Role of secondary phases in the scaling of stained glass windows exposed to rain

•Weathering in real atmospheric conditions of Si-K-Ca model and ancient stained glasses.•Formation of alteration layers without protective role.•Precipitation of secondary phases inside altered layers causes separation of scales from the bulk.•Scaling responsible for the rapid decay. The cyclic nature of atmospheric conditions triggers different weathering responses. Although studied in the past in solution, the complex alteration mechanisms were not adequately described. In the present work, atmospheric weathering of medieval SiO2-K2O-CaO glass was studied via field exposure of model-glasses and ancient stained glasses. Local dissolution and precipitation of secondary minerals, mainly calcite, create a fracture network, allowing water to move into the glass enhancing alteration processes and scaling of the surface. In fine, the renewal of glass surface by removal of altered scales helps accelerate the alteration rate and prevents the formation of a protective layer.