A Conservation Case Study
||The repaired railings in Bedford Square, London
Bedford Square dates from the
second half of the 18th century and
is acknowledged as one of the most
significant and best preserved Georgian
squares in London. Important attributes
include the nearly symmetrical, palatial
composition of the terraces on all four sides,
all of which are Grade I listed. The private,
landscaped gardens in the middle are included
in the English Heritage Register of Historic
Parks and Gardens, and listed Grade II*.
Severe damage was caused to some of the
square’s original railings in 2010: following
heavy snowfall in London in January, a car
spun out of control and hit the railings of
No 37 Bedford Square, part of the terrace
which forms the square’s western side.
Fortunately, the car came to a halt before
it could fall into the lightwell, which the
railings enclosed. This article describes
the conservative repair and reinstatement
of the damaged Georgian railings.
ASSEMBLY AND CONDITION
The railings of No 37 are typical of the square
and of the period in both their materials
and their ornamental detailing: square
wrought iron balusters are fixed at regular
intervals into a Portland stone plinth and
secured with molten lead in the plinth
sockets. At the top, the balusters are held by a
horizontal cast iron coping rail. The railings
were constructed in two panels, defined
by wrought iron standards decorated with
cast iron urn finials. The badly damaged
panel included a pivoted gate. The whole
structure is braced to the front wall of the
building. The return section to the entrance
door has the cast iron horizontal coping
fixed into the brickwork, also using lead.
The impact of the car, as well as
severely damaging one of the panels, had
caused movement to the whole enclosure,
including the gate standard and pivot. The
wrought iron balusters in the damaged
panel were buckled and dislodged, and
the stone plinth sockets had been broken
as a result. The cast iron coping remained
attached to the collapsed balusters as
the coping itself broke into two pieces,
consistent with the brittleness of cast iron.
||The damaged railings following the impact of a car in snowy conditions
Assessment of the damaged railings
revealed two key maintenance problems, both
Firstly, the metalwork had
only a thin protective covering of paint. It
was clear that the railings had been stripped
and repainted in the past few years and
that repainting had not provided adequate
protection as there were several areas of
Secondly, the Portland
stone plinth had been painted white. The
impact from the accident had caused the
paint to flake and the stone underneath to
delaminate but this also revealed areas where
the stone had turned to powder. This was
probably the result of frost damage, as water
seeping in through capillary action could not
evaporate but remained trapped behind the
impervious paint. The painting of the Portland
stone plinth was problematic both in terms
of its appearance, which was not faithful to
the original 18th-century design, and its long
term compromising effect on the integrity
and strength of the plinth and its capacity
to support the main fixings of the railings.
Remedial works were carried out in the
following sequence of like-for-like repairs:
1 Railings stage 1: the damaged cast
iron coping rail and wrought iron
square balusters were dismantled by
the metalwork contractor and taken to
the contractor’s specialist blacksmiths’
workshop. There, the balusters were
stripped of paint, straightened and
repaired using traditional forge
techniques and the cast iron coping
was stripped of paint using light bead
blasting. The restored metalwork was
then primed ready for delivery to site.
2 Portland stone stage 1: according to the
assessment it was not feasible to carry out
mortar repairs to the Portland stone plinth
section which had supported the collapsed
metalwork because the original sockets
in the plinth were too badly damaged.
It was therefore decided that this part of
the plinth would be replaced with new
matching Portland stone and a workshop
drawing was accordingly produced
by the mason and checked on site.
Once the railings had been taken away,
the paint on the retained Portland stone
section was removed using a chemical gel
application and the DOFF system. This
uncovered past repairs in grey cement,
which were carefully chiselled out. Finally,
a sample mortar repair was carried out
using hydraulic lime and Portland stone
dust as the main aggregate and sponged
off to produce a smooth finish. This
approach was designed to achieve an
appropriate strength and to match the
colour and texture of the surviving stone.
3 Portland stone stage 2: the
relevant stone plinth sections were removed and the new stone was bedded in on a sand and
non-hydraulic lime mix with the minimum amount of water.
Mortar repairs were carried out to wetted stone to ensure that water
was not drawn too rapidly from the repair mortar, which would
have caused shrinkage. The repairs were built up in layers no more
than 12mm deep, as required. Mechanical keys were formed on the
undercoats to achieve better adherence of the overlay by combing
or scratching to produce evenly spaced lines. The top coat was
wiped with a sponge after the mortar had set but before it became
too hard, to produce a seamless joint with the stone proper.
Finally, all stone joints, both new and old, were re-pointed.
|Partial replacement of the Portland stone plinth
4 Portland stone stage 3: round sockets were diamond
drilled in the new Portland stone plinth by the metalwork
contractor and squared off by the stonemason.
5 Railings stage 2: the primed and repaired railings were returned
to site, refitted in the new plinth sockets and fixed in place with
molten lead. The molten lead fixings attaching the wrought iron
balusters to the cast iron coping were also reinstated. The reconnection
of the coping to the cast iron section that had remained
in situ was carried out using a combination of a discreet mechanical
fixing in the form of a connection plate to its underside and brazing
of the broken joint. This is standard practice: because cast iron does
not lend itself to welding, repairs to broken sections make use of
mechanical connections and patent methods such as cold stitching.
In contrast, wrought iron can be forged, allowing weak sections
to be cut out and new pieces of matching profile and material
(such as recycled wrought iron) to be welded to the original.
6 Railings stage 3: once the damaged section was reassembled, the
rest of the railings were rubbed down to recover healthy metal
behind rust patches, primed and prepared for re-painting.
It is important that all rust is removed from any piece of ironwork
and a rust inhibitor is applied straight away to avoid re-oxidisation
of the cleaned surface. If the rust is not removed completely it will
continue to develop through electrolytic action beneath the paint
until all the metal returns to its original state (iron oxide, the raw
material from which iron is obtained). Left unprotected, all ferrous
metal corrodes rapidly. When lead fixings to stone fail, water collects
in the sockets causing the ferrous metal to corrode and expand. This
may explain the cracks in the retained original stone plinth sockets
and associated earlier cement repairs in the Bedford Square railings.
7 Railings stage 4: all railings were painted using a two-pack zinc
phosphate primer followed by a micaceous iron oxide intermediate
coat finished with a high build, high gloss polyurethane top coat.
8 Portland stone stage 4 (pending): the lead in the
new Portland stone sockets will be concealed with
caulking to match the colour of the stone.
In the 18th century, railings were rarely painted black, a practice
introduced by the Victorians. English Heritage is currently
encouraging a return to authentic colour schemes and it is hoped
that a future agreement can be reached with Bedford Estates to
adopt a Georgian colour for the ornamental ironwork and other
elevational elements of the listed buildings in Bedford Square.