First, LIBS technology and its advantages
LIBS (laser-induced breakdown spectroscopy) is a laser induced breakdown elemental spectroscopy technique. The principle is as follows: the laser pulse is concentrated on the surface of the sample (solid, liquid) or internal (gas, liquid), and a very small amount of sample is ablated to generate a laser-induced plasma (LIP). Detecting LIP light radiation and performing spectral analysis to obtain qualitative and quantitative information on the chemical composition of the sample - each chemical element has its own characteristic line, so the detected LIP spectrum is the so-called "chemical fingerprint" of the sample.
Schematic diagram of LIBS technology
Therefore, LIBS has more unique advantages than traditional elemental analysis techniques (such as ICP acidification digestion): the instrument is simple to operate, and the result is obtained in 1 second; real-time online analysis is possible for solid, liquid, and gas state samples. It can perform element space distribution analysis and profile distribution analysis; quasi-no-damage measurement; no sample pretreatment is required; one measurement can simultaneously detect, qualitatively and quantitatively analyze almost all elements in the periodic table; enable in-situ telemetry.
Therefore, for gemstones, antique artifacts, ancient buildings and other samples, LIBS technology can qualitatively, quantitatively and spatially analyze the elements without damage to the samples, thereby identifying their composition, age, process, maintenance, environmental effects, Origin, impurities and authenticity are rapidly gaining popularity in this field.
Second, the application plan and case
1. LIBS technical intervention in the repair process of the bust of St. Gregory
S. Castromediano, the provincial museum in the ancient city of Lecce, Italy, was ordered to repair the bust of St. Gregory. In order to determine the corrosion and determine the most suitable recovery plan, the museum applied the LIBS technology to carefully study the elemental composition of the various parts of the bust.
Repair the previous bust of St. Gregory (1716) |
The bust material is precious. Experts speculate that the face, hands and hair are made of silver; the cloak is silver foil covering the wooden conical surface; the Mitra and the cross are brass, and the shawl is made of copper and decorated with gold-plated copper. A glass sacred box is placed in the center of the shawl. Its silver, copper and brass parts have varying degrees and forms of corrosion. Based on the residue on the bust, the expert concluded that the bust had been repaired before.
Prior to this restoration, experts applied LIBS technology to analyze their hair, clothing, streamers and coatings. From the measurement line, the bust of the bust is not the silver that was previously thought, but the traditional silver alloy, silver-copper alloy.
In order to detect the main body of the ribbon and the coating (preferred to be gold-copper, ie copper-zinc alloy) elements, the inner and outer sides were measured by LIBS technique. The spectral line data shows that the main part of the ribbon is copper. Very interesting and unexpected, the coating part is completely free of zinc, but gold – that is, not the brass that people used to be, but gold!
LIBS spectrum of the bust at the surface of the hair (wavelength range: 250–560 nm)
In addition, by comparing the intensity of the LIP emission lines of the silver and copper materials in the entire bust, the compositional contents of silver and copper in the alloy are substantially equal. Also, unlike the assumption, the foil has not been used throughout the process of making the bust.
(a) St. Gregory's bust profile of the copper body main surface at the same position LIBS continuous excitation (b) St. Gregory's bust silver hair surface layer LIBS continuous excitation profile spectrum
By continuously measuring at the same location, the surface corrosive composition and corrosion depth can be known. For example, by measuring the surface profile of the continuous excitation of the ribbon, it is found that the main component of the corrosion is Ca. The main elements of corrosives in the hair are Ca and Al, but their lines disappear after several successive excitation measurements, so experts concluded that the impurities are from the air. Another expert hypothesized, or can come from the water---religious rituals, people often give holy water to the icon.
Case 2 : LIBS elemental analysis technology detects the repair of ancient weapons
Source: S. Acquaviva a, ML De Giorgi a, C. Marini b, R. Poso b. Elemental analyses by laser induced breakdown spectroscopy as restoration test on a piece of ordnance. Journal of Cultural Heritage 5 (2004) 365 – 369
A. Before cleaning; B after cleaning; C after cleaning details |
The gun in this case was discovered in the Adriatic Sea in the 1970s and later preserved in the provincial museum S. Castromediano in the ancient city of Lecce, Italy. Due to long-term immersion in seawater, it has been tightly wrapped by marine sediments, salt weathering, metal oxides and the like. Experts have concluded that the gun may have come from the merchant ships of Genoa in the 16th and 7th centuries. After a merchant ship sank in a trade voyage, it was gradually eroded and destroyed by long-term immersion in seawater. The damage usually depends on the soaking water quality, air humidity, radiation and air pollution in the environment where the cultural relics are located. The salt in the seawater attacks the metal crystals and chemically reacts to form other substances, which is the main reason for the corrosion of the gun surface.
When repairing it, the LIBS elemental analysis technique was used to analyze the seawater deposition shell composition and corrosion type of the gun. Further experiments were carried out based on the experimental results to determine the cleaning agent, cleaning tool and protective material, and repair the surface layer. The repair steps are as follows: (1) Mechanically remove the seawater corrosion layer on the surface of the gun and restore the original shape of the gun. (2) Clean with a special chemical reagent such as bronze or iron to remove the seawater corrosion layer. (3) Prevent corrosion afterwards. (4) Surface protection to avoid contact with air. Prior to (3), the surface material of the gun was analyzed using LIBS technology to determine if there was still rust residue remaining.
The LIBS line of the surface material of the gun. Gray is before repair. White for repair
It can be seen from the emission line that the harmful elements are mainly calcium and magnesium, so it can be used as a key parameter in the repair process to confirm whether the seawater corrosion and deposits on the gun surface are effectively cleaned.
In short, LIBS technology can not only detect the elemental composition and distribution of cultural relics, but also tell us about its natural corrosion state and repair. Help us find the most suitable repair method and evaluate its effectiveness. In this case, the cleaning method of the gun surface is successful - it indicates the parameter calcium, which has a clear line before cleaning and disappears after cleaning.
Case 3 : LIBS elemental analysis technology applied to historical document paper maintenance
Source: A. Kaminska a, M. Sawczak b, K. Komar b, G. S Ì liwin Ìski b, Application of the laser ablation for conservation of historical paper documents. Applied Surface Science 253 (2007) 7860–7864
LIBS spectral data for laser cleaning of 17th document paper:
(a) the first laser cleaning laser pulse; (b) the second laser cleaning laser pulse; (c) the third laser cleaning laser pulse;
Repair historical documents such as wood pulp paper in the 15th and 6th centuries. If water or drying method can only remove some stains, it will cause discoloration of the paper. In the past 20 years, laser cleaning technology has achieved rapid development in paper protection applications. This technology can be targeted for paper texture and stains, accurately removing stains, especially for old, fragile documents. Laser spectral element analysis techniques such as LIBS and LIFS have been successfully applied to the research of laser cleaning technology---in the laser cleaning, spectral analysis of laser ablation plasma, determination of samples and their stains, pigments in samples, surface The distribution of impurities and spots in the spots and the laser cleaning effect, and can determine the effect of reagent cleaning and physical cleaning on the surface of the sample.
Blue pencil traces on the back of the 16th century Leopolita Bible: (A) before laser cleaning;
(B) After laser cleaning and its corresponding LIBS spectrum: (a) color traces; (b) spectra after cleaning
In this case, laser cleaning technology and LIBS elemental analysis techniques were used to measure the stains (dust, color points) and pigments of the 14th century paper surface and the blue pencil traces on the back of the Leopolita Bible.
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