Pre-consolidation of decayed Angera columns with ammonium phosphate : a pilot study in a Milanese courtyard of XVI Century

Angera stone is a lithotype widely employed in Lombardy (north Italy) for ornamental elements/frameworks of historical buildings. Owing to decay processes it is affected by detachments, exfoliations, pulverizations with consequent loss of material. The traditional application of synthetic organic products on the Angera surfaces had a significant role in the decay processes as well (Colombo et al., 2006). Over the last years, the boundaries of Conservation Science have been pushed toward the use of inorganic-mineral treatments that are stable, durable, highly transpirable and more compatible with the stone substrates. Ammonium phosphate [DAP, (NH4)HPO4], one of the most recent inorganic-mineral treatment, is based on the reaction between a water soluble precursor with carbonatic substrates; the newly formed crystalline phases are able to restore the lost cohesion of the microstructure. Recent studies investigated the interactions of DAP solutions with calcite of marbles and limestones (Matteini et al., 2011; Possenti et al., 2016). To the best of our knowledge, no data are available on the application of DAP to: i) the conservation of dolostones; ii) the pre-consolidation of decayed stones. Therefore, in this study we explored the potentialities of DAP treatments as a pre-consolidating agent for white Angera stone. DAP solutions were vaporized on vertical specimens to avoid mechanical contact with decayed surfaces. The research evaluated the effects induced by the molarity and the application method (repeated applications, wetted or dry substrate, comparison with paper poultice) on the surface and the influence of pre-consolidation on the subsequent consolidation with DAP. The formation of specific crystalline phases and their diffusion inside the first few microns of the stone matrix were investigated following a multi-analytical approach (SEM-EDS, Raman and IR spectroscopies, XRD). The research outcomes supplied outstanding indications for restoration, supporting the application of DAP pre-consolidating treatments to Angera columns strongly decayed in a Milanese courtyard of XVI Century. Colombo, C., Conti, C., Realini, M., Sansonetti, A. (2006): Diagnostic studies aimed to conservation works in S. Fedele church (Milan). Proceedings of HWC Congress (Heritage, Weathering and Conservation), Taylor & Francis Group, 917-922. Matteini, M., Rescic, S., Fratini, F., Botticelli, G. (2011): Ammonium phosphates as consolidating agents for carbonatic stone materials used in architecture and cultural heritage: preliminary research. Int. J. Archit. Herit. Conserv. Anal. Restor., 5, 717-736. Possenti, E., Colombo, C., Bersani, D., Bertasa, M., Botteon, A., Conti, C., Lottici, P.P., Realini, M. (2016): New insight on the interaction of diammonium hydrogenphosphate conservation treatment with carbonatic substrates: a multi-analytical approach. Microchem. J., 127, 79-86.