Conservators in Croatia have completed a ten-year project to remove more than 1,700 years of grime from the courtyard of the palace of the Roman Emperor Diocletian (AD244-311), in the coastal city of Split. Lasers were used as the primary method to clean the peristyle of the fourth-century imperial residence—an innovative technique that is normally reserved for cleaning individual sculptures or details of larger architectural elements, as opposed to whole structures. According to the architect Goran Niksic, who works for the city, this is the first time lasers have been used on this scale in Croatia to clean stone.

The peristyle was covered not only in soot, but also in cement dust from a nearby plant that was active in the late 19th and the first half of the 20th centuries. In some parts, the build-up of grime was up to a centimetre thick. The late Antique palace boasts a mix of Western and Oriental architectural styles and inspired many later architects, including the Neoclassicist Robert Adams. “You couldn’t read the architecture or the decoration because of the large patches of soot,” Niksic says, comparing the structure’s appearance to looking at a photographic negative. “It was thick and very difficult to remove, so we opted to use lasers to clean the stone. Normally, lasers are just used for small details; I don’t know anywhere else in Europe where this has been done,” he says. Niksic stresses that “the enemy was the dirt deposit” and that one of the reasons they chose laser treatment was that it “is the only technique that will not touch the [stones’] original patina”....