Wyatt’s spectacular Egyptian dining room at Goodwood Park, which was completed
by 1806 and covered up
in 1906, reputedly following the objections of Edward VII.
The scheme was preserved behind panelling and
over-painting, and has recently
been restored. (Photo: Courtesy of the Trustees of the Goodwood Collection)
The term scagliola (pronounced scălliōla,
with a silent g) derives from the Italian
‘scaglia’, a local name used in the Italian
Alps for limestone (Oxford English Dictionary,
2012). Scagliola is a form of plaster, traditionally
gypsum, which imitates decorative stone and
has been widely used for the decoration of
walls, columns, floors, fireplaces and table tops.
Artificial forms of decorative stone
based on gypsum or lime plaster have a
long history and were widely used in the
ancient world. Recipes for their production
can even be found in the works of the
great Roman architect, Vitruvius.
The technique was rediscovered in the
Renaissance and flourished on the continent
in the Baroque and Rococo periods as a
medium for the imitation of exotic marble
and precious stone inlays. First introduced to
Britain during the 17th century, rare surviving
examples include a fireplace in the Queen’s
Closet at Ham House, Surrey.
In Britain, however, the use of scagliola
is more commonly associated with the
imitation of the finest marbles on a much
larger scale, without any joints or defects,
and for columns and pilasters in particular.
This use reached its zenith in the Regency
period, typified by the extravagant bright
scarlet and imitation lapis lazuli columns
introduced by John Nash at Buckingham
Palace in the 1820s (illustrated overleaf) and
by the spectacular staircase hall of Charles
Barry’s Reform Club, London in the 1830s.
In the latter part of the 19th century
marezzo scagliola was developed in the
US. It advanced the traditional methods
of making scagliola by using Keene’s
cement, a gypsum-based cement patented
in 1838, rather than plaster. Both marezzo
scagliola and traditional scagliola in
America was prolific in the 19th century,
although their use was generally confined
to public buildings, such as state capitols,
courthouses, churches and railway stations.
The difference between marble and scagliola
can be established by feeling the surface – if
it is cold, it is likely to be marble. Scagliola
also produces a hollow ring when tapped.
In appearance, marble is more translucent,
although scagliola sometimes has pieces of
alabaster or other semi-precious stones added
to the mix to give localised translucence.
Looking carefully at any damage holes or at
the rear surface if this is accessible, a marble
piece will look like marble throughout, whereas
scagliola will have a distinct plaster backing
behind the colour coat. The presence of
masonry joints would also normally indicate
marble, as scagliola is often used expressly to
create continuous surfaces. Scagliola can also
be identified if the surface has hairline cracks.
Chemical analysis may be used to confirm
whether or not the material is made of gypsum,
but it should be borne in mind that it will
reveal information about the current material
as it is now, rather than what it was. Mass
spectrometry for example will reveal whether
it is a gypsum plaster by showing up calcium,
sulphate and oxygen. In the case of with
Keene’s cement, there will also be a percentage
of aluminium, potassium, sulphur and oxygen.
Chemical analysis would also reveal the
presence of pigments, animal glue, isinglass,
linseed oil or some other organic material.
CARE AND MAINTENANCE
||A 17th century scagliola fireplace in the Queen’s
Closet at Ham House, Surrey: this is one of the
earliest known examples of the technique in Britain.
Scagliola is vulnerable to water ingress. Being
made of gypsum, it will quickly return to
its natural state when water is added. Water
running over the surface will etch runs and
pitting as it washes out the gypsum. If soaked,
salt crystals will effloresce on the surface of
the scagliola as it dries, as crystals of calcium
sulphate migrate and form on the surface.
Water will also loosen the bond between the
surface and deeper layers of the build-up,
leading to delamination and an abnormally
hollow sound when tapped. Prolonged
exposure to moisture will eventually weaken
and destroy scagliola.
Scagliola is quite brittle. Because it is
made up of many separate elements joined
by variable strengths of adhesion, scagliola
is not a structural material and will crack
along the line of least resistance. Care should
be taken to allow tolerance for thermal
expansion and contraction when restoring a
piece, for example between a scagliola column
and the superstructure. Cracking caused by
compression is often seen in columns and
frequently occurs at the base. Superficial
cracking can be considered to be part of the
patina of age, which gives scagliola part of its attractiveness as a finish. It is important
to identify the type and extent of cracking
and to determine whether the cracks indicate
pieces have delaminated and are ready
to detach, or whether it is just superficial
‘crazing’. Establishing a benchmark for future
comparison in this regard is one of the most
useful conservation techniques.
As the skills required for the manufacture
of scagliola had largely died out by the
1930s, a number of fine examples have been
inappropriately restored using plain plaster or
other fillers and by painting in the colour. Care
needs to be taken not to expose such patches,
by abrasion for example, unless this is the
intention. Where over-painting has disguised
the presence of a decorative finish, extensive
damage has often been caused by contractors
who are unaware of the sensitive nature of the
underlying material. The filling of cracks with
inflexible fillers can also lead to deterioration if
done in such a way as to restrict movement.
In order to conserve and maintain this
sensitive material, it is useful to have a general
knowledge of the methods of construction.
The first and older technique is described
as fine or traditional scagliola, the other is
referred to as marezzo or American scagliola
(the latter because of the large amount of
marezzo scagliola seen in the US).
|Pink Regency scagliola which had later
to look like Sienna marble
|The restored lapis lazuli scagliola columns
Music Room at Buckingham Palace
With fine or traditional scagliola which
imitates marble, the craftsman tried to imagine
how the earth compressed, broke and twisted
the various elements to produce the appearance
of a particular type of stone. Geological
processes which took millions of years were
recreated on the bench by making large lumps
of soft marble which are arranged to reflect
the required brief and then allowed to set.
This method is further expanded to
produce a mix of soft material with lumps of
the hard set material within it. The simplest
example of this is a Porphyry type scagliola
which is made by mixing a thick slurry of red
pigment with small chips of hardened plaster.
A Verte Antique scagliola is made by combining
lumps of set plaster and pieces of alabaster into
a slurry mix. An experienced scagliolist will
have many different recipes and techniques for
creating different types of decorative stone.
Marezzo scagliola is made wet, getting the
effects required by fixing the flow of pigments
and plaster mixes at a particular stage in
the manufacture. Marbling and veining is
produced by dipping tangled lengths of silk
thread in a liquid pigmentation then stretching
them over a bench. Over these coloured silk
threads a thin skin of coloured Keene’s cement
is poured or spattered, transferring the pigment
from the silk threads to the thin skin of cement.
This form of scagliola is usually made to the
required thickness right from the start, and this
is often as little as one eighth of an inch thick.
Both traditional and marezzo scagliola are
finally cut back to expose the different colour
changes within the body of the material, then
smoothed and polished with progressively
finer abrasives. Finally, the surface was usually
sealed and polished with oil or a mixture of
oil, wax and sometimes other materials.
Inspection should include a close examination
of the surface, noting any efflorescence, signs
of mould or runs of varnish, and identifying
the various coatings that may have been added
such as polyurethane. At this point the careful
removal of a surface sample should help to
determine the most appropriate treatment.
It is essential to make sketches and scale
drawings to record the condition of the
piece so that any defects can be measured
and recorded accurately with comments.
Attention should be paid to the thickness
of the colour coat, as this is often not great
and may already have been polished to
negligible thickness. Further polishing may
result in non-reversible blank patches.
Chemical analysis of both the surface
and the substrate can help, but there is no
substitute for a trained eye in identifying a
particular scagliola recipe. A knowledge of the
pigments, materials and techniques available
when a particular piece was made will aid
both an understanding of how it was made
and how it could be replicated.
With experience, a scagliola piece can
be sounded with a gentle tap to identify the
differences between delaminated and firmly
attached areas. As scagliola is a surface
application it has a variable thickness, usually
physically keyed into the background by
scratching and application of a slip of plaster,
if this adhesion is failing, sounding will give a
different tone. To a trained ear this can help
determine whether cracks go right through the
piece or are superficial. When the sounding is
complete, it is helpful to use the drawing as a
map to indicate suspect areas. This should give
an indication of how much of the piece is still
securely attached to the substrate.
It is important to treat the inspection as
part of an ongoing maintenance programme,
providing a snapshot of the condition at a
particular time. This allows others following
on to gauge rates of deterioration and the
success or otherwise of any intervention.
||Marezzo marble columns at Stamford Old Town Hall, Connecticut: over-painting
has been stripped and the underlying surfaces are being cleaned and repaired
prior to polishing.
As with the conservation of any historic
material, the intervention should always
be fail-safe and should only be attempted
after non-invasive testing and close
inspection, or preferably a full condition
survey, including visual inspection,
sounding and historical research.
Where surface delamination has occurred
it may be possible to re-attach the layers by
injecting one of a variety of specialist modern
adhesives, selected according to the properties
required, such as gap filling, reversibility and, of
Different depths of separation
need different treatments, but the principle is to
clean out the void either by flushing or blowing,
inject a suction-reducer (which may be water
or diluted adhesive) and then inject sufficient
liquid adhesive to reattach the surface layer to
the substrate. Any holes made in the process
are then plugged with appropriately coloured
material prior to refinishing and polishing.
Colour fading can occur when heat
sources are too close to scagliola or where
regularly exposed to bright sunlight. Fading is
usually almost impossible to reverse, although
minor surface fading can be rectified by
cutting back the surface and refinishing.
Scagliola has often been over-painted
at some time, either to match a new colour
scheme in the room or because the client
disliked the original colour. To revive a scagliola surface, for example
after paint stripping, it is advisable to carefully
start with a small trial area to find the best
restoration method for a particular scagliola.
Most chemical strippers can erode gypsum,
lime or Keene’s cement very quickly if applied
in the wrong concentration.
When the most suitable cleaning and restoration methods have
been chosen and the paint removed, the surface can be inspected again
for further defects. At this stage any hidden filling will come to light.
Dirt on the original surface can now be carefully removed.
|A new catalogue of Hayles and Howe’s work in this field is available from the author (click image for details).
If the decision has been made to cut back the surface of the
scagliola and restore it, the next step is to start cutting back the surface
with a coarse abrasive paper, gradually moving to fine. As the work
proceeds, all loose and friable material is removed, and damaged areas
are repaired with fresh scagliola mix to match. These repairs must
be done wet to reduce suction and to enable adhesion. Once set, the
process of cutting back restarts, gradually moving to the finer grit and
even finer powders if required, retracing the final finishing process
of the original manufacture as far as can be determined. This cycle
is repeated as many times as necessary until a sheen starts to appear,
before carefully and delicately applying the specified finish.
Matching the highly polished surface of scagliola requires careful
selection of abrasives with the appropriate particle size for each stage
of the procedure, as well as a suitable polish. The abrasives range from
very rough plaster planes of 80 grit through to 1600 grit, finishing
with fine powders. French polish, linseed oil, milk and a number of
modern waxes have all been used to achieve a high gloss. Sadly, modern
urethane coatings have also been widely used in the past, and these may
need chemical removal.
The stunning lapis lazuli scagliola columns in the Music Room at
Buckingham Palace posed a very interesting problem. The columns
which were originally made in 1836 by William Croggon and his team
included a mix of filings of brass, tin and copper, to give a gold and
silver flecked appearance to the blue sheen. The filings immediately
began to corrode and expand, blowing pits in the surface. This process
continued slowly for 150 years, but when the Palace State Rooms were
recently opened to the public, the decay began to accelerate as a result
of the additional moisture generated by visitors.
To restore the columns
to Nash’s original design it was decided that a treatment using beeswax
mixed with some rare, specially selected pigment and finished with a
lighter grade of wax to give a shine would protect the columns without
obscuring them from view.
A Zecchini, Arte Della Scagliola sul Lario, Hoepli, Milan, 1997
G Beard, Craftsmen and Interior Decoration in England 1660-1820,
Bloomsbury Books, London, 1986
W Millar, Plastering Plain and Decorative, 1st Edition 1897,
facsimile edition, Donhead, Shaftesbury, 1998
G Worsley, ‘Buckingham Palace, London’, Country Life, 5th August, 1993
The Building Conservation Directory, 2013
DAVID HARRISON is managing director of Hayles and Howe Ltd in
the UK and Hayles and Howe Inc in the US. Working in
traditional fibrous plaster as well as specialist scagliola enables the
two companies to provide a wide range of design specifications and
finished products. A book by his colleague David Hayles, The Magic of
Scagliola, is being prepared for publication.
Lime Mortars and Renders
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