The occurrence of gypsum in the scaling of stones at the Castle of Chambord (France)

The main cause of degradation in tuffeau, a soft siliceous and clayey limestone extensively used in the construction of the castles of the Loire Valley in France, is scaling. The most damaging form of weathering is spalling. Over time, a plaque several centimetres in thickness gradually forms under the stone surface and eventually falls away, leaving the resulting new surface powdered. Hypotheses regarding the initiation and development of such degradation have not yet met scientific consensus. The objective of the present paper is to improve existing knowledge of scaling degradation by analysing in situ samples representing different stages of the same degradation process. The analysis of the stones at the Castle of Chambord, revealed the presence of gypsum (CaSO₄·2H₂O), mainly located in cracks parallel to the stone surface. This gypsum was then studied in terms of its crystal morphology and distribution with depth. For comparison purposes, another form of scaling identified at the Castle of Chambord but rarely studied, flaking, was also examined. A first attempt at explaining the origin and formation of the observed gypsum in both scaling processes was conducted by interacting and comparing the characterisation results with historical and environmental data.