Petrographical investigations were performed on a naturally badly decayed Carrara marble statue covered with a black crust. On thin sections (parallel nicols) the sulphation zone was found to be constituted of two layers: the outer one (0.1-3 mm thick) contained allogenic particles that gave it a dark aspect; the inner one, 150-200 μm thick, looked clear because no allogenic particle could be seen within it. The texture of this clear gypsum layer suggested that it was the consequence of a pseudomorphism process, i.e. transformation of calcite into gypsum without any change in shape or volume of the crystals, even cleavages being conserved. This specificity suggested that the only part of the sulphation zone that could be called "black crust" was the outer dark gypsum layer. Different morphologies observed within the clear gypsum layer were assumed to represent different steps of its formation. These steps would be the following: (1) marble thermal microfracturing; (2) highly localized inward formation of gypsum rims lining the boundaries and cleavages of the calcite crystals; (3) dissolution of calcite grains by sulphuric acid and subsequent formation of hollow gypsum boxwork; (4) precipitation of gypsum within all the voids left, i.e. gypsum boxwork and thermal cracks. The occurrence of pseudomorphism just beneath the stone surface has important implications for stone conservation. Owing to the volume and shape conservation related to pseudommorphism, the surface details of the original sculpture were well preserved under the black crust, despite substitution of calcite by gypsum. Thus, it is suggested that if a clear gypsum layer occurs underneath black crusts on marble statues, it should be conserved when the stone is cleaned. © 1994.
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