Petrographic and physical investigations on geomaterials for conservation of Las Plassas Castle (Marmilla, Sardinia, Italy)

The Castle of Marmilla, located near the village of Las Plassas (central Sardinia), represented a strategic military stronghold of great importance in the war between the kingdom of Arborea and the Aragonese Kingdom of Sardinia in the fourteenth century. Archaeological documents highlight the use, albeit partial, of some environments even in the modern age, during the times of feudal Sardinia (XV-XVI century), when the castle was an integral part of the possessions of the Barony of Las Plassas. It was used as a district prison until the nineteenth century. Built on top of a conical hill at 270 meters above sea level the castle has an irregular hexagonal shape, elongated along the north-west and south-east. Its foundations are excavated in a layer of Miocene sandstones and currently occupy the entire hilltop. What remains of the castle of Marmilla denotes a series of construction phases. A wide circle of walls delimiting the fortification of 550 square meters: inside there are the remains of a cistern and two towers, to the N and S sides, near the entrance. At present, the main tower and the walls, perimeter and internal, are preserved, albeit affected by a profound degradation, and they allow us to observe the construction techniques and choices made in the ongoing renovations, restorations and strengthening interventions. The structure is made of cantons sandstone cut with precision; at the base of the north tower was used ashlar stones. The cistern, located below ground level, is carved into the rock and covered with sandstone cantons; a second tank is located outside the masonry, near the northern side of the fortification. By minero-petrographic and physical-mechanical analysis of geomaterials (i.e., stones, mortars) used in the castle construction, the alteration degree of the materials, in order to identify a correct restoration for conservation purposes, has been defined