Analysis and study of corrosion products of copper yak in the Yuan Dynasty
The author of this article: Zhao Jiaying, Deputy Research Librarian, National Museum of China.
Published in Cultural Relics, No. 7, 2011.
The "4 Figures 4 Tables" shown in the article showing experimental data are deleted because they are more professional and do not affect reading.
In 1976, a Piece of Yuan Dynasty Copper Yak, 118 cm long, 77 cm high and 75 kg, was excavated in Youai Village, Haxi Town, Tianzhu County, Gansu Province, and was collected by the Tianzhu County Cultural Center in June 1980 as a national first-class cultural relic. When the relevant departments copied this copper yak in 1995, after the side of the copper yak was turned over, it was covered with a layer of gray-white and light green and thick 2-3 mm hard knot on the side of the mold because it was not cleaned, the ears, neck, buttocks, tail and other places of the copper yak were covered with light green tumors, there was layered rust accumulation on the back, back, hind legs, etc., there were hard knots at the ankles, flying joints, hooves and knees, and the degree of corrosion in the cheeks, nose and forehead was different.
Copper yak before restoration
1. Disease analysis and detection
Before repairing the copper yak, we conducted a comprehensive analysis of it, systematically understood its material, buried environment, corrosion products and related diseases on the basis of scientific analysis, and formulated a restoration plan for it with the support of test data. Systematic analysis consists of chemical wet analysis and instrumental analysis.
(1) Qualitative analysis of corrosion products silver nitrate
Qualitative analysis of silver nitrate is an effective detection method for identifying aggressive ions in rust, we conducted a qualitative analysis of 14 different color surface rust samples of copper yak, and the analysis results showed that most of the rust contained aggressive chloride ions (Table 1).
(2) Ion chromatography analysis
The application ic to analyze the anions in the soluble salts in the surface cements of copper yak can understand the distribution of aggressive ions in the soluble salts in the surface cements, and the analysis results show that the detection samples contain aggressive Cl_, SO42_ and NO3_, and the analysis spectra are shown in Figure 1 (omitted).
(3) Analysis of matrix components of portable energy spectroscopy cultural relics
The analysis of the copper yak matrix using portable energy spectroscopy can understand the texture and alloy ratio of the matrix in various parts. The analysis of the substrates of 13 parts of the copper yak shows that its matrix is a copper, tin and lead ternary alloy, and the content of copper, tin and lead in different parts is quite different (Table 2 Omitted).
(4) Analysis of XRF components of corrosion products
X-ray fluorescence microscopy can analyze not only the composition of metal elements commonly found in bronzes, but also the aggressive elements (chlorine, sulfur) in rust. By understanding the composition of different corrosive product elements on the surface of copper yaks and the distribution of aggressive elements on them, effective protection methods can be developed. In order to fully understand the composition of the elements and phases in the corrosion products of the copper yak, we used X-ray fluorescence microscopy to analyze the elements and structure of the 14 rusts (the same sample in duplicate) on it, and the XRF analysis results showed that the content of calcium and sulfur in the white cement at the side of the copper yak was as high as 95%, and the dark green corrosion products in the ear contained a large amount of chlorine (table 3), and the analysis spectrogram was shown in Figures 2 and 3 (omitted).
(5) Analysis of XRD components of corrosion products
With the help of X-ray diffraction, it is necessary to conduct phase analysis of different levels and colors of rust of cultural relics, and the analysis results can directly provide harmful rust and harmless rust phase composition, providing technical support for the formulation of cultural relics protection and restoration programs. The XRD analysis results show that the corrosion products on the copper yak are composed of hematite, green salt copper ore, water gall alum and malachite (Table 4), and the analysis spectra is shown in Figure 4 (omitted).
2. Summary
Before the protection and restoration of the copper yak, we used different detection methods to systematically analyze it, and the analysis results are as follows.
The qualitative analysis results of silver nitrate show that most of the rust samples contain aggressive chloride ions, and the experimental phenomenon indicates that the acidified specimen dissolves without white precipitation at all, and the degree of turbidity of the solution after adding the silver nitrate reagent has a certain relationship with the color and generation state of the rust.
The results of ion chromatography showed that the inhydrated liquid of the cement on the surface of the copper yak was weakly alkaline, with a pH of 7.6, and the sample contained Cl_, SO42_ and NO3_ aggressive ions.
The test results of portable energy spectroscopy on the substrate after the rust removal of copper yak show that the matrix is a ternary alloy of copper, tin and lead, and the content of tin and lead is high, and the content of copper, tin and lead elements in different parts of the matrix is different.
In the copper yak rust samples detected by X-ray fluorescence microscopy, the main elements were Cu, Sn, Pb, Fe, Ca, CI, S, and the contents of calcium and sulfur in the white cement on the side of the mold were 55.06% and 40.62%, respectively, and chlorine was detected in diseases such as tumors.
X-ray diffraction analysis showed that cu2Cl(OH)3Cu4 was contained in the light green rust, Cu2(CO3)(OH)2 was the main phase of the green rust, and harmful substances were contained in the black rust. The main phase of the white cement about 3 mm thick on the side of the copper yak is Ca(SO4)(H2O)2 gypsum, and it is speculated that the formation of this corrosion product is related to the gypsum and release agent used in replication. When turning the mold, first coat a layer of soft soap on the surface of the utensils, that is, sodium stearate (sodium stearate has good permeability, soluble in water or alcohol, the solution is alkali due to hydrolysis), and then the gypsum is applied to its upper mold, and after the mold is turned into a mold, because the operator has not cleared it clean, it is stored on the side of the copper yak; in the following decades, gypsum and sodium stearate as a carrier have been attached to the surface of the copper yak, and the physicochemical properties of its own residue and the constantly changing preservation environment make the copper yak side corrosion seriously. Surface cementitious matter and heavy rust are all over the mold.
III. Conclusion
The content of tin in copper yak rust is negligible, and most rust samples contain aggressive chloride ions. The aqueous solution of the cement on the surface of the copper yak is weakly alkaline and contains aggressive Cl_, SO42_ and NO3_. The copper yak matrix is a high tin bronze alloy. The main elements in copper yak rust corrosion are Cu and Pb, and the content of chloride ions in samples collected at pitting, stratification and vesicles is high. The main phases of copper yak rust are copper oxide (hematite), basic carbonate (malachite), basic chloride (green salt copper ore), and basic sulfate (water gall alum).
In the restoration of the copper yak in the Yuan Dynasty, we used scientific detection methods to systematically analyze its related diseases, and under the premise of organically combining with practical experience, we proposed a practical restoration plan and successfully restored this national first-class cultural relic.
Restored copper yak