Application Cases | Application Of Raman Spectroscopy In Identification Of Cultural Relics

Application Cases | Application Of Raman Spectroscopy In Identification Of Cultural Relics

1. Introduction

Due to factors such as age, preservation environment, material characteristics, etc., many cultural relics are in decay and serious damage. How to accurately and non-destructively identify the material of cultural relics is the basis and important work of cultural relics protection. At present, most of the material analysis of cultural relics focuses on inorganic substances, and less involves the analysis of organic substances. For example, X-ray fluorescence analysis and X-ray diffraction analysis are common methods for analyzing the composition and structure of inorganic cultural relics. However, the identification and characterization of organic matter in cultural relics is still a difficult point in this field. Although Fourier transform infrared spectroscopy technology is often used to identify organic compounds in cultural relics, the detection needs to be carried out in a specific environment, and the results are not single-directional, making it difficult to accurately identify organic components in complex cultural relic systems. Chromatography-mass spectrometry technology has higher sensitivity and excellent separation effect than infrared spectroscopy technology, but the pre-treatment processes such as sample extraction, hydrolysis, and derivatization are lengthy and cumbersome. Raman spectroscopy technology is not only used for the analysis of inorganic cultural relics such as paints, metals, and ceramics, but is also increasingly used for the analysis of organic dyes, paint glue, residues and other organic cultural relics.

2. Advantages of Raman spectroscopy technology for identification of organic matter in cultural relics

Raman spectroscopy is an analytical technology that is based on the analysis of scattering spectra with different frequencies from the incident light to obtain relevant information such as molecular vibration and rotation, and is applied to the study of molecular structure. As a kind of "fingerprint spectrum", Raman spectroscopy has the characteristics of simple and fast measurement in the identification of organic matter in cultural relics, and can realize non-destructive and micro-area analysis. It has become a common method for studying the material structure of cultural relics.

2.1 Non-destructiveness

The ideal cultural relic analysis technology should be based on maximizing the amount of information obtained from cultural relics and minimizing damage to cultural relics. Raman spectroscopy testing requires a very small amount of sample (can be as little as ~1 μg), and can even be done without sampling, which is especially suitable for the analysis of precious cultural relics that cannot be sampled. With the application of portable Raman spectrometers in on-site analysis of cultural relics, real-time, in-situ non-destructive testing of cultural relics has been realized. For example, Perez-Alonso used a portable Raman spectrometer to non-destructively analyze the materials of Spanish murals in the 16th century, and obtained comprehensive information on the use of mural pigments. He also detected the presence of corrosion products such as calcium oxalate and calcium sulfate, which provided scientific basis for the evaluation of the later preservation status of the murals. Although portable X-ray fluorescence spectroscopy technology has been used for online non-destructive analysis of cultural relics, this method can only obtain relevant element information of cultural relics, and the test results are easily affected by instrument sensitivity and on-site testing conditions.

2.2 Micro area detection

Raman spectroscopy has high spatial resolution. When characterizing the material of painted cultural relics, it can effectively avoid the mutual interference of pigment particles in adjacent areas, thereby accurately identifying each single component in the mixed pigment. In particular, the combination of microscopic analysis technology and Raman spectroscopy can reduce the laser detection spot to the order of microns, directly and flexibly obtain the Raman spectrum information of a specific area of the sample, and realize the micro-area detection of cultural relic samples. In the field of cultural relic analysis shows great vitality. For example, the Raman spectrum results of painted pottery from the 6th to 7th centuries BC unearthed in Turkey not only provide information on the material of the cultural relics, but also detect anatase, hematite and other substances in the micro-region of the matrix, which further reflects the firing environment of the painted pottery. This result has important guiding significance for the study of the firing technology of early Turkish painted pottery.

2.3 Easy to measure

When detecting and analyzing with Raman spectroscopy technology, there is no need for sample preparation or complex sample pretreatment, and the sample is directly placed under the detector. This method is simple and fast, and it only takes a few minutes to obtain relevant information of the tested object. In contrast, chromatography-mass spectrometry analysis is cumbersome and time-consuming, generally taking several hours.

3. Application of Raman spectroscopy technology in identification of organic matter in cultural relics

3.1 Dye identification

Natural dyes have been used in murals, textiles and other cultural relics for a long time due to their bright colors, easy availability of raw materials, and wide range of applications. Therefore, through the analysis and research of dyes related to cultural relics, it will help people to more accurately understand the production process, raw material sources and dyeing techniques of cultural relics to obtain relevant information such as the living environment and technological level of ancient ancestors, which will provide important information for archaeological and historical research.

The blue color in many architectural paintings of the Forbidden City, a World Heritage Site, is the indigo used on a large scale when initially painted. This result has an important guiding role in later research on the fading and discoloration mechanisms of architectural paintings and their restoration and protection. Similarly, indigo also has a long history of use abroad. Andreev detected that the blue dyeing materials on textiles from the 2nd to 9th centuries included indigo, Mayan blue and Prussian blue. Indigo has been used as a dye as early as in ancient Egypt. Mayan blue only began to be used in the 1st century BC, and Prussian blue was first synthesized in 1704, making it the latest among the three dyeing materials. Therefore, the identification of dyes on cultural relics is helpful for the dating analysis of cultural relics. Although indigo has been used as a dye for textiles since ancient Egypt, it is rarely detected in Egyptian murals. Abdel-Ghani used Raman spectroscopy combined with energy spectroscopy for the first time to discover that indigo was found on Egyptian panel paintings from the 18th century. He also deduced that indigo came from the genus Indigo instead of the genus Woad, which provided relevant information on the source of the material of the cultural relics. The purple dye on textiles unearthed from the Athens cemetery in the 5th century BC (Figure 1) was analyzed and determined to be dibromoindigo (C16H8N2O2Br2) extracted from the body of the mollusc snail. It has common names: Tyrian purple and noble purple. It first appeared in the Mediterranean basin. It was a symbol of the aristocratic class at that time. The discovery of this dye further confirmed the special status of the tomb owner.

Figure 1 Raman spectra of purple dye fragments (a) excavated objects and contemporary fibers (b).

3.2 Textile material research

As a symbol of the progress of human civilization, textiles undoubtedly demonstrate the wisdom and talents of the ancients. However, textiles are prone to aging and deterioration, which greatly affects the preservation and inheritance of their historical and cultural value. Raman spectroscopy technology has been applied to the identification of fiber types, preservation status and aging research of textile cultural relics, providing a reference for the later selection of the preservation environment of textile cultural relics museums, which has important guiding significance for the scientific preservation and protection of this type of cultural relics.

The fluorescence effect produced by the aging and degradation of textile cultural relics interferes with the measurement results to varying degrees, even making the Raman spectrum peak of flax masked and unable to be accurately identified, while cotton fiber is less affected by fluorescence interference. Fiber aging will also cause the loss, shift and width change of the Raman spectrum peak. For example, after the mulberry silk simulated sample is irradiated with UV light, the 1232cm-1 amide III Raman peak shifts to 1218cm-1. At the same time, the intensity of the 1666cm-1 Raman peak weakens significantly, and in the characteristic Raman frequency range of amide III in the secondary amide association molecule, two peaks at 1259cm-1 and 1297cm-1 appear, reflecting the α-helical conformation of amide III. (Figure 2). The above changes indicate that the silk has undergone structural changes under UV irradiation. When Decai Gong studied the corrosion mechanism of three pieces of silk (numbered YZ, LA, and JZ) buried underground more than 2,000 years ago, he found that although the Raman spectrum peaks of the actual cultural relics were basically consistent with those of modern standard silk, two special peaks D and G were added. (Figure 3), which is caused by carbonization of silk. On this basis, combined with the results of electron paramagnetic resonance spectroscopy, it was concluded that the carbonization process of silk is based on the release of free radicals from silk protein.

Figure 2 Raman spectra of silk without (a) and after 23 hours (b) and 47 hours (c) UV irradiation.

Figure 3 Raman spectra of modern standard silk and ancient silk (No. YZ, LA, JZ).

3.3 Identification of painted glue materials

Glue is the binding substance of the painted layer of cultural relics. Its function is to tightly bind the pigment or dye to the surface of the cultural relic matrix. Most of the glue used for painted cultural relics is a natural organic substance, which is susceptible to aging and loss due to environmental factors, causing the paint to fall off and pulverize, which is a key factor affecting the stability of the cultural relics and their artistic beauty. Therefore, the identification and research of glue materials for ancient painted cultural relics have attracted more and more attention from scholars. However, the cultural relics rubber is seriously aged, has complex composition, and is in a system that coexists with a large number of pigments, so it is very difficult to accurately identify it.

Vandenabeele systematically established a Raman spectrum database of standard glue materials for painted cultural relics at the end of the last century. The types of glue materials include proteins, polysaccharides, fatty acids, resins, etc. It is a relatively comprehensive Raman spectrum database of organic glue materials for painted cultural relics, on this basis, the glue material of medieval manuscripts was studied and it was determined that the glue contained beeswax. Raman spectra of gold and silver ink in religious manuscripts from the 10th to 18th centuries of the Byzantine Empire show the presence of terpenoids, suggesting that frankincense may have been used as a glue. However, the Raman spectral characteristics of pigments for painted stone carvings in the 13th century indicate that resin was used as glue. material.The Fourier transform Raman spectrum of the shuttle rubber material in a French art collection shows that there are Raman vibration peaks of C-H bonds at 1305cm-1, 1445cm-1 and 2900cm-1, the vibration peak of C=C is at 1602cm-1 , consistent with the vibration peak of linseed oil. Linseed oil may be used as a glue, and the presence of organic dyes such as alizarin and carmine has been detected.

The research team used confocal Raman microscopy to analyze common natural organic compounds on the surface of ancient Chinese painted cultural relics, including hide gum, peach gum, beeswax, etc. (see Table 1). The results show that: (1) The characteristic vibrations peaks of protein natural organic compounds are located near 1657cm-1, 1305~1252cm-1, 1033cm-1 and 1003cm-1; (2) The characteristic vibration peaks of polysaccharides are few and only exist wider vibration peaks in the 1500~1200cm-1 and 1200~950cm-1 regions. (3) The characteristic vibration peaks of waxes are in the range of 1470~1350cm-1. If vibration peaks appear at 1659cm-1 and 1303cm-1, the wax is animal wax. If vibration peaks appear at 1636cm-1 and 1610cm-1, the wax is a vegetable wax. (4) The resin has a characteristic vibration peak in the range of 1650~1660cm-1, and a strong vibration peak in the range of 1460~1440cm-1.

Table 1 Raman characteristic peaks of common natural organic compounds on the surface of ancient Chinese painted cultural relics.

3.4 Organic residue analysis

Raman spectroscopy has been used in the analysis and identification of organic residues in cultural relics, providing people with useful information such as the functions and production processes of cultural relics. Edwards used Fourier transform Raman spectroscopy to analyze the yellow residue on the outside of Canadian Inuit leather boots (Figure 5). The results showed that the main component of the yellow sediment was pinaceous resinous substances, inferring that it may have been caused by early causes. Inuit used it as a waterproofing agent. Interestingly, this type of rosin substance is not produced in Inuit residential areas. Therefore, the application of resin as a waterproofing agent is of great significance to the study of Inuit social exchanges and trade. Analysis of the residue on the mouth rim of a jar unearthed from a Vietnamese tomb 2,300 years ago shows that the residue contains terpenoids, the composition of which is similar to abietic acid and rosinic acid, but its composition has been partially changed due to aging. Although the unearthed incense has aged to a certain extent, its Raman spectral characteristics can still better reflect that the main component is frankincense. The Raman spectrum of the underlying material of the Mesopotamian pottery paintings in the two river basins (Euphrates and Tigris) has a characteristic peak of fatty acids at 1870cm-1. It is inferred that oil may be used as the underlying material of the paintings.When Raskovska was studying the firing temperature and glaze composition of glazed pottery fragments unearthed from the Republic of Macedonia, she was surprised to find that sample 16-2 (Figure 5a) had a strong Raman spectrum scattering peak near 1400cm-1 (Figure 5b), inferring that these are traces of organic residues, which may be oleic acid substances. It is further inferred that 16-2 glazed pottery may be used as a container for storing food or a cooking vessel.

Figure 4 Residues on the outside of leather boots.

Figure 5 Ceramic samples and their Raman spectra.

5 Conclusion

As an ideal cultural relic analysis method, Raman spectroscopy technology can quickly and accurately obtain structural information of cultural relics without sampling or with less sampling. It has obvious advantages in the identification of organic dyes, painted glue and residues of cultural relics. The advantage is that it realizes the non-destructive or minimally destructive identification of cultural relic materials. The future development direction of Raman spectroscopy technology in the identification of organic matter in cultural relics: (1) Elimination of fluorescence interference: Due to the aging of some cultural relics themselves, strong fluorescence background interference will be produced, which will have a certain impact on the analysis results. Therefore, the fluorescence interference of Raman spectrum should be eliminated as much as possible to improve the accuracy of analysis results. (2) Combining Raman spectroscopy technology with other separation technologies: On the basis of improving detection sensitivity, the separation and determination of organic matter in complex cultural relics can be completed simultaneously. For example: Thin layer chromatography and Raman spectroscopy are combined for the analysis of amino acid mixtures to accurately obtain the "fingerprint information" of different substances, and complete high-sensitivity separation and structure determination of material components in one go. The emergence and application of these combined technologies will further expand the application scope of Raman spectroscopy technology. (3) Application of remote Raman spectroscopy analysis technology: With the development of optical fiber technology and its combination with Raman spectroscopy technology, testing can be carried out within a range of several meters to hundreds of meters, achieving remote measurement and online analysis of immovable cultural relics.

The following is the pigment spectrum of the ATR8300 Raman microscope test: the Raman characteristic peak of the substance can be clearly detected.