How to clean a cathedral

The science behind a thorough church service.

St Mary’s Cathedral in Sydney is under attack. The Gothic Revival church, as long as a football field and with spires more than 20 storeys high, is built from tens of thousands of sandstone blocks that are being destroyed by acid rain in Sydney’s windy, damp, salty climate.

The Sydney Catholic diocese knew its spiritual home needed help so, in 1996, a preservation program was begun. “Facade conservation involves sandstone replacement and repair, and cleaning is the first step,” says Ralph Morris, conservation project manager for St Mary’s.

The soft texture of sandstone crumbles under traditional cleaning methods such as handwashing and water blasting, so a dry stonework technique known as facade gommage, pioneered in France and used on the Louvre Museum, Paris, has been applied at St Mary’s. Never before used in Australia, facade gommage involves hand blowing the exterior of the cathedral with dry, low-pressure air mixed with organic and mineral powders. “It’s a very gentle process that removes only the dirt and not the fabric of the stone,” Ralph says. The Australian cleaners are led by a highly skilled technician from France, who operates the hand-held nozzle, moving it continuously backwards and forwards at eye level, 25 cm from the stone surface. The work is carried out within airtight plastic so all the airborne dust can be recovered by vacuum units.

Cleaning the cathedral will take about 35 years – half of a projected 70-year, multi-million-dollar stonework conservation program.

Source: Australian Geographic Jul - Sep 2008