Salt Weathering Risk Under Hygrothermal Cycling in Stele 17, Qianlong Stone Classics, Beijing

The Qianlong Stone Classics are the largest and best-preserved ensemble of officially commissioned stone inscriptions of Confucian classics extant, yet their stele bases are currently threatened by salt efflorescence. Fluctuations in ambient temperature and humidity contribute significantly to this deterioration. Taking Stele 17 as a representative case, this study assesses the risks of surface condensation and moisture-induced salt phase transitions through integrated temperature–humidity monitoring, infrared thermography, and soluble salt analysis. The risk of condensation remains low under typical conditions, as the stele base surface temperature exceeds the dew point by at least 0.5 °C. However, risks of salt deliquescence and hydration are substantial. The stone surface contains elevated levels of soluble salts, including four highly soluble species (sodium sulfate, calcium nitrate, sodium nitrate, and sodium chloride) and one moderately soluble species (calcium sulfate). Deliquescence phase transition humidities are approximately 50.5% for calcium nitrate, 74.3% for sodium nitrate, and 75.4% for sodium chloride, while sodium sulfate exhibits a hydration phase transition near 81%. Exhibition Hall humidity fluctuates around these critical thresholds, driving repeated dissolution–crystallization and hydration–dehydration cycles that progressively erode the stone microstructure. These hygrothermal cycles exhibit pronounced seasonal patterns, with frequent air-conditioning operation in summer amplifying thermal and humidity impacts. This study elucidates an air-moisture-driven salt deterioration mechanism distinct from classical capillary rise, clarifies the persistent progression of efflorescence in transitional seasons, and provides a scientific basis for optimizing environmental control strategies.