Analysis Of Pigments Based On LIBS And Raman Techniques

The Dunhuang Mogao Grottoes are renowned for their exquisite murals.However,due to their ancient age,the surface pigments of the murals have undergone severe discoloration,causing many murals to become unrecognizable.The discoloration of the surface pigments made it difficult for people to truly appreciate the original colors and details of the murals,thereby lowering their artistic value and increasing the difficulty of restoration.Especially the orpiment and realgar mineral pigments in the murals,which are similar in color and have the same elemental composition,cannot be accurately distinguished from each other after they have been discolored and overlaid in multiple layers.This made it is challenging to identify the orpiment and realgar pigments and the multiple layers of murals painted by them.This can not only provide a reference for identifying other pigment types with similar colors and elemental compositions in ancient murals,but also has great significance for the protection and restoration of the murals.The specificity of the orpiment and realgar pigments in the murals requires a new analytical technique capable of obtaining multiple information atomically and molecularly to carry out the identification of the orpiment and realgar pigments in the Mogao Grottoes.Laser-induced breakdown spectroscopy(LIBS)and Raman spectroscopy techniques have irreplaceable position in the field of cultural heritage mural analysis due to their unique advantages of being minimally or non-destructive,rapid and in situ.The LIBS technique is an atomic emission spectroscopy,which enables layer-bylayer ablation for the analysis of murals,but it is not sufficient to distinguish between homogeneous heterogeneous mineral pigments with the same elemental composition;Raman spectroscopy is a molecular scattering spectroscopy,which enables the analysis of the molecular structure of substances,but cannot be performed on multi-layer murals.By combining LIBS and Raman spectroscopy techniques can acquire both atomic and molecular spectra of substances,which not only enables rapid detection of homogenous isomorphic mineral pigments but also provides information about pigments on different layers of multi-layered murals.In this work,we used LIBS and Raman spectroscopy techniques to identify the orpiment and realgar pigments with the same elemental composition and conducted layer recognition analysis on multi-layered simulation blocks painted with these pigments.The main research contents are as follows:1,In response to the problem that the yellow mineral pigments orpiment and realgar,which are commonly found in murals,cannot be identified because of discoloration,a LIBS experimental device was built to collect spectra of both and perform spectral line identification.Several common normalization methods were used to normalize the LIBS spectra,and the best standardization pre-processing method for spectral data of pigment layers in murals was determined based on the relative standard deviation(RSD)value.Then,a clustering analysis model for realgar and orpiment was established based on LIBS and principal component analysis(PCA).The results showed that LIBS technology cannot distinguish between the two pigments.2,In response to the problem that LIBS technology cannot distinguish orpiment and realgar mineral pigments,Raman spectroscopy experimental setup was built,and the spectra of orpiment and realgar mineral pigments were collected using 532 nm,785 nm and 830 nm lasers,respectively,and the optimal excitation wavelength was determined to be 830 nm,and the corresponding vibrational modes of the Raman characteristic peaks for both orpiment and realgar were assigned.The results showed that Raman techniques can successfully identify and classify the two minerals.In addition,the Raman spectra of orpiment and realgar mineral pigments before and after laser ablation were measured and compared,and it was found that the realgar pigment underwent structural changes before and after laser ablation.3,The use of LIBS and Raman spectroscopy techniques were studied on multilayered wall painting simulation blocks made from the pigments realgar and orpiment with different layer thicknesses.Firstly,the layer-by-layer ablation ability of the LIBS technique was used to obtain the fitting relationship between the number of laser ablation shots and the ablation depth on the simulation blocks with different thicknesses.Secondly,the morphology of the laser ablation craters on the simulation blocks was analyzed,and the Raman spectra of the ablation craters produced by different laser ablation shotes on the simulation blocks with different thicknesses were analyzed and combined with microscopic images.Finally,pigment information for each layer of the multi-layered wall painting simulation blocks was successfully obtained.This paper presents a preliminary study on the identification of homogenous but heterogeneous minerals,namely orpiment and realgar,in murals as well as the identification of pigments from different layers of a multi-layered simulated mural block made from the two pigments.The study utilizes LIBS and Raman techniques,providing a reference and guidance for the analysis and identification of other homogenous but heterogeneous mineral pigments of different colors in murals.