Preserving the shipIn
1970, the ss Great Britain returned to the dry dock, in the
Great Western Dockyard, where she was built between
1839 and 1843. The Great Western Dockyard is a vital context for understanding the ship, and the conjunction of ship in
original dock is of international importance. The ss Great Britain’s historic association with this site therefore makes it the most fitting place to conserve her.
Conserving the more than
160-year-old iron hull on this site is exceptionally difficult as the dock was a
damp environment, and moisture in the air made the process of
corrosion continue quickly. Exposure to
rain and moisture in the air (humidity) in Bristol allowed the ship to continue to corrode, particularly accelerated by salt infestation in the wrought iron structure.
Iron corrodes when it comes into contact with water and air. It forms new substances called
‘corrosion products’. Iron oxide – or common rust – is one of these. There are many others, such as iron chloride, magnetite, and akaganeite.
Salt, or chlorides, make the problem of corrosion much worse. This is partly because they are
hygroscopic, which means that they strongly attract water, even out of the air. When salts build up and bond with iron or steel they make the metal rust faster. The lower parts of the hull are worst affected because of this process and their exposure to
salty seawater for
127 years. These salts are extremely difficult to remove, and they cause very severe corrosion.
Saving the ss Great Britain demanded
expertise from research scientists, iron conservators, architects, structural engineers and others.
New research at
Cardiff University demonstrated that corrosion could destroy the ss Great Britain within a few years. This research also showed that the iron ship could survive if she is protected from humidity in a very dry environment. At
20% relative humidity (RH) or less this kind of
salt driven corrosion virtually ceases. The RH of Bristol’s historic harbour is typically around
80%. 20% is more akin to the Arizona Desert.
Due to her
fragility the ship could not be moved. Working together with the curators and specialists at the Trust, the team implemented
two complementary strategies to protect the iron hull from humidity.
Firstly, they enclosed the most
fragile parts of the ship, below the waterline, in a giant
dehumidification chamber, made by roofing the dry dock with
glass. The
glass ”sea” is therefore watertight and airtight.
This protects the hull below the waterline, where the corrosion problem is worst because the iron is most heavily contaminated with salt.
Two special dehumidification machines dry the air both in the dry dock and inside the ship. The hull is protected from moisture below the waterline on the exterior. On the interior it is protected all over, from top to bottom.
The dehumidification machine works by
sucking in air and dries it by forcing the air through a very water-absorbent chemical powder, before blowing the now dry air up over the surface of the ship and through its interiors. There are ducts that collect the blown air to then recycle it through the machine.
This system means the air beneath the glass roof is kept at a constant
20% RH, and corrosion can no longer continue.
The second element of the strategy was to protect the
less vulnerable upper section of the hull. This was treated in a different manner as there is not as much salt contamination in the topsides. These could be protected from corrosion without drying the surrounding air.
Conservators cleaned the upper part of the ship, above the waterline, using ultra-high pressure water jets. Then they covered these ‘topsides’ with several coats of
anti-corrosion paint, shielding them from the weather.
This method could not be used below the waterline as there are salts bonded in the iron. The salts attract moisture, and draw it into the iron, and this drives the corrosion process. This process makes the iron corrode even if it is painted. The only realistic way to tackle the problem below the waterline is to remove the moisture from the air.
The conservators then carefully cleaned the hull,
removing fibreglass patches used in the 1970s to fill holes in the hull plates. There are
hundreds of holes in the iron hull although most of them cannot now be seen. Most of the holes have been filled with
resin, to recreate the shape of the hull without altering the original fabric.
It took a team of seven conservators
three years to complete this work.
Conservation is the process of
caring for objects in order to keep their historic significance. Different methods such as
preservation, restoration, or reconstruction might be used. These are three different and distinct parts of conservation. It is important to use them carefully as they can change people’s reactions to historic objects. Caring for historic objects can involve difficult decisions.
Because of the ss Great Britain’s importance, a decision was taken to
preserve all parts of the ship which existed before her return to Bristol in 1970, when she became a museum. These parts make up the
original working ship, and tell us so much about her history. They help us to understand her design and to interpret the changes and events of her working life.
Visit the site to learn more about the ship:
http://www.ssgreatbritain.org/Home.aspx