Optimization of conditions for the cleaning of overpaint from the stone carvings of the Potala Palace, Tibet using enzyme-based cleaning agents

This work focuses on the study of the removal of overpaint from the stone carvings of the Potala Palace, Tibet. The removal of historical mortar and overpaint is a significant challenge for the conservation of stone carvings and requires careful optimization. Paints applied in the past were made of a mixture of hydraulic lime, water and milk, and are causing damage to the carvings of the Potala Palace. The protein content of lime-based paint samples collected from the Potala Palace was determined by enzyme-linked immunosorbent assay (ELISA). To optimize the removal of the overpaint without causing abrasion or other damage to the underlying stone carvings, we prepared a simulated paint with the same protein content. Removal rate and color difference were used to select the best cleaning agent from ionic surfactant, ethanol/aqueous solutions, five proteases and Twin-20 (non-ionic surfactant). With color difference and gloss as evaluation indexes, the influencing factors (temperature, pH, enzyme concentration, and enzyme hydrolysis time) were measured. Results suggest that alkaline protease was the most suitable cleaning agent for removing surface paint, and the best working condition is an enzymatic dosage of 1.92 mg/mL under 42 °C, pH of 9.5, with a hydrolysis time of 5.4 min. This method generated the desired color difference of 2.25 and the minimal gloss (0.1 ± 0.1 GU) on the underlying stone, with no significant variation from the predicted value (P < 0.05). Before and after cleaning, the change in surface roughness was less than 10%. This study provided a feasibility plan for removing the paints from the outer wall of the Potala Palace by using enzymatic hydrolysis.