New experimental studies can be focussed to understand the specific conditions (measurements of micro-climatic and thermodynamic parameters) and mechanisms for black crusts formation in situ. The problem of the kinetic of the sulphation process of marble, the assessment of black layers formation in the case of different carbonate stone materials and the study of acid attack in presence of surface protecting layers deserve further investigation.

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The paper describes the analysis of a particular kind of plaster from the walls of the Roman Sanctuary (first half of the first century BCE) in the centre of Brixia (now Brescia, Italy), which is an outstanding example of Roman Republican architecture. The walls were plastered and painted with different patterns, imitating marble panels and curtains. Optical microscopy on thin sections, X-ray diffraction and infrared spectroscopy were performed on several samples of the plaster in order to reveal the execution technique. The palette consisted of glauconite, celadonite, Egyptian blue, and red and yellow ochres. In some cases, an organic compound, possibly a lipidic compound, was present in the external paint layer, as a surface treatment. The plaster contains two superimposed coats: the render coat with lime binder and sandy aggregate; the finish coat with a clay fraction (illite, chlorite, kaolinite), together with calcite from slaked lime and grains of quartz, silicate and carbonate rocks. Although Vitrivius' De architectura reported the use of creta (clay) as daub smeared on reed vaults, the Sanctuary of Brixia represents the first documented use in Roman buildings in a painted plaster laid on a stone masonry wall.

The use of dark metamorphic stones in architecture is common through the centuries. Such materials were traditionally employed mainly for decorative purposes, both indoor and outdoor, and for architectural cladding. Unfortunately, after prolonged outdoor exposure their characteristic surface colour tends to fade. In the framework of the pilot conservation site of the Renaissance façade of the Cathedral of Monza (Italy), a thorough study of the features and state of conservation of an example of this lithotype, Oira stone,was conducted. This stonewas employed during the 19th century restoration of the façade, and the on-site evaluation of the stone blocks after more than a century of exposure showed a distinctive chromatic alteration. The formation of a fragile superficial layer with scaling and detachments was also observed. Samples of the stone were studied using a multi-analytical approach to identify the deterioration mechanisms involved. The results showed that the colour variation is associated with the chemical alteration of the stone, resulting in the selective leaching of magnesium ions from the phyllosilicate structure of the most external material. At the same time, a surface recrystallization of mainly low-ordered SiO2occurred. The role of chemical leaching by atmospheric interaction in stone discoloration was also investigated by laboratory accelerated ageing of serpentinite specimens