Decay of Pietra Serena and Pietraforte, Florentine Building Stones: Petrographic Observations

Abstract

Pietra Serena and Pietraforte, the two stones most frequently used in Florentine architecture, have been considered in this study. Through mineralogical-petrographical study and by determining the physical properties of these materials it has been possible to show clearly that the principal factors of their decay are seepage of water and changes of temperature. The simultaneous appearance of these two factors seems to induce the decay with different characteristics in the two stones. Gypsum, thought by some authors to be one of the main factors in the decay of stones, has been detected on materials already deteriorated; it is a product of decay apparently brought on by natural change, and may take part in a process which accelerates the decay.

A Multidisciplinary Methodology for Technological Knowledge, Characterization and Diagnostics: Sandstone Facades in Florentine Architectural Heritage

“…The main material of the facade is Pietraforte, a sandstone used as a building material, with a typical color ranging from grey to yellow-ochreous, employed in many historic Florentine buildings. From a geological point of view, Pietraforte is a sedimentary rock of the Upper Cretaceus, belonging to the turbiditic formation present in the allochthons External Ligurid units (Calvana Supergroup or Calvana tectonic unit), which thrust on the Sub-Ligurian (Canetolo unit) and Tuscan Nappes (Tuscan Nappe and Cervarola-Falterona unit) [24][25][26][27]. The Pietraforte macroscopic characteristics are convolute laminations and calcite veins [28,29] that represent zones of weakness, where preferential detachments of blocks or scales can occur.…”

“…The dissolution of the calcium carbonate occurs in calcite veins causing a decrease of the "weld" with a progressive reopening of the pre-existing fractures and consequent detachment and loss of blocks (even of considerable size). The stone matrix is affected by the dissolution of the stone's carbonate cement and the expansion of the clay minerals [25,28], giving rise to intense intergranular decohesion, superficial disintegration, and exfoliation. In this case, the detachments involve the shallowest rock layers of a significantly lower thickness.…”

Stone sustainable protection and preservation using a zein-based hydrophobic coating

“…In this study, Serena stone, a fine-medium grain sandstone characterized by a grey-bluish color, was chosen to investigate the protective performances of a zein-based protective coating. It is mainly made of quartz feldspars and phyllosilicate grains and white micaceus lamellae (muscovite) with minor amounts of siliciclastic rock fragments, calcite and dolomite [3,42] Porosity of Serena stone was measured by helium picnometry and mercury intrusion porosimetry (MIP), performed with Pascal 140 Evo and Pascal 240 Evo mercury porosimeters (Thermo Fisher Scientific) [43,44]. Porosity was found 3.11%, and a bimodal pore size distribution was measured, with median pore size in the ranges 0.03-0.02 μm and 6.30-3.98 μm.…”

Monitoring and Evaluation of Sandstone Decay Adopting Non-Destructive Techniques: On-Site Application on Building Stones

“…As mentioned above, in general, the decay of stones depends on intrinsic parameters, like composition and textural/structural characteristics, and on extrinsic agents, like climate and anthropic work. Changes in temperature are a very important factor to be considered, since they can cause microfractures that can increase sandstone's porosity and enable penetration of rainwater, water vapor etc., which further contributes to the degradation, thus triggering a self-feeding loop [19,20].…”