Digital mapping, building technique and petrographic characterisation of stones from Opera Punta Rossa fortification (Caprera, north Sardinia)

Stefano Columbu

First

Writing - Original Draft Preparation

;Sofia Pieri;Giorgio Verdiani;Pierluigi Cianchetti

2018-01-01

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Abstract

The Opera Punta Rossa fortification was built at the end of XIX century (1887) on the south of Caprera island (north Sardinia), when returned to thinking of La Maddalena archipelago as a strategic military centre, no longer determining in relation to the only French border, but to the much larger chessboard of the Western Mediterranean. It is of singular interest not only for the functional parameters but, above all, for its imposing appearance with wide dimensions, and other many interesting technical-architectural aspects: laying modes of wall structural elements (i.e., for access portals, intrados of vaults, moldings, architraves, jambs, thresholds, cantons), use and working technics of the granite stone expertly molded for the thick masonry, characterized by the presence of loopholes and continuous modulation of materials with moldings and worked stone ashlars to highlight the wall surface. The geomaterials used are local rocks outcropping in the area of Opera Punta Rossa, belonging to the Paleozoic granitic pluton of Gallura and to the associated filonean bodies (Upper Permian - Carboniferous, between about 320 and 270 million years ago). In the fortification have been mainly used the "granite" rocks (substantially biotitic granites and granodiorites) and subordinately also the filonean rocks (especially for irregular ashlars) with variable colours and dacitic-rhyodacitic composition. Generally, the cornices and the openings were made with the lighter greyish granite facies. The stone elements were laid in place using lime or hydraulic / pozzolanic based mortars. Conglomerates and cement-based concretes were used for throwing pitches in even thicker castings. In some subsequent renovations, bricks were also used. The photographic and digital relief was carried out using laser scanner methodologies, which has allowed to define the conservation status of the Opera Punta Rossa complex, producing a 3D model of the current state of building.