Page 107 - 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 0 7
3.2
Structure & Fabric :
Masonry
In this case it was concluded that the masonry
was beyond any consolidation as all strength
had been lost in the cement matrix, and the
sandstone was rapidly reverting to a pile of
sand grains. It had to be renewed entirely.
Sandstones generally pose particular
challenges when it comes to conservation
and repair due to this variation in their
constitution. The methods of repair and
philosophy of conservation that are
appropriate depend, therefore, on a detailed
analysis and understanding of the stone
characteristics involved, the physical and
environmental conditions that prevail and
detailed historical research into previous
repairs or other influences that may have
affected the chemical integrity or equilibrium
of the material.
Other factors affected by the physical
properties of the sandstone include
vulnerability to staining by natural weathering
and atmospheric pollution. If they are
also vulnerable to inappropriate cleaning
techniques, changes to their appearance can
be permanent.
Some sandstones may also be highly
porous, making them vulnerable to over-
saturation and subsequent frost damage.
In view of these characteristics and
the fact that weaker beds of stone are very
vulnerable to salt crystallisation decay and
subsequent wind erosion or other chemical
attack, it is clear that replacement stone
requires careful selection. However, harder
beds of stone may be more durable to erosion
or chemical attack, but they also tend to be
more brittle and consequently more vulnerable
to mechanical damage or fracturing, often
due to corrosion (rusting) of ferrous materials
embedded within them.
This issue of ironwork corrosion within
the masonry is a prime example of where it
Typical defects at St George’s Hall prior to conservation
is essential that proper research and analysis
is essential so that a full understanding of
the risks involved is gained before making
any decisions regarding appropriate repair
techniques and well before any action is taken.
Some particular periods or stylistic traditions
in architecture incorporated iron or steel
elements in the masonry construction and
some didn’t, so an intimate knowledge of their
differences is essential when assessing the risk
of long term damage and decay.
Sandstone conservation at
St George’s Hall, Liverpool
Repairs carried out at St George’s Hall in
Liverpool illustrate a pragmatic but informed
approach to sandstone conservation. This
highly significant 19th century Grade I listed
Neo Classical civic building was suffering from
all of the most common problems associated
with sandstone, and a major conservation and
restoration project was carried out in three
phases between 1997 and 2008 under the
direction of Purcell and with the substantial
support of the Heritage Lottery Fund. These
included extensive sensitive repair and
conservation measures to the entire external
fabric, which consisted of mainly sandstone
faced masonry.
Cramp repairs
The illustration top right shows one of several
hundred similar fracture defects before the
stonework conservation and repairs. The
corroding cramp set into the top face of the
ashlar course below the cornice had corroded
and shards of hard, brittle stone were about
to fall off the building. In contrast however,
the moulded course above was selected from
a softer bed to make the working of the stone
face easier for the banker masons; this course
had suffered from salt crystallisation decay
around the vertical joint due to over-saturation
and continual evaporation cycles caused by
an open joint in the sky surface of the cornice
above. The black patch was a previous cement
repair which had been coloured to match the
pre-existing pollution-soiled facade. This patch
was almost detached due to the salt action
behind and the characteristic permanent
picture-framing stain pattern caused by the
exacerbated evaporation at the joints can be
seen on the ashlar and fascia stone faces.
The conventional repair solution adopted
widely beforehand, both on this building
and many other similar cases where rusting
St George’s Hall after completion of the conservation and cleaning project