T W E N T Y F I R S T 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 4
1 2 1
3.3
STRUCTURE & FABRIC :
ME TAL,
WOOD & GLASS
The high performance wax coating penetrates and protects the whole metal surface, and its distinctive lustre
reveals the laminations of the historic material. It requires only relatively simple maintenance every five to
seven years to ensure protection.
sponges are dry cleaning sponges for use on
soiled surfaces. Dirt particles are absorbed
into the sponge’s surface which, in turn,
crumbles away during use.)
Three different coatings were tested:
tannic acid suspended in polyvinyl acetate
(PVAc), microcrystalline wax, and Waxoyl,
a hydrocarbon suspension of wax and
phosphoric acid rust inhibitors made by
Hammerite Products. These coatings offer
a good barrier layer between the ironwork
and the moisture in the atmosphere and are
relatively easy to maintain. Unlike flaking
paint, they do not require complete removal
before re-application. During the testing
process it was found that Waxoyl gave the best
result and appeared to be the hardest wearing.
During the cleaning tests a small amount
of linseed oil putty was applied to the coated
areas, with the purpose of assessing how well
the putty would stick to the various coatings.
The putty needed to provide a weather seal
between the external environment, the
wrought iron frame and the leaded glazing.
As expected, the tests showed that the putty
would not stick to the Waxoyl and therefore
another approach had to be considered for the
inside face of the ferramenta.
One solution was to treat internal
surfaces with tannic acid, with linseed oil
putty applied on top. Tannic acid is a well
established corrosion inhibitor for iron.
It works by transforming the destructive
iron (III) oxides into iron tannates, which
are stable compounds. However, the tannic
acid is known to cause corrosion of lead. To
ensure that the tannic acid did not affect the
lead matrix of the stained glass panels, two
layers of Paraloid B72 (20 per cent weight by
volume in a solvent of acetone and industrial
methylated spirits) were applied over the
tannic acid. Linseed oil putty could then be
safely applied to the sealed surface.
During the conservation process it was
noted that a number of the glazing lugs were
broken or extremely thin and delicate. It was
agreed that 21 new lugs would need to be
fabricated and fitted. However, the original
material, high-quality charcoal wrought iron,
is no longer available commercially, so
alternatives had to be considered. The
medieval bloomery technology of iron
smelting, where the metal was never melted,
leads to an inhomogeneous microstructure
as found in the samples taken from the
oculus. Variation was evident within the few
millimetres of a single sample. This type of
charcoal or bloomery iron smelted at low
temperatures, is relatively pure, with only
small amounts of carbon and phosphorus
absorbed into the metal, the former from the
fuel, and the latter from the ore.
One option was to make the lugs from
modern rolled/recycled wrought iron. The
recycled wrought iron available today is
generally puddled iron from the industrial age.
It contains slag inclusions and laminations,
and has a similar grain structure to charcoal
wrought iron. Charcoal-smelted bloomery
iron is surprisingly resistant to corrosion
in contrast to more recent iron and steel
in which sulphur from the use of mineral
fuels is retained within the metal and acts to
accelerate the corrosion process. The original
samples examined were also very low in slag
inclusions, which may serve as easy routes
for corrosion penetration. Furthermore,
due to the reclamation process modern
wrought iron varies widely in quality and
would be visually and metallographically
indistinguishable from historic repairs,
which might confuse future investigations.
The alternative option was to use modern
pure iron. Metallurgically there is little
difference between pure iron and wrought
iron other than the fact that the pure iron
does not contain fibrous slag inclusions. The
conservation benefits of using pure iron are
that it is close to wrought iron, of consistent
quality, and easily distinguishable from the
original material. After considering the
options the decision was made to use pure
iron. A historic repair to one of the glazing
lugs was used as the design basis for the new
lugs. A small design change was incorporated
into the new lugs, and all the new components
were also date stamped to make them easily
identifiable in the future.
Considering the slight chemical difference
between the wrought and pure iron, it was
important to minimise the risk of galvanic
corrosion. To do this, at least one of the three
corrosion causing factors – oxygen, water, or
direct contact between dissimilar metals –
had to be eliminated. Using techniques that
were originally used in the manufacture of
the oculus (surrounding the copper wedges),
lead paste (a compound of linseed oil and red
lead containing, by weight, 95% lead (II, IV)
oxides) was applied to the ferramenta to act as
a jointing compound and barrier between the
wrought and pure iron. The new lug was then
placed over the paste and locked into place
using iron wedges. The lead paste would act as
a seal preventing water ingress.
This brightly coloured lead paste was
allowed to harden for two weeks, and was then
toned down with a coating of black Waxoyl.
LOOKING AHEAD
The south oculus ironwork had clearly
undergone some deterioration: localised areas
of more advanced corrosion were evident in
places, and substantial repairs had been made
to the ferramenta. However, it is a tribute
to the ingenuity and skills of the medieval
craftsmen that the iron window framing had
continued to fulfil its role for over 800 years. It
even survived damage received during World
War II bombing raids, when a section of the
ferramenta was effectively removed under
enormously high stress loadings, but the
integrity of the frame held.
Having carefully considered our options,
a high-performance wax coating (Waxoyl)
was chosen rather than either a traditional
or a modern paint system. The maintenance
of this coating could be carried out every five
to seven years, using a cherry-picker to gain
access to the oculus. The ferramenta and grille
would require a simple wash-down, followed
by re-application of Waxoyl. Paint systems, by
comparison, generally require more invasive
maintenance, including shot-blasting in the
case of the two-pack systems commonly
used, and all paint systems can trap moisture
when they crack or flake. We believe that high
performance wax coatings provide a good
balance of low-intervention treatment and
manageable maintenance. It is hoped that this
sympathetic approach to the conservation of
its 800-year-old ironwork will preserve the
oculus for many more centuries.
BRIAN HALL
ACR is managing director and a
senior conservator at
.
He has worked in the field of conservation
for the past 28 years specialising in sculpture
and architectural metalwork. He is also a
trustee for the National Heritage Ironwork
Group and tutor on the SPAB courses ‘The
Repair of Old Buildings’ and ‘Metalwork
Masterclass’.
The conservation and repair works to the south
oculus window were carried out as part of the
ongoing conservation works to the cathedral’s
stained glass windows and was assisted by a
generous donation by the Worshipful Company
of Ironmongers.
