Road salt

In detail

De-icing salt is mostly sodium chloride, sometimes with calcium or magnesium chloride added for lower-temperature performance. Carried indoors on shoes it dries as a pale, crusty tidemark on carpet, matting and leather, and the mark is salt crystals that came out of solution as the water evaporated. Two things follow. The first is that it dissolves readily, so the treatment is genuinely simple — the carpet institute's snow-melting-chemicals entry is one of the shortest in the whole database, a detergent solution and then a professional, with no solvent and no acid. Blotting with plain warm water repeatedly does most of the work. The second is that salt is not merely a mark. Chloride ions attack the passive oxide layer that protects stainless steel and aluminium and accelerate the corrosion of ordinary steel, which is why vehicle sills, garden tools and outdoor metalwork suffer disproportionately in winter, and why rinsing matters far more than appearance suggests. On leather the salt also draws moisture and oils out of the hide, leaving it stiff and pale, so a conditioning step belongs after cleaning rather than instead of it.

Why

• Road salt is explained by Chloride attack
• Road salt is explained by Evaporative deposition