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ARTICLES
Conserving Decorative
Plaster
Richard Ireland
From plain
walls and ceilings, through elaborate architectural panelling
and complex cornices to the most exquisite figurative modelling,
plaster has adorned and decorated buildings for centuries. Work
in this most basic of materials spans the full breadth of achievement,
from coarse craft to highest art. Conservation can be required
where decay or failure threatens its survival and often entails
preservation, repair and frequently reinstatement within the context
of existing historic fabric. To achieve an appropriate methodology
and specification of materials it is vital that all variables
are given due consideration. Though ideologically sound, employment
of traditional materials and techniques on a like-for-like basis
may have an unwanted impact. Plaster may itself be fragile, be
dependent upon a fragile substrate, or even provide the ground
for delicate decorative coatings and wall paintings, all of which
may be archaeologically and historically important.
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| Crumbling
frieze and cornice at Killua Castle, County Westmeath, 1780s |
This article
is not intended as a prescriptive 'How To' manual, but rather
as a means of raising awareness to the intricacies and complexities
that can surround such an apparently narrow subject. Techniques
should be tailored to each particular situation and adjusted accordingly.
Too often, for a variety of reasons, techniques are implemented
without due thought and consideration. Success depends on a thorough
understanding of materials and techniques, both old and new, so
as to minimise intervention, maximise preservation and, where
applicable, reinstate aesthetic integrity. A thorough primer on
lime and gypsum plasters and their use can be found in many articles,
pamphlets and textbooks detailing the subject in the technical
and conservation press. A basic guide is outlined below.
PLASTER
Plastering
materials and methods remained largely unchanged over several
centuries in Europe until the older methods were gradually ousted
during the 19th century by faster setting alternatives including
sand and cement based renders and fibrous plaster. These new technologies
were coupled with the use of cast gypsum plaster enrichment which
was ushered in during the late 18th century. Two distinctive material
types are generically termed 'plaster': lime and gypsum. Broadly,
till the latter part of the 18th century, it was lime plaster
that predominated. This was typically produced by the calcination
of limestone rock (calcium carbonate) at temperatures in excess
of 1,000°C forming quick lime (calcium oxide). Slaking with water
produced a non-hydraulic lime putty - that is, a form of lime
(calcium hydroxide) which does not set on contact with water.
The lime slowly reverts to a chemically identical material as
its parent rock by slow absorption of carbon dioxide during carbonation.
The addition
of set-enhancing pozzolans such as crushed brick to non-hydraulic
lime plasters reduces setting time at the expense of malleability,
as does the use of naturally hydraulic lime. Ultimately artificial
cements made pure limes all but redundant in new building construction,
and gypsum plaster emerged as the predominant finish for interior
walls and ceilings. Lime's unique properties, when intimately
combined with aggregates like sand, include plasticity and controlled
setting. These selfsame traits restrict maximum coat thickness
to some 18mm and necessitate several days between applications
to allow for shrinkage and development of adequate strength. The
inclusion of differing grades of aggregate and of organic ingredients
like cattle hair modify and adjust performance to suit the work
in hand. The resulting mixture may be used to render walls and
ceilings, run mouldings, press ornament and model in situ.
Internal flatwork
on walls and ceilings is traditionally comprised of three layers:
a render or pricking coat, floating coat and setting coat applied
to solid or lathed backgrounds. Coarse sharp sands reduce the
effects of shrinkage whilst inclusion of cattle hair increases
tensile strength. A physical key for each successive coat is formed
by scratching the partially set surface of the preceding one,
and the final coat is given a finer finish applied around 3mm
thick. External rendering is usually carried out without hair.
19th century experimentation led to the common use of very dense
hard finishes often over softer coarse textured base coats. Run
mouldings are built up layer by layer in the same manner as flatwork,
the shapes formed with pre-cut metal profiles incorporated on
a timber frame run onto the flatwork.
By the 20th
century, casting of large flat and curved plain faced sections
became increasingly economical and heralded today's extensive
use of the techniques from domestic interiors through to the largest
shopping malls. Modelling of lime plaster is an additive process
by which material is gradually built up from the surface by the
craftsman. This individual hand working of each element enabled
the deep undercut and layering which enriches so many buildings
of the late 17th and 18th century and distinguishes it from the
later mechanical repetition of cast plaster produced from moulds.
Large projections such as limbs, foliage or instruments required
the use of an armature till adequately carbonated. These can be
ferrous or organic, such as wood and bone - indeed anything capable
of providing suitable support for the carbonating plaster was
used and often became a significant element in later deterioration.
While lime
is inherently weather resistant and could be used inside or outside,
the same is not true of gypsum, otherwise known as plaster of
Paris or casting plaster (calcium sulphate). Calcined at no more
than 120°C to produce a white powder, it typically sets rapidly
and rigidly within 15 minutes of mixing with water. This is a
material for internal use only. It was often used from the latter
half of the 18th century as an admixture combined with lime plaster
to achieve an early setting and to counteract shrinkage.
Mixed on its
own with water to a creamy consistency, gypsum is particularly
suited to pouring into low relief moulds. Exploitation of these
attributes in the late 18th century, together with the rise in
popularity of the neo-classical style, enabled large quantities
of repetitive low relief ornament to be churned out in a fraction
of the time taken to model lime in situ. This led to the rapid
decline of the lime plaster modeller. By the mid 19th century,
flexible gelatine moulding materials allowed a degree of undercut
to be achieved in a single cast. However, cast enrichment cannot
capture the organic vivacity of the hand modelled lime it superseded.
SUBSTRATES
The plaster
surface frequently forms the last built layer in a complex interrelationship
of building materials from stone, timber and brick through a variety
of plasters and renders. Internally a timber substructure behind
ceilings and built out elements like arches is constructed to
reflect the topography of the design and minimise excessive thickness
and weight of features such as running mouldings, ribs and cornices.
Externally, substrates are often brick or stone. Well into the
20th century elements such as cornices could combine both solid
running and planted cast elements, but eventually fibrous plaster
lengths of decorative cornice became the norm. Each material has
intrinsic qualities which make it best suited to a specific role
in a building. However, no material is without some drawbacks
to its use, and where these are not fully appreciated at the time
of use, they may well cause problems later. Inadequate structural
timber, poor loadings and hard inflexible plasters are typical
problems associated with historic work. In all cases they will
have an impact upon the means of conservation.
INSPECTION
AND INVESTIGATION
Whilst the
splendid enriched and decorated architectural surfaces of house
interiors are much admired, the conservator should also bear in
mind the artifice frequently employed to represent one material
by another. Papier mâché, paint and plaster frequently masquerade
as materials as diverse as wood, metal and stone. Successful conservation
(of plaster and of the structural skeleton behind it) depends
on thorough investigation. Quick fixes are rarely satisfactory
in the long term. Programme and cost constraints as well as availability
of skills are very real factors which must be carefully appraised
and accounted for in developing solutions.
Ceiling backs
and salvaged debris may provide evidence both of failure as well
as previous repair campaigns which may only be detectable from
above. Changes in plaster colour, integrity of plaster key, changes
of lath and evidence of intervention in supporting structures
all suggest potential problems impinging on plaster stability.
Mortar analysis breaks down the components of plaster and identifies
the proportions of binder (lime and/or gypsum), the type and quality
of aggregate and the presence of any organic components. Care
is needed in interpreting results as often crushed limestone or
old mortar can be included in the mix which can skew the apparent
binder to aggregate ratios. The analysis can also shed light on
the durability or failure of the existing plaster.
Whilst not
a dating tool in itself, mortar analysis can provide useful comparisons
with both the plasters used in different areas and on suspected
remodelled sections of buildings, as well as between different
schemes created in the same geographical area in the same period.
It is especially helpful where colour and texture need to be accurately
matched for bare plaster or render surfaces. Where salts appear
as a white efflorescence or otherwise causing a problem, analysis
using a laboratory flame photometer will indicate the type of
salts and help trace the cause which in turn is vital to treatment.
Sulphate salts are a product of air borne pollution but are also
present in cements and hydraulic lime and a component of gypsum
from which they may be leeched to cause decay. Nitrates tend to
be associated with decaying organic matter whilst sodium chloride
can come from a variety of sources, including road salt and, in
the case of a garden temple, the source has been traced to the
urine of livestock.
X-Ray and
other remote non-interventionist techniques such as metal detectors
and infra red thermography are also invaluable in the right circumstances
for the detection of structures such as ferrous armatures that
may be rusting and splitting plaster sculpture and modelling.
Plasterwork is rarely signed by the modeller, which can make dating
and provenance difficult. Stylistic and structural context may
be the only guides in the absence of bills and other archival
sources. Dendrochronology of related structural timbers and even
nail chronology can be useful in assisting dating.
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| Detail
showing damage by salts and water at Russborough House, County
Wicklow |
DECAY
The agents
of decay wreak havoc behind the scenes. Ferrous armatures rust
and expand, salts effloresce, insects and fungi take hold of the
timber substructure. In the British Isles water is the single
most destructive agent. Much damage is due to poor water management
and neglect. Leaks, penetrating damp, rising damp, overgrowth
of flora, frost, poor maintenance and erosion are just some of
the ways it innocuously permeates buildings. While lime plaster
itself is generally not directly affected by water, many of the
materials it is applied over are more vulnerable. To make things
more challenging, these materials are often buried within or beneath
the plaster.
Persistent
exposure to water will cause gypsum, alone or combined in a lime
plaster matrix, to gradually decay and soften until physical failure
occurs. Some loss of strength is of little consequence, but for
later 19th century plasters which may contain high proportions
of gypsum, this can be catastrophic on ceilings. Heavy and repeated
applications of decorative coatings to hide, freshen and 'hold
back' signs of damp on plaster have the reverse effect. Moisture
is withheld in the plaster and may be spread over a much wider
area, driven by capillary action and steep moisture gradients
which actively draw moisture to drier air. Typical methods employed
include the use of impermeable oil based decorative coatings,
thick lead carbonate paste primers, varnishes and even bitumen.
Persistent
problem areas may be exacerbated by the introduction of harder
and 'better' modern plasters or cements applied in the belief
that their durability will provide both repair and barrier. Sadly
this generally leads to accelerated decay of original fabric at
boundaries where moisture and salts become more concentrated.
FIRST
AID
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| The
same ceiling after consolidation of the surviving plasterwork.
The white dots show where shallow inset washers have been
inserted and tied off on perforated straps secured between
the joists behind. |
Once a problem
has been identified, whether catastrophic physical failure like
a ceiling collapse or noting a spreading leak, the subsequent
action has a large influence on the success of any conservation
and repair. Ceilings are frequently propped in a mad panic with
heavy timbers and large sheets of plywood. Whilst effectively
restraining the remaining ceiling, it considerably compounds the
difficulty of assessment, and may cause more breakage, particularly
around perimeter areas that have been forced back against falling
debris from the ceiling back. In addition, it can be difficult
to remove the props without causing still further damage to fragile
plaster.
Provision
of evenly spaced free standing props with manageable platforms
of no more than a metre square, topped with blanket or foam held
just shy of the surface enables easy access and avoids wholesale
removal of supports prior to undertaking work. Furthermore, any
fragments which subsequently fall off will be safely collected
and their source pinpointed, aiding their reinstatement and indicating
the areas still at greatest risk.
Where decay
has rendered plaster particularly friable or where armatures are
splitting modelling, the plasterwork can be 'faced' using acid
free tissue applied with reversible conservation grade adhesives
to form a temporary protective skin pending consolidation and
repair. Where sections of plaster need to be removed, a rigid
'case' of applied plaster of Paris or fibreglass is required to
provide sufficient support for the object once removed from its
backing substrate. When using this technique, additional protection
can be achieved by adding layers of canvas or hessian. Temporary
props may be needed to allow for the added burden on a ceiling
section, and strengthening battens may be incorporated in the
protective case to support the combined weight of object and protective
case. Handles may also be added to the case to assist carrying.
TECHNIQUES
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| Inappropriate
propping of a ceiling at Portland Place, London: boarding
such as this makes it impossible to assess the condition of
the plasterwork, and removing it can cause further damage. |
Good craftsmen
acquire and refine the knowledge and skills particular to their
job. The conservator is no different. Wherever possible it benefits
the conservator to develop as simple technique as possible for
any given task, knowing that it is the breadth and depth of their
knowledge and its application with skill and experience that distinguish
their work, not the complexity of their solution. Frequently,
over-complication in an effort to justify 'specialist' work can
have the reverse effect and, worse, may be later replicated by
operatives with less knowledge and skill, to the detriment of
the object.
Once a deteriorating
object has been made physically stable, a strategy must be formulated
for the next step. Though factors vary in importance for any object
and situation, they will include historical value, contextual
importance, causes of decay, physical integrity and state of repair,
accessibility practicality and need of conservation and repair,
cost and programme. Other important factors will relate to impact
of techniques on the object itself, as well as adjacent existing
fabric. Sometimes all that is needed is environmental change or
control to inhibit or arrest decay. A non-interventionist approach
is generally preferable, though the need for conservation often
arises as rescue rather than prevention.
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| The
staircase ceiling at Belvedere College, Dublin |
Many strategies
are used to stabilise collapsing ceilings. Some are logical, some
prescriptive, and others potentially disastrous. Attempts to straighten
sagging ceilings is often asking for trouble. Unless painstakingly
cleared, debris caught beneath laths and joists can only increase
stress in those areas as attempts are made to lift or push areas
back. Likewise, the levelling of plasterwork that has sagged over
a long period will only move stress and strain onto other hitherto
stable areas providing the potential for later disaster. The layered
nature of the typical ceiling construction can result in intercoat
separation, though generally collapse results from deflection
of the timber substructure from which the plaster depends, coupled
with failing key and laths.
The method
I have developed and favour for securing relatively coherent sections
of existing or barely intact ceiling plaster provides a simple,
dry, lightweight, flexible, cost effective and readily reversible
means of restraint. Shallow inset washers are inserted into the
ceiling face and tied off on perforated straps secured between
the joists behind. Treatment can be localised and specific to
major cracks and delaminating areas. The use of thin wire ties
suspended from flexible band introduces a degree of suspension
and flexibility absent from the use of more rigid methods using
solid studding or thick wire. It also allows for movement of the
ceiling/floors whilst limiting point loadings. Drawbacks are the
need for intervention at the ceiling face and the need for full
access from above. Nonetheless, using wall painting conservation
techniques, even decorative paintwork can be removed from restraint
points and replaced afterwards with minimum retouching required.
Less desirable,
due to inflexibility, is the use of inset washers fixed through
the ceiling face into the joists behind with screws. However,
this may be the only solution where access above is impractical.
The use of resin embedded studding into the back of plaster is
often used in conjunction with significant resin replacement of
failed laths.
At the opposite
end of the spectrum, the often used prescribed method of pouring
plaster of Paris across the ceiling back over wire reinforcement
is a potentially disastrous and unsuitable means of repair. This
introduces large quantities of wet material setting as rigid monolithic
blocks. These in turn accentuate any deflection forces at boundary
edges, destabilising otherwise coherent existing plaster. Adhesion
to an already limited plaster key, combined with the difficulty
of ensuring adequate bonding of material, further compromises
this potentially destabilising treatment. It is only partially
mitigated when incorporating wire loops through the ceiling face.
The incorporation of large quantities of water into a small area,
often closed over as soon as complete, can only enhance the possibility
of fungal attack and decay. Moreover, removal is virtually impossible
without considerable damage.
The use of
modern resins and adhesives need to be carefully considered. All
will gradually harden and lose flexibility as they oxidise and
cross link over time. Loss of performance in common commercial
products will be greatly exacerbated by temperature fluctuations,
humidity and light compared to conservation grade equivalents.
Nevertheless, selective use of resins can be extremely beneficial.
For example, where patching moisture sensitive existing fabric,
such as a decoratively painted surface, an impervious resin can
be used to minimise suction and transfer of water into adjacent
fabric from the new plaster.
Some consolidation
techniques employ copious amounts of resin to increase structural
integrity, often saturating the object. Careful consideration
should be given before using a method that may complicate long
term conservation options. Removal of soluble materials from porous
objects is rarely easy or practical, and becomes increasingly
difficult with age. Grouting of unseen non-structural voids should
be carefully tested to determine suitability and success of the
method. Much time and effort can be wasted to little or no effect
using a technique that cannot be readily measured or checked.
Thought is
needed when specifying internal crack repairs to ceilings and
walls. Whilst likefor- like repairs are often ethically desirable,
they may not always be appropriate. Repairs should never be harder
than the adjacent existing fabric. Too often cracks are filled
without considering the consequences of restricting the natural
expansion and contraction of the building fabric - not least on
a seasonal basis. Hard fills act as wedges and physically erode
adjacent softer material or transfer and increase loads across
to other areas which can be further destabilised.
]The use of
lime-based plaster to ensure like-for-like compatibility can also
be counter productive. The wider the crack, the more shrinkage
may be anticipated together with poor lateral adhesion. Shrinkage
cracks may be impossible to avoid without addition of gypsum.
Either way, care and tending will be required. Narrow cracks may
have to be opened and raked out to provide sufficient purchase
for the lime and so cancel the benefits of any friction interference
helping to retain structural integrity of plaster sections. The
necessary wetting may also be undesirable on sensitive or easily
stained surfaces.
Some modern
gypsum based fillers are particularly hard and are marketed on
the strength of their water resistance and durability. These are
unsuitable for the repair of soft historic plaster. Others are
especially soft and thus eminently suitable. Easily weakened further
with the addition of whiting, they should be matched or be weaker
than the surrounding fabric. Benefits include good adhesion, rapid
drying to a neutral surface, fine texture and soft matrix which
makes gentle smooth and controlled sanding an easy task.
CONCLUSION
Frequently
the greatest challenge facing the conservator is the treatment
of flawed technology of the period. Use of the selfsame materials
would perpetuate the problem whilst modern alternatives may not
be ethically appropriate. Each and every situation must be assessed
on its own merits with a thorough awareness of the consequences
of each and any action on that object.
Seldom will
any material or coating possess every desirable trait without
drawbacks. It is the duty of conservation specialists, whatever
their discipline, to ensure a responsible and sympathetic treatment
of the fabric they are dealing with that encompasses as many favourable
conditions as possible and minimises negative attributes. The
challenge is identifying the key requirements, minimising compromise,
to adopt a sympathetic and effective solution.
Strict adherence
to the original materials and techniques does not always achieve
the long term goals of conservation and preservation, especially
where buildings are outside the specialists control. Solutions
must be considered both in the conservation of the past for the
present as well as their suitability and maintenance for an unknown
future.
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Communications Limited 2009 |
