Page 183 - The Building Conservation Directory 2013

T w e n t i e t h a N N i v e r s a r y 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 3 1 8 3
5
INTERIORS
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.
Manufacture
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).
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.
Visual inspection
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.
Conservation
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
course, adhesion. 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.
Pink Regency scagliola which had later been painted
to look like Sienna marble
The restored lapis lazuli scagliola columns in the
Music Room at Buckingham Palace