White finishing of the Potala Palace: material system, deterioration patterns and considerations for sustainable conservation

Abstract The Potala Palace is among the most important historical structures in Tibet and features massive stone masonry walls finished with various earthen slurries. Among them, the white exterior finish—commonly known as the ‘sweet wall or milk wall’ because of its inclusion of milk and sugar—covers most of the surface of the palace and has symbolic and ritual significance. Despite its cultural value and annual renewal tradition, the material system of this finishing has remained scientifically underexplored. This study systematically investigates the white earthen plaster in the Potala Palace, integrating field documentation, sample analysis, and material characterisation. The results confirm that the primary component is kaolinite clay obtained from Yangbajain town, mixed onsite with organic additives such as sugar, milk, and cowhide glue to create a splash-treated slurry. The finish produces a textured surface with distinct morphological characteristics, including the stalactite-like ‘reba’ formations. This study also identifies three main forms of deterioration—erosion from precipitation, chalking related to moisture and salt, cracking and delamination, and biological colonisation—all of which are exacerbated by the shifting climate conditions of the plateau. In response, the research team elucidated the material system and deterioration patterns of the white earthen finish and outlined preliminary considerations for sustainable, value-based conservation that balanced technical enhancement with the continuity of local materials, techniques and public participation. These findings provide a foundational reference for the sustainable preservation of earthen heritage under environmental pressure.