| As an ancient country, there are countless stone cultural relics distributed in China, such as cave temple, cliff inscription, tablet inscriptions, statues, stone architecture. These stone cultural have excellent high value and significance both on the study of social formation and on inheriting Chinese traditional culture. It is common that there are varying degrees of contamination on stone artifacts in China, especially on large outdoor preserved stone relics. Moreover, with the intensification of human activities and air pollution, this contamination witnessed a gradual increase. These contaminants accumulating on stone surface, obscure the ornamentation and style of stone relics. Simultaneously, most of the pollution will imposed different degrees of impact on the stone artifacts, even exacerbate the occurrence of disease and lead to potential hazards. Thus the scientific recognizing surface contamination of stone artifacts, taking scientific and reasonable cleaning methods, has become an unavoidable problem in conservation work.Stone conservation is a scientific system of integrated process, choosing appropriate cleaning methods not only can remove or ease the surface contaminants, but also the art value can be rehabilitated and visual impact can be improved. Meanwhile, cleaning implementation can remove the harmful surface contaminants, such as soluble salt and biological disease, to ease or remove the disease to extend the life of stone cultural relics. Because cultural relics are "precious and nonrenewable", stone relics cleaning must ensure the safety of its promise and carry out in scientific and reasonable manner. Inappropriate methods and materials will easily lead to irreparable damage. With the increase of conservation awareness and the improvement of science and technology, chemical cleaning especially the acid and alkali corrosive reagents are forbidden in stone artifacts conservation. Therefore, the search for nonwater physical cleaning technique such as micro-particles blast and laser have become the technology frontier and central issue on the removal of surface contamination of stone relics.Micro-particles blast cleaning technique has the superiority of having no chemical residues, no water(solvent) operation, operability, and fast etc, it has gradually become the new advantage technique for cleaning contaminant of the stone artifacts especially the outdoor large stone artifacts. But in china for the micro-particles blast cleaning technique there is almost no relative research of removal mechanism, operational safety and evaluation of the effect of implementation. This leads to the micro-particles blast cleaning technique can’t apply to surface contaminant cleaning of stone cultural relics widely, and also restrict the improvement of conservation technologies.This paper relies on the "Eleventh Five-Year" key project of the National Science and Technology Support Program "Research on surface harmful contaminants removal technology for cultural relics Grotto" (No.2009BAK53B05). During the research, in accordance with the principles and the research program of cultural relics protection, we took the investigation of surface contamination on stone relics surface. We summarized the mechanism and removal mode of micro-particles blast cleaning technique. We analyzed the technique safety and the cleaning effect, and also we give out the optimal operation parameters of micro-particles blast cleaning technique used on stone cultural relics. The study of this paper Filled the technology gaps in micro-particles blast cleaning technique filed, and will meet the real needs of stone cultural relics protection in china, and promote the study of stone cultural relics protection technology system.The major research tasks completed and their achieved results are introduced following:(1) Finished the investigation of classification, chemical composition, formation mechanism, hazardous and the necessary for cleaning research of stone cultural relics.During the research, base on the Investigation of the world cultural heritage Yungang Grottoes (outdoor sandstone grotto), Guangxi Huashan rock paintings (outdoor limestone rock paintings), Shaanxi Tang tomb stone carvings (Qianling-Outdoor large stone modeling), Longmen Grottoes (outdoor limestone grotto), Shaanxi Beilin stone carvings (moveable collection stone relics), we finished investigation and research of contamination and their common species types on stone relics surface. For the first time, this paper analysis and demonstrate the chemical composition, disease causes binding mode, activity (hazardous), the necessary for cleaning of typical stone contaminants in China. Combined with other research, we finish SACH national industry standard "Brick Heritage Collection Diseases and Illustration "(in press, recommend execution).The results show that surface contamination is widespread phenomenon especially on outdoor stone relics. Common contaminants can be divided into typical natural factors such as dust, rain sagging, microbial corrosion, calcareous crusts can be precipitation of dissolved salt, rust, animal contamination; Human factors such as cement contamination, resin sagging, paint stains, ink pollution, graffiti and other surfaces. Pollution caused by human factors will affect the visual effects and reflect artistic value, the vast majority of surface contamination caused by natural factors affect not only reflects the artistic value, but also the contaminations are active. The application of scientific and effective cleaning techniques surface cleaning is not only beneficial reflect cultural artistic value, but also conducive to the further implementation of the post-termination and other protective measures for the disease, researchers cleaning technology is imperative.The formation of pollutions stone on cultural relics surface is directly related to the material of the stone, the atmospheric environment, preserved geologic environment and the humanities environment. The different kind of surface contamination have the different way to combinate with stone surface. From the micro structure surface contamination can be divided into three types, they are loose type, close type and soaking type contamination. Theoretically loose type contamination is the most easy to remove, followed with the close combining, soaking contamination due to the contamination Immersed into the rock internal gap are most difficult to remove.(2) Took the research on mechanism and controlling critical factors of micro-particles blast cleaning, builded an Particle jet simulation model and calculated Jet pressure, which laid a theoretical foundation for micro-particles blast cleaning technology used on stone relics.The advantages of micro-particles blast cleaning are fast and having no residue, but its mechanism and doubt towards the implementation controlling process have limited its application and promotion. In this paper, the commercial CFD software Fluent tool is used to describe the process of compressed air’s flowing with micro-particles by Lagrangian method and Euler method relatively, and we build a numerical simulation model of micro-particles blast cleaning used to clean stone relics. And with this model, we simulate the behaviors of sandstone and limestone materials depending on different operating conditions (such as injection pressure, angle of incidence, the operating distance) and different spray material (material, particle size, shape, etc.) and define removal functional mode of blast cleaning and its variation pattern under different working situation. For the first time, we explicit the mechanical transfer and accelerated process of micro-particles spray in the airflow and summarize the critical influencing factors and the variation trends of micro-particles blast cleaning process, which laid a theoretical foundation for micro-particles blast cleaning technology used on stone relics.The results show that micro-particles blast cleaning is mainly use the effect impact and grinding to clean up the sueface contamination. Result of modeling analysis show that after abrasive mixture flow jet from the nozzle the flow formed a Shape similar to "bell", and its radial distribution of particles display the tendency of central Concentration, when the Particles leave the nozzle, there still exist some differences between the velocity of particle and carrier air, In general for the spraying distance Within 30mm velocity of particle will still increase, and then gradually reduce, so at spraying distance within 30-50mmwe can obtain the best removal effect. When we adopt soft walnut sand as abrasive Material, The deformation caused by collision is mainly concentrated in the soft material spraying particle, the impact force caused by Collision is small, Mainly by the friction of soft material to clean the surface contamination, In this process is advised to use smaller injection angle to get a better removal effect. While hard materials such as glass beads is adopt, stress can be more effectively conducted to the stone surface, The removal effect is overall better than soft material spraying, But the security and operation will reduced, at this time larger injection angle should be adopted to improve the impact force. At the same time increase the injection pressure and abrasive particle sizeare beneficial to increase the impact force and improve the cleaning ability, But too high the injection pressure and particle size increased will lead to a sharp increase of disturbance and air resistance, cause unnecessary energy loss, at the same time them affect the realization of fine operation also. Therefore, if we want to enhance the removal capacity effectively, these two parameters can only be ascension at certain range.(3) Accomplished micro-particles blast cleaning implement safety assessment for stone cleaning and controlling threshold determination.Because of the heritage properties and non-renewable of stone cultural relics, protecting methods must be implemented ensuring the safety of its promise. This paper takes Yungang Grottoes sandstone, Huashan cliff paintings limestone as the main objects, Take into account of granit, emarble and other common stone materials, completed damage experiment, by the measurements of micro morphology, mass loss and appearance of change, for the first time in China accomplished micro-particles blast cleaning implement safety assessment for stone cleaning and controlling threshold determination, Eliminate the technical challenges and bias, Provide application theory and technology support micro-particles blast cleaning technology using in stone conservation field.The results show that:①The major damage mode on stone cultural relics is Micro damage, which is caused by the Elastic brittle deformation on the stone surface when micro-particles blast on the stone. The primary factor of safety indicators is abrasive hardness, which decide the damage occur or not. Using the soft abrasive which Mohs hardness of less than 4 such as sponges, walnut sand can ensure the safety in the process of cleaning.②The operation factors, such as injection pressure, spray distance and jet angle are the secondary factor of safety indicators, they only infect the damage extent, and the shape or structure of damage.③The size and shape of particle also have certain influence on the damage on the surface of stone. Generally speaking, the larger particle size, the shaper and more irregular edges of particle, mains the more damage on stone. Conversely choosing smoother or smaller particles is beneficial forperation safety.Overall, The influence of operating parameters contribution on safety control is in accordance with the following order:abrasive material hardness >> injection pressure and particle size> particle shape and injection angle. Comprehensive results show that the general safety threshold of micro-particles cleaning is using soft abrasive with Mohs hardness of less than 4 such as sponge and walnut sand etc. Under the same conditions, it is suggested to choose a smaller diameter of injection spray materials, and smaller incident angle to implement the cleaning work.(4) Finished the research on effection of different contaminations removal and its demonstrationBy indoor simulated sample test and outdoor standard area test, we identified removal capacity of different Typical pollutants by micro-particles blast, evaluated its applicability, and given the suggested value of parameters which used to set join the micro-particles blast process to get a good job in cleaning. And provide reference and guidance to the cleaning work in the field of the protection of cultural relics stone objects.①For the inorganicform pollutants such as soil, dust, rust and calcified crust, because of its hardness and structural strength is generally less than the rock base, so their removal process mainly for grinding and thinning the layer of contaminant by abrasive micro-particles. Removing contaminants start from the first layer, then the bonding layer is removed. To remove this kind of contaminants, the hardness of abrasive micro-particles should be selected less than the stone base, but close to or greater than contaminants. And with smaller angle of incidence of 30°-60° can cause almost no damage to the stone base. The ejection pressure only cause some degree of influence on the speed of cleaning only.②For oil stains, ink marks, smoked traces and other immersion contaminants, using soften microparticles abrasive for jet cleaning only have removal effect on deposited contaminant of stone surface. To complete removal them, we must choose abrasive materials which hardness must be close to or exceed the rock base, along with immersion layer removed, the damage of stone surface occlude too. Therefore, for this kind of pollutants is only recommended to do some thinning treatment with soft abrasive, not recommended for complete removal.③For modern organic coatings such as paint, because it has an elastic surface, the removal effect of micro-particles blast cleaning is unsatisfactory, the abrasive materials only with a sharp angle may cause a certain degree of thinning processing.④For biological contaminants, due to the obvious crisp transition layer between biological contaminants and stone, using soft abrasive materials such as walnut sand as the ejecting material, with a smaller incidence angle of 30°-60° can rapidly removing the contaminants and do not hurt the rocky base.The innovation of this paper is as follows:(1) Use the CFD software we established jet ejected model of micro-particles blast cleaning, and use Euler and Lagrange method to Simulat the flowing process of micro-particles and compressed air flow, and construct numerical simulation model of micro-particles blast cleaning, give out its mechanism and influence factors. Laid the theoretical foundation for micro-particles blast cleaning technique introduced into the field of cultural heritage conservation.(2) Through damage experiment, for the first time in China we comprehensive analysised the change in microstructure, appearance and quality loss on the common culture stone objects such as sandstone or limestone, which could caused by micro-particles blast cleaning. And put forward the safety guarantee conditions for the micro-particles blast technology used in the protection of stone cultural relics, and gives out the concrete control parameters. This work fills the gaps in the stone relics cleaning technology, and has important theoretical and practical significance to enhance our technology and the advancement of technology related industries.(3) In this paper, we finish systematical study on the applicability of micro-particles blast technology used to clean the common contaminant on the cultural relics stone object’surface, and gives the best operating parameters or the recommended setting. Inaddition, appropriate standards demonstration areas are established. And expecting to provide guidance and reference for cultural heritage related restoration work. |
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