Study Of Cleaning Deposit Crusts On Excavated Polychrome Stone Buddha Statues

With the development of archaeology,more and more polychrome stone Buddha statues have been excaveted.These statues contain extremely important information of the Buddhism,Buddist art and craftmanship,so it is great significance to conserve them.However,polychrome stone Buddha statues is widely covered by deposit crust with the effect of burial environment.It is difficult to clean these deposit crusts,because not only the composition of them is similar to that of stone matrix,but also the polychrome painting is fragile due to the degradation of binding media.Therefore,this study started from invesgating the composition and the formation mechanism of deposit crust.In order to remove the deposit crust,polyvinylamine(PVAm)gel combined with acetic acid was used to clean controllablly and effectivly.In addition,a set of standard system was established for evaluating the cleaning work for excavated stone cultural relics.The main work is as follows.First of all,the polychrome stone statues excavated from the ruin sites of Linzhang,Heibei province and Qingzhou,Shandong province were taken as the research objects.The materials and craftmanship of these statues were investigated by comprehensive application of analysis methods.According to the literatures and simulation experiments,the craftmanship of polychrome paintings was explored.The formation mechanism of deposit crust was researched.This provides a wealth of information not only for the study of cultural relics,but also for the study of cleaning.Based on the research above,chemical cleaning was selected because it can achieve targeted and effective cleaning,while avoiding mechanical damage on the painting layer.Through testing the cleaning efficiency and the surface morphology changes of the mock-up samples,acetic acid was superior to other cleaning agents.Further more,simulating cleaning experiments were carried on the mock-up stone samples with adjusting the concentration and pH value of acetic acid.Because the main compositions of the deposit crust are calcite and dolomite,which are similar to the stone statues,the cleaning efficiency difference of acetic acid on three kinds of mock-up samples,shows that there are chemical effect and physical effect during the cleaning progress.The chemical effect is that the weak acid dissolves and deposits the crust to remove,while the physical effect is due to the permeability of the cleaning agent,which lead to the crystal falling off.In addition,the physical effect is greater than the chemical effect in the cleaning process.In order to control the penetration and diffusion of the cleaning agent and reduce the physical effect,PVAm gel was added.Results of the peelability and rheological properties of PVAm gel demonstrated that it is suitable not only for cleaning on rough surface,but also for acid cleaning.By further characterizing the chemical structure,viscosity,pH and controllability of cleaning agent of PVAm gel,it is clear that the peelability of PVAm gel is caused by the structure change with pH of hydrolytic solution.It is concluded that the gel was easier to peel off in weak acid solution.In a certain range of ratio,PVAm gel could be maintained on the surface of stone statues with a certain inclination angle.The pH of PVAm gel will be the same as that of the acid solution,and the addition of gel material will not reduce the cleaning effect of the solution.In addition,the chemical structure and cleaning mechanism of PVAm gel combined with acetic acid were also characterized.The cleaning experiments were carried out on simulated samples by using PVAm gels hydrolyzed by acetic acid with different concentrations and pH.The optimal ratio of PVAm gel cleaning material was obtained by evaluating the cleaning efficiency,the changes of morphology and aberration on surface of mock-up sample,the porosity and water absorption of stone matrix,and the cleaning method was defined.The cleaning experiment carried out on small pieces of cultural relics obtained ideal results.Finally,in order to exlpore the standardization of evaluating the cleaning work of excavated stone relics,it was tried to construct the evaluation system.The system was based on analytic hierarchy process and fuzzy grey clustering method.This is a scoring system,which provided a reliable basis to evaluate the cleaning work quantitatively,as well as selecting and improving of cleaning methods.Further more,this evaluation system was applied on the cleaning work of the stone statues in Linzhang,and the results could obiectively and accurately demostrate the overall effects and deficiencies of the cleaning.