The Investigation On The Application Of Laser Cleaning On Bronze Cultural Relics

The cleaning of bronze relics is one of the main problems in the protection and restoration of cultural relics.It is of great research value and significance to use advanced laser technology to clean bronze relics without damage.At present,laser cleaning,as a new cleaning technology,can quickly and effectively remove the dirt covering the surface of cultural relics on the basis of protecting cultural relics themselves,which has great advantages over traditional technology.After comparative screening,this paper selects EOS Combo Nd:YAG laser to perform the laser cleaning experiment of actual bronze fragments.The instrument has short free running(SFR)pulse mode and long Q(LQS)pulse mode.First of all,the cleaning effect of different energy densities of the laser in two modes on the remnants of bronze relics in various rust layers was studied.It is found that laser cleaning starts from the surface layer to the inner layer.If there is no strong force between the cleaning rust layer and the lower rust layer,or the damage threshold of the two is close to each other,the cleaning effect is poor.Fine surface particles,loose rust,speckled rust and"powder rust"cleaning effect is better.And for the green(dark green),red,regional block,thick rust,in two modes,the cleaning effect is poor.Laser cleaning rust layer is mainly dependent on the laser pulse mechanical damage,thermal damage will make the surface"burning"phenomenon.When cleaning in SFR mode,thermal damage is the main factor.It is appropriate to use SFR mode to clean the rust layer that contains protective layer and does not need to be cleaned from 100 mJ energy.When cleaning in LQS mod,mechanical damage is the main factor.It is appropriate to use LQS mode to clean the rust layer that needs to be cleaned from 150 mJ energy.Secondly,based on the existing equipment and instrument materials of the museum,this paper proposes methods to analyze and evaluate the cleaning effect under the two modes from the macro level to the micro level,and analyzes the applicability of the method used.At the macro level,the evaluation was made from four aspects:visual observation under a keensee microscope at 5 0X,chromaticity analysis,calculation of effective cleaning area and effective cleaning rate by super-resolution HDR mode,and calculation of cleaning thickness by 3D depth of field.At the micro level,the Raman spectrometer is used to analyze and evaluate the changes of the main material structure in the micro area before and after cleaning.Portable X-ray fluorescence analysis to evaluate the change of main element content before and after cleaning.SEM + EDS was used to analyze and evaluate the changes of microstructure and element content before and after cleaning.Finally,the laser instrument is used to clean the ancient copper COINS in the actual collection of cultural relics and to evaluate the cleaning effect.Intuitive observation results show that the cleaning effect of LQS mode is better than that of SFR mode.The colorimetric analysis showed that the large area of the surface was slightly damaged by heat after LQS mode cleaning,while the small area was seriously damaged by heat after SFR mode cleaning.The effective cleaning area under LQS mode is 3329044.3 m2,and the effective cleaning rate is 97.78%,far higher than the effective cleaning area of 2648103.4 m2 under SFR mode,and the effective cleaning rate is 46.15%.Microscopically,the changes of surface microstructure and composition before and after cleaning were analyzed by SEM-EDS.The results showed that the surface of base copper alloy was exposed after cleaning by LQS mode,while the rust layer was not thoroughly cleaned by SFR mode.The final result is that LQS mode is suitable for cleaning the loose rust layer under the energy density of 1.87 J/cm~2.