A special type of road grit is being used outside London's National Gallery to prevent irreparable damage to its priceless oil paintings.
Experts say normal grit, walked in by visitors, contains high levels of salt which can have an adverse affect on the works of art.
Westminster Council switched to using salt-free grit this week after the problem emerged in recent bad weather.
The gallery will pay for the grit which costs ten times more than normal.
The problem first came to light when equipment detected high levels of calcium chloride from de-icing mixtures trodden into the exhibition room floors by visitors.
Experts say once this goes into the air as a fine dust, it can contribute to a blackening affect, creating irreversible damage to the paintings.
And they believe as much as 15% of its permanent collection could be put at risk by exposure to high salt levels.
The connection with road gritting and high levels of sodium in the atmosphere at the Trafalgar Square venue had been made in the recent cold spell.
Dr Ashok Roy, Director of Science at The National Gallery, said: "We really started to notice a problem at the end of last winter but it's only after facing similar challenges with the snow this year that the issue became particularly acute."
He said they were doing all they could to protect their paintings and ensure the danger to what are irreplaceable works is significantly reduced.
Gallery experts said any level of chloride-bearing material could be particularly damaging to paintings containing the traditional red pigment vermilion.
Commonly used in the nation's old master pictures, it was the only bright red opaque pigment available to artists in oil and egg tempera for a period of 750 years from around 1270.
Other art galleries, including Tate Britain and The National Portrait Gallery, are being contacted by the council to warn them of the damage that can be done by grit and salt.