Sandstone of the Macigno Formation of Montereggio (Lunigiana). Applied petrographic studies

The sandstone of the Macigno Formation quarried along the Fontanella stream near Montereggio (Lunigiana - NW Italy) has been used in XVI-XVII century for the construction of important architectural elements such as ashlar portals in the fortified palace of the Malaspina and in Montereggio village. It is a fine-to coarse- grained feldspathic-lithic sandstone with scarce carbonatic cement and sporadic clay minerals consisting of illite. When exposed to the ravages of time, these artifacts undergo a physical degradation mainly consisting of a superficial exfoliation due either to direct causes i.e. thermoclastic and cryoclastic process or to physical indirect causes related to the presence of ashlar workmanships which leads to an increase of surface microfissuring and water absorption. Phenomena of disgregation with intergranular decohesion are not observed; electron microscopy analysis reveals only superficial corrosion of the mineral phases that acquire rounded morphology and become covered by neck-shaped, thread-like and rose-cut patinas of exclusively Si-Al composition. These latter, together with hollow micrometric spheres of analogous composition, represent the product of firewood combustion. This produces also carbonaceous matter, but not in such quantity to induce the formation of black crusts; conversely, the products of combustion of coal and mineral oils, are very scarce or almost totally absent. Although we do not observe significant effects of weathering, the increase of sandstone microporosity can be chiefly attributed to biotic-related agents such as lichens and musks. The sandstones of the ancient quarry located along the Fontanella stream have been submitted to physical-mechanical tests in order to compare them with known litotypes and to choice the most appropriate products for consolidation and protection in view of possible preservative restoration. These tests show that the sandstones of Montereggio are characterized by low porosity and low water absorption coefficient, excellent values of compressive strength, both under dry conditions and after cycles of freezing, and excellent resistence to flexural strength under concentrated load. All these characteristics make these rocks suitable in load-bearing structures such as the studied portals. Our study led us to conclude that the most important physical parameter is the thermal dilatation coefficient, because it qualifies the choice of reinforcing and waterproofing products used to prevent further alteration of the stone surface. We recommend that this coefficient is kept constant, otherwise the detatchment of the superficially consolidated layer from the substratum would be unavoidable.