In the corner of a vast store was a navigation instrument, whose history deserves to be revisited. It has not been used in any major battles, nor did it belong to any great commander. It served ordinary seamen on-board a trawler and it tells the story of the bravery of ordinary men who volunteered to protect British waters during the Second World War.

Admiralty sextant (NAV1229)

As the war in Europe progressed and the enemy started creeping closer and closer to the British shores, the Admiralty appointed a large number of fishermen to form the crews of additional minesweepers. The trawler HMS Royalo was one of those civil vessels converted to use by the Royal Navy in 1940. Its service history was short, for it struck a mine and sunk in September 1940 just off the shore of Cornwall, near Penzance.

The position of the wreck was known to the local community and in 1962 a group of divers recovered a wooden box. As it turned out, it contained a sextant, used for navigation, made in 1939 by famous Hughes & Son Ltd. of London. The Royal Museum Greenwich acquired the sextant in the 1970s.

During a recent inspection of scientific instruments it was decided the object should come to conservation for further investigation and possible treatment. As an intern at the Museum, I saw the instrument as an excellent learning opportunity, because it presents many intriguing conservation issues.

The sextant has the most fascinating combination of colourful corrosion products I have ever encountered. The instrument is made of different copper alloys, originally covered with a layer of paint. Sea water contains large amount of chloride and sulphide ions, which react with the metal. Different shades of green, blue, grey and yellow corrosion products covered large areas of metal surfaces. The paint remained unaffected by the elements, causing an ethical dilemma when considering any conservation treatment. Original paint is part of the object and should not be damaged by conservation treatment. The dilemma is whether to sacrifice the original finish for the longevity of the instrument or to do as much as possible without damaging the surface. A number of methods were tested to implement the second option. The best results were achieved using chelating agents, which chemically bond and remove the particles of corrosion products. The surface was then covered with a coat of wax to prevent oxygen and moisture reaching the metal and causing further corrosion. The sextant needs to be monitored regularly to make sure the treatment was sufficient.

Treatment of the sextant

Had it not been for the wooden box however, the preservation of the instrument would be considerably worse. Worm holes cover large area of the lid, causing the wood to become fragile and soft. Any loose dirt was removed mechanically from the box. The surface was washed with special soap and the most fragile fragments were carefully consolidated chemically using a syringe. Afterwards, a protective layer of wax was applied to all surfaces.

The main purpose of conservation treatment is to make sure the instrument survives intact for as long as possible. The damage sustained during its burial at sea is now part of its history.