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BCD SPECIAL REPORT ON
HISTORIC CHURCHES
24
TH ANNUAL EDITION
19
THE ENTRANCE SCREEN
The main entrance screen of The City
of London Cemetery was designed
by William Haywood (1821–94),
architect and chief engineer to the
City Commissioners of Sewers
and
best remembered for his work on
the Holborn Viaduct (1863).
The cemetery was built in 1853 at a
time when London’s churchyards were
in crisis due to a population explosion
coupled with an increase in cholera-
related deaths. The City of London
Corporation moved to build a new
‘super-cemetery’ that would fulfil the
needs of all the City churchyards and it
was Haywood’s job to instigate this. The
cemetery remains the largest municipal
facility of its type in the UK.
The screen is a traditional
construction of Kentish Ragstone walling
with Caen stone quoins, copings and
decorative carved elements. Caen stone
is a soft French limestone which was
ideally suited for fine carving and it
was widely imported by the Victorians:
the down side is that it performs
poorly in a polluted environment.
The rain during the 19th century
and up to
The Clean Air Act
of 1956
was much more acidic than it is today,
accelerating the decay of limestones.
Carbonates in the stone are turned
into sulphates which are more soluble,
allowing acid rain to penetrate further
into the stone. Sulphate-crystallisation
occurs in the drying cycles, and as
these crystals grow they expand within
the pore structure, causing the stone’s
structure to break down, forming voids.
Because the entrance screen faces
south west it is subject to frequent
rain and direct sun. This results in the
stonework receiving extremes of weather,
including freeze-thaw and wet-dry cycles.
At the start of the conservation and
repair programme, the conservators
(London Stone Conservation) were faced
with the damage caused to the structure
by numerous interventions. Premature
decay of the stone elements just 43 years
after building completion was evident in
the many Victorian cementitious mortar
repairs, including those to a pair of
decorative panels inscribed ‘Erected 1855’
and ‘Restored 1898’.
These repairs present a major threat
to the future of the screen. Portland
cement is both hard and impervious, so
it does not allow the stone to breathe.
Moisture trapped by the cement is forced
to evaporate from the softer more porous
stone around the edges of the repairs.
Here, the same natural decay mechanisms
described above are accelerated due to
the greater accumulation of soluble salts.
Eventually the cement repair de-bonds
from the surrounding stone. The resulting
friable surfaces are most prominent in the
ornate central panel due to the number of
cement repairs, and because the increased
surface area of all the sculptural carving
amplifies their wetting and drying cycles.
FINDING A SOLUTION
Repairing the screen posed a number of
challenges. A mix of stone replacement
and lime mortar repair was proposed, lime
mortar being much softer and far more
permeable than Portland cement. As the
stone copings had been entirely encased
in cement, they all had to be replaced with
new stone. Instead of the original Caen
of Normandy a similarly fine-grained
limestone was specified from Lavoux near
Poitier. This is the stone currently used
as a Caen replacement at Canterbury
Cathedral due to its superior weathering
characteristics and frost resistance.
On dismantling, the structure was
found to be saturated in areas below the
cement copings, especially under the
canopies of the central panel. The new
stone replacement copings would allow
the structure to dry out and slow the
decay process because, unlike the cement,
the stone will allow moisture to evaporate.
After close examination of the central
panel it was realised that the condition
was far worse than originally thought and
it was necessary to carry out extra work
to improve the structural stability and
integrity of the stone elements. It was
decided to treat the whole central panel
with nanolime to improve the friable
surfaces of the stone.
A series of nanolime trials was
undertaken to ascertain the best type
of Calosil to use. The main aim was to
ensure maximum penetration so the
consolidating effect of the lime will have
an effect through the body of the stone
instead of just at the surface.
A four-pronged approach was adopted
to the repair of the central panel:
1 Cement repairs were selectively
removed to slow down decay of the
carved stone.
2 The entire surface was treated with
a combination of Calosil E5 (which
contains 5g nanolime per litre) and
E25 (25g per litre), the rationale
being that the E5 will give greater
penetration to start with, followed
by the E25 to deliver maximum
amounts of lime to the structure. The
Calosil was applied with a pressurised
garden spray; five applications of
E5 followed by three of E25 over
a two-day period. The area was
covered with cling-film between
applications to prevent premature
evaporation of the ethanol and so
promote maximum penetration. It
is important that water is sprayed
onto the treated area for a period
afterwards because the lime requires
both air and water to carbonate.
3 Selected areas of previous mortar
repairs were reinstated in a hybrid
mortar to slow down further stone
The bottom of the crest before work: note the cementitious repair (left) and
adjacent erosion.
The bottom of the crest completed: note the retained original fine carved
medallion and the new shield above it. The friable areas of stone were first
consolidated with nanolime, then mortar repaired and limewashed.