surface finishes. Those commissioning each new piece
must recognise the ongoing responsibility to annually maintain
it, and be mindful of the ongoing budget this requires. They must
then actively implement good policy. Conservation usually involves
significant cost and disruption: regular maintenance on the other
hand is cost effective and, most importantly, protects the sculpture
and its aesthetic and financial value.
article is reproduced from The
Building Conservation Directory, 2006
HARRIS is a conservator of fine metalwork and sculpture. He
has held the position of Metalwork Conservation Advisor to
The National Trust for England and Wales since 1982, and was
appointed Architectural Metalwork Consultant to English Heritage in 2003.
Statuary and stone carving
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