A Survey of Electromagnetic Techniques Applied to Cultural Heritage Conservation

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Abstract

Cultural Heritage (CH) represents the identity of populations; it is a heritage not only for the culture that produced it, but also for the entire human civilization. Still, preserving it is not an easy task; several factors hinder its preservation, from time and natural disasters to wars and neglect. Science can play a leading role in preserving CH, and among the different techniques available, Electromagnetic (EM) techniques are particularly suitable for this purpose because of their efficacy, safety for both people and materials, and their applicability to artifacts made from different materials and of complex and irregular shapes. Although usually associated with diagnostic applications, EM techniques also have a crucial role in restoration applications thanks to EM radiation treatments for the recovery and consolidation of materials such as wood, paper, parchment, stone, ceramics, and mummies. The state-of-the-art of radiation technologies shows efficacy for the elimination of pests, mold, fungi and bacteria, and for the consolidation of damaged or weakened artifacts. This paper aims to provide a useful tool for a first yet rigorous understanding of the contribution of EM techniques to CH recovery and lifetime extension, also comparing them with traditional methods and highlighting main issues in their application, such as lack of protocols and distrust, and potential risks in their application.

1. Introduction

CH represents the identity of populations, and preserving it assumes special relevance for countries in war or in the case of natural disasters [1], to the point that the United Nations Educational, Scientific and Cultural Organization (UNESCO) keeps the List of World Heritage in Danger updated on its official website [2]. UNESCO speaks of World Heritage not only in terms of CH, stating that “What makes the concept of World Heritage exceptional is its universal application” [3], i.e., a World Heritage property belongs to the entire world independently of the territory in which it is located. Even though not all CH properties are declared World Heritage, UNESCO is very clear regarding this point: “The fact that a property belonging to the cultural or natural heritage has not been included in either of the two lists mentioned in paragraphs 2 and 4 of Article 11 shall in no way be construed to mean that it does not have an outstanding universal value” [4] (Article 12).

CH also has significant economic value; in fact, the art market is a global one, involving private collectors, auction houses, and insurance companies. The identity value, together with the economic one, has brought about the need for dedicated regulations [5,6]. An emblematic case in this sense is represented by Italy, where there is an export veto (a unique case in the world) on cultural goods of particular interest to the country [7].

As things stand, CH preservation is a must, but it also represents a real challenge that requires close collaboration among art historians, conservators, museum directors, and, last but not least, scientists. The fact that scientists could be a fundamental asset to the world of art started to manifest itself in the first half of last century with activities focused on the analysis of works of art; it was what we now commonly call diagnostics. The only country bringing to attention the conservation issue together with the diagnostic one at the time was France. The literature reviewed confirms France’s excellence in irradiation techniques for disinfection and consolidation of CH artifacts, particularly because of the experience accumulated in the last 50 years [8]. The Louvre hosts the Accélérateur Grand Louvre d’Analyse Élémentaire (AGLAE), which is “the only particle accelerator in the world fully dedicated to the physio-chemical characterization of heritage materials” [9]. Romania [10], Belgium, the Netherlands, and Croatia [11] are gaining an emerging role in the use of EM techniques applied to CH conservation, but also Brazil [12] and Tunisia [13], probably due to an open-mindedness towards these techniques, which are often viewed with suspicion in different countries even today.

The state of the art reveals a development in this type of technique mostly for the application to wood, paper, and parchment artifacts, but also stone and ceramics, and for artifacts that have a strong identity value, either religious or historical, but also related to the local community life and that have suffered major damages due to war, natural disasters, or pest attacks, or that experience particular wear [14].

Many publications are dedicated to the effect of radiations, mostly gamma radiation, on organic material. Several publications regarding paper feature deterioration issues, comparing natural deterioration with that eventually induced by radiation exposure for study or treatment purposes [15,16,17,18,19,20] or in any case investigating the effects of radiation on paper durability [21]. Studies have been conducted on the effect of gamma radiation on wood structures [22,23,24,25], parchment [26,27,28,29], textiles [30,31], and a rare example for the effect of irradiation on canvas oil paintings [32]. A particularly relevant case in the field of CH conservation is represented by mummies, and Ramses’ mummy, commonly known as The Mummy, probably represents the best example [8,14,33,34,35] of the relevance that radiation technologies may have in CH preservation. As a conclusion to this Introduction, we would like to point out some crucial issues that emerged from the literature reviewed:

• There is distrust towards irradiation techniques applied to CH conservation, and in some countries, there is a prohibition on their use;
• The coexistence in some artifacts of different organic materials makes conservation procedures a real technical challenge;
• As for the EM techniques applied to diagnosis, even for those applied to conservation techniques, there are no real standards or best practices.

Based on all this, the aim of our work is to provide a useful tool to introduce the reader to the deterioration processes that have the greatest impact on works of art, and to the state of the art in the EM techniques used for their conservation according to a review of the most relevant publications in the field, pointing out advantages and disadvantages with respect to traditional methods and assessing the risks in their application to CH artifacts. The paper is organized as follows: Section 2 is dedicated to the general principles of EM techniques, their comparison with traditional techniques, main aspect of dosimetry in the context of CH, hence the potential risks associated with an improper control of the fundamental parameters involved in the irradiation process. Section 3 provides the reader with the EM techniques for restoration that have been proven to be the most efficient, yet safe, providing detailed irradiation parameters and results of the case studies present in the literature reviewed. Section 4 provides the reader with an overview of the most common types of degradation processes on different types of artifacts, namely organic and inorganic. Section 5 is dedicated to conclusions and future perspectives.

2. EM Techniques Applied to CH

In this paper, with the term CH, we refer to tangible cultural heritage, and we consider those objects that have been highly damaged by natural causes (pest attacks, floods) or wars, as well as artifacts subjected to intense use, buried, and under water. Our focus is on the EM techniques applied to conservation/restoration of CH, meaning by conservation/restoration the direct interventions to extend the lifetime or to recover the integrity of CH artifacts, and considering as EM techniques those based on ionizing radiation produced with electromagnetic waves (EMWs). The techniques presented are those that have proven themselves to be efficient yet safe.

Table 1. Irradiation techniques of wider application for decontamination/consolidation purposes and relative most relevant references.

Technique	Application	References
Gamma/X-ray/ion beam irradiation	Decontamination of:
	wood	[10,22,23]
	parchment	[26,27]
	paper	[11,15,16,17,36]
	textile	[30]
Gamma irradiation	Wood consolidation	[14,37]
Microwave (MW) heating	Decontamination of:
	Stone	[38,39,40]
	wood	[41,42,43,44]
Radiofrequency (RF)	Biofilm removal	[45,46,47]

summarizes the most consolidated techniques at the moment, their applications, and the main references reviewed.

We do not intend to provide a complete and in-depth knowledge of each technique but rather to offer a preliminary understanding that may serve as a starting point for further and more detailed studies on the matter. Each technique will be investigated based on its physical principles; this approach provides immediate comprehension of the results that may be achieved through its application, the materials/artifacts to which it is best suited, and its advantages and limitations.

At the base of the various techniques we will consider, there is the interaction between matter and EM radiation, which is the principle underlying both diagnosis and restoration [48]. As anticipated in the Introduction, the first attempt to apply Science to Cultural Heritage occurred for diagnosis purposes. The first countries to embrace the challenge in the first half of last century were France, Germany, and the USA, while in Italy, there was mostly resistance to the application of scientific methodology and techniques in the field of CH [49]. In the USA, scientific and innovative research was already conducted in the 1920s-1930s at the Fogg Art Museum at Harvard University [50], and at the time, Munich (Germany) could already boast a century of activity [51]. The first country that considered the application of EM radiations for conservation purposes was France. Cellier, director of the laboratory established at the Louvre in 1923, during the first International Conference for the Study of Scientific Methods in the Examination and Conservation of Artworks (Rome, October 1930), brought to attention the conservation issue together with the diagnostic one; in particular, he spoke of the possible treatment of wooden statues against parasites and of precautions to be taken for stone artifacts against atmospheric agents, as well as precautions to preserve artworks from ventilation, heating, and illumination in the exhibition rooms [49]. Since our focus in this paper is on EM technique for conservation/restoration of CH artifacts, we are not interested in the science of spectroscopy, which is mostly related to diagnostic applications. For an overview of this science, the interested reader may refer to publications [52,53,54]. However, spending a few words on spectral absorption is due. The father of atomic absorption is Alan Walsh [55,56,57]. In his famous paper [58], he outlined the relevance of absorption in determining the concentration of an element. This is particularly relevant in the field of CH, because it allows for a deep insight into the chemical composition of samples, which is essential for understanding present deterioration processes and/or preventing them if they are not yet active.

2.1. Radiation Parameters and Dosimetry for CH Application

When speaking of EM radiations for CH restoration, it must be clearly stated that radiation leads to the breaking of chemical bonds in the irradiated object, and to the release of free radicals or excited species from it [59]; this is the starting point to provide a valuable risk assessment of the use of EM technique for CH preservation. The most vulnerable bonds are the covalent ones, which are typical of living creatures. This is why irradiation is so efficient for disinfection and pest decontamination, but it is also why the irradiation of CH artifacts may be responsible for side effects [14]. When an object is irradiated, two parameters are of the greatest relevance:

• The absorbed dose;
• The absorbed-dose rate.

The absorbed dose is defined as “the mean energy imparted by ionizing radiation to the matter in a volume element divided by the mass of that volume element” [60] (p. 27), while the absorbed-dose rate refers to “absorbed dose per unit time” [14] (p. 44). The importance of these parameters depends on the fact that the radiation must be efficient, yet safe, for the artifact, the environment, and the people around it; hence, the dose of irradiation must be properly evaluated.

In practice, the absorbed dose considered is an average value, and it is peculiar to the specific material under treatment; the success of the treatment depends on the dose delivered, and this is why dosimetry systems are so important. The classification recalled by IAEA [14] for dosimetry system is ISO/ASTM 51261:2013 [61]. According to this standard, dosimetry systems with the highest metrological qualities are the primary standard dosimetry system, followed by the reference standard one. But the dosimeters used for routine dose measurements are those of the third category, the routine dosimetry system category. These dosimeters are recognized as reliable worldwide for dose mapping and process monitoring. Examples of routine dosimeters are reported in ISO/ASTM 52628 [62] and for each of them, the useful absorbed dose range is reported; this range is an indication of which dosimeter is the best suited for a specific field of application.

Originally intended for the medical, agricultural, industrial, biological, and food [63,64,65,66] sectors, dosimetry systems are now available for CH applications. As an example, at the gamma irradiation Calliope facility of Casaccia Research Center (Rome, Italy), ionizing radiation treatment are performed for CH objects and different dosimetry systems are available: Fricke solution (20–200 Gy), Red Perspex (5–50 kGy), radiochromic (1 kGy–3 MGy), alanine—ESR (1 Gy–500 kGy), Thermo Luminescent Dosimetry TLD (0.1 mGy–100 Gy), and electronic RADFET (0.01–Gy) [67].

For a correct use of dosimetry systems for CH applications the following elements must be considered:

• Size, density, homogeneity or heterogeneity of the artifact;
• Balance between reliability and harmlessness of treatment.

As for the first point, this has an impact also on the number and location of dosimeters, while the second point results from the dose range window determined by the minimum and maximum values to be considered. The minimum value is related to the concept of reliability, i.e., it is the value which guarantees that the decontamination or consolidation effect is achieved; the maximum value is the one that still preserve the integrity of the irradiated object. According to L. Cortella et al. [68] the maximum safe dose considered is the one valuable for cellulosic materials (among the most sensitive material present in CH objects), while for the minimum value, two different scenario are considered: insect eradication (500 Gy) and fungicidal treatment (6 kGy).

2.2. Ionizing Radiation vs. Traditional Methods

If compared with traditional methods, ionizing radiation does not leave chemical residuals; hence, there is no need of quarantine; moreover, their efficacy is preserved longer, and they act simultaneously on different type of biodeteriogens and at different stages of the life cycle. In the “Irradiation Protocol for cultural heritage conservation treatment”, M.L.E. Nagai et al. [69] specify the most evident disadvantages of traditional disinfection methods, namely:

• Alcohol 70% eliminates fungi but it has no effect on spores;
• Fumigation is effective but carcinogenic;
• Anoxia does not guarantee penetrability;
• Freezing is effective on fungi, but not on spores;
• Dry sanitization performs only a superficial removal [70].

Fumigation is conducted by means of poisonous gases, usually ethylene oxide, which may be emitted over time from treated objects, and methyl bromide, which is carcinogenic and extremely flammable. Moreover, gases hardly penetrate materials; hence, the effectiveness of treatment is compromised [71].

Ethyl and isopropyl alcohol act quickly on fungi but, as said above, they are ineffective against bacteria spores. An effective liquid against fungi, bacteria, and spores is formaldehyde, but it is a carcinogen and may cause respiratory problems and skin irritations. Moreover, any liquid should not remain in contact with a CH artifact for a prolonged period [72].

Anoxia requires long exposure time under strict environmental conditions (low oxygen level), and is ineffective on fungi [73].

One advantage of ionizing radiation over traditional methods is the penetrating capability. This allows treatment of thick and dense objects, but because CH objects may have complex shapes, a critical parameter to manage together with the absorbed dose is the uniformity of radiation delivered, which is highly dependent of the type of irradiation facility. These are two key factors that make the success or failure of the application.

Also, the IAEA outlines the validity of radiation techniques in CH applications, even in comparison with more traditional methods like chemical and physical ones. “Chemical and physical methods have been developed to treat and restore cultural heritage artifacts. However, chemical methods may leave undesirable chemicals in the material, whereas physical methods are generally quite extreme and therefore not suitable for all types of materials. In contrast, radiation technologies do not cause any harm to the artifacts themselves. They can be used to destroy parasites […] and to assess any internal damage that was caused by them” [74]. (Treatment and restoration, © IAEA, https://www.iaea.org/topics/cultural-heritage-restoration, accessed on 3 December 2024).

2.3. Ionizing Radiation Risks

Apart from the failure risk associated with the improper control of the two parameters mentioned in the previous subsection, the major risk in the use of ionizing radiation are secondary effects like damages or unacceptable changes in the artifact due to modifications induced by the radiation on the object. The eventual modification of the object may be a direct consequence of the interaction with the radiation, or an indirect one produced by the free radicals induced by the irradiation process. The worse side effects are those that bring changes to the constituent polymers of the artifact, i.e., cellulose, lignin, and proteins.

Cellulose is the main component of paper, textiles made of cellulosic fibers and wood, which also contains lignin. The principal cause of cellulose deterioration is oxidation, and the second one is acidification. Radicals are a determining factor in the oxidation process, and they may be induced by ionizing radiation [75,76,77,78], but cellulose stands doses of up to 10 kGy. Details of the levels of irradiation used to evaluate changes in the artifact, and the results obtained are reported in Section 3.

Lignin has an antioxidant effectiveness; for this reason, wood, which contains lignin in addition to cellulose, is less prone to oxidative degradation than paper or cotton textiles; hence, ionizing radiation is particularly suited for wood, which can stands repeated treatments.

Concerning protein fibers, silk is less stable than wool, but both typologies of textiles resist exposure to doses of up to 10 kGy of gamma rays, without the loss of essential properties such as elasticity and mechanical resistance [30].

The possible side effects of irradiation treatments may be summarized as follows [79]:

• Color changes and surface erosion;
• Compromising of the mechanical integrity of material.

These side effects typically appear in the following ways for three of the most common typologies of CH materials:

• Transparent materials tint easily;
• Opaque materials do not manifest visible changes;
• Paper may structurally collapse.

As is clear from previous subsections, the risks associated with the use of radiation technologies for the artifact depend on the radiation dose absorbed by the object, but it must be highlighted that, although side effects may occur, they are usually far less damaging than the deterioration process.

In next section, we will go through the EM irradiation techniques summarized in

Table 2. EM techniques for CH restoration ¹ covered in Section 3.

EM Irradiation for Restoration
Gamma/X-ray
Radiofrequencies
Microwave heating

¹ By restoration, we mean decontamination and solidification.

. References [14,35,48,74,80,81,82,83,84,85,86] are the core references for the fundamentals of the EM techniques presented; the other references will be addressed as needed.

3. EM Techniques for CH Restoration

Decontamination and consolidation are the most relevant conservation/restoration treatments used to recover and extend the integrity of a CH artifact over time, and these can be performed using EM techniques. For this type of application, the most authoritative reference source is the International Atomic Energy Agency (IAEA), as the techniques used are based on ionizing radiation. Radiation techniques consist of EMWs or charged particles irradiating the artifact and interacting with its matter in different ways [14]. In the case of EMWs, the important factors during the irradiation process are as follows:

• Irradiating photon energy;
• Atomic number of the absorbing material.

Because the atomic number of the absorbing material is a key parameter in the irradiating process, it follows that these techniques do not yield to the same result on different materials. According to the state of the art reached in France over more than 50 years of experience in the field [8], and to the information provided by the IAEA [14,87], artifact types for which radiation treatment are not recommended are glass, gemstones, amber, basketworks, bone, horn, ivory, and tortoiseshell. On the opposite site, materials for which radiation treatments are most successful are all kinds of wood and paper artifacts, composite ethnographic collections, natural history collections, and mummies. Concerning photographs and film stock, the suitability for radiation techniques depends on the substrate material and, in the case of cellulose nitrate or acetate, these techniques must be avoided.

Although different treatment typologies exist (see

Table 3. Main disinfection methods and their principles.

Methods	Principles
Mechanical 1	Physical removal of pathogen factors
Chemical	Elimination of pathogens by chemicals
Biological	Bio-cleaning/bio-mineralization through microorganisms
physical	Based on anoxic treatments

¹ Note that some authors [83] consider mechanical methods as part of the physical ones, and IAEA does not consider radiation as a physical method, as it is easy to verify from the quotation at the beginning of Section 3.

), all ionizing radiation produces chemical effects in the irradiated material by breaking the chemical bonds. In fact, “ionizing radiation is a radiation with enough energy to remove tightly bound electrons from the orbit of an atom, causing that atom to become charged or ionized” [88]. Covalent bonds, typical of living creatures and organic materials, are the most affected ones. Irradiation affects important parts of the DNA structure; it actually acts in two main ways:

• Direct interaction with the DNA;
• Indirect modification, inducing free radical production through water radiolysis [59].

A rule of thumb to keep in mind is that the more complex the organic molecules are, the less energy is required to damage them and eventually inhibit their replication [14]. This means that dose required to decontaminate CH objects from insects is lower than the one required in the case of mold; the dose range usually used for insect eradication is 0.5–2 kGy vs. a dose of up to 10 kGy for mold. A second major aspect to outline is that some biodeteriogens are not only dangerous for artifacts, but are also toxic to humans [86], while irradiated materials do not retain radiation; hence, they cannot irradiate people working on them after treatment, nor visitors.

Since the techniques investigated in this section are strictly related to how the deterioration of CH artifacts begins and evolves, you may refer to Section 4 for an insight into the main causes and principal processes leading to CH deterioration.

3.1. EM Techniques for Decontamination of CH

According to S. Romeo and O. Zeni [89], disinfection methods (we prefer the term decontamination for CH, as disinfection is more related to medical and industrial procedures) are of four different types; Table 3 summarizes the typologies and principles.

Apart from the different classifications, the treatments of interest to us are those based on ionizing radiations produced by EMWs, which are particularly effective for the elimination of insects, bacteria, fungi, and molds—the most dangerous causes of CH deterioration.

Table 4. Relevant methods and their advantages [14,86].

Methods	Advantages
Gamma/X-ray 1 radiation	Can penetrate complex, volumetric shapes
	Rapidity of treatment
	No temperature increase
UV-C irradiation	Disinfection of buried/waterlogged
	stone artifact

¹ X-ray, to some extent.

reports the methods of interest to us and their main advantages.

3.1.1. Application of Gamma and X-Ray for Decontamination of CH

Regarding the physical principle, we have already outlined what is necessary for the scope of this paper in Section 2 and in the introduction of Section 3; here, we immediately go through the application of gamma and X-ray for decontamination treatment. The biological vectors for deterioration process of CH objects are mainly insects and fungi; ionizing radiation is an effective solution in both cases. It must be noted that it is gamma radiation that is primarily used in practice, mainly because of its higher penetration power and, hence, homogeneous irradiation of the artifact. The important parameter to control in order to achieve an efficient biocidal effect while ensuring the artifact’s integrity is the absorbed dose. Moreover, the efficacy of irradiation is not invalidated by keeping the object inside a package or container; this is, instead, a way to preserve the object in a decontaminated environment after treatment. The dose used to decontaminate CH artifacts from insects is usually in the range of 0.5–2.0 kGy and 10 kGy for microorganisms, while for the sterilization of medical devices, a dose of 25 kGy is commonly used [14]. The difference in the dose range between medical and CH application may seem huge, but it is easily explained by considering the fact that surgical equipment is meant for a one-time use while CH objects should be preserved for as long as possible. To have a comprehensive view of specific irradiation parameters and of the post-treatment effect associated with them, the experience of Grenoble with its ARC—Nucléart (Atelier de Recherche et de Conservation) in biocidal and consolidation treatments is extremely useful. The data collected in the Center since the early 1970s shows that a dose of 500 Gy is deterministically enough to kill any insect at any life stage within 20 days from treatment and, in the meantime, they are not able to reproduce, and it also seems that their appetite is reduced; hence, higher doses are required only if the degradation process must be immediately stopped [90]. For fungi and molds, the approach is probabilistic; hence, it is not possible to determine such a sharp threshold, but most of them have a D₁₀ (the necessary dose to reduce their population by a factor of 10) in the range of 0.1–1 kGy.

It must be noted that most of the available publications in the field of ionizing radiation for CH are IAEA documents, and are not focused on the disinfection or consolidation process per se, but on research undertaken to understand if, and to what extent, there are induced damages or, better, “changes” in the aesthetic and structural characteristics of the artifact [19,22,24,25].

Although gamma radiations are suitable for basically all objects, negative opinions about their use for transparent objects are presented in [8], recalling how the sensitivity of transparent materials to irradiation was already noted by Curie in 1899, and has been confirmed over the years, culminating in the works by Cortella et al. [8,68,79,90,91]. Adverse effects on glass, not only from gamma rays, but also from X-rays and electron radiation, include color fading—reversible or irreversible—and the formation of defects known as color centers; for these reasons, the use of commercial silicate glass as a routine dosimeter has been investigated [63,64]. Farah et al. [63] demonstrated that, for measuring doses in the range of 0.1–17 kGy, this type of glass can be a reliable routine dosimeter if calibration is properly performed and environment conditions are controlled. The same results have been obtained by P. G. Fuochi et al. [64], but considering a wider range of doses, 1–50 kGy. Although side effects on transparent materials are confirmed, Ponta et al. [91] report that the Color Centers Activation starts for doses above 1 kGy which, as previously recalled, is the maximum value of D₁₀ for most fungi and molds.

Gamma irradiation has also been used for disinfection on an “oil painting experimental model” of 1995 by Romanian artist, George Alexandresku, [32]. The painting has been irradiated at an average dose of 33 kGy, three times higher than the dose of 10 kGy generally effective for decontamination, and at an average dose rate of 3.1 kGy/h. The painting’s layer structure and color changes have been monitored by Fourier Transform Infrared Spectroscopy (FTIR) and Fourier Transform Raman (FT-Raman) spectroscopy. Manea et al. [32] report that, despite the high irradiation dose, insignificant spectral changes were observed in the oil painting after irradiation.

Beyond these examples, the use of ionizing radiation for decontamination is mostly related to the treatment of organic artifacts made of paper, parchment, wood, and textiles, and of a particularly fascinating type of CH object, mummies. The eyes of the world are focused on them because of their imaginary power, and because they are made of different highly perishing organic materials, from human tissues and bones to textile materials; this fact represents a great challenge from a technical point of view as, according to what stated in the introduction of Section 3, the irradiation dose needed for decontamination is inversely proportional to the complexity of the organic molecule to be irradiated. For mummies’ disinfection, gamma radiation turned out to be the best choice because of its efficiency and jet safety, provided that the right dose rate is applied.

Even for one of the most important cases of CH object decontamination, the mummy of Ramses II, it is not easy to find information apart from those provided by IAEA [14,33]. The image reported in the upper part of Figure 1 is a photo from 1912.

The mummy of Ramses II not only has incredible historical value, but its treatment is also incredibly challenging from a technical point of view, since it is made of different organic materials:

• Hair;
• Skin;
• Teeth;
• Textiles.

The species of fungi present on the mummy at the time of disinfection were more than 60 (as reported by IAEA [14]); hence, the minimum dose approach was impracticable [33]. Moreover, due to the extremely irregular shape and density of the mummy, a calculus model has been necessary to design the gamma ray source in a way that irradiation parameter of a minimum dose rate of 18 kGy was guaranteed to all parts of the mummy. The irradiation time was 12 h and 40 min, with a rotation of the mummy at half of this period. Notice that the mummy, as well as the sarcophagus, had been previously sealed in a plastic envelope in order to preserve it in a sterilized environment during manipulation. The result was a real success, with the mummy still in very good condition nowadays.

A case, similar in certain aspects to that of mummies, because it deals with organic material, is the case of the frozen baby mammoth. The selected dose was 20 kGy of gamma radiation with a maximum level of 40 kGy; hence, much higher than the maximum dose of 10 kGy usually reached in CH treatments. These levels of irradiation caused lesions to the DNA, although the integrity of the protein structure of the animal was preserved. Even in this case, the only information we could find are those reported by the IAEA [14].

In [10], the importance of radiation treatment is outlined when large collections are affected by serious biological attacks; this is usually the case in libraries and paper or textile collections.

An example of mold disinfection through gamma radiation in the case of large collections is the disinfection of the Peace Palace Library in the Netherlands. The very high level of humidity, combined with dust—which further increases the water absorption capacity of paper—and the presence of spores everywhere in the room made the situation of the library critical. The dose applied was up to a maximum of 10 kGy, and the collection was exposed to gamma irradiation for 150 consecutive days [14]. After this, the dust was removed, and then all of the books were vacuum cleaned. Even in this case, the treatments together with monitored environment conditions turn out as a success.

Gamma radiation treatments for the recovery of CH artifacts from biological damage are considered concerning paper/books in [15,16,17,36], and parchments in [26]. A. Gimat et al. [18,21] were interested in the effects of X-ray radiation on paper during irradiation, and on the short and long term. They outline how possible side effects induced by irradiation exposure are influenced by intrinsic factors, like the presence of ink and additives in the paper, as well as by the degradation state. They exposed different types of paper (laboratory model aged paper, and 18th and 19th century archival documents) to different radiation doses, 4–36 kGy. The results showed that aged papers have the lowest degree of depolymerization, and that doses below 4 kGy have no side effects, even on the long run, while doses in the range of 26–36 kGy turn into a slight UV luminescence after 21 months and yellowing after three years. Patten [29] investigates evidence of eventual changes in parchment collagen due to X-ray exposition not because of decontamination treatment, but because of reading necessities through X-ray microtomography (XMT) in the case of particularly fragile parchment which cannot be unrolled. Modern (10 years old) and ancient (1769–1832) parchment samples were studied, and the conclusion is that, for fragile (but not highly gelatinized) samples, the dose of X-ray absorbed with the XMT does not cause significant changes.

Investigations on gamma rays’ effects on wool and silk have been conducted in [30], using radiation doses from 10 to 25 kGy. The research confirmed that a maximum dose of 10 kGy does not cause “significant changes” in the proteins of textiles. In [31], the effects of gamma irradiation on color changes in textiles were investigated. Samples of different textiles (wool, silk, cotton, linen) were divided into three groups: not dyed, naturally dyed, and chemically dyed. All examples were irradiated with gamma doses in the range 0.5–25 kGy. The results show that all samples darkened, but at different levels. The greatest effects have been found on linen and naturally dyed samples.

3.1.2. Microwave (MW) Heating and Radiofrequency (RF) for Decontamination of CH

Although MW heating and RF treatments are not that common, we will spend a few words on them, for the sake of completeness. Microwave heating has been widely used for industrial and medical applications, but because of the peculiarity of CH artifacts in terms of the variety of materials within the same object and irregular shapes, this technique may lead to overheated areas, while others remain scarcely irradiated [89].

For these reasons, most applications may be found for stone treatments, particularly building walls, which means artifacts mainly made of one material and with a relatively regular shape compared to the majority of CH objects [38,39,40]. O. A. Cuzman et al. [39] used a 2.45 GHz microwave generator to eradicate a fungal colony isolated from cultural stone artifact. The temperature rate was increased depending on the material characteristic and power level of microwave radiation. Experiments show an evident influence of microwave heating on three different fungal colonies; in all cases, an exposure time of more than 9 min at 55 °C was enough to inhibit growth or kill the colony. Nevertheless, there are different cases in which this technique has been applied to other types of CH manufacture for decontamination purposes. R. Olmi et al. conducted a study on the use of localized MW heating treatment for biodeterioration control not only on stone, but also on wooden objects [93].

Wood artifacts are among the most treated by means of the MW heating method [41,42,43], especially for the eradication of woodworms in all metamorphic stages (eggs, larvae, and pupae), including in painted boards [34]. A. T. Z. Ortiz et al. [42] assessed a procedure to eradicate woodworm larvae from wild pine. The aim was to find the right combination of time and power to eradicate the pest, preserving the integrity of the wood. According to the authors, the right combination in their experiment was a microwave-power of 1 kW for 20 s.

Regarding MF, it has been applied to remove biofilms from ceramic surfaces. The studies were mainly conducted by P. Cennamo et al. from the University of Federico II, Naples. Biofilms not only produce aesthetic alterations, but also induce physical damage to the surfaces on which they grow. Biodeteriogens identified by P. Cennamo et al. on terracotta pots fall into three main types [45]:

• Eubacteria (five different species isolated in their research);
• Cyanobacteria (10 different species isolated in their research);
• Green algae (13 different species isolated in their research).

Shards of broken spots were inoculated with biofilm samples of the three different types of biodeteriogens, and exposed to RF treatments: two hours of exposure every other day for a week. The average reduction was more 81% for all three species. The authors also report that the RF power level applied (although not specified) did not produce any significant temperature increases in the treated surfaces.

Other research has been conducted by P. Cennamo et al. on the use of EMWs in the range of RF for biofilm removal [46,47], but these are still niche studies. Figure 2 shows a confocal laser scanning image of biofilm on a Candoglia marble, even after chemical cleaning combined with a mechanical brushing treatment [80].

3.2. EM Techniques for Consolidation of CH

Ionizing radiation can be used to strengthen weakened materials by forming a new polymer inside the weakened structure. Artifacts made of porous materials are the most vulnerable, and irradiation is meant to improve their mechanical properties.

The method consists of vacuum impregnation with liquid resin, followed by a polymerization process called radiation curing, obtained through gamma irradiation [37,64]. The result is a very stable compound.

The main applications regard not only waterlogged archeological wood, but also dry wood subjected to a particular stress, e.g., flooring.

The application of ionizing radiation to improve wood mechanical properties performed at the ARC—Nucléar, also called the “Nucléart” method, is so efficient that it is used to produce high-quality parquet, even with woods usually not appreciated for this type of application. A total dose of 20–30 kGy is required.

Densified parquet is used in places subject to high pedestrian traffic, such as museums and airports [14]. In the IAEA document [14], cases of parquet consolidation using gamma radiation are reported in Grenoble (the Old City Hall, which was becoming the Stendhal Museum) and in Viviers. The Grenoble floor is still in very good condition after four decades, despite the number of visitors to the museum.

It must be said that, differently from what happens with decontamination, consolidation of porous material changes the mechanical properties of the artifact, as well as the physicochemical ones, and these changes are irreversible. Also, the appearance may change. For all of these reasons, although extremely efficient, this process is performed only when it is the only way to preserve the artifact [90].

Unlike wood, there is no radiation technology applicable to improve the mechanical resistance of paper and textiles.

4. Deterioration of CH Artifacts: Causes and Process

First of all, we want to focus attention on the fact that many CH artifacts are made with different materials. This means that deterioration causes and processes may be different for different parts of the artifact, and their responses to decontamination or restoration treatments are also different. It may be the case that the use of some restoration material or procedure on certain components works as a catalyst for water absorption, or directly as an attractor of biodeteriogens for other components (the case of glue in book manufacturing). It may also be the case that the best practices in preserving some materials are inefficient or even dangerous for others.

Materials inevitably degrade; we cannot avoid this, but we can interfere with the speed of this process. To do so, we need an in-depth knowledge of the following:

• The constituent(s) of the material(s) the manufacture is made of;
• The level of deterioration;
• How the constituents interact with the deterioration agent(s) (hence the mechanism of deterioration processes).

While point 3 is part of the acquired scientific knowledge [94,95], points 1 and 2 are specific to each case, and that knowledge must be acquired during the study and preliminary diagnosis phase. Since our interest is in CH artifacts, we will consider only materials, deterioration processes, and their causes circumstantial to our field of interest. It must also be noted that the archeological excavations themselves alter the stability of CH manufacturing, since they expose them to solar radiation, atmospheric phenomena, fine dust, and CO₂ [96]. Additionally, restoration interventions usually present recurrent phenomena, such as sulfur and iron-containing efflorescence. Figure 3 shows a case of recurrent efflorescence on the pinewood of a 14th-century vessel stored indoors (Museu Marítim de Barcelona) [97].

A consideration needs to be made: together with the material itself (its elemental composition) and the manufacturing process, the most relevant element to be considered is the surface. Porous materials are more penetrable and, hence, intrinsically more sensitive to external factors, whether they are damaging factors or preservation treatments and investigations.

Different causes interact synergistically, to the point that P. Querner et al. speak of Integrated Pest Management (IPM) [98].

4.1. Inorganic Materials

Inorganic does not mean not interacting with living matter, but inorganic natural materials, either raw or obtained through human manipulation, are generally particularly stable from a chemical point of view. Even when subjected to aggressive agents like wind or rain, they preserve their chemical composition despite visible mechanical damage.

Although considered eternal, stone may be attacked by different kinds of pathogens but, usually, it is the synergistic action between atmospheric agents and microorganisms [94,95,99,100,101,102] that deteriorates it. Biodeteriogens take advantage of the stone deterioration produced by wind, water, and other atmospheric agents in terms of cracks and increased porosity in the material. The consequence of the biodeteriogens’ metabolic activity is the production of acid, which dissolves the minerals of the stone; this facilitates the degradation produced again by weather events and by the growth of vegetation, and the synergistic cycle keeps going [84,94].

4.2. Organic Materials

As in the inorganic case, when dealing with organic matter degradation, processes are due to a synergy of multiple factors. Even in this case, environmental conditions prepare the soil for the deterioration process. Particularly, moisture plays a leading role, since fungal attacks require humidity to thrive, and proteins degrade through hydrolysis. It is important to note that, although all of the different factors interact together, the presence of pests (insects/biopathogens) is the most detrimental, being the factor that accelerates the degradation process the most.

Biopatogens may be present in a dormant condition in the artifact, waiting for the right environmental conditions to emerge, and the manufacturing procedure used to create the artifact highly affects the development of the deterioration process.

The organic materials of interest, due to their abundance in indoor artifacts of artistic value, are as follows:

• Wood;
• Paper;
• Parchment;
• Leather;
• Textiles.

Their worst biodeterioration factors are fungi, bacteria, and insects.

4.2.1. Wood

Insects, like termites, destroy the wood, but they do not destroy the cellulose, which is instead metabolized by microorganisms; different biopathogens mostly work in synergy.

The most important component of wood is cellulose, and the second is lignin; their structure and behavior towards water are opposite, as you may immediately see from

Table 5. Cellulose and lignin characteristics.

Component	Structure	Behavior
Cellulose	Highly organized	hydrophilic
Lignin	amorphous	hydrophobic

.

Biological deterioration of wood may start at humidity levels of 20%. At this humidity level, fungi become active. There are mainly three different types of fungi of interest for CH artifacts, and they are named by the color of the decomposition material they produce; in fact, they are recognizable by the naked eye: white rot, brown rot, and soft rot [103].

Although fungi and bacteria may also have high toxicity for humans, the worst, in terms of physical destruction of CH artifacts made of organic materials, are insects, because they tremendously speed up the degradation process. Holes and tunnels are the results of insect activity, which become visible only when the damage is at an advanced state.

Different insects prefer different foods; some are able to eat anything, while others need to live in symbiosis with microorganisms.

4.2.2. Paper

The manufacturing process of paper varies across different historical periods. This means that the structure and composition of paper, and hence its quality and susceptibility to deterioration, differ. Two main events signify a sharp change in the manufacturing process of paper, namely:

• 1799, the Fourdrinier machine;
• 1844, groundwood pulp.

Moreover, the end of the eighteenth century saw the beginning of the use of bleaching agents to whiten paper. What primarily determines the strength and quality of paper are the fibers. With the utilization of groundwood pulp, fiber sources shifted from cotton or linen rags to wood. This change results in different percentages of cellulose and lignin, leading to a different fiber structure in the final paper.

Table 6. Percentage of cellulose for different fiber sources [94].

Fiber Source	Cellulose %
Cotton	95
Linen	80
Wood	45
Grasses	30

reports the percentages of cellulose in the various raw materials utilized in the evolution of paper production.

Paper of different ages undergoes distinct deterioration processes and responds differently to irradiation; old paper exhibits better quality in terms of mechanical resistance and chemical stability, making it less prone to biodegradation than modern paper [14,18,21,104]. Because it is highly hydrophilic, paper, in general, is less resistant than wood to biodeterioration. Due to its hydrophilicity, paper is particularly vulnerable to microscopic biodeteriogens. Some fungi can grow even at humidity levels as low as 8%. Speaking of paper, insect attacks are a secondary effect related to the presence of other materials needed, e.g., for the creation of books (glue, leather); these materials are the real targets of insects. The results of insect presence on paper are similar to those observed on wood, such as holes and tunnels.

4.2.3. Leather and Parchment

Leather and parchment are produced from the skin of animals; hence, they are strongly biodegradable. The part of the skin used to create leather goods is the dermis, because only this layer of the skin provides the good mechanical properties of the final product [14]. The leather manufacturing process is not an easy task; it involves different steps, among which tanning is a particularly critical one, and it is crucial for determining the lifetime of the manufactured leather. Tanning implies chemical bonds between tanning agents and collagen, while inhibiting hydrophilic chemical groups [14,94]. This reduces the amount of water retention to a level of 15%, which significantly decreases the possibility of microorganism development, although it is still more likely to occur in vegetable-tanned leather.

Aging factors accelerate the biological deterioration process, since they modify the water intake capacity of leather. The principal aging factors are listed in

Table 7. Principle leather aging factors [14,105].

	Aging Factor Typology
Physical	Chemical	Mechanical
Oxidative breakdown of collagen	Catalyzation of hydrolytic degradation of collagen	Mechanical degradation
Photochemical deterioration of collagen	Pollutants Metallic ions	Temperature and humidity fluctuation 1

¹ Together with use.

.

4.2.4. Textiles

Textiles can be divided into two main groups, depending on the origin of their fibers: vegetable and animal textiles. The main components are keratin for wool and fibrin for silk.

Even for textiles, the manufacturing processes are crucial for determining their deterioration and disposal. Loosely woven textiles accumulate dust more easily, and dust is hydrophilic; hence, its presence facilitates water retention and increases humidity levels, which creates a favorable environment for any biodeteriogens. On the contrary, the presence of precious metals, typical of ancient precious textiles, has a microbiocidal and bactericidal action.

While cellulose is more vulnerable to fungi, proteins (the main component of textiles of animal origin) are more vulnerable to bacteria. However, insects pose the greatest danger of all to textiles.

5. Conclusions and Future Prospective

For preserving CH for future generations, the capability to recover and increase the durability of works of art is fundamental. EM techniques may be of the greatest help, combining effectiveness and safety. The main obstacles to the widespread use of these techniques are distrust of their applications and a lack of standards.

Regarding the first point, the distrust towards these techniques is mostly related to a lack of knowledge. When compared to more traditional techniques like chemical methods, irradiation techniques are safer, and the results are long-lasting. Dissemination of knowledge will probably be the best solution to this issue. Regarding the second point, the lack of a reference point for accessing scientific information in preserving CH, and the need for official standards and best practices to which the different actors involved in the conservation of CH may refer, is outlined by the International Atomic Energy Agency (IAEA) [14,35]:

• “There is broad agreement across networks that although attention is paid to the dissemination of sound scientific developments […] improvements can be made in terms of bringing the results to where they have the most impact” [14] (Introduction, p. 2, © IAEA https://www.iaea.org/publications/10937/uses-of-ionizing-radiation-for-tangible-cultural-heritage-conservation (accessed on 2 January 2025));
• “Besides networking, there is a continuous need for establishing good practice procedures and standards in the field of safeguarding cultural artifacts” [14] (Introduction, pp. 1–2, © IAEA https://www.iaea.org/publications/10937/uses-of-ionizing-radiation-for-tangible-cultural-heritage-conservation (accessed on 2 January 2025));

Experience gathered in recent decades all over the world has proven the validity of applying EM irradiation techniques for the conservation of CH artifacts. Further developments will surely arise if actions are taken to convert distrust into conscious trust through knowledge acquisition.