The artificial stone in Florence between the XIX and XX centuries: problems of durability

Up to the Renaissance the idea to imitate the stone was absent: the masonries were rather covered by plasters which smooth aspect emphasized the architectonic elements realized in stone. During the Baroque time complex relief decorations (stuccoworks) were realized with poor materials like gypsum and lime which in the exterior wall could not guarantee a good durability. Only from the second half of the XIX century the diffusion of the new hydraulic binders made it possible to manufacture elements of particular hardness and durability with the aim to imitate the stone. The use of these artificial stone material spread out all over Europe with a maximum diffusion reached at the beginnings of the XX century. These materials satisfied the request of architectonic ornamental elements making possible a notable speed of realization of the decorative objects, simplifying their installation with a significant economic benefit. The use with structural function is not documented. The Artificial Stones were realized with a binder (Roman Cement, Portland or White Cement) and an aggregate (sand, gravel or grits), often embellished with addition of crushed stone and pigment. The mixtures were generally lean with a low percentage of binder in comparison to the other components. These materials were produced in workshop with the use of casts or directly in the building with the use of "modani" (outlines cut out in galvanized plate, reproducing the negative of the profile) in order to make particular workmanships with different shapes and dimensions. Once manufactured and after a suitable aging, the Artificial stones were finished up with tools typical of the mason or chisel, washed and brushed to remove the superficial cement in order to emphasize the presence of the aggregate. The production was realized by artists-artisans working into workshop-laboratories, where the "teacher" was owner of the knowledge of the art production, setting "recipes" of mixtures that changed according to the request of the client. The Artificial Stones are generally more durable than the natural stone materials but nevertheless some of them decay more than others. With respect to sandstones, the higher durability depends on the absence of clay minerals, which are responsible of the well known exfoliation and discontinuities like laminations and veins. Taking into account the kind of binder, usually the Artificial Stones produced with the Roman Cement are more durable than those produced with Portland cement because the former do not contain soluble salts. Conservation problems can be observed also when the Artificial stones elements (above all decorative) are realized with the help of an internal iron framework and/or installed with iron anchorages: these elements can oxidize increasing in volume and giving rise to fissuring phenomena. In Florence the use of artificial stones both as original material and as substitution material in the restoration of ancient buildings, made it possible to realize artifacts very close to the original stone materials (Pietra Serena, Pietra Bigia and Pietraforte). In the paper the composition of the artificial stones from different buildings is investigated through petrographical, mineralogical and chemical analyses in order to determine particularly the binder/aggregate ratio, the nature of the binder and the present of soluble salts. These data, together with the kind of realization (in mould or laid directly in the façade) will be discussed in order to explain the problems of conservation of the artifacts.