The discoloration and decay of stonework in smoky atmospheres is a worldwide problem and enormous sums are spent on cleaning and restoration. It is surprising, therefore, that little research has been done on the nature of the process of decay, it being widely assumed that this is simply the result of the progressive coatings of sulfur dioxide on wet stone. However, investigation in northern Italy and particularly in Venice (where the rate of decay is unusually high) indicates that carbon particles derived from oil fuels have a powerful catalytic effect on the decay of marble.