The Arch of Septimius Severus, which dates to AD 203, was built to celebrate imperial victories over the Parthians. In the Roman Forum it represented the ultimate milestone in imperial triumph before the ascent to the Capitoline hill. The four large sculpted scenes and figures on the column bases narrate with descriptive precision the principal military feats of Septimius Severus, who was intent on conquering the capitals of the Parthian empire.
The Proconnesian marble monument showed signs of extremely serious deterioration linked to urban pollution, which leads to aggressive acid rain and so-called ‘black crusts.’
Over the centuries, the original rainwater disposal system had been damaged, and this factor allowed acid rain to attack the sculpted marble surfaces with devastating results.
An image used at the time of treatment to describe the marble’s dire condition was that of a packet of sugar, where the packet was the black crust of marble-pollution interaction and the sugar the original marble.
Conservators of the Center for Archaeological Conservation (CCA) struggled for eight years with the difficult task of removing the black crust (which was still chemically active) while also preserving the pulverized marble underneath.
To accomplish this task, new equipment and treatment techniques were devised, based on revisiting the original construction and maintenance techniques as deduced from study of the monument’s surfaces.
 

Arch of Septimius Severus, Rome

Arch of Septimius Severus, Rome

Arch of Septimius Severus. Cleaning