1 Introduction

The metal cultural relics of the past are endowed with a rich source of information about the technological evolution of a country. China has earned its special place in the world as a treasure of archaeological and cultural heritage. Many historical relics gifted with the Thousand Buddha Iron Tower [1], the Square Zun with Four Sheep [2], the Silver dragon-shaped cha [3], the golden staff [4], etc. have been found across the China bearing eloquent testimony to the masterly skills of ancient Chinese craftsmen. Due to the strong activity and easy corrosion of metals, the existing metal cultural relics are faced with the severe challenges of poor preservation status and great difficulty in protection [5, 6]. Therefore, how to scientifically protect metal cultural relics has become the research focus of cultural relics conservators and material workers.

Cleaning is the first step in the treatment and protection of metal relics, which usually refers to the removal of contaminants (e.g. dust, old coatings or burial deposits) and corrosion layers. Traditional cleaning methods include mechanical method, sandblasting method, ultrasonic method and chemical method [7]. Mechanical method refers to the use of physical friction contact principle to remove the attachment and loose rust on the surface of cultural relics with tools. This cleaning method consumes time and energy, and has low efficiency. Sandblasting method applies the injection of metallic or ceramic powder to clean the surface of metal cultural relic, operated in a special sand storehouse, and after sandblasting material surface roughness is larger [8]. Ultrasonic method uses the micro-mechanical shock wave generated by ultrasonic wave to shake off the dirt attached to the surface of metal cultural relics, so as to achieve better cleaning effect and improve the efficiency of rust removal [9]. However, due to the limitation of the interior space of the ultrasonic equipment, this method is only suitable for small metal relics. Chemical method uses chemical reagents to react with the rust layer on the surface of metal cultural relics to achieve the purpose of rust removal. However, some chemical reagents have certain toxicity and corrosion, which may damage the surface of metal cultural relics and pollute the environment [10]. It can be seen that the traditional cleaning methods cannot be effectively guaranteed in terms of cleaning quality, cleaning efficiency and environmental protection because of technical limitations. A new cleaning technology is urgently needed to solve this series of problems. The emergence of laser cleaning technology gives hope to cultural relics conservators.

Laser cleaning is to locate the laser beam to the surface of the metal cultural relic, and remove the pollutants through the radiation effect of the laser to achieve the cleaning effect [11]. Laser cleaning technology has the characteristics of non-abrasive, non-contact, high efficiency and suitable for various materials, and its application in the field of cultural relic protection has attracted increasing attention [12]. To improve the application of laser cleaning technology on metal cultural relics, lots of works have been carried out. Therefore, based on a large number of literature research and long-term tracking of the research trends of laser cleaning technology, this article provides a comprehensive review of the application of laser cleaning technology on metal cultural relics. Firstly, the paper introduces the history and mechanism of laser cleaning technology. Secondly, the paper summarizes the corrosion mechanism of the most commonly encountered metal cultural relics (copper relics, iron relics, silver relics, gold relics) and the application achievements of laser cleaning technology in the field of metal cultural relics. Finally, the paper points out the key problems and the development prospects of laser cleaning technology of metal cultural relics.

2 The history of laser cleaning

Laser was produced and applied in the 1960s, because of its good unidirectionality, high brightness, pure color, large energy and other characteristics, it is widely used in military, medical, industrial and other fields [13]. In the 1970s, American scientist J.F. Asums carried out a series of cleaning experiments on the hard shell pollutants on the stone surface of Venice for the first time by using laser, opening the application of laser cleaning technology in the field of cultural relics protection [14]. However, due to the limitations of laser technology at that time, laser cleaning was not widely used in the field of cultural relics, but only limited to the laboratory stage. Until 1997, the first International Conference on Laser for Art Protection (LACONA) held in Crete became a milestone in the development and dissemination of laser technology in the field of art protection [15]. With the rapid development of laser, its application in cultural relics has been promoted. In Europe, America and other countries, laser cleaning, as a mature technology, has gradually replaced the traditional cleaning method and become the mainstream.

The research of laser cleaning technology in China started late. Since 2003, the Chinese Academy of Cultural Heritage began to pay attention to and devote to the application research work in this field, and has the first laser cleaning equipment in the domestic cultural heritage protection industry [16]. Qi, Mao and Ye et al. [17,18,19] studied the cleaning technology of stone cultural relics and achieved good results. Jiang, Ye and Li et al. [20,21,22] used laser technology to clean metal cultural relics and obtained good cleaning effect. As an ancient civilization with a long history, China has a large number of metal cultural relics that need to be cleaned and protected, and the application prospect in the future is considerable.

3 Laser cleaning mechanism

Laser cleaning is generally divided into two processes, including gasification and vibration, as shown in Fig. 1. Figure 1a shows the gasification process when the laser acts on the rust layer on the surface of the metal cultural relics [23]. The laser can generate a heat source with high energy density. When high-energy laser beam irradiates the rust layer on the surface of the metal cultural relics, due to the different absorptivity of the base material and the rust layer to the laser (see the Table 1 for the common metal laser absorption coefficient), the laser energy is mainly concentrated on the surface rust layer. The temperature rises rapidly and volatilizes when it exceeds its melting point or gasification point, so that the base material can be cleaned [24,25,26,27]. Figure 1b shows the vibration process generated when the laser acts on the rust layer on the surface of the metal cultural relics [23]. As the high-power and high-frequency pulse laser acts, most of the energy generated by the laser is absorbed by the rust layer, and the other part of the energy will be transmitted to the interface between the base material and the rust layer in the form of sound waves. The energy generated at the interface will explode and impact the rust layer on one side of the interface. The impacted rust layer will be broken and separated from the surface of the base material. In the actual process, the two processes usually occur together and complement each other [28]. The most commonly used lasers are Nd: YAG laser and Nd-YVO4 laser [29, 30]. The cleaning methods mainly include dry cleaning, wet cleaning, laser plasma shock wave, inert gas method, non-chemical corrosion method, et al. [31,32,33,34,35,36,37]. The specific content is shown in Table 2.

Fig. 1
figure 1

Schematic diagram of laser processing: gasification process (a); vibration process (b) [22, 38]

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Table 1 The common metal laser absorption coefficient [39]
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Table 2 Action mechanism, advantages and disadvantages of laser cleaning methods [31,32,33,34,35,36,37]
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4 Application of laser cleaning technology on metal relics

In order to achieve the purpose of cleaning, it is necessary to understand the corrosion mechanism of metal cultural relics and the composition of rust. Metal cultural relics are eroded by various factors such as water, oxygen, various anions and cations, and microorganisms in the atmosphere and soil, which can generate layers of rust on the surface [40]. It is a cumulative chemical reaction. The structure of corroded metal artifacts is shown in Fig. 2 [41]. Cultural relics protection workers need to perform laser cleaning on corroded objects to achieve the purpose of protection [42].

Fig. 2
figure 2

The structure of corroded metal artifacts [41]

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4.1 Bronze cultural relics

4.1.1 Corrosion mechanism

Bronze relics are precious historical relics, which are the basis for the study of human history, culture, art and metal smelting. Most of the bronzes were rusted to varying degrees after being unearthed. The corrosion mechanism of bronze relics is complex, which is not only related to the environment and preservation, but also related to the nature of the relics [43]. Take bronze dagger-axe for example, the micrographs are shown in Fig. 3. White, green corrosion products can be observed [44].

Fig. 3
figure 3

The micrographs of bronze dagger-axe [44]

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In the absence of chlorine, copper reacts with oxygen to form red cuprous oxide (Cu2O). Cu2O will generate dark green basic carbonic acid [Cu2 (OH) 2CO3] under the conditions of water vapor and carbon dioxide. Cu2O and [Cu2 (OH) 2CO3] are harmless rust, which can play a definite role in protecting bronze cultural relics [45]. The reaction formula is as follows:

$$\mathrm{Cu }+ {{\text{O}}}_{2}\to {{\text{Cu}}}_{2}{\text{O}}$$
(1)
$${{\text{Cu}}}_{2}\mathrm{O }+ {{\text{CO}}}_{2} + {{\text{O}}}_{2}+ {{\text{H}}}_{2}\mathrm{O }\to {{\text{Cu}}}_{2}{({\text{OH}})}_{2}{{\text{CO}}}_{3}$$
(2)

Once the bronze relics contact with the chloride in the soil or atmosphere, the bronze relics will be further corroded to form white cuprous chloride (CuCl) [46]. In a humid environment, CuCl reacts with water:

$$\mathrm{Cu }+{\mathrm{ Cl}}^{-}\to \mathrm{ CuCl}$$
(3)
$${\text{CuCl}}+{{\text{H}}}_{2}{\text{O}}\to {{\text{Cu}}}_{2}{\text{O}}+{\text{HCl}}$$
(4)

The further reaction is that the generated hydrochloric acid reacts with copper rust:

$${{\text{CuCO}}}_{3}\bullet {\text{Cu}}{({\text{OH}})}_{2}+\mathrm{HCl }\to {{\text{CuCl}}}_{2}+{{\text{H}}}_{2}{\text{O}}+{{\text{CO}}}_{2}$$
(5)

If the rust of bronze relics already contains CuCl2, it will react with H2O and O2 in the air to produce green basic copper chlorid [47]:

$${\text{CuCl}}+{{\text{H}}}_{2}{\text{O}}+{{\text{O}}}_{2}\to {{\text{CuCl}}}_{2}\cdot 3{\text{Cu}}{({\text{OH}})}_{2}+{\text{HCl}}$$
(6)

CuCl and CuCl2·3Cu(OH)2 play a major role in the corrosion of bronze cultural relics, which need to be removed to protect bronze cultural relics [48]. Otherwise, the bronze cultural relic will be pulverized, destroyed, and its life will be shortened. In serious cases, the whole cultural relic will be pulverized or even completely destroyed.

4.1.2 Application of laser cleaning technology

Jiang et al. [49] used Q-switched laser to clean the tripod, ancient coins and ancient bronze mirrors of the Western Zhou Dynasty under different power densities. The experiment proved that Q-switched laser can effectively remove the rust spots and dirt layers on the surface of bronze relics. A thin and dense alloy passivation layer was formed on the treated surface, which has a good protective effect on bronze relics and does not change the natural color of the relics. Mateo et al. [50] successfully used a Q-switched Nd: YAG laser to remove decorative ink and corrosion products from the surface of brass artwork without affecting the surface finish of the artwork. Buccolieri et al. [51] used a UV laser to clean an outdoor copper bell, dating from the second half of 600, and performed nondestructive analysis on the bell jar with an EDXRF portable device before, during and after the cleaning process to assess changes in concentrations of sulfur, chlorine, calcium, copper, lead and tin during the laser cleaning process. It was found that the concentration of tin and lead increased, the concentration of sulfur and calcium gradually decreased, and the concentration of copper almost unchanged. The experimental results show that UV laser cleaning technology is an effective tool, which can achieve a better cleaning of the rust on bronze objects. DiFrancia et al. [52] carried out laser cleaning on two Roman Imperial archaeological bronze coins that had been buried in the soil for a long time. One was characterized by stable corrosion products and the other by dangerous corrosion products (bronze disease). Both the laser cleaned and uncleaned samples were systematically chemical-physical surface characterized. It is found that complex stratified corrosion layers can be subjected to a low invasive and surface laser cleaning procedure by setting the correct treatment parameters. Cai et al. [53] found that under the wet state short free running (SFR) pulse mode Nd: YAG laser cleaning equipment, bronze cultural relics can get a higher removal rate of loose green rust within the safety threshold of 2.83 ~ 8.49 J/cm2, which conforms to the rules of "not changing the appearance of cultural relics" and "minimum intervention" for cultural relics protection. Large bronze sculptures were also lately laser cleaned such as the Etruscan statue “Arringatore” or the sculptural group “Decollazione del Battista” by Vincenzo Danti from the Baptistery of Florence, the Samuel F. B. Morse statue created by Byron Pickett and the bronze Napoleon as Mars the Peacemaker by Antonia Canova [54, 55].

The application of laser cleaning to bronze relics sometimes changes the surface color and other physical and chemical properties [56]. Shen et al. [57] cleaned the bronze cultural relics samples by means of ordinary wet laser cleaning, mechanical cleaning, gel-laser cleaning. The results showed that the gel can reduce the laser energy slightly, reduce the changes in the surface microstructure of bronze cultural relics caused by laser, and obtain ideal cleaning effect. The cleaning effect is shown in the Fig. 4. In order to limit the micromelting phenomenon, Shen et al. [58] continued to research and found that irradiation with pulsed Nd:YAG 1064 nm laser in LQS regime (100 ns) followed by chemical cleaning using a low-toxicity solvent mixture proved to be respectful towards the original patina of the bronze as well as of high efficiency. Different parts of cleaning of the Gu vessel is shown in the Fig. 5. Combining laser and other methods will be demonstrated noteworthy practicability in future applications.

Fig. 4
figure 4

Surfaces of sample treated with different cleaning methods [57]

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Fig. 5
figure 5

Different parts of the Gu vessel: before (a, c, e) and after cleaning with laser cleaning and residue removal with solvent mixture (b, d, f) [58]

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4.2 Iron cultural relics

4.2.1 Corrosion mechanism

The essence of iron cultural relics is iron carbon alloy [59], when an oxygen containing water film is formed on its surface and a certain amount of electrolyte ions exist (mainly Cl−), the ferrite(α- Fe) and cementite (Fe3C) will form innumerable micro galvanic cells, thus forming micro-battery corrosion. Figure 6 shows the corrosion of iron cultural relics with a mode of micro-battery [60]. The surface electrolyte water film is an external circuit, the internal Fe is used as the anode, and the Fe3C is used as the cathode. They will have electrochemical reaction, greatly accelerating the corrosion of iron [61].

Fig. 6
figure 6

Micro-battery corrosion reaction on iron surface: hydrogen evolution corrosion (a); oxygen absorption corrosion (b) [60]

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The reaction formula of iron cultural relics subjected to micro battery corrosion is as follows [62]:

$$\mathrm{Anodic reaction}:{\text{Fe}}-{2{\text{e}}}^{-} \to {{\text{Fe}}}^{2+}$$
(7)
$$\mathrm{Cathodic reaction}: {{\text{O}}}_{2} +1/{2{\text{H}}}_{2}\mathrm{O }+{2{\text{e}}}^{-} \to {2{\text{OH}}}^{-}$$
(8)
$${{\text{Fe}}}^{2+} + {{\text{OH}}}^{-} \to {{\text{FeOH}}}^{+}$$
(9)
$${{\text{FeOH}}}^{+} +\mathrm{OH }\to {\text{Fe}}{\left({\text{OH}}\right)}_{2}$$
(10)

As the Fe(OH)2 film formed on the surface is not compact, the electrolyte is easy to penetrate into the Fe (OH) 2 film layer, causing further corrosion of the base metal, and the Fe(OH)2 corrosion product film is unstable. As the corrosion proceeds, Fe (OH) 2 will react, with the reaction formulas as follows:

$$\begin{array}{c}2{\text{Fe}}{\left({\text{OH}}\right)}_{2} +1/{2{\text{O}}}_{2} \to 2\\ \mathrm{FeOOH }+ {{\text{H}}}_{2}{\text{O}}\end{array}$$
(11)
$$\begin{array}{c}2\mathrm{FeOOH }\to {{\text{Fe}}}_{2}{{\text{O}}}_{3}+{{\text{H}}}_{2}\\ \mathrm{FeOOH }+ {{\text{H}}}_{2}{\text{O}}\end{array}$$
(12)

In addition, reduction reaction occurs in the cathodic reaction area:

$$\begin{array}{c}6\mathrm{FeOOH }+{2{\text{e}}}^{-}\\ \to {2{\text{F}}}_{3}{{\text{O}}}_{4} +{2{\text{H}}}_{2}\mathrm{O }+{2{\text{OH}}}^{-}\end{array}$$
(13)

Because Fe3O4 has a dense structure, it blocks the invasion of H2O and O2 in the air and inhibits the initial electrochemical reaction process. However, when drying, the local battery of the rust layer and metal substrate is open circuit, and the immersion of O2 will promote the oxidation of Fe2+ in Fe3O4 to Fe3+, resulting in γ- FeOOH and α-FeOOH [63]. The reaction formulas are as follows:

$$\begin{array}{c}{4{\text{Fe}}}_{3}{{\text{O}}}_{4} + {{\text{O}}}_{2} + {6{\text{H}}}_{2}\mathrm{O }\to \\ 12\upgamma -{\text{FeOOH}}\end{array}$$
(14)
$$\begin{array}{c}4{{\text{Fe}}}_{3}{{\text{O}}}_{4} + {{\text{O}}}_{2} +{6{\text{H}}}_{2}\mathrm{O }\to \\ 12\mathrm{\alpha }-{\text{FeOOH}}\end{array}$$
(15)

Many archaeological iron objects were unearthed from the Nanhai I ship from the Southern Song Dynasty that sunk in the South China Sea [64, 65]. Hu et al. [66] studied many iron pot artifacts and found that the material of the iron pot was hypereutectic white cast iron. Jia et al. [67] researched some archaeological iron objects and found that the layer was mainly composed of Fe2O3, α-FeOOH, γ-FeOOH and Fe3O4. There are many cracks inside by comprehensive corrosion, which makes the iron objects unstable (see Fig. 7).

Fig. 7
figure 7

Archaeological iron artifacts from the Nanhai I (a); Graphs of rust layer (b) [67]

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4.2.2 Application of laser cleaning technology

There are several case studies of iron corrosion removal: in (Ruan 1980 [68]; Liu et al. 2018[69]) experiments on cleaning artificially corroded coupons from modern steel were carried out; in (Pereira et al. 2007 [70]; Yandrisevits et al. 2017 [71]) the CO2 and Nd:YAG lasers were used to clean iron objects covered with corrosion, naturally formed in atmospheric conditions, the cleaning effect is shown in the Fig. 8; in ( Koh 2006 [72]; Chamon et al. 2008 [73]) laser cleaning of archaeological iron objects was carried out.

Fig. 8
figure 8

Corroded artefacts before (a, c) and after Q-Switched Nd:YAG Laser cleaning (b, d) [71]

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However, some researchers investigated the applicability of laser cleaning of iron cultural relics and found that the iron surface would become black after irradiation (see Fig. 9) [74, 75]. The side effect causes the cultural relic protection to be more cautious about the application of laser cleaning to iron cultural relics. Prokuratov et al. [75] studied the possibility of laser cleaning to control the iron corrosion on the gold and silver foil plated on the medieval nomad’s belt pad. They found that laser cleaning can be improved by combining laser cleaning with mechanical cleaning (scalpel or sandblasting). In practical application, the cleaning process of iron cultural relics requires multiple cleaning methods to achieve the best cleaning effect. Laser cleaning of iron relics is worth further exploration. Special equipment is typically used to display and store iron cultural relics in museums and galleries, which is very costly to purchase and maintain.

Fig. 9
figure 9

Laser ablation craters [62]

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4.3 Silver cultural relics

4.3.1 Corrosion mechanism

Silver is a white and shiny "metal aristocrat". Because of its soft texture, strong castability and other advantages, it has become the ideal material for people to make all kinds of fine silver jewelry, practical objects, currency and other items since ancient times [76]. However, silver cultural relics often change color, seriously affecting the aesthetic value. Tarnishing is produced by reduced sulfur gases, principally H2S, and other organic molecules from atmospheric pollution, such as carbonyl sulfide (OCS) and dimethyldisulfide ((CH3)2S2) [77,78,79,80]. Tarnishing is originated in the first stage by the reaction of the environmental oxygen with the silver surface, forming a thin oxide film. Then, the presence of reduced sulfur species in the atmosphere can displace these oxides, leading to the formation of silver sulfide (Ag2S). Humidity, NO2, ozone and ultraviolet radiation can act as accelerators of the process [80, 81]. Relevant chemical reactions involved in the tarnishing mechanism are:

$$2\mathrm{Ag }+ {{\text{H}}}_{2}\mathrm{S }+1/{2{\text{O}}}_{2} \to {{\text{Ag}}}_{2}\mathrm{S }+ {{\text{H}}}_{2}{\text{O}}$$
(16)
$$2\mathrm{Ag }+\mathrm{OCS }\to {{\text{Ag}}}_{2}\mathrm{S }+{\text{CO}}$$
(17)

Ag2S is a black compound and its formation on the silver surface produces a loss of the shine and a change of color to a dark appearance, which is unacceptable for silver artifact [82]. Zhang et al. [83] studied the effect of immersion of pure silver in 0.1 mol/L sodium sulfide solution for different times (see Fig. 10), and found that the longer the time, the darker the color, which seriously affected the value of the silver.

Fig. 10
figure 10

Immerse silver tablets in 0.1 mol/L sodium sulfide solution for different times [83]

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4.3.2 Application of laser cleaning technology

In 2010, Kholodova et al. [84] cleaned the corroded layer and contaminants on the surface of the silver shotgun, and the cleaning effect was relatively excellent. In 2016, Palomar et al. [85] used a nanosecond Q-switched Nd: YAG laser at 1064, 532 and 266 nm to clean the sterling silver products, and found that the color and quality of silver products would change differently at different wavelengths, but this phenomenon did not occur at 532 nm visible light wavelength, thus determining the best cleaning threshold for the sterling silver products. Here are the SEM images of sterling silver coupons after the first (a, b, c) and the sixth (d, e, f) tarnishing-laser cleaning at the three indicated wavelengths (see Fig. 11). In 2017, Bojana et al. [86] used Nd:YAG laser to clean the silver plated copper wire ethnographic fabric, which proved that the laser cleaning technology is more effective than traditional methods. In 2019, Raza et al. [87] used excimer lasers to clean the black scale on the surface of silver cultural relics, which is mainly silver sulfide, very effectively, and will not significantly remove the silver substrate. They believed that this is a new method to remove silver sulfide crusts on the silver surface.

Fig. 11
figure 11

SEM images of sterling silver coupons after the first (a, b, c) and the sixth (d, e, f) tarnishing-laser cleaning at the three indicated wavelengths [85]

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Fig. 12
figure 12

Optical image of red corrosion products on surface of the gold coin (a); cubic structure of crystals of red products (b) [88]

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4.4 Gold cultural relics

4.4.1 Corrosion mechanism

Gold is extremely stable in the air and is not easy to corrode. For pure gold cultural relics, they generally do not need special protection. However, most of the gold will be mixed with some other metal elements, such as copper, iron and silver, and corrosion will occur [89, 90]. When copper is mixed, green thin rust will appear, and when iron is mixed, red rust will occur. When the silver content in gold exceeds 20%, the alloy will change color. Red rust on the surface of gold coins discovered by researchers, as shown in Fig. 12 [88].

Fig. 13
figure 13

Silver gilded pin in the form of a figure of Nike holding a wreath before (a) and after Er:YAG laser cleaning (b) [91]

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4.4.2 Application of laser cleaning technology

Kono [92] cleaned the military gold knitting of the nineteenth century without any harmful effects on the gold foil or the underlying silk thread structure, proving the effectiveness of laser cleaning. Elnaggar et al. [93] reported successful cleaning of gilt-silver threads using an Nd:Van laser at 1064 nm with ps pulses. Siano and Salimbeni [94] obtained excellent results with the long Q-switched laser to remove corrosion from the gildings (applied using the fire-gilding technique) of the famous bronze “Gate of Paradise” from the Baptistery of Florence. No damage was observed using scanning electron microscopy. The removal of a thick layer of corrosion from a gilded silver Roman pin using an Er:YAG laser (Fig. 13) was presented at the ICOM-CC Triennial Conference in 2017 [91]. Singh et al. [95] used an Nd:YAG laser emitting 100 ns pulses to remove a 20 nm thick layer of carbon from gold and measured a decrease in the surface roughness of gold compared to before gold was contaminated with carbon. Zhang et al. [96] cleaned a gilt bronze statue using the Nd: YAG laser developed by Italy's Quanta System company, which is specially developed for art protection. After laser cleaning, the face and body lines of the statue are clear, and the gilt luster is exuded. The traces of processing technology are well retained, without residue and new scratches. Pawita et al. [97] also demonstrated the potential for Er:YAG laser radiation in the controlled removal of brass-based overpaint from gilded wood.

Different metal cultural relics of application of laser cleaning from reviewing the literature published is summarized. There are many successful application achievements on metal cultural relics. Nd: YAG nanosecond lasers are widely used in most cases. Due to the non-renewable and complex structures of cultural relics, laser cleaning cannot be directly applied to the surface of metal cultural relics. Many researchers are observing surface structure through simulation before experiments to predicted the effect of laser parameters, and therefore provided the theoretical reference for experiments [98,99,100,101,102]. The simulation model, the gaussian distribution of laser beam and the ablation depth under different laser parameters are shown in Fig. 14. Simulation models can be considered for application in the protection of metal cultural relics.

Fig. 14
figure 14

Simulation model (a,b); diagram for Gaussian distribution of laser beam (c,d); ablation depth under different laser parameters (e) [98, 99]

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5 Current problems and future research directions of laser cleaning

5.1 Existing problems

Laser cleaning technology has made great progress in the protection of metal cultural relics, but still faces some problems.

  1. (1)

    The laser cleaning process is very complex. Due to the different components and structures of the attachments on the surface of metal cultural relics, the mechanism of laser action is also different, and the cleaning mechanism is also different due to the different cleaning parameters, which reduces the cleaning efficiency in the application of cultural relics and delays the progress of comprehensive promotion of laser cleaning.

  2. (2)

    Limitations of laser cleaning equipment. The research and development of laser cleaning technology and equipment in China started late. Although some achievements have been made in a short time, there are few mature laser cleaning equipment in China, most of which are still in the research stage. And the cost of laser cleaning equipment is high. Due to the limitation of equipment use, the process of laser cleaning replacing traditional cleaning gradually slows down.

  3. (3)

    On the combination of laser cleaning and other methods is less. The protection procedures for metal cultural relics are complex, and laser cleaning needs to be combined with other cleaning methods to achieve good results.

  4. (4)

    Hidden danger to human health. Although laser cleaning is a green and environment-friendly technology, lasers will inevitably develop towards high power in the future. The direct light and reflected light generated by lasers will cause damage to human eyes and skin, especially to eyes. At present, the band used for laser cleaning cannot be seen by human eyes, which also increases the risk to human health.

5.2 Future research directions

For the current problems of laser cleaning, we can start from the following directions in the future:

  1. (1)

    Laser cleaning technology is now mostly applied to some relatively stable cultural relics, and more attention should be paid to the cleaning of vulnerable cultural relics in future research. In addition, simulation can be used to analyze in detail the components that need to be cleaned on the surface of cultural relics to determine laser parameters and improve cleaning efficiency [100,101,102].

  2. (2)

    Mobilize the enthusiasm of scientific research institutions to research and develop laser cleaning equipment, promote the development of cultural relics protection industry, and strive to overcome the impact of expensive equipment on popularity.

  3. (3)

    Strengthen the composite research of laser cleaning and other processes. The protection of cultural relics needs to be coordinated and interdisciplinary, and belongs to the international frontier of scientific research. We should give full play to the supporting and leading role of modern scientific and technological means in the protection of cultural relics [103, 104].

  4. (4)

    Conduct in-depth research on the safety of laser cleaning technology. Security should be considered while researching and promoting new technologies.

6 Conclusion

To sum up, laser cleaning technology is one of the main development technologies for cultural relics protection in the future. Although there are still shortcomings, the successful application of laser cleaning technology in the protection and restoration of cultural relics shows its unique advantages and has very broad development and application prospects.