Salt efflorescence in historic wooden buildings

Abstract

Efflorescence and subflorescence are frequent phenomena occurring in buildings of porous inorganic materials such as bricks, stone, and concrete. Since the above phenomena have detrimental effects, they were the subject of detailed physicochemical investigations. Efflorescence on wooden material was commonly observed on collection artefacts treated with preservatives such as inorganic salts or organic pesticides. Occurrence of efflorescence in historical wooden buildings was less studied although it could cause serious conservation concerns. This paper describes an examination of chemical and mineralogical composition of salt efflorescence in wooden buildings. In situ analysis by X-ray fluorescence spectrometry was followed by investigation of plain efflorescence by infrared spectrometry, X-ray diffraction, electron microscopy, and electron probe microanalysis. Quantitative analysis of purified efflorescence was performed by plasma atomic emission spectrometry, ion chromatography and absorption spectrophotometry. In addition, depth-profiling of characteristic ions in wood was performed. The objects of study were a series of log houses of the Sverresborg Trøndelag Folk Museum in Trondheim, Norway. It was found that the efflorescence in these buildings includes a series of metal ions, the most abundant being Al3+, Mg2+, Zn2+, K+, Na+, and Fe2+. In addition the ammonium ion is present in a large amount. The single detected anion is sulfate. Actually, the efflorescence consists of a mixture of simple and double sulfate salts of alum and Tutton’s compound types. Identified Tutton’s compounds are mohrite ((NH4)2Fe(SO4)2·6H2O), ammonium zinc sulfate hexahydrate ((NH4)2Zn(SO4)2·6H2O)), and boussingaultite ((NH4)2Mg(SO4)2·6H2O)). Among the alums, both tschermigite (NH4Al(SO4)2·12H2O), and potassium alum (KAl(SO4)2·12H2O)) were detected. The composition of efflorescence suggests that the wooden material was treated with a blend of sulfate salts ((NH4)2SO4, Al2(SO4)3·18H2O, MgSO4·7H2O, FeSO4·7H2O, ZnSO4·7H2O, and Na2SO4·10H2O)) for protection against fire and biological degradation. The efflorescence appeared as a consequence of water leakage through the roof that led to the dissolution of the salts contained in the wood, followed by evaporation and salt crystallization at the surface in the form of double sulfate salts. Although the occurrence of efflorescence in this case was accidental, a natural and more frequent cause of efflorescence could be the deliquescence of inorganic preservatives impregnated in the wood.