Analysis of Mineralization and Disease Characteristics of Bronzes Unearthed in Archaeology

Bronze artifacts unearthed by archaeologists are affected by underground burial environment and other factors, and most of them have different degrees of corrosion. Bronze artifacts unearthed by archaeologists have different degrees of mineralization. Establishing a data analysis database for basic structure and disease analysis of mineralized bronze artifacts is a necessary prerequisite for developing targeted reinforcement materials and realizing accurate protection. Based on the characteristic analysis of the mineralized bronzes, this paper systematically analyzed the mineralization and disease characteristics of the mineralized bronzes by Confocal Super Depth-of-Field Microscope(CSDTM), scanning electron microscopy (SEM), energy spectrum analysis(EDS) and XRD, and compared and analyzed the microscopic morphology, composition and phase structure of the bronzes under the same burial environment, so as to provide support for subsequent targeted protection and reinforcement.


Introduction
After thousands of years of burial, archaeological bronze ware has been gradually damaged or deteriorated by the erosion of various factors in the buried environment, which is usually called "corrosion" or "mineralization" [1].Metal corrosion is considered to be destructive erosion caused by chemical or electrochemical reactions on the surface of materials and the surrounding environment [2].However, some scholars believe that bronze corrosion is a process in which bronze alloys are eroded by environmental media, copper, tin, lead, etc., are transformed into compounds and reduced to minerals [3].That is, generalized mineralization is the concept of a corrosion process characterized primarily by the gradual transformation of bronze metal alloys into nonmetallic minerals.
The narrow sense of bronze mineralization usually refers to a kind of corrosion disease of bronze.According to the definition of the disease of "whole body mineralization" in the cultural relics protection industry standard "Bronzes Diseases and Illustrations", the whole body mineralization refers to the overall mineralization of bronzes is loose and brittle due to excessive corrosion degree [4]."Loose and brittle" is the main characteristic of high mineralization of bronzes, this expression is relatively vague, and there are many terms around mineralization, such as "fragile bronzes", "stable highly mineralized bronzes", "tin fossilization" and so on.Wan believes that mineralized bronzes refer to bronzes without any physical properties of metal, which are characterized by loose body from the outside to the inside, easy to break off powder, and easy to break under force, and can also be called fragile bronzes [5].
Zhang et al. believe that the degree of rust accounts for more than 50% of the volume, and the brittness is serious, but most of the shape and pattern are still maintained, and no powder rust appears, which can be called stable highly mineralized bronzes [6].Using density as a measurement index, Jiang et al. concluded through data analysis that when the density index is less than 7, there is a possibility of vulnerability, and when the density index is less than 3, the vulnerability reaches 100% [7].In 2014, the State Administration of Cultural Heritage promulgated the national standard "Diseases and Diagrams of Bronze and Iron Cultural Relics in the Collection".The new standard made some adjustments to the terms and definitions of diseases, and "mineralization" was defined as "the corrosion phenomenon in which bronze and iron cultural relics retain their original surface and lose their rigidity" [8].Stiffness refers to the ability of a material or structure to resist elastic deformation when it is stressed, which can be manifested as structural strength on bronze cultural relics, and the loss of rigidity means the decline of structural strength.In combination with the national standards and relevant scholars' research, this paper will more specifically describe the mineralized bronzes as the bronzes that retain the original surface, the alloy matrix is seriously corroded, or even completely corroded, the physical properties of the metal are damaged, the structural strength is reduced, and the brittleness is serious.Bronze corrosion mineralized products can only be characterized as minerals after mineralogical and geological analysis to confirm that they can correspond to naturally occurring minerals.Based on published research data [9].The main corrosion mineralized products of bronze ware are copper oxide, tin oxide, lead oxide, basic carbonate, chloride and basic chloride.
At present, the research on the corrosion causes of mineralized bronzes is still in the exploratory stage [10].There are few researches on the application of reinforcement materials for mineralized bronzes, and the research and development of reinforcement and protection materials for mineralized bronzes needs to be targeted based on the basic structure and performance characteristics of mineralized bronzes.Therefore, this paper summarizes the main characteristics of mineralized bronzes by testing and analyzing the samples of mineralized bronzes and combing the characteristics of mineralized bronzes unearthed in archaeology.The corrosion mineralization products and structure of bronze ware were analyzed, and the mineralization and disease characteristics of mineralized bronze ware in the same burial environment were mainly compared and analyzed, which provided data and theoretical basis for the subsequent research and development of reinforcement materials and reinforcement technology.

Mineralized bronze samples
Mineralized bronzes analysis and detection samples were from 3 typical mineralized bronzes with almost complete corrosion of alloy matrix, among which samples Z1, Z2 and Z3 were from Han Dynasty tombs in Chenxiao Village, Huojia County, Xinxiang, Henan Province.Details of the samples are shown in table 1. Experimental conditions: 20 times grating 1200l/mm eyepiece, power 0.5-50W, excitation light source 532nm, 633nm.

Results and Discussion
3.1 Microstructure analysis of samples Z1, Z2 and Z3 were placed under ultra-depth microscope and scanning electron microscope to observe the specific characteristics of the surface and cross section.
As shown in figure 1, the surface of Z1 sample is densely covered with green corrosion mineralites, among which there are reddish-brown and blue corrosion mineralites (figure 1a).An obvious layered structure was observed in the section, which can be divided into two layers.The surface layer is a blue-green mineralized product layer, the inner layer is a red mineralized product layer, and the inner layer is partially mixed with light yellow material (figure 1b).The surface corrosion mineralized products were relatively loose under electron microscope (figure 1c).The section has few holes and is relatively dense (figure 1d).Z2 samples were mainly green corrosion mineralized products with a small amount of yellow and dark green materials (figure 2a).The visible stratified structure is observed in the cross section, which is mainly divided into two layers, the surface layer is green mineralized layer, and the inner layer is red mineralized layer (figure 2b).Under electron microscope, the surface corrosion mineralized products are loose and there are a lot of pores (figure 2c).The cross section has fewer pores and is relatively dense (figure 2d).
Under the ultra-depth of field microscope, it can be clearly observed that the surface of Z3 sample is mainly green corrosion mineralized products, and a large number of holes and cracks are densely covered (figure 3a).An obvious layered structure can be observed in the cross section, which is mainly divided into two layers.The surface layer is the green mineralized product layer, and the inner layer is the red mineralized product layer.The boundary of the corrosion mineralized layer can be clearly observed (figure 3b).It was observed under electron microscope that the structure of surface corrosion mineralized products was relatively loose and there were a large number of cracks (figure 3c).There are a large number of small holes in the section (figure 3d).

Component analysis
SEM-EDS was used to detect each mineralized bronze, and on the basis of the morphology observation information, energy spectrum analysis was used to deeply understand the differences in the composition and proportion of elements on the surface and cross section of mineralized bronze samples with different morphology characteristics.
As shown in figure 4, the green surface layer of Z1 was rich in copper compounds, no tin and lead elements were detected, and a small amount of soil elements such as silicon, aluminum and iron were present.The results of sectional composition analysis show (figure 4b) that copper content is very high in the red area of the inner layer, tin element is not detected, and there is a small amount of lead element.The inner red mixed light yellow area contains higher copper and lead elements, and a small amount of tin elements are detected.No chlorine was detected in surface and section composition analysis.The results of surface and cross section energy spectrum analysis are shown in table 2.  The results of surface composition analysis of Z2 (figure 5a) indicate that the green outer layer is rich in copper compounds, with a significant amount of copper and a small amount of lead elements, while tin elements were not detected.The results of cross-section composition analysis (figure 5b) show a higher content of copper elements in the red inner layer, with a considerable amount of lead elements and a small amount of tin elements detected.Lead is more prevalent in the near surface transition zone, whereas the proportion of copper elements decreases from the inner to the outer layers.Tin elements are mainly distributed internally.Chlorine elements were not detected in both the surface and cross-section analyses.The results of surface and cross-section energy spectrum analysis can be found in table 3. The surface composition analysis results of Z3 indicated (figure 6a) that the green surface layer was rich in copper, and no tin and lead elements were detected, and a small amount of soil elements were contained.The results of sectional composition analysis (figure 6b) show that the red inner layer has a higher content of copper, more lead and a small amount of tin.The content of Pb was the highest in the transition zone.Tin is mainly distributed in the interior; No chlorine was detected in surface and section.The results of surface and cross section energy spectrum analysis are shown in table 4.

Phase analysis
The different corrosion mineral layers of various samples were detected using an X-ray diffraction analyzer (XRD).Based on the analysis of spectral composition, the phase composition of various mineralized products in the structure was further identified.The results of the XRD analysis are shown in table 5.The XRD spectra are shown in figures 1-6.

Conclusion
Z1-Z3 samples are all fragments of copper coins of the Han Dynasty.The surface corrosion and mineralization products are relatively loose, and the Z1 section has few holes and is relatively dense.The corrosion mineralized structure can be divided into two layers, the surface is blue-green, mainly malachite and azurite, and red chalcopite is mixed locally.The inner layer is red, mainly chalcopatite, and partially mixed with white lead ore.A small amount of tin was detected in the inner layer of the sample, which should contain a small amount of corrosion mineralized products of tin.No chlorine was detected in the sample and no harmful rust was found.Sample Z2 has a large number of pores, few cross section pores, relatively dense.The corrosion structure can be divided into two layers, the surface is green, mainly malachite; The inner layer is red, mainly copper; The transition zone between the inner layer and the surface layer contains more white lead ore.A small amount of tin was detected in the inner layer of the sample, which should contain a small amount of corrosion mineralized products of tin.No chlorine was detected in the sample and no harmful rust was found.There are a large number of small holes in section Z3.The corrosion structure can be divided into two layers, the surface is green, mainly malachite; The inner layer is red, mainly copper.The transition zone of the inner and outer layers and the inner layer have more lead elements, which should contain a small amount of lead corrosion mineralization products.No chlorine was detected on the surface and section, and no harmful rust was found.

Figure 6 .
Figure 6.EDS analytical position of: (a)Z3 Surface; (b)Z3 cross-section Energy spectrometer: EDAX APOLIO-X type energy spectrometer.Experimental conditions: point scan, surface scan.Ultra depth of field microscope: Experimental conditions: The radiation source is Cu/Kα, the wavelength is 15.4nm, the voltage is 40kV, the current is 40mA, and the diffraction measurement range is 10 ~ 70°.Laser Raman spectrometer: British Renishaw company INCIA-REFLEX laser Raman spectrometer.
2.2 InstrumentsScanning electron microscope: QUANTA-650 environmental scanning electron microscope of FEI company.Experimental conditions: high vacuum mode; The operating voltage is 25 kV; Secondary electron image and backscatter image.

Table 3 .
Composition analysis of Z2

Table 4 .
Composition analysis of Z3

Table 5 .
phase analysis of the mineralized bronzeware samples