Sulfate and carbon compounds in black crusts from the Cathedral of Milan and Tower of London

Atmospheric deposition, chemical transformation, dissolution and loss of sulfate and carbon compounds within black crusts or damage layers are examined at the cathedral of Milan and the Tower of London. Major components of dark layers suggest that contemporary fluxes to crusts deliver more organic materials than those typical for deposition of coal smoke that characterized atmospheres a hundred years ago. This is reflected in the ratios between organic and elemental carbon and in the appearance of the layers, which shift from dark tones to lighter and more brownish colors at present. Such changes should influence the way monuments are managed. Minor constituents of the crusts provide information about local conditions, processes and transformations. Low solubility compounds such as oxalate have a long residence time in the crust and are transformed only slowly, so are likely to be conserved. Conversely more soluble ions, such as formate and acetate are removed from the layers relatively quickly by rainfall although can be maintained in the crusts at reasonable concentrations by biological activity. This work also reminds us also the differences in the way we manage air pollution to protect human health and preserve the built heritage. In particular the standards for particulate material refer to human health and are based on particle diameter whereas composition is a key issue for cultural heritage. Elemental carbon (EC), for instance, is responsible for blackening of historical buildings, but the organic compounds may be relevant for the way it adheres to surfaces and changes color over time