Britain's National Gallery is taking swift action against a corrosive force that threatens to damage masterworks by some of the world's greatest painters—road salt. With London blanketed with unusually heavy snowfall, authorities have spread tons of salt to clear the roads. But that salt, tracked in on the shoes of visitors to the National Gallery, is causing a chemical reaction in the air which can cause "blackening" on certain paintings, the Telegraph reports.
The chemical issue is that the rock salt, even in tiny amounts in the air, reacts with the pigment vermilion, a common red pigment used by old masters. Works by Cézanne, Michelangelo, and Monet stand to be ruined if the salt assault is not stopped. To that end, Westminster Council is buying sodium formate—at 10 times the cost of normal rock salt—to sprinkle around the city's art galleries.