Identification of secondary salts and their sources in deteriorated stone monuments using micro‐Raman spectroscopy, SEM‐EDS and XRD

Abstract

Salts crystallisation is a cause of deterioration of stone monuments. Very small sizes of salt crystals, often a multiphase composition and usually low concentrations hamper their identification. Therefore, micro‐Raman spectroscopy, scanning electron microscopy and, in some cases, X‐ray diffraction have been used to identify the salts that crystallised within and on the weathered limestone and sandstone architectonic details of the Blessed Salomea sculpture and its arch in the cloister of the Poor Clares in Kraków, Poland. Possible sources of salts, i.e. regarding the plaster around the details, the mortar of joints of the arch and previous conservation procedures, have been discussed. The limestone sculpture reveals the presence of black crusts and the sandstone arch of white efflorescences, both stones crumble and exfoliate. Gypsum and barite were the only salts in the black crust and subflorescences of the limestone. The salts in the pore space of the sandstone include mainly aphthitalite, niter, nitratine and burkeite, while efflorescences consist of thenardite and syngenite. Such differences probably result from various precipitation conditions in narrow pore spaces and on the unconfined sandstone surface. The deterioration products have been mainly assigned to air pollution, stone itself in the case of the limestone, and migration of ions from structural elements of the arch in the case of the sandstone. The impact of the chemicals utilised for stone conservation also seems to be possible. Characterisation of the salts present in deteriorated stones can help in selecting the methods of stone desalination and conservation. Copyright © 2016 John Wiley & Sons, Ltd.