FOR TWO CENTURIES, London’s buildings were black. Blanketed in sulphurous soot from coal fires – the famous London “pea souper” fogs – a thin layer of carbon coated every surface in the city. London was so dirty that there was no memory that it might ever have been any other way. During the restoration of 10 Downing Street in 1954, it was discovered that the familiar dark facade was not actually black at all, but originally yellow brick. The shock was considered too much for the country to take and the newly clean building was painted black to maintain its previous, familiar appearance.
But then, in the late 1980s and early 1990s, there was a great cleanup. For more than a decade, scaffolding surrounded landmarks like St Paul’s Cathedral, as power washers hosed the grime down into the sewers and out of sight. These days the city is russet and pale grey, silver-mirrored and blue green – the colours of brick, limestone and glass. The pollution is now polychrome: the primary residue adhering to buildings is not the black of carbon soot, but a warmer browny-yellow colour from the organic hydrocarbons in petrol and diesel fuel. As sulphate emissions from traffic fall, buildings may yet turn green as mosses and lichens grow back.
Yet you cannot just blast dust and grime off all of London’s landmarks. Westminster Hall is the oldest building in parliament, built about 900 years ago by William Rufus, son of the Norman conqueror. In 2007, architectural conservators found that its walls were being corroded by air pollution and penetrated by moisture. They reckoned it had not been cleaned in 200 years. It was time.
But how to do this while maintaining respect for the building’s fabric? Limestone is porous, soluble stuff, which could dissolve under strain from high-pressure washing. Fortunately, more subtle methods are available. Delicate carving can be cleaned using poulticing, akin to a clay face mask
