Keeping Brunel’s SS Great Britain alive
The SS Great Britain returned to the place she was built on 19 July 1970. In the 127 years between her launch and her recovery from the Falklands she had a rich and varied working life. At first as a luxury liner, then running aground in 1846, next carrying 15,000 emigrants to Australia, then being converted to sail as a cargo ship and finally acting a floating warehouse. In the thirty years between her scuttling and recovery the elements had taken their toll on her iron hull. At the time of her rescue she was in such a poor state, and her hull so corroded, it was feared she would not last another six months.
Race against time
The dry dock in Bristol was a fitting location for the return of Brunel’s SS Great Britain. But it held a potentially devastating threat, namely water. Despite its name the dry dock is far from dry. High levels of moisture in the air, and the rain, meant that once home the ship continued to corrode. Time was running out. Researchers from Cardiff University estimated that corrosion could destroy the ship within a few years. But they also found that the ship could survive if kept in a very dry environment, protected from humidity.
The most fragile parts of the ship, below the waterline, were sealed in a giant dehumidification chamber. This is created by a glass plate, fitted around the ship, keeping it water and air tight. Two special dehumidification machines dry the air in the dock and in the ship. They keep the air at a relative humidity of 20% meaning that corrosion cannot take place. The air in the dry dock is now as arid as the Arizona Desert!
With the upper sections of the hull less vulnerable, a different conservation approach was needed. These were cleaned using high pressure water jets and then covered with anti-corrosion paint to shield it from the weather.
Conservators cleaned the hull, removing the fibreglass patches used in the 1970s. Most of the hundreds of holes were then filled with resin to recreate the hull’s shape.
A team of seven conservators took three years to complete the work.
Corrosion: the SS Great Britain’s greatest enemy
Iron corrodes when it comes into contact with water and air. It forms new substances including iron oxide – or rust. Salt makes corrosion much worse as it attracts water even out of air. When salts build up and bond with iron or steel they make the metal rust faster. So, after 127 years in the water, the salts that built up in the SS Great Britain’s iron hull were extremely difficult to remove. And were causing it to corrode at an alarming rate.
By keeping the air as dry as possible corrosion can be prevented. The ship’s bespoke dehumidification machine sucks in air and dries it by forcing it through a water- absorbent chemical powder. It then blows dehumidified air onto the ship’s hull. Ducts below the glass waterline plate collect the blown air and recycle it. This keeps the air at a corrosion-busting 20% relative humidity.
The glass plate surrounding the ship acts as the roof of the giant dehumidification chamber. A layer of water gives the impression that the ship is afloat and acts as an insulating blanket – saving up to £20,000 per annum in the Trust’s energy bills.