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\bibitem{Leysen_etal1990} {\textquotedblleft}\textit{Characterization of the weathering crust of various historical buildings in Belgium}{\textquotedblright}. Leysen LA, Roekens EJ, Van Grieken RE, De Geyter G, The science of the total environment \textbf{90}, 117 (1990).   http://doi.org/10.1016/0048-9697(90)90190-6

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