| Stone archaeological finds such as grottoes, sculptures, steles, and other forms play an extremely important role in the cultural heritage of China because they have features of high value, are widely distributed in every corner of China, and have a long history. These precious stone artifacts that comprise high historic, artistic, and scientific value are suffering different kinds of erosion caused by long-term natural weathering (including physical, chemical and biological weathering), for example, air pollution, acid rain, or man-made damage. The traces in stone with the most valuable information have disappeared and the losses cannot be estimated. So it is very important to do research on stone conservation.Grottoes, a typical representation of stone heritage site, are rock-cut temples with religious sculptures and wall paintings. China’s grottoes began in the third century and flourished in the 5th-8th century. Although they were initially an imitation of India’s grottoes, they formed their own unique grotto style through cultural exchange. China’s four grottoes famous for their ancient Buddhist sculptural sites are Dunhuang Grottoes, Yungang Grottoes, the Longmen Grottoes, Dazu Rock Carvings. They are outstanding representatives of Chinese grotto temples and are all UNESCO World Heritage sites. The conservation technology of grottoes is the most complex and comprehensive one in stone conservation because it considers not only the stone itself but also the geological environment which is associated with almost all aspects of conservation technology.The Yungang Grottoes, the large immovable cultural heritage site, are ancient Chinese Buddhist temple grottoes and an excellent example of rock-cut architecture. The prolonged exposure of the grottoes to the environment will eventually lead to deterioration of the stone due to the joint action of light, biological effects, and air pollutants as sulfur dioxide, nitrogen dioxide and dust. One example of a contaminant is the dirt on the surface of stone. These deposits and encrustation can cause further degradation processes of both a physical and a chemical nature, and affect the further protection treatments and their effects. So surface cleaning must be done to remove harmful materials from the surface.Laser cleaning has the advantage of selectivity, non-contact, versatility and reliability and is a new tool that has been used as a main cleaning technique in conservation. But it has been used little in stone conservation in China, although it has been used for over 20 years, mainly in Europe. It is the obligation of conservators to study the mechanisms of laser cleaning and apply it to conservation projects, like Yungang grottoes. It is also important for promoting stone conservation techniques and very useful for other conservation fields.This dissertation is based on the project, "The Research of Cleaning Techniques for Removing the Harmful Deposits on the Surface of Grottoes" supported by National Key Technology R&D Program During the "Eleventh Five-Year" Plan Period"(No.2009BAK53B05). On the basis of survey of equipment, literature of theory and applications of laser cleaning, the dissertation takes Yungang grottoes as an example to study the laser technique for cleaning harmful deposits of sandstone. In the dissertation, the author inspected and experimented on mechanical properties of sandstone, the types of the deposits on the sandstone surface, carried out experiments on laser cleaning to remove the contaminants, the damage of sandstone during laser cleaning, laser cleaning equipment design and manufacture for stone cleaning, the treatment procedure of laser technique for cleaning contaminants on stone surface and its applications on heritage sites. The author also developed a study for the laser cleaning mechanism and safety of sandstone heritage.This dissertation first introduces the modes of laser cleaning systems, cleaning methods, the laser-material interaction process and the possible mechanism of laser cleaning. For stone conservation, the main laser system used is Q-Switching (QS) Nd:YAG lasers, parameters of these lasers are:wavelength (k):1064 or 532 nm, pulse energy (E):250-1000 mJ, and pulse width:5-20ns, repetition frequency>5-20Hz. The laser cleaning methods are dry cleaning and wet cleaning. Laser dry cleaning method has the laser irradiation directly on the surface. The contaminant will absorb energy and will be removed from the surface through thermal diffusion, photo-decomposition and vaporization. In comparison, in laser wet cleaning, a thin layer of suitable liquid (water or sometimes alcohol) is sprayed onto the surface, and then the laser irradiates the surface. With the assistance of the liquid’s explosive vaporization, particles are removed from the surface. Laser-material interaction depends on laser parameters, material properties, and environment. The incidence of a laser beam on a material is accompanied by absorption and scattering phenomena, which depends on the optical properties of the material. Linear laser-material interaction will produce heating, vaporization, and photoacoustic effects. For non-linear laser-material interaction, the phenomena of a multiphase flow including solid particles, vapour, gas, and in some cases also a plasma phase, which characterises the so called ablation plume will be produced. These all indicate the complex mechanism of the interaction between laser and material.The physical model used to explain phenomena of laser cleaning is that the particle thermal expansion generated by the laser leads to high acceleration of particles that removes them from the material surface, the selective laser ablation, the photoacoustic effect, the mechanical action, etc.Due to the principle of laser-material interaction, the cleaning process depends on the irradiated material, and therefore the basic physical and mechanical properties of sandstone of Yungang Grottoes and the characteristics of the polluted layer on the surface must be known. Combined with previous research, the basic physical properties of sandstone Yungang Grottoes were tested. The experiment results show that the Yungang sandstone is composed mainly of quartz and feldspar. The average natural density of Yungang Grottoes sandstone is 2.50 g/cm3, the average dry density of 2.49 g/cm3, the saturated density of 2.55 g/cm3, the average porosity of 5.63%. The sandstone’s compression strength is around 65～128 MPa, the elastic modulus is 16～34 GPa, the deformation modulus is around 11～24 GPa, Poisson’s ratio is 0.15-0.3, the tension strength is 2.5-5 MPa, the average thermal conductivity is 3.338W/(m·K), the specific heat capacity is 850～950 J/kg·K, and the coefficient of linear expansion a is about 7.3-9.5×106(1/K).The pollutants on the surface of Yungang Grotto can be classified into 12 types. They are dust deposition, salt crystals, smoked black dirt, cement traces, yellow mark, residual resin, writing and carving, paint stains, grease, water spots, animal contamination, and biological dirt. Dust deposition and smoked black dirt are the main pollutants from an external source which account for over 91% of pollutants from external sources. Other major pollutants with external sources include graffiti carving, residual cement and resin. Dust deposition is the largest pollutant type in Yungang Grottoes, accounting for more than 50%. The main source of it is air particulates, and its composition is gypsum, mineral particles and carbon. We find that dust deposition is a loose structure with fine particles and fit closely on the base rock layer. Smoked black dirt is another major pollutant accounting for more than 13% of pollutants with an external source. The main component of it is gypsum and carbon. Smoked black dirt is generally more uniformly covered on the rock surface with a clear boundary.According to the features of contaminants on the surface of Yungang Grotto, we chose four typical types of pollutant:dust deposition (imitated by lime), smoked dirt, black ink, and paint as target pollutants and test samples were made. The sandstone is the local sandstone from Yungang grottoes. The laboratory experiments on sandstone samples have been performed with a Q-switched Nd:YAG laser operated at 1064 nm with a pulse width of 10 ns and spot diameter 3mm. The cleaning mechanism is also analyzed and discussed. The experiment results showed that the laser can effectively remove four types of contaminant on the sandstone surface. The laser cleaning thresholds are obtained. For the black ink, the dry cleaning threshold is 21mJ, wet cleaning threshold 21mJ; smoked black dirt, dry cleaning threshold is 20.5mJ, wet cleaning threshold is 20.5mJ; for paint, dry cleaning threshold is 32.5mJ, wet cleaning threshold 32.5mJ; for lime, dry laser cleaning threshold 49.5mJ, wet cleaning threshold 33.5mJ. The distribution of temperature and stress of deposition layer is simulated and calculated by ANSYS software, where the parameters of standard graphite is using for representing the black ink and smoked black dirt (carbon), lime plaster mortar parameters for limes, and parameters of paint itself for paint. The results show that for black ink and smoked dirt with the incident laser energy 20mJ, the carbon black surface temperature could be up to 3047℃, equivalent stress is up to 237MPa, its tensile stress is 0.029 MPa. The dry cleaning mechanism of black ink can be described as when irradiated by the laser beam the carbon black is heated and thermal strain is generated. The carbon black could reach its melting point, evaporate and combust and be removed from sandstone surface. When liquid is absent, on the one hand, it could resolve black ink and make the adhesion lower, on the other hand, the black ink will be heated and then evaporated explosively. All these will accelerate the spallation speed. Of course, the thermal stress has a certain function during the process. The cleaning mechanism of black smoked dirt could be due to thermal stress which causes the smoked dirt to be heated and evaporated explosively. The smoked dirt consists of many oil substances. These oil substances could be heated and evaporated explosively and have the same function as wet cleaning. The mechanisms of wet cleaning are the same as dry cleaning, in this circumstance water cannot resolve oil substances, and the oil substances take the role of water instead. For paint, the calculation results show that when the irradiation laser is set to 40mJ, the temperature of paint could reach at 1977K, equivalent stress on the center of the irradiation area is up to 155 MPa, and the maximum tensile stress is 0.364MPa. The laser removal mechanism of paint is mainly due to ablation from the high temperature, and the paint could evaporate and combust. Also mechanical stress could be generated due to thermal expansion of the paint. For lime, the calculation results show that under 40mJ laser irradiation, the surface temperature is up to 3388K, the equivalent stress of the beam center is up to 8.38MPa, and the maximum tensile stress is 0.012MPa. The mechanism of removal, under dry conditions is mainly due to ablation and thermal stress. In the case of wet cleaning, its mechanism could reduce the effect of ablation because the water could absorb more heat and make the temperature lower. In addition to the thermal stress function, there is the function of water evaporating explosively and generating pressure waves etc. These functions could make the cleaning threshold lower and cleaning more efficient.The most important and primary thing for laser cleaning stone artifacts is that the operation does not damage the artifacts. If it does, the laser cleaning technique cannot be applied in the conservation of artifacts. So the safety of sandstone during laser cleaning and the maximum energy that could be used in the operation was studied. For this purpose, an interaction model is presented and the finite element analysis software ANSYS was used to simulate distribution and changes of temperature and stress on irradiated area. The calculated results were discussed and an experiment was conducted to verify these. The calculated results show that the center of the irradiation zone is the highest temperature, and it reaches 878K at a radiation energy of 100mJ. The temperature is gradually reduced away from the center. At a radiation energy of 200mJ, the sandstone surface temperature rose to about 1500K. At this temperature, sandstone could be damaged because the temperature is higher than the melting point. The changes in temperature over time at the center of the irradiation zone showed that the surface temperature increased rapidly up to the maximum temperature in 0.2s, then temperature drops rapidly to room temperature after one pulse irradiation. At the frequency of 1Hz, laser radiation does not exhibit cumulative thermal effects on the sandstone’s surface. The stress changes after laser irradiation show that at laser energy of 74mJ, and number of pulses at 10, the stress generated on the sandstone surface is greater than the compression strength and tension strength of Yungang sandstone, so 74mJ could be regarded as the theoretical damage threshold of laser cleaning for the sandstone of Yungang grottoes. An experiment of laser irradiation on sandstone was carried out to test damage threshold and verify the feasibility of the theoretical model. The microscopic observation and precise measurement of loss of sandstone quality by balance was used. The results show that laser energy of 73.5mJ is measured as the sandstone damage threshold that quite closely to numerical value.The purpose of conservation of heritage is to solve the actual problems in practical project. A laser cleaning system for stone artifacts was invented and the operational technical regulation was studied. The laser cleaning system is designed with a real-time monitoring part and a spot position indication part for the special needs of conservation. It is based on conservation principles, cleaning processes of cultural heritage and combines with laser cleaning experiments. The technical regulation includes measurement of reflectivity of surface contamination, damage threshold tests of the substrate of rock, cleaning threshold tests of pollutants, cleaning tests, carrying out the cleaning, assessment of the cleaning effects, recording of the process, and final project report of cleaning. The site experiments of laser cleaning are undertaken on Yungang Grottoes in Shanxi Province, Dishui Temple in Mianyang, Sichuan province, Leshan Grand Buddha, Sichuan province and Rock Paintings of Hua Mountain, Guangxi province. The results show that laser cleaning system for stone and the operation’s technical regulation accord with site conservation projects and could be used on a real conservation project.The innovation points are as follows:(1) The laboratory experiments of laser cleaning on sandstone samples of main of Yungang grottoes have been conducted. The cleaning threshold was obtained and cleaning mechanisms of those pollutants by temperature changes, thermal stress and chemical changes are discussed. An interaction model of laser-sandstone is presented and the finite element analysis software ANSYS was used to simulate this situation. Also an experiment for testing the safety threshold is carried out. The results show that model is reliable.(2) A laser cleaning system for stone artifacts is developed with independent intellectual property rights, which functions include a real-time online monitoring surface morphology of stone artifacts, laser energy indicator, visible indication of the laser spot and adjustable parameters. The system could fix on a position and be effectively controlled depending on the circumstances of the stone artifacts and dirt on site. It can be adjusted during the cleaning process to ensure the safety of artifacts. The device has received national invention patents, patent Number:No.1113791.(3)The technical operation regulation is presented including the laser cleaning process and the technical methods of laser cleaning, which provides an integrated and methodological approach to laser cleaning of stone artifacts and a reference for cleaning of other artifacts. |
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