Three local sandy limestones have been used as historical building materials in Flanders: the Gobertange, Balegem, and Ieper stone. They show more or less the same components, but their relative abundance and arrangement are different. As a result, they take a different patina. Their decay by atmospheric pollution starts below an external gypsum crust. Gradually a gypsum layer is formed around the quartz and glauconite grains; the latter are finally fragmented by invading crystals. Part of the calcite matrix is also transformed into gypsum. The resistance against gypsification is stronger when the calcite matrix is well developed (Gobertange), and lower when the sand fraction (Balegem) or the glauconite fraction (Ieper) is important. Large scale substitution of original material should be avoided for esthetic and technical reasons. -- AATA