Page 10 - The Building Conservation Directory 2013

1 0 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 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
electrolysis, with small containment tanks
constructed around the metal frame. The
process was originally developed for ferrous
archaeological artefacts recovered from
shipwrecks. In essence it involves attaching
the negative terminal of a battery to the
ferrous artefact (in this case the frame) and
the positive terminal to a separate metal object
such as titanium mesh. Both are immersed in
water so that the artefact becomes the cathode
and the titanium mesh becomes the anode.
Salt (sodium chloride) breaks down in the
electrically charged solution (or ‘electrolyte’),
with chloride ions drawn to the anode and
sodium ions drawn to the cathode.
In the event the process proved
impractical for a structure as complicated
as this, as well as being expensive and time
consuming, and adequate results were found
to be achievable through the cleaning process
alone. After the timber boards of the hull
had been carefully removed, the wrought
ironwork was therefore cleaned, first with a
low-pressure air abrasive to remove corrosion
products, and then by grit-assisted water
jetting, again at low pressure. The surface was
then tested for residual salts using a salt-
contamination meter. If the level exceeded
acceptable levels, further cleaning was carried
out until a satisfactory level was reached.
The metalwork was painted immediately
after cleaning to prevent further corrosion
using two-pack epoxy zinc phosphate
primers, two-pack epoxy micacious iron
oxide intermediate coats and two-pack acrylic
urethane gloss-finish top coats. Film thickness
of the paint system was meticulously
controlled throughout.
Original ironwork was painted white,
as originally, and new steelwork which
was introduced alongside to supplement
the originals without altering them, was
painted grey.
Raising the hull
To prevent further distortion it was essential
to remove all the props and take the weight off
the keel. The chosen solution was to suspend
the hull, once repaired and consolidated, from
a series of beams that pass though it from side
to side just above her waterline. Each beam has
two cables just inside the hull on either side
of the ship which are fixed to the keel at the
bottom – in essence forming a series of giant
inverted coat hangers within the hold of the
ship. Each beam is supported off the sides of
the dry dock by struts.
The system raised the keel off the floor of
the dry dock so that it was held in suspension,
and the decision was taken to raise Cutty Sark
by a further three metres, creating a large
visitor facility below and enabling the sleek
shape of the hull to be admired from below.
However, from outside the appearance of the
raised ship now surrounded by Grimshaw’s
glass bubble is bizarre, and the design was
derided by both the architectural press and
the historic ship fraternity.
Inside the ship the new structural
support does not detract from the original
architecture. Much of the wrought ironwork
of the hull is heavily corroded, and the use of
the additional ribs has enabled the original
wrought ironwork to be retained.
Keeping out the rain
Although the new roof over the dry dock
enables the environment surrounding the
hull to be controlled, the environment inside
depends on keeping out the rain. In particular
leaks in the deck and the drainage system
had meant that salt used in frosty weather to
keep the main deck safe had contributed to
metalwork corrosion below. The integrity of
the main deck was essential.
As the fire destroyed what little remained
of the teak stored for reuse on the deck, Cutty
Sark’s main deck was replaced with a modern
composite material with a teak veneer.
This, combined with traditional leadwork
particularly in the scuppers, and repairs to the
structural framework ensured its integrity.
The degree of renewal necessitated by the
fire and by the loss of fabric during her latter
years in service means that some parts of
Cutty Sark’s fabric have been added to make
sense of those elements that are historic.
The rigging and masts for example are a
reconstruction of her days as a clipper. Other
elements have been added simply to maintain
its structural integrity and to avoid further
loss of fabric, or for fireproofing. The results
may leave the visitor feeling a little uneasy
at times, unsure exactly what is original
and what isn’t. Below deck, the history is
clearer. Like it or not, the reconstruction
and restoration work does provide an
extraordinary illustration of the different
philosophical and practical considerations of
conservation.
The author: this article was prepared by
executive editor Jonathan Taylor with the
help of Cutty Sark Trust director Richard
Doughty, Steve Perryman of English
Leadworks (see page 83) and conservation
engineer Jim Solomon of Buro Happold.
Historical research was carried out by
Aysha Taylor.
The raised hull after completion with information panels in the converted dry dock
(Photo: National Maritime Museum)
Section through the hull at the entrance to the ship
showing one new timber and a conserved original,
packed out to restore the shape of the hull. The bolts
were fitted into the original holes to avoid damaging
original work. Caulking was left out to enable the
structure to breathe. (Photo: Jonathan Taylor)
Designed as a tea clipper, Cutty Sark was a dry
ship. The reinstated lead flashings and the new
composite main deck ensure that rainwater is
once again excluded from the structure below
(Photo: Steve Perryman, English Leadworks)