Characterisation of salt efflorescences in cultural heritage conservation by thermal analysis

Nitrates, oxalates and sulphates are known to be among the most harmful soluble salts in the held of wall paintings and conservation of porous materials. Under the influence of the environmental conditions these salts are subjected to cycles of crystallisation-dissolution in the porous matrix of the building material, leading to mechanical stresses and chemical alterations that can result in the flaking and powdering of both paint layer (when present) and support. Often the identification of these salts is not very easy, since the experimental techniques usually employed (FT-IR spectrometry, X-ray diffractometry, X-ray fluorescence spectrometry, etc.) are not suitable because of various problems like reciprocal interferences among the different nitrates, hygroscopicity, and low melting points. The aim of the present paper was to set up a method, based on thermal analysis, allowing the quantitative determination of sodium nitrate, calcium nitrate tetrahydrate, magnesium nitrate hexahydrate, calcium sulphate dihydrate (gypsum), and calcium oxalate monohydrate in samples of porous building materials. The idea was to analyse the thermal parameters associated to either first-order (Ca(NO3)(2). 4H(2)O, Mg(NO3)(2). 6H(2)O), or second-order (NaNO3) phase transitions, and to the dehydration (CaSO4. 2H(2)O and CaC2O4. H2O) as a function of the salt content in a plaster matrix. The study showed that by submitting plaster samples containing different amounts of these salts to thermal analysis, it was possible to determine the type and content of the salts, even in the simultaneous presence of the degradation products mentioned above. The experimental calibration curves plot weight of the salt in the plaster versus thermal parameter of the transition were tested and the behaviour was shown to be linear in the limit of the experimental errors. The method was applied during recent restorations of a wall painting and a historical plaster in Florence. (C) 1998 Elsevier Science B.V.