Abstract Recrystallized grains are potentially useful as indicators of palaeostress in naturally deformed rocks, providing that well-calibrated relationships (palaeopiezometers) exist between the recrystallized grain size and stress. Rocks can exhibit microstructures that are heterogeneous, that is, containing recrystallized as well as deformed grains, and showing subgrains within grains that differ in size and character from the grain core to the mantle. Previous studies on palaeopiezometers only rarely took into account such heterogeneous microstructure. We used electron backscattered diffraction (EBSD) to accurately quantify the heterogeneous microstructures in experimentally deformed Carrara marble (flow stress 15–85 MPa, temperature 700–990 °C and natural strain 0.15–0.90). The sizes of bulges, recrystallized grains and deformed grains have been measured. We found that the overall character of the microstructures varies as a function of deformation conditions. In heterogeneous samples showing core-mantle microstructures, the sizes of the bulges and recrystallized grains are independent of strain and show an inverse dependency on stress. The recrystallized grains have been found to nucleate at grain boundary bulges. Our study illustrates that very different microstructures may develop in relation to the complexity of the recrystallization mechanisms. We therefore suggest that piezometers should be calibrated and applied for a single type of overall microstructure.

Abstract Carrara Marble, from Italy, probably one of the most famous dimension stones in the collective memory and in ordinary people, is here nominated as a suitable ‘Global Heritage Stone Resource’. Quarried since pre-Roman times, the Carrara marble is the testimonial of an area/industry that was able – for a variety of reasons not easily repeatable in future stone history – from the dawn of the stone sector, to trigger a flywheel effect on a global scale. Suitable for any environment and cultural context, this versatile marble has been appreciated in almost any field of application – from building to architecture, from fine arts to urban landscape, from funerary art to modern 3D design – probably reaching its highest point in the eternal works of famous sculpturers, artists and architects such as Michelangelo, Donatello, Jacopo Della Quercia, Canova, Bernini, and many others. It is currently excavated in more than 100 quarries, processed almost everywhere and sold all over the world.

Abstract Strain localization and associated rheological weakening are often attributed to grain size reduction resulting from dynamic recrystallization. Most studies investigating rheological changes due to dynamic recrystallization regard recrystallized grain size as a single value that is uniquely related to stress during steady-state deformation. However, rock materials invariably exhibit a grain size distribution with distribution parameters that may be altered by dynamic recrystallization during transient deformation, affecting the rheological behaviour. This study aims to investigate the effect of deformation conditions on the evolution of grain size distribution and rheological behaviour during dynamic recrystallization in the approach to steady state. To study this, we have deformed Carrara marble to natural strains of 0.15–0.90 in axial compression at temperatures of 700–990°C, stresses of 15–65 MPa, strain rates of 3.0 × 10 −6 –4.9 × 10 −4 s −1 and a confining pressure of 150 or 300 MPa, and analysed the grain size distribution of each sample. The results show that during dynamic recrystallization, grain size distributions evolve by a competition between grain growth and grain size reduction. The relative importance of grain-size-sensitive creep increases as the average grain size is reduced with strain. Minor weakening is observed, which is probably insufficient to cause strain localization in nature.

Abstract The outdoor statuary of the city of Venice is being lost at the rate of approximately 5 per year. As in most areas of the industrialized world, this phenomenon stems from accelerated weathering of stone caused by air pollution. In the conservation of these artistic works, it is usually necessary to remove the surface encrustation that forms during the deterioration process; this procedure is required for both aesthetic appearance and physical-chemical stability. Unfortunately, much of the Renaissance statuary of Venice is exceedingly friable and highly soluble, so encrustation divestment has been largely a tedious manual effort. Experiments are described here in which pulsed laser radiation was used to selectively evaporate black encrustations from highly altered Carrara marble statuary. Scanning electron microscopy was employed to determine thresholds for microcracking of the calcium carbonate grains as a function of laser-beam energy. Long-pulse xenon flash-lamp radiation and the air-abrasive technique are found to induce considerably more surface damage than optimum laser cleaning. However, limitations of laser technology suggest that at present it will be impractical to clean marble at a rate greater than a few square metres per day. On the other hand, xenon flash-lamp banks may be readily scaled to rates of tens or hundreds of square metres per day.