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Publisher Summary Pollution linked to the combustion of wood is characteristic of the atmosphere of the cities in the past. It led to the development of thin grey crusts on the surface of the stones of monuments. The grey crusts were discovered on the Heads of Kings of Juda Statues, present on the western facade of Notre-Dame in Paris from the 13 th Century to 1792. These thin grey crusts are the material witnesses of the effects of the ancient air pollution, prevalent during the French revolution and the period immediately before it. This chapter focuses on the research conducted on the character and evolution of urban air pollution in the past; and the comparison of results with those derived from studies on the modern industrial black crusts. Several analytical techniques were used to highlight the main physicochemical and mineralogical characteristics of the grey crusts: colorimetric technique, X-ray diffraction, and photon microscopy. Many of the characteristics of the ancient grey crusts of the Heads of Kings of Juda directly contrast with those of the modern day black crusts: color, texture, thickness, and chemicomineralogical nature of the crusts and of the prismatic or spherical particles embedded in the matrix. Diffractometric analysis of the modem black crusts highlights the presence of the same minerals as those found in the ancient crusts of the Heads, but in different proportions greater amounts of gypsum than calcite, followed by quartz and weddellite. A further major difference between the ancient grey crusts and the modern black crusts lies in the kind of particles contained in the gypsum-calcite cement. While the grey crusts only contain the residues of incomplete wood combustion, the modern crusts embed microspheres, which turn out to be fly-ash released into the atmosphere by the combustion of coal or heavy fuel.

Publisher Summary Carbonaceous fly-ash, particularly those emitted by heavy fuel oil combustion, has a strong physicochemical reactivity. They contain many sulfured chemical species, in particular sulfates along with metals such as vanadium, iron, and nickel. This chapter describes the role of carbonaceous fly-ash in the nucleation of gypsum crystals and in the development of incipient sulfated black crust on carbonate surface. This study is conducted by comparing simulation chamber data with those obtained in the field. Embryonic black crust forming over 12 months through interaction between carbonaceous fly-ash and samples of Jaumont limestone in the controlled atmosphere of a simulation chamber are compared with those forming in the field on the same carbonate stone, over the same time period in a real polluted atmosphere. The Jaumont limestone samples were placed inside the LASC, either naked or sprinkled with fly-ash. Similarly, Jaumont field samples were collected and examined with ASEM. The chapter discusses the results from both samples and compares them to draw conclusions. On the basis of simulation chamber and field data obtained over 12 months, the study of the morphology, mineralogy, and chemical composition of the deposits and associated neocrystallizations indicates that they represent the first stage of black crust development. The fly-ash appears to play a crucial role in the formation of black crusts because they facilitate the precipitation of gypsum, which constitutes the predominant mineral in the black crusts.

Summary The limestones (“tuffeau”) of the ”Cloitre du Grand Moutiers” and the “Salle Capitulaire” in the Fontevraud Abbey (Loire Valley, France) contain two types of mineral microspherules : - in the non-altered substrate stone : microspherules (10 μm in diameter), only siliceous in composition and probably biogenic in origin. - in the altered black crust : alumino-silicate or metallic microspherules (1-5 μm in diameter) associated with gypsum crystals. A study by means of Analytical Scanning Electron Microscopy has been performed on the microspherules both in non-altered and altered stone. The chemical composition of the microspherules associated with gypsum is the same as that of industrial fly-ash emitted by coal-burning power plants. Their presence in the altered crusts of carbonatic rocks is in agreement with a previous study. Their role in the formation of the sulphate crust is probably of major importance. The origin of fly-ash has been studied by back trajectories of air masses during the sampling period.