By Jo Marchant

State-of-the-art X-ray scans have revealed the internal mechanisms of a corroded, barnacle-covered pocket watch recovered from a seventeenth-century wreck. The watch looks little more than a lump of rock from the outside, but the scans show that the mechanism inside is beautifully preserved, from delicate cogwheels and Egyptian-style pillars to the maker's inscription.

Researchers from the National Museums Scotland in Edinburgh used the images to construct a three-dimensional virtual reconstruction of the watch's lost workings (see video flythrough). They hope to use the technique to probe the internal structure of other archaeological artefacts, especially those salvaged from underwater sites. The watch and three-dimensional reconstruction are now on display in the Treasured exhibition of the National Museum of Scotland until 2011.

The watch was found on a wreck believed to be the Swan -- a small warship that sank off the west coast of Scotland during the English Civil War. She formed part of Oliver Cromwell's forces that attacked the Royalist stronghold of Duart Castle in Mull, UK, but succumbed to a violent gale on 13 September 1653.

A naval diver discovered the wreck in the 1970s, and it was excavated in the 1990s. Salvaged artefacts including the pocket watch, a hoard of silver coins, cast iron guns and an ornate sword hilt were taken to the National Museum of Scotland.

Rusty remnants

The watch itself was barely recognisable. Conventional X-ray images taken at the museum showed cogwheels inside, but didn't provide any useful information about the workings of the watch, or how well it was preserved.

Museum researchers Lore Troalen, Darren Cox and Theo Skinner saw a paper in Nature that described how X-ray computed tomography (CT) had been used to image an ancient Greek device called the Antikythera Mechanism1. This artefact had also been salvaged from a shipwreck.

CT involves taking a series of slices through an object at different angles, and combining them using a computer to produce a three-dimensional reconstruction of the object's internal structure. Andrew Ramsey and his colleagues at the company X-Tek Systems in Tring, Hertfordshire, UK, had developed an improved CT technique using small yet high-voltage X-ray sources, which enabled them to obtain very high-resolution images, even when penetrating dense metal.

Troalen and Cox took the Swan watch to X-Tek (now owned by Nikon), and the resulting images taken by Ramsey and his colleagues have a resolution of 63 micrometres2 and show that much of the mechanism inside the watch is perfectly preserved ( see slideshow ).

Any parts made of steel, including the watch's single hand as well as the studs and pins that originally held the mechanism together, have corroded away. But most of the components are brass, and in excellent condition. "The results surpassed all of our expectations," says Troalen. "We never thought that so much of the mechanism would have survived."

Detailed dream

The resulting reconstruction of the watch shows plenty of decorative touches and exquisite attention to detail. The top and bottom plates are held together by square-section Egyptian tapered pillars, first used around 1640. Other parts of the mechanism are engraved with a floral design. The clockface itself is marked in Roman numerals with what appears to be a fleur de lys on each half-hour, and an English rose in the centre.

The researchers even found the maker's name. An engraving on the watch's top plate reads "Niccholas Higginson of Westminster". Clockmakers' directories attest that Higginson was making watches in Chancery Lane and in Westminster, London, in the years before the Swan went down.

Colin Martin, a retired maritime archaeologist who was at the University of St Andrews in Fife, UK, and led the original excavation of the Swan, describes the work as "brilliant". "We know more about the thing than we would have done if we had taken a hammer to it," he says. "And the object is still intact."

This study and the imaging of the Antikythera Mechanism show the level of detail that can be preserved inside objects even after hundreds or thousands of years under water, and demonstrates the potential of CT for probing artefacts that are encased within layers of corrosion products. For imaging small-scale structures such as inscriptions and mechanical components, "it's a dream," says Cox. The researchers now hope to scan a door lock from the Swan.