Many aggressive gases can destroy or damage the surfaces of bronzes. Particularly destructive is sulfur dioxide derived from the combustion of fossil fuels. It destroys and disfigures bronze sculpture placed in the open. Its effect on bronzes indoors is not well known, whereas, in recent years, damage to prehistoric bronzes caused by another gaseous sulfur compound has been observed. The phenomenon appears as small black spots, occurring either singly or in groups, and either lying flat on the surface or rising from it like tiny tufts. Analysis has revealed that they consist of copper sulfide they are thus due to a reaction between copper and hydrogen sulfide or some other form of reduced sulfur. Hydrogen sulfide is formed mainly by microbiological breakdown of organic material, but the corrosion under discussion, which has been recorded everywhere in European and American museums, may possibly also be attributed to combustion of fossil fuel.