LA CONSERVAZIONE DEL “DEGRADO” DI SUPERFICI LAPIDEE ALL’ESTERNO: UN CONTRIBUTO SPERIMENTALE SULLA FORMAZIONE DELLE PELLICOLE AD OSSALATO DI CALCIO DA MATERIALE ORGANICO NATURALE.

Giovanna Alessandrini1, Dario Paolo Benedetti2, Davide Del Curto3, Andrea Griletto4

1 Politecnico di Milano, Scuola di specializzazione in Beni Architettonici e del Paesaggio, via Golgi, 39 - 20133 Milano, giovanna.alessandrini@hotmail.com

2Università degli Studi di Brescia, Centro di Ricerca per l’Archeometria - via Branze 38 - 25123 Brescia, dario.benedetti@unibs.it

3 Politecnico di Milano, Dipartimento di Architettura e Pianificazione, via Golgi, 39 - 20133 Milano, davide.delcurto@polimi.it

4 Assorestauro, via Londonio 15, 20154 Milano, andrea.griletto@assorestauro.org

*referring author: E-MAIL: andrea.griletto@assorestauro.org

ABSTRACT:

Calcium Oxalate films are often detected on the external surfaces of many historic buildings. These films have been studied as a memory of ancient technologies, for their role as a protective layer of the stone surface, and to set a proper way to preserve themselves. From the first surveys upon the “yellow patinas” made by Dr. Liebig on the marble surfaces of the Parthenon, researchers have directed their interest towards the evaluation of the chemical composition of these films and their origins. The origin of the oxalate films have been attributed both to a natural or environmental cause both to the mineralization process of the organic compound originally contained in the surface treatments intentionally applied on the outdoor (and generally stone made) surfaces of historic buildings, with the meaning of protect, decorate and restore them.

This paper aims to contribute to the discussion by presenting some results of an experimentation oriented to evaluate how an organic treatment applied on the external surfaces of a historic building has been transformed during three years. Two different materials of the façade of the Broletto palace in Brescia (Botticino's limestone and brick masonry) were treated with three organic based mixtures based on traditional receipts (casein, albumen and linseed oil). The stone surfaces have been characterized before and after the treatment by means of an analytical protocol (colorimetry, XRD, FTIR, optic microscopy) and monitored during the experimentation. The basic environmental parameters (T, UR) have been monitored too. The results are discussed emphasizing the methodological asset.

KEY-WORDS: Calcium Oxalate Film, Conservation, Diagnostics, Maintenance