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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 4
T W E N T Y F I R S T E D I T I O N
3.3
STRUCTURE & FABRIC :
ME TAL,
WOOD & GLASS
Using a portable XRF analyser to document the chemical composition of the copper wedges as they age
The back of the ferramenta with surviving lugs
riveted through the iron bars to provide the
connection point to support the stained glass
Detail of a past repair to secure one of the lugs: 21 of
them had to be renewed entirely.
carried out on the structure to determine the
effects of wind load using ambient vibration
tests. Once the modal response had been
identified, the structure could be digitally
modelled using DIANA finite element analysis
software. It was established that 17 per cent of
the structure had been lost through corrosion
and that it could afford to lose a further 75 per
cent before it became structurally unstable.
The grille junctions consist of wrought
iron horizontal bars passing through a square
punched hole in the vertical bars. These joints
were tightened with one or more copper
wedges driven in, and then sealed with molten
metallic lead and red lead putty to prevent
water ingress.
The stained glass leaded panels are
supported off the rear of the ferramenta by
iron wedges and lugs (see illustration) riveted
through the ferramenta. Again, putty was used
to create a watertight seal.
When the South transept was scaffolded
as part of a wider programme of repairs, the
Dean and Chapter required an Icon-accredited
metal conservator with suitable experience
to advise the consultant team made up of the
surveyor of the fabric, the head of stained glass
conservation, the head of conservation and
masonry, and the structural engineer. As the
appointed metalwork consultant, I was to put
forward suggestions/options for investigations,
carry out treatment trials and provide a report
showing treatment specification.
The client needed a long-term, low-
maintenance solution, as once the scaffolding
to the south transept was taken down, this part
of the cathedral might not be easily accessible
for another 50 years.
CONDITION
As its general condition was stable, the
ferramenta was able to cope with the
forces acting upon it. However, the north
oculus external grille had already been lost,
demonstrating that there was a potential for
failure here too. Caution had to be exercised.
In the ferramenta’s recent history it had
clearly been painted with several layers of a
lead based paint, which was now flaking and
peeling. The paint had not been consistently
maintained, and had broken down, enabling
water ingress and corrosion. Failed, broken
down and degraded paint on ironwork
promotes differential aeration corrosion, and
can be more detrimental to substrate iron
than no coating at all.
On the internal, upper and lower faces of
the ferramenta, in addition to the paint layers,
there were the remains of old window putty,
which had dried out and was now absorbing
moisture. This moisture sitting against the
iron surface for extended periods of time
had resulted in small areas of extensive flaky
corrosion.
Corrosion products covered the exposed
surface of the iron and could be seen beneath
the flaking paint. However, the rust streaking
and staining of the stone appeared to be light
considering the volume of exposed iron.
Lead-filled grille junctions were also
a cause for concern, as dissimilar metals
(such as lead, copper and iron) in direct and
electrochemical contact could easily become
sites for electrochemical (galvanic) corrosion.
However, lead caulking and putty seemed
to adequately protect the joints from water
ingress, and air flow around these areas was
slowing down the rate of corrosion. There
was no discernible difference in the amount
of corrosion at these junctions compared to
other parts of the grille.
There were various old repairs and breaks
to the grille and ferramenta, but they were
not detrimental to the overall stability of
the structure. Sections of metal pipe had
been fixed around the horizontal bars in
areas of breaks, but they were now loose,
performing no structural function, and as
they risked forming water traps they could
lead to corrosion. Once these additions were
removed, it was determined that underlying
breaks had been stable for a long time. They
did not compromise the structural integrity
of the grille and did not require interventive
treatment. Strap repairs on the ferramenta
were left in place.
CONSERVATION PROPOSAL
The conservation treatment to the metalwork
was to be carried out in situ. A minimum
intervention approach was adopted. All parts
of the metalwork were to be conserved as
found wherever possible. Interventions would
be restricted to stabilising, and repairs would
not be undertaken unless they were absolutely
necessary. Where necessary, repairs were to
be made with compatible ferrous materials
to prevent galvanic corrosion. No effort was
to be made to straighten distorted metal
components.
We needed to decide on a coating system
that would offer a low level of intervention,
provide adequate protection against
weathering, be easily maintainable or re-
treatable, and be compatible with the original
materials.
The use of both traditional and modern
paints was ruled out as our brief was to offer
a treatment that could last up to 50 years with
little maintenance. This led us to favour the
trialling of oils and waxes.
We had to consider an adequate coating
for both the external and internal faces of the
ferramenta and grille, as well as a sealant for
the interface between the ferramenta and the
leaded stained glass panels. Compatibility
between these materials was to be explored
during the trials.
TREATMENT
Very few archaeo-metallurgical studies have
been carried out on comparative architectural
ironwork in the United Kingdom. The
conservation of the south oculus provided
an opportunity to study the extensive
iron components of the window to better
understand its manufacture and the history
of changes during its lifetime. Small samples
were removed for metallographic analysis,
and sections of both original and repair
material were examined with a portable X-ray
fluorescence (XRF) reader.
Following investigation and cleaning
trials, it was decided to adopt a low-
intervention cleaning strategy: this would
involve the removal of the paint, putty and
corrosion products using scalpels, soft wire
brushes and Wishab sponges. (Wishab
