# Natural and Artificial Aging Methods For Silver Mock-ups In Recent Conservation And Heritage Studies: A Short Review

Qing Wu, Han Zhou, Loïc Bertrand, Maartje Stols-witlox, Laura Brambilla, Edith Joseph

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Qing Wu, Han Zhou, Loïc Bertrand, Maartje Stols-witlox, Laura Brambilla, et al. Natural and Artificial Aging Methods For Silver Mock-ups In Recent Conservation And Heritage Studies: A Short Review. Chemistry–Methods, 2025, 5 (5), pp.e202400055. 10.1002/cmtd.202400055. hal-04965618

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Submitted on 25 Feb 2025

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# Natural and Artificial Aging Methods For Silver Mock-ups In Recent Conservation And Heritage Studies: A Short Review

Qing Wu,*[a] Han Zhou,[b] Loïc Bertrand,[c] Maartje Stols-Witlox,[b] Laura Brambilla,[a] and Edith Joseph*[a]

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Silver has been widely used for art and heritage objects throughout history, due to its excellent ductility, malleability and brilliant metallic lustre. However, silver objects tarnish over time and must be regularly treated to maintain their appearance. In conservation and heritage studies, naturally or artificially aged silver mock-ups are often used to simulate the tarnishing conditions of valuable historical silver objects. Over the last few decades, many research groups have proposed different aging protocols. In this paper, we reviewed the silver aging protocols employed in recent studies in the field.

Although the chemistry-based aging methods are most widely used, issues regarding alterations of the tarnish stratigraphy and negative influences on environment and operator safety should not be neglected. This review aims to provide important references for conservators, scientists and researchers to select safe and suitable aging methods for building their own mock-up systems, and to encourage the development of more sustainable aging protocols for future conservation of historical silver objects.

# 1. Introduction

Silver (Ag) is a noble and precious metal with a very high electrical and thermal conductivity. Silver and silver alloys have been extensively employed in human history to produce artifacts such as coins, jewellery, liturgical vessels, medals, tableware, ornamental works, etc., that exhibit high economic, aesthetic, cultural and historical value.[1,2,3] The main disadvantage of using silver lies in its vulnerability to pollutants such as sulphur-containing compounds in the atmosphere, causing surface darkening and loss of the metal lustre.

Historical silver artefacts are typically exhibited and stored in indoor environments such as museums or private collections. In the context of metal objects in indoor environments, the term 'tarnish' refers to the formation of a thin corrosion layer on the metal surface.[4] Previous studies have revealed that in museum environments, the gaseous sulphur-containing compounds such as hydrogen sulphide (H₂S) and carbonyl sulphide (COS) are the main cause for the tarnishing of silver objects.[5] The formation of silver tarnish (on a pure silver substrate) causes a colour change by the conversion of lustrous metallic silver into greyish black silver sulphide (Ag₂S) via dark brown/black silver oxide (Ag₂O) under the influence of moisture from the air.[5,6] Thin film interference effects can also cause the tarnish layers to appear blue or reddish.[7] The tarnishing mechanism can be described as: when exposed to air, the top atomic layer of silver quickly oxidizes and lowers the surface energy.[8] This oxide monolayer is stable at ambient temperatures in the absence of water. However, in the presence of humidity, water is adsorbed on the silver surface, providing a

medium for the dissolution of the silver oxide layer which makes it reactive to gaseous compounds such as H₂S and COS, and results in the precipitation of corrosion products such as acanthite (Ag₂S).[5,9,10] The thickness of the adsorbed water layer increases with humidity, thus enabling an increase in the rate of corrosion.[10] Therefore, relative humidity (RH) is a critical factor for the silver tarnishing, as it controls the thickness and extent of the water layer on the silver surface.[11] An overall reaction of the most common silver tarnish product is presented in the equation below:

$$4 \text{Ag(s)} + \text{O}_2 + 2\text{H}_2\text{S} \rightarrow 2\text{Ag}_2\text{S(s)} + 2\text{H}_2\text{O}^{[4]}$$

It is worth mentioning that it is impractically difficult to fully avoid such invasive gases even within museum showcases, which causes a certain tarnish rate and further leads to a continuous program of recleaning silver artefacts.[12] The consequences of repeated cleaning can cause the loss of some fine details on the silver artefacts, and result in silver-enriched surfaces in the case of silver alloys.[12,13]

Since silver is a soft material, alloying elements, especially copper (Cu), are often added to improve the material's hardness and resistance to mechanical wear and deformation.[14] Many historic silver objects were therefore made of silver alloys with an elemental composition close to sterling silver (92.5 wt.% Ag, 7.5 wt.% Cu).[6,15] Despite its small amount, copper can play a crucial role in the tarnishing process of silver alloys due to its preferential oxidation.[6] This leads to tarnishing that differs from that of pure silver, in terms of mechanisms, composition, and surface microstructure.[15] It is reported that a tarnish layer on historical silver objects can have a yellow brownish to blue-black colour, dependent on the thickness and composition of such layer.[7]

For the conservation of historical silver objects, it is essential to understand the tarnishing mechanisms and factors influencing the nature of tarnish products, which are the scientific foundation for the design and development of conservation concepts. For example, the removal of silver tarnish is a common task in the conservation of silver objects. Due to the high cultural, historical and economic values of historical silver artefacts, it would be an unacceptable risk to apply untested cleaning protocols directly on them. Furthermore, some research, especially those studying the dynamic tarnishing or corrosion phenomena, requires silver samples that can be systematically aged to different stages of corrosion. Therefore,

[a] Haute École Arc Conservation-Restau ration, HES-SO University of Applied Sciences and Arts of Western Switzerland, Espace de l'Europe 11, CH-2000, Neuchâtel, Switzerland

[b] Conservation and Restoration of Cultural Heritage, University of Amsterdam, BG2 Turfdraagsterpad 15-17, 1012 XT, Amsterdam, The Netherlands

[c] Université Paris-Saclay, ENS Paris-Saclay, CNRS, PPSM, 91190 Gif-sur-Yvette, France

Correspondence: Dr. Qing Wu and Prof. Edith Joseph, Haute École Arc Conservation-Restau ration, HES-SO University of Applied Sciences and Arts of Western Switzerland, Espace de l'Europe 11, CH-2000, Neuchâtel, Switzerland.

Email: qing.wu@he-arc.ch and edith.joseph@he-arc.ch

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© 2025 The Author(s). Chemistry - Methods published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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in many conservation and research studies, a mock-up system containing silver or silver alloy coupons, either naturally or artificially aged, is employed to simulate or reproduce the tarnished condition of historical objects. To achieve this goal, many research groups have proposed different aging protocols over the last few decades. However, some such protocols have issues of toxicity and thus create concerns for both operators and the environment, while some others can affect the evaluation of cleaning efficacy. In the current study, we present a review of aging methods employed in recent studies in the conservation field. Factors influencing the tarnish formation, individual aging methods and protocols employed, as well as corresponding aging results, are discussed in detail. It also presents the risks and consequences of specific aging methods.

This review study aims to provide important references for practitioners and researchers to select safe and suitable aging methods for their own practice or studies. At the same time, we expect that this study will bring inspirations into the field and encourage the development of innovative, green(er) aging protocols, which should meet the satisfaction of the sustainability requirements in the conservation field while still resembling the real tarnishing situations on historical silver objects with a high reproducibility.

## 2. Application Of Aging Methods In Conservation And Heritage Studies

In general, the formation of tarnish on silver mock-ups, more specifically silver coupons, is affected by the composition and surface treatment of the metal substrate and the aging that was

achieved naturally or with artificial acceleration. These factors often influence the aging results in an integrated manner.

### 2.1. Metal Substrates

#### 2.1.1. Substrate Composition

Although historical silver objects were mainly made from silver alloys, coupons made from pure silver (i.e., Ag999) continue to be used,¹⁴,¹⁶,¹⁷,¹⁸,¹⁹,²⁰,²¹,²²,²³ as they allow a focused study of silver tarnishing products without complicating factors from other metals such as copper.¹⁸ This is especially beneficial for the investigation of dynamic tarnishing processes using specific analysis techniques such as Raman spectroscopy¹⁹ and can provide important references, supporting the identification of tarnishing products on silver alloys. In general, the aging results from pure silver coupons cannot be representative for historical silver objects, in which the presence of copper, even in a small amount, results in a crucial difference in the tarnishing process due to its preferential oxidation.⁶,¹⁵ Nevertheless, some historical silver-alloy objects have undergone repeated cleaning treatments, which has often resulted in silver enrichment, especially on highly polished surfaces.¹³ In such cases, the tarnishing products could be similar to those formed on pure silver.

Compared to pure silver, sterling silver has an alloy composition closer to historical objects and thus coupons made from sterling silver are often selected by research groups.¹,⁷,¹⁵,²³,²⁴,²⁵,²⁶,²⁷,²⁸,²⁹,³⁰,³¹ Additionally, in some studies coupons made from specific silver alloys are also produced for mimicking the tarnishing performances of objects from certain regions and epochs. For example, Guidera *et al*. produced silver

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Qing Wu studied her BA and MA in Art Conservation at the Bern Academy of the Arts, Switzerland, specializing in painting and sculpture. Her Ph.D. investigated medieval gilding systems at the University of Zurich. After her PhD, she worked at the Cologne Institute of Conservation Science, Germany. She is now using her skills in the intersection of conservation, art technology, and materials analysis as a postdoc at the Haute École Arc, University of Applied Sciences and Arts of Western Switzerland, to develop innovative green methods for metal cleaning in the “GoGreen” EU project.

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Edith Joseph received a PhD degree in chemistry from University of Bologna, 2009. From 2010 to 2023, she was granted with a Marie Curie IntraEuropean Fellowship at the Swiss National Museum (BAHAMAS, 2010–2012), and a Swiss National Science Foundation (SNSF) Ambizione grant (MAIA, 2013–2016) followed by a SNSF Professorship (MICMAC and Get on Board 2016–2023) at the University of Neuchâtel. Currently, she is Full

professor at the Haute Ecole Arc Conservation Restauration, University of Applied Sciences and Arts HES-SO. Her research interests focus on the development of green and sustainable conservation strategies for cultural heritage.
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sheets with an alloy of 80% Ag and 20% Cu, representing the Chimú silver.[32] Ingo et al. produced two types of silver-copper-lead (Pb) alloys to imitate the silver artifacts made from the Phoenician Punic ages to Roman periods, including the CNR91 type (97.0% Ag, 1.5% Pb, 1.5% Cu) and the CMR141 type (92.5% Ag, 7.3% Cu, 0.2% Pb).[33] Ingo's study shows that due to the different alloy compositions, the tarnishing appearances and tarnish compositions differ: tarnish on the CNR91 alloy (lower Cu content) mainly consisted of Ag₂S and chlorargyrite (AgCl), while additional corrosion products such as covellite (CuS) were observed on the CMR141 alloy (higher Cu content).[33] Vassiliou et al. employed three types of coupons for the imitation of ancient silver alloys, including Ag970 (97.0% Ag, 1.5% Cu, 1.5% Pb), Ag920 (92.0% Ag, 6.5% Cu, 1.5% Pb) and Ag965 (96.5% Ag, 3.5% Cu).[25] The tarnish layer created was composed of AgCl and some paratacamites (Cu₃(Cu,Zn)(OH)₆Cl₂) on the Ag970 alloy, AgCl and cuprous chloride (CuCl) on the Ag920 alloy, and AgCl and a very small amount of Cu₃(Cu,Zn)(OH)₆Cl₂ on the Ag965 alloy.[25] De Caro et al. studied the tarnishing processes of pure silver and six types of Ag–Cu alloys with Cu contents ranging from 1% to 92.5% (details presented in Table 1).[14] Depending on the Cu content of the substrate, the tarnishing products created on these alloys contained one or several of sulphide compounds such as Ag₂S, jalpaite (AgₓCuᵧS), mckinstryite (AgₓCuᵧS), stromeyerite (AgCuS), CuS and chalcocite (Cu₂S).[14]

Regarding the sample dimension, most coupons reported were rectangular or square with side lengths ranging from one to a few cm, and the thickness usually less than 2 mm (see Table 2). There are a few exceptions where the coupons were made in the shape of coins with a thickness of 2 mm.[14,25,33]

While most studies used silver coupons as test samples, the use of sacrificial objects is also observed. For example, De Figueiredo et al. used out-of-circulation Brazilian coins that were made of an alloy of 50% Ag and 50% Cu[34] and an alloy of 90% Ag and 10% Cu.[35] The surface details on these coins allowed the evaluation of the aesthetic impact of a proposed cleaning method.

### 2.1.2. Substrate Finish

Before being subjected to aging, silver samples often receive surface treatments. The purposes of such treatments are (i) to create a standardized and reproducible surface condition; (ii) to remove any potentially existing tarnishing layer; (iii) to mimic the surface conditions of target objects. Surface polishing is a common form of surface treatment, for which various materials can be used, including grinding paper of different grits,[1,19,24,27,33] polishing slurries or pastes,[1,15,17,18,20,29,34,35,36] as well as abrasive powder (e.g. calcium carbonate CaCO₃) of different particle sizes.[17,18] In most cases, a mirror-like surface finish is aimed for, which is considered similar to the highly polished areas of historical silver objects.[15,20,29,33] In some cases, a matte surface is desired for mimicking rougher surface areas on the objects. For example, Tissot et al. polished sterling silver coupons with emery paper of different grits up to 1000 mesh to obtain a

rough surface that resembled 'normal' silver heritage objects.[24] In addition to mechanical polishing, other research groups also employed some unusual processes with the purpose of a thorough removal of the existing tarnish before performing the artificial aging. For example, Águas et al. cleaned the surface of pure silver coupons with electrolytic reduction, and the resulting surfaces were described as "shining".[16]

In some instances, special surface treatments are carried out to achieve desired surface effects. For example, van Bennekom et al. employed a traditional method of repeatedly annealing and pickling silver alloy coupons in a 5% sulfuric acid solution to create a silver enrichment layer on the surface, with which the tarnishing situations on the coupons were close to those on the objects studied.[26] Wolfe et al. electroplated sterling silver coupons with 24 carat gold to resemble gilded silver objects.[28] Tobisch et al. compared the aging results of samples treated with machine-polishing, hand-polishing, hand-hammering, and untreated samples.[7] Through such treatments, the surface conditions of the samples are altered, which could further affect the formation of tarnish.

After surface polishing, samples are often rinsed or swabbed with water or solvents (e.g., ethanol) to remove the residues of polishing materials. This process is often aided with an ultrasonic bath.[15,19,20,22,24,33,35]

## 2.2. Aging Methods

### 2.2.1. Natural Aging

Natural aging can provide genuine tarnishing products in real-life situations. Procedures involving natural aging are often used in studies of silver corrosion mechanisms or those investigating environmental impacts on the preservation of silver objects.[12,15,16,17,18,33,36] For instance, Águas et al. exposed pure silver coupons on selected museum displays at different height levels and in areas outside of the museum for one-, three- and six months.[16] The authors stated that with natural aging the corrosion mechanisms were strongly dependent on the exposure location and time. In Águas's study, the main corrosion products included Ag₂S, AgCl and Ag₂O.[16] The tarnish layer thickness was observed to increase with exposure time, and the increase was faster on the coupons placed outside of the museum compared to those inside. The concentration of Ag₂S decreased with the increase of thickness, indicating that Ag₂S was not stable and tended to be confined towards the external layer of the tarnish.[16] For those coupons placed on the museum displays, the concentration of AgCl increased with tarnish thickness, while for those outside of the museum the AgCl concentration remained high, indicating that chlorine was the main pollutant responsible for the difference in tarnish thickness between the coupons exposed inside and outside of the museum.[16] The formation of Ag₂O was observed to be slower but more stable than Ag₂S. The concentration of Ag₂O increased with tarnish thickness and exposure time and with a decrease of Ag₂S, suggesting the possible replacement of Ag₂S itself.[16] Compared to the interaction between silver and H₂S,
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silver reacted with chlorine more slowly, but the reaction product AgCl was more stable than Ag₂S and more important to the kinetic of the growth of the tarnish layer.[16]

Fukuda et al. emphasized that when using natural aging methods, the tarnishing mechanisms of silver (and copper) were strongly dependent on the environment where the samples were aged.[36] In their study of the indoor corrosion of copper and silver in Japan and Asian countries, pure silver and pure copper coupons were exposed at 23 sites in four Asian cities (i.e., Bandung, Singapore, Bangkok and Tokyo) for one to six months.[36] By comparing the tarnishing phenomena on the coupons at each site, the authors concluded that air pollution considerably influenced corrosion even in indoor environments: in environments containing a low content of H₂S pollution, silver reacted not only with H₂S but also with other pollutants such as nitrogen dioxide (NO₂), hydrogen chloride (HCl) and sulphur dioxide (SO₂), producing Ag₂S, silver nitrate (AgNO₃), AgCl and silver sulphate (Ag₂SO₄) respectively, while in environments with severe H₂S pollution, the reaction of silver was limited to that with H₂S and only Ag₂S was produced.[36]

An interesting tarnish complex has been observed by Hallett et al. on their pure silver samples, which has been exposed in different showcase and gallery locations in the British Museum as well as in an outdoor location protected from rainfall.[12] SIMS and XPS measurements of such naturally tarnished samples showed that silver sulphide was a minor component in the tarnish complex while some organic species were the majority in the tarnish composition.[12] This phenomenon was not only observed on the tarnish surfaces but in the whole tarnish layers that were formed in the early stages of tarnishing.[12] However, the SEM results showed that the species other than silver sulphide were formed over much longer time scales.[12] The presence of organic species in the tarnish complex may explain why the tarnish on some museum silver objects can be removed with acetone.[12] The authors also mentioned that the source and identity of such organic species in the museum environments was an interesting topic and worth further study.[12]

In contrast to those studies in which pure silver samples were used, Ingo et al. exposed Ag–Cu–Pb alloy coupons to indoor natural aging.[33] The coupons were placed in a museum showcase, together with silver and gold artefacts, and in the open air of a museum corridor for six months.[33] The authors noted that both manufacturing techniques (i.e., cast ingots and mechanically worked ingots in this case) and aging environments were critical for the tarnishing mechanisms and tarnish appearances of silver alloys. In the open air, coupons were observed to gain a thin light brownish tarnish layer, while a blueish patina, constituted by platelets, scale-like protuberances or clumps, was observed on the surface of coupons stored inside of a showcase.[33] The coupons made from cast ingots with a lower Cu content seemed to be more reactive than those made from the mechanically worked ingots with a higher Cu content; the main tarnishing products on both types of coupons were Ag₂S and AgCl, but with some CuAgS and Cu₇,₂S₄ associated with a lower Cu content and with Cu₂S and CuAgS associated with a higher Cu content.[33] The authors furthermore

mentioned that the deposition of particulate matter (e.g., sodium chloride NaCl, crustal elements Ca and Si) could also significantly affect the tarnishing behaviour, and the particulate aggressive species were responsible for the modification of the tarnishing products nature. The formation of Ag₂S and AgCl showed different corrosion kinetics: Ag₂S seemed to follow a linear corrosion law, while AgCl could form a more protective tarnish layer and thus presented a passivating behaviour.[33]

In their study regarding the sulfidation process of sterling silver in different corrosive environments, Storme et al. carried out two natural aging processes on sterling silver coupons, exposing the samples in (i) an indoor ambient atmosphere at a private residence and (ii) a chemical laboratory environment.[15] After six months of exposure, almost no visible colour change was observed on the samples exposed in (i), while in (ii) the coupons were observed to be covered with a brownish tarnish layer, which gradually darkened over time.[15] The authors furthermore noted that both exposure sites may result in a strong formation of oxides.

The above-mentioned cases of natural aging share some similarities in the aspect of tarnish formation. For example, Ag₂S is the major or at least one of the major products in the tarnish layers. However, since the implementation of natural aging is often site-specific and sometimes involves uncontrolled environmental conditions, the reproducibility of such methods is rather limited. Another critical issue with natural aging is exposure time. Natural aging processes involved in the above-mentioned studies were carried out within a few months, or up to a couple of years. Using natural aging on silver samples to reach a tarnishing level close to historical objects is impractical for most conservation research projects with a limited time frame.[15] In the context of historical silver objects, the results of natural aging are only representative for the early stages of tarnishing.[15]

### 2.2.2. Artificial Aging Processes

Compared to natural aging, artificial aging methods are more commonly employed in conservation projects, especially in those focusing on the investigation of tarnishing mechanisms or evaluation of conservation treatments such as cleaning.[1,7,14,15,17–32,34,35,39] Although the tarnish obtained with artificial aging may slightly differ from that produced in the natural aging processes, artificial aging provides the important benefit that it can be conducted within a manageable timeframe while still providing referential insights into the tarnishing mechanisms. Moreover, within an exposure time ranging from a few minutes to several days, tarnish can be generated in a well-controlled environment. From this aspect, artificial aging is a highly reproducible process. Since the major tarnishing reagents for historical silver objects are sulphurous gases,[5] creating a sulphur-rich environment, specifically H₂S atmosphere, is the main mechanism employed in artificial aging methods.

A commonly used reagent for creating a H₂S-rich atmosphere is thioacetamide (TAA) with a chemical formula of
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$C_2H_5NS$. [37] According to the DIN EN ISO 4538:1995, TAA can slowly decompose and release $H_2S$ vapour at a constant relative humidity (RH) of 75% and room temperature, with the aid of a super-saturated sodium acetate trihydrate ($CH_3COONa·3H_2O$) solution in an enclosed container.[38] However, saturated sodium sulphate ($Na_2SO_4$) solutions (with which even higher RH can be created during the TAA aging process) find more frequent use, possibly for a better control of the tarnishing rate. For example, Storme *et al.* employed a saturated $Na_2SO_4$ solution to create a constant RH of 91%, at which sterling silver coupons were exposed to an $H_2S$ atmosphere created by TAA for a maximum time of one week,[15] while Selwyn and McKinnon used a $Na_2SO_4$ solution to create a RH of 97–98%, at which pure silver coupons were tarnished by TAA for 24 h.[20] Selwyn and McKinnon reported that with the TAA method the pure silver coupons underwent a colour change from brown (with a tarnish layer thickness of 12.4 nm) to blue (with a tarnish layer thickness of 25.7 nm), within this exposure time.[20] This colour change was mainly due to the increase in thickness of the tarnish layer, in which $Ag_2S$ was the main product, but was also related to the tarnishing rate.[20] The latter can be explained as follows: the tarnish that was formed rapidly in an artificial atmosphere showed interference colours over a narrower range of thickness than that was formed slowly in natural environments,[20] In other words, the artificially aged silver appeared darker than the naturally aged silver, given the same tarnish thickness. The authors also mentioned that in comparison to their previous study in which freshly hard-boiled eggs were used, the TAA method produced a thinner tarnish layer.[20]

The colour change of sterling silver during the aging process with TAA was also reported by Storme *et al.*[15] After one week's exposure with TAA, the sterling silver coupons exhibited a greyish and matt surface, which was close to the appearance of natural tarnish after 5 months' exposure in a residential area, using the same type of samples.[15] The tarnish was composed of Cu-rich and Ag-rich sulphide compounds and the tarnished coupons displayed a layered structure including (from bulk metal towards surface): (i) a bulk silver with copper inclusions (ii) a porous interface (i.e., presence of voids) between the bulk metal and a sulphide layer, and (iii) a compact sulphide tarnish layer with a thickness of 0.3 µm.[15] In addition to the TAA method, the authors also implemented two other artificial aging methods: aging with $H_2S$ and $SO_2$ gases, and aging with sodium ($Na_2S$) solutions. They observed that the tarnished surfaces resulting from different sulphidation environments appeared very similar but showed significant differences at a microscopic level.[15] For example, the surface morphology resulted from TAA exhibited a needle-like structure, while after immersion in $Na_2S$ solution the presence of small and coarse grains obscured all surface features.[15] Furthermore, through scanning electron microscopy (SEM) imaging on sample cross-sections, it was observed that the tarnish layers formed with different aging methods showed significantly different morphology and thickness.[15] Based on such observations, the authors warned that the silver tarnish formed with artificial aging methods was not necessarily representative of naturally tarnished objects, and different aging methods can lead to

different tarnishing phenomena and products.[15] Tobisch *et al.* have performed similar aging procedures on sterling silver as Storme *et al.* but with samples treated with different metalworks or surface finishes.[7] It showed that both metalworks and aging methods were critical for the subsequent aging phenomena, i.e., the machine- and hand-polished samples received stronger corrosion attacks during the natural aging process, while the corrosion phenomena were more profound on the original (i.e., no metalwork or surface treatment applied) and machine-hammered samples.[7]

The void-forming issue noted with the TAA method is worth some further attention. Using SEM imaging on the sample cross-sections aged with TAA, a porous interface with a substantial number of voids between the tarnish layer and the metal substrate can be observed, which could lead to a poor adhesion of the tarnish layer to the base metal.[15] According to Storme *et al.*, the formation of voids continued during the aging process and the tarnish layer started to peel off at a certain stage. Therefore, the employment of TAA to create tarnish could be problematic if cleaning treatments need to be assessed, because of the poorly adhered tarnish layer created.[15]

In addition to TAA, some metal-sulphide salts including sodium ($Na_2S$) and potassium sulphide ($K_2S$) are used for producing a sulphur-rich atmosphere since $H_2S$ is released with the addition of water.[27,31] A long aging process using barium sulphide (BaS) powder together with water was implemented by Grabow *et al.* and van Bennekom *et al.*[6,30] Sterling silver coupons were hung above a petri dish containing BaS powder and water for a total exposure time of 3000 hours (125 days). The total amount of $H_2S$ was measured to be 6 ppm.[30] This method was also employed by Scheepers *et al.* for aging pure silver coupons, with a reagent/coupon area ratio of 5 g BaS per $20 \text{ cm}^2$ coupon surface.[17,18] In addition to metal-sulphide salts, some research groups also used sulphuric acid ($H_2SO_4$) vapour to tarnish their samples.[39]

In some studies, $SO_2$, which is a naturally occurring atmospheric pollutant as well as an artificial pollutant in atmospheres near heavy industry is used to age silver samples and compared with $H_2S$.[15,19] The employment of high concentrations of $SO_2$ and $H_2S$ allows to investigate the effect of these two sulphuric gases on the formation of tarnish on both pure silver and silver alloys in detail. Marina *et al.* reported that pure silver had a relatively low reactivity towards $SO_2$ at a concentration of 500 ppb at 50% and 90% RH; secondary atmospheric corrosion processes such as hydration reaction and chemisorption occurred during an exposure time of 24 h.[19] In comparison, the reactivity of pure silver towards $H_2S$ at the same concentration and same RH was much higher and could be further enhanced by increasing RH.[19] The major tarnishing product after 24 h exposure was observed to be $Ag_2S$ for both gases, which appeared as a blue-brownish layer on the metal surface.[19] Storme *et al.* studied the tarnishing mechanisms of sterling silver exposed to 500 ppb of $H_2S$ and $SO_2$ at 90% RH.[15] The samples were first exposed to $H_2S$ for 24 hours and then to $SO_2$ for 48 hours. During this 72-hour exposure time, the formation of oxides, especially copper oxides, predominated the sulphidation, which is similar to the tarnish created by two
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natural aging methods, using the same type of coupons.[15] It was also observed that some tarnishing product islands agglomerated on top of a uniform tarnish layer.[15] Compared to the TAA method and the immersion method with Na₂S solutions, there were fewer voids at the interface between the base metal and the tarnish layer.[15]

Regarding the immersion method with aqueous solutions of sulphurous salts, the most widely used salt is Na₂S;[15,21,22,24,27,28,29] some other metal sulphides such as K₂S are also employed, but with a much lower frequency.[23,31] For example, Bernard *et al.* dipped pure silver plates into a 0.1 M Na₂S solution for 1 h and reported that the concentration of the solution and the exposure time were sufficient to create a blackish tarnishing layer on the sample surface.[21] Evesque *et al.* exposed their silver specimen to an aqueous 0.5 M NaCl solution containing 10 mM Na₂S for 1 h.[22] In their depth-probing study of aging results for sterling silver, Schalm *et al.* exposed coupons in subsequent 60 second cycles consisting of a short immersion (12 seconds) in a Na₂S solution with a 0.1 M concentration, followed by a 48-seconds exposure to air.[29] They observed several surface states of tarnishing during the aging process: metallic appearance (State 1), then a colour change going from yellow (State 2), through blue (State 3) and grey (State 4) to a dark grey (State 5).[29] At the same time, the dominance of the tarnishing products correspondingly shifted from cuprite (Cu₂O), tenorite (CuO) and chalcocite (Cu₂S) (State 2), through Cu₂O, CuO, Cu₂S and CuS (State 3), to mainly CuS with some Cu₂S (State 4), and finally Ag₃CuS₂ and Ag₂S (State 5).[29] The authors stated that due to the preferential oxidation of copper in sterling silver, copper was displaced from the bulk towards the surface, where it accumulated and produced a copper-rich tarnish layer; a silver enrichment layer can therefore form beneath the copper-rich tarnish layer.[29] Furthermore, at the same time, the Cu inclusion<fnref n="2" /> beneath the surface were transformed into voids. Following the oxidation of silver, such voids became larger, resulting in a weakened attachment of the tarnish layer to the base metal, which would eventually lead to the exfoliation of the tarnish itself.[29] Schalm *et al.* also noted that at a later stage of the aging process (e.g., after 20 min), when the silver oxidation started to be more dominant than the copper oxidation, a thin film of Ag₂S was formed on the surface.[29] According to the authors of this article, the presence of a Ag₂S film may be an important factor for the assessment of cleaning efficacy. Indeed, copper chelating agents are used in the conservation field to remove copper-related products from tarnished sterling silver.[29] An Ag₂S film on the surface would act as a barrier, hindering the reaction between the copper-specific chelating agents and the copper products beneath the Ag₂S film.

De Caro *et al.* implemented an interesting experiment using a dilute ammonium sulphide ((NH₄)₂S) solution for immersing Ag–Cu alloy coupons with different elemental ratios for 15 min.[14] Using micro-Raman spectroscopy, various Ag–Cu–S products were identified, including Ag₁.₅₅Cu₀.₄₅S, Ag₁.₂Cu₀.₈S and

AgCuS, along with CuS and Cu₂S.[14] The authors mentioned that for pure silver and pure copper, the tarnishing products Ag₂S as well as, respectively, Cu₂S and CuS were stable in ambient atmosphere, while for Ag–Cu alloys the sulphidation process followed the pseudo-binary system of Ag₂S-CuS, in which three defined ternary phases containing 33.3% of S corresponded to AgCuS, Ag₁.₂Cu₀.₈S and Ag₁.₅₅Cu₀.₄₅S.[14] Each of these compounds could coexist with CuS or with silver, but only Ag₁.₅₅Cu₀.₄₅S can coexist with Ag₂S.[14] De Caro's study disproved a previous theory, namely that the tarnishing process of Ag–Cu alloys only involves the formation of silver sulphide and copper sulphide, and thus provided valuable evidence for the understanding of the tarnishing mechanisms of Ag–Cu alloys.

Another interesting immersion solution, alkaline thiourea (SC(NH₃)₂), was employed by De Figueiredo *et al.*[34,35] It is worth noting that acidic thiourea is a major ingredient of commercial silver dips, because of its complexing property to silver ions.[20,41] In strongly alkaline environments, thiourea decomposes, releasing sulphide ions.[34] The authors immersed Ag–Cu coins (50% Ag, 50% Cu) in an alkaline thiourea solution at 0.01 M concentration for 3 h[34] and other coins (90% Ag, 10% Cu) in an alkaline thiourea solution at 0.1 M concentration for 5 h.[35] With this method, a brownish-black tarnish layer was obtained.[34,35]

A specific artificial aging method was employed by Vassiliou *et al.* for samples mimicking ancient silver alloys.[25] The silver-copper-lead alloy samples were first chemically aged with solutions of NaCl, copper chloride (CuCl₂) and HCl, and then buried in archaeological sites, with an addition of a 5% NaCl solution to the soil.[25] After being buried for six months, the sample surfaces had become covered with a uniform dark brown layer.[25] The tarnish composition was observed to be strongly dependent on the copper content: for alloys with a lower Cu content (Ag970 and Ag965), the main tarnishing products were Ag₂S and AgCl, while for alloys with a higher Cu content (Ag920), the main tarnishing products included CuS as well.[25] This method provided important references for understanding the corrosion mechanisms of ancient silver alloys, which can be interpreted as: exposed copper acted as an anode and silver as cathode; silver was attacked by Cu(II) ions in the presence of chlorine ions and dissolved, forming AgCl, while Cu(II) ions were reduced to Cu(I), forming CuCl.[25] Due to its poor solubility, AgCl precipitated and the amount of silver in the alloy decreased, pushing the reaction towards the formation of AgCl and CuCl.[25]

Artificial aging methods, especially those using chemicals, are usually based on chemical compounds containing S that emit H₂S as tarnishing reagent. In very rare cases elemental S was used as tarnishing reagent, taking advantage of the property that elemental S sublime in a sealed container.[32] The silver samples aged with elemental S showed a low level of corrosion.[32]

It is worth noting that some chemicals employed are toxic, which causes severe concerns in relation to environmental aspects and operational safety. For example, thiourea is a suspected carcinogen inducing dermatitis and can cause skin allergies.[41] As references, GHS labels (Globally Harmonized

<footnote n="2">²Due to the low solubility of Cu in Ag, Cu is present in Ag–Cu alloys in form of pure Cu inclusions or islands in Ag matrix.[6]</footnote>
<figure><img src="image_16.png" /></figure>

<table><caption>**Table 1.** GHS classifications and labels for chemicals that are commonly used in the artificial aging of silver.</caption><thead><tr><th>Chemical</th><th>GHS Classification</th><th>GHS labels</th></tr></thead><tbody><tr><td>Hydrogen Sulphide (H<sub>2</sub>S)</td><td>Flammable; Gas Under Pressure; Acute Toxicity; Hazardous to the Environment</td><td></td></tr><tr><td>Sulphur Dioxide (SO<sub>2</sub>)</td><td>Gas Under Pressure; Acute Toxicity; Corrosive</td><td></td></tr><tr><td>Barium Sulphide (BaS)</td><td>Acute Toxicity; Corrosive; Hazardous to the Environment</td><td></td></tr><tr><td>Sodium Sulphide Na<sub>2</sub>S hydrate (Na<sub>2</sub>S·9H<sub>2</sub>O)</td><td>Flammable; Acute Toxicity; Corrosive; Hazardous to the Environment</td><td></td></tr><tr><td>Ammonium sulphide ((NH<sub>4</sub>)<sub>2</sub>S)</td><td>Flammable; Corrosive; Hazardous to the Environment</td><td></td></tr><tr><td>Thioacetamide (C<sub>2</sub>H<sub>5</sub>NS)</td><td>Serious Health Hazard; Health Hazard/Hazardous to the Ozone</td><td></td></tr><tr><td>Thiourea (SC(NH<sub>2</sub>)<sub>2</sub>)</td><td>Serious Health Hazard; Health Hazard/Hazardous to the Ozone; Hazardous to the Environment</td><td></td></tr></tbody></table>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>

System of Classification and Labelling of Chemicals) for chemicals commonly used in the artificial aging methods for silver are presented in Table 1. It can be seen that most

chemicals that are commonly used in the current silver aging protocols have serious safety issues to both operators and environments. In this context a more promising method is the
<figure><img src="image_39.png" /></figure>

<table><caption>Table 2. Overview of artificial aging methods on silver samples based on literature references. shaded rows: natural aging; unshaded roles: artificial aging.</caption><thead><tr><th>Sample type</th><th>Sample composition</th><th>Sample dimension (length×width×thickness)</th><th>Surface treatment</th><th>Aging method and conditions</th><th>Tarnish appearance</th><th>Tarnish composition</th><th>Analytical techniques<sup>[a]</sup></th><th>Reference</th></tr></thead><tbody><tr><td>High purity silver</td><td>99.9985% Ag</td><td>25×25×1 mm</td><td>Coupons were ground with SiC paper of various grits up to 4000 mesh and then polished with diamond paste (1 µm); after polishing, coupons were washed with ethanol in ultrasonic bath.</td><td>Coupons were weathered with H<sub>2</sub>S and SO<sub>2</sub> gas respectively at 50% and 90% RH for 24 h</td><td>Blue-brownish patina appeared on coupon surfaces.</td><td>Mainly Ag<sub>2</sub>S in H<sub>2</sub>S environment</td><td>OM, Raman</td><td>[19]</td></tr><tr><td>High purity silver</td><td>99.9% Ag</td><td>14×10 mm (thickness unspecified)</td><td>Coupons were electroytically reduced in a 0.5 M HCl solution (to remove existing tarnish) and then washed with distilled water and in an ultrasonic bath with acetone; cold air dried.</td><td>Coupons were exposed inside of selected museum displays and outside of museum for 1, 3 and 6 months.</td><td>It was difficult to visually distinguish between Ag<sub>2</sub>S, AgCl and Ag<sub>2</sub>O, especially when they are mixed - they all appear dark.</td><td>Ag<sub>2</sub>S, AgCl, Ag<sub>2</sub>O</td><td>SE</td><td>[16]</td></tr><tr><td>High purity silver</td><td>99.9% Ag</td><td>30×30×0.3 mm</td><td>Coupons were polished with a gauze and aluminum paste, and then cleaned with distilled water and immersed in ethanol; air dried.</td><td>Coupons were exposed indoor on 23 sites of 4 Asian cities for 1–6 months.</td><td>Coupons tarnished on every site and showed very little pitting.</td><td>When coupons disposed in severe H<sub>2</sub>S polluted environments, tarnish contains only Ag<sub>2</sub>S. When coupons disposed in slightly polluted areas, tarnish contains Ag<sub>2</sub>S, AgNO<sub>3</sub>, AgC1 & Ag<sub>2</sub>SO<sub>4</sub></td><td>OM, EPM, AES, IRS, XPS, XRD</td><td>[36]</td></tr><tr><td>High purity silver</td><td>99.99% Ag</td><td>18×5 mm (thickness unspecified)</td><td>Surface polished with chalk and cotton swabs, cleaned using 50% ethanol</td><td>Coupons were placed above a petri dish containing ca. 5 g barium sulphide per 20 cm<sup>2</sup> coupon for 63 h. The desiccator was sealed using petroleum jelly.</td><td>Coupon surfaces appeared yellow to brown after aging</td><td>Unspecified</td><td>Unspecified</td><td>[17, 18]</td></tr><tr><td>High purity silver</td><td>99.99% Ag</td><td>18×5 mm (thickness unspecified)</td><td>Surface polished with chalk and cotton swabs, cleaned using 50% ethanol</td><td>Coupons were placed in various bathrooms for 166 days.</td><td>Unspecified</td><td>Unspecified</td><td>Unspecified</td><td>[17, 18]</td></tr><tr><td>High purity silver</td><td>99.99% Ag</td><td>13×38×0.25 mm</td><td>Coupons were rinsed with acetone, then polished with a slurry (volume 1:1) of water and 1 µm alumina (Buehler MicroPolish II) on both sides, then attached to Plexiglas (5×5×1 cm) with double-sided tape</td><td>Coupons were aged by suspending in a 9 L glass desiccator containing thioacetamide (TAA) for 24 h. The aging chamber was set up according to the standard ISO 4538:1978, except that</td><td>Dependent on exposure times, coupons appeared light brown, dark brown, dark blue, and dark grey.</td><td>Ag<sub>2</sub>S</td><td>SLV</td><td>[20]</td></tr></tbody></table>
<figure><img src="image_40.png" /></figure>

<table><caption>Table 2. continued</caption><thead><tr><th>Sample type</th><th>Sample composition</th><th>Sample dimension (length×width×thickness)</th><th>Surface treatment</th><th>Aging method and conditions</th><th>Tarnish appearance</th><th>Tarnish composition</th><th>Analytical techniques<sup>[a]</sup></th><th>Reference</th></tr></thead><tbody><tr><td>High purity silver</td><td>Ag content unspecified</td><td>Unspecified</td><td>Undescribed</td><td>and rubbed over a polishing cloth (Buehler MicroCloth) and slurry in a circular motion for 2 min. After polishing, the tape residue was removed with acetone, and coupons were ultrasonically cleaned for 1 or 2 min in Milli-Q water.</td><td>Coupons were exposed to sulphuric acid vapour (aging conditions unspecified)</td><td>Not described</td><td>Ag<sub>2</sub>S</td><td>XRD, EDXRF</td><td>[39]</td></tr><tr><td>High purity silver</td><td>99.95% Ag (from Good-fellow)</td><td>10×10×1 mm</td><td>Surface polished with alumina powder on a rotating felt sheet to obtain a mirror surface.</td><td>Silver plates dipped in a 10 mM Na<sub>2</sub>S aqueous solution was used as the tarnishing solution for 1 h.</td><td>Blackish surface</td><td>Unspecified</td><td>UV-VIS-IR EQCM, chro-nopotentiometry</td><td>[21]</td></tr><tr><td>High purity silver</td><td>99.95% Ag (from Good-fellow)</td><td>10×10×1 mm</td><td>Surface polished by emery-paper up to 4000 grade and finished with alumina on felt, and then ultra-saturated in ethanol for 6 min.</td><td>The silver plate exposed to an aqueous 0.5 M NaCl solution containing 10 mM Na<sub>2</sub>S for 1 h.</td><td>Unspecified</td><td>Unspecified</td><td>Impedance spectroscopy, EQCM, Raman</td><td>[22]</td></tr><tr><td>High purity silver</td><td>99.98% Ag (from Fisons)</td><td>Circular coupons, thickness of 0.12 mm</td><td>Samples abraded with a glass bristle brush and soaked in acetone for 2 min.</td><td>Coupons were exposed in various showcase and gallery locations in the British Museum for 28 days, along with diffusion tubes to measure H<sub>2</sub>S and COS concentrations; exposures and measurements were also performed at an outdoor location protected from rainfall.</td><td>Unspecified</td><td>A large range of ions on the silver surfaces were observed through SIMS, including SO<sub>3</sub>, S<sub>2</sub>, S<sup>-</sup>, Cl<sup>-</sup>, C<sup>-</sup> and a whole range of organic species up to C<sub>6</sub>, including some oxygenated ions. XPS confirmed several species other than silver sulphide including sulphate, chloride, oxide, organic carbon, oxygenated organic carbon, carbonate or carbonyl.</td><td>Colorimetry, SIMS, XPS, SEM-EDX</td><td>[12]</td></tr><tr><td>Silver alloy</td><td>Sterling silver (92.5% Ag, 7.5% Cu)</td><td>40×20×1.3 mm (for flat coupons)<br>60×40×1.3 mm (for curved coupons)</td><td>Flat coupons were punched with hammer and steel puncher with a rounded tip (Ø 1.5 mm); coupons were</td><td>Coupons were exposed in an aging chamber containing freshly boiled eggs for 24 h</td><td>Coupons showed heterogeneous tarnishing appearances ranging from yellow to blue.</td><td>Unspecified</td><td>LSV</td><td>[42]</td></tr></tbody></table>
<figure><img src="image_41.png" /></figure>

<table><caption>Table 2. continued</caption><thead><tr><th>Sample type</th><th>Sample composition</th><th>Sample dimension (length×width×thickness)</th><th>Surface treatment</th><th>Aging method and conditions</th><th>Tarnish appearance</th><th>Tarnish composition</th><th>Analytical techniques<sup>[a]</sup></th><th>Reference</th></tr></thead><tbody><tr><td>Silver alloy</td><td>Sterling silver (92.5% Ag, 7.5% Cu)</td><td>coupon area of 1.5 cm<sup>2</sup>; thickness of 2 mm</td><td>wet ground manually with abrasive paper (first 1000p, then 2500p) in circular movements. Coupons were curved with a vice and steel tube (Ø 30 mm), then wet ground manually with abrasive paper (1000p, 2500p, 4000p) and then wet polished with 3 µm diamond paste in circular movements.<sup>1</sup></td><td>Coupons were ground with emery paper of different grits up to 1000 mesh for a rough surface to resemble normal culture heritage objects; grinding residues were removed with deionised water in ultrasonic bath.</td><td>Colour change of coupons was similar to those published for pure silver sulfidation; with exposure time, coupons appeared light yellow, orange, red, violet, blue-grey, blue, grey, and dark grey.</td><td>From 1 min: mainly Cu<sub>2</sub>O, with some Ag<sub>2</sub>S and AgCuS. From 15 mins: mainly Cu<sub>2</sub>O, with some AgCuS. From 30 mins: Cu<sub>2</sub>O. From 60 mins: Ag<sub>2</sub>CuS<sub>2</sub>. From 480 mins: mainly Ag<sub>3</sub>CuS<sub>2</sub>, with some Ag<sub>2</sub>S.</td><td>Micro-XRF, UV-VIS-IR, SEM-EDX, goniometry, ellipsometry, XRD</td><td>[24]</td></tr><tr><td>Silver alloy</td><td>CNR91 (97.0% Ag, 1.5% Pb, 1.5% Cu) CMR141 (92.5% Ag, 7.3% Cu, 0.2% Pb)</td><td>Coupon diameter of 15–18 mm, thickness of 2 mm</td><td>Coupons were polished with carborundum papers and then diamond pastes up to 1/4 µm to obtain a mirror-like finish and then cleaned with ethanol in ultra-sonic bath; dried and stored in a dry nitrogen atmosphere.</td><td>Coupons were im-mersed in a 0.1 M Na<sub>2</sub>S aqueous solution in an open container for different durations (1 to 7 mins, and then 10, 15, 30, 60, 120, 240, 480 and 1020 mins). After immersion, coupons were washed with de-ionised water and dried in a N<sub>2</sub> stream.</td><td>Dependent on the locations and alloy compositions, coupons were either covered by thin light brownish patina (in the open atmosphere), or dark bluish patina (in the show-case) constituted by platelets, scale-like protuberances or clumps distributed on the alloy surface.</td><td>For alloys with a lower Cu content, tarnish mainly contains Ag<sub>2</sub>S and AgCl. For alloy with a higher Cu content, Ag<sub>2</sub>S, AgCl and CuS are main products, with small amounts of CuAgS and Cu<sub>2</sub>S<sub>4</sub> in a few cases.</td><td>SEM-EDX, OM, XRD, XPS, Raman</td><td>[33]</td></tr><tr><td>Silver alloy</td><td>Ag970 (1.5% Cu, 1.5% Pb), Ag920 (6.5% Cu, 1.5% Pb), Ag965 (3.5% Cu)</td><td>Coupon diameter of 15 mm, thickness of 2 mm</td><td>Silver ingots were thermally treated at about 300°C and then mechanically worked in order to simulate the shaping or coinage process.</td><td>Coupons were first chemically aged with solutions of NaCl, CuCl<sub>2</sub> and HCl, then buried in archaeological sites for 6 months with addition of 5% NaCl solution to the soil.</td><td>Both sides of coupons were uniformly covered with dark brown patina.</td><td>For Ag970 alloy, tarnish contains AgCl and small amount paratac-mite. For Ag920 alloy, tarnish contains AgCl and CuCl. For Ag965 alloy, tarnish contains AgCl and very</td><td>SEM-EDX, GAXRD, OM</td><td>[25]</td></tr></tbody></table>
<figure><img src="image_42.png" /></figure>

<table><caption>Table 2. continued</caption><thead><tr><th>Sample type</th><th>Sample composition</th><th>Sample dimension (length×width×thickness)</th><th>Surface treatment</th><th>Aging method and conditions</th><th>Tarnish appearance</th><th>Tarnish composition</th><th>Analytical techniques<sup>[a]</sup></th><th>Reference</th></tr></thead><tbody><tr><td>Silver alloy</td><td>Sterling silver (92.5 ± 0.03% Ag, 7.42 ± 0.02% Cu)</td><td>70×25×1 mm</td><td>Coupons were manually polished with abrasive paper of increasing granulometry (400, 500, 1000, 1200, 2000). Polishing was always performed in the same direction to obtain a homogeneous and recognizable pattern.</td><td>total time of 3000 h (125 days... The BaS and water were renewed each time the coupons were placed back into the H₂S chamber... The total measurement of the amount of H₂S with a Dräger diffusion tube measures 6 ppm H₂S<sup>*</sup></td><td>Unspecified</td><td>The sulphuration layer was very thin (ca. 1 µm) and not homogenous.</td><td>OM, SEM-EDX</td><td>[27]</td></tr><tr><td>Silver alloy</td><td>Ag/Cu content unspecified</td><td>Unspecified</td><td>Unspecified</td><td>Coupons were exposed to H₂S vapours generated by a Na₂S·9H₂O solution in a tightly sealed glass chamber. The solution was prepared by dissolving 0.3 g of Na₂S·9H₂O (Flu-ka) in 80 ml distilled water and adjusting pH to 7.0 with HCl. The solution was renewed twice after 72 h of exposure, un</td><td>Unspecified</td><td>Ag₂CuS₂ and Ag₂S</td><td>XRD, EDXRF</td><td>[39]</td></tr><tr><td>Silver alloy</td><td>Sterling silver</td><td>Unspecified</td><td>Coupons were exposed to sulphur acid vapour (aging condition unspecified).</td><td>Coupons were sub-merged in a 0.2% Na₂S solution at pH 3 (pH adjusted by H₂SO₄) for 17 h, then rinsed in de-ionized water and cleaned with a soft toothbrush.</td><td>Unspecified</td><td>Mixture of Cu/Ag sulfides</td><td>ESEM-EDS</td><td>[28]</td></tr><tr><td>Silver alloy</td><td>Out-of-circulation Brazilian coins made from alloys of 50% Ag and 50% Cu</td><td>Unspecified</td><td>Coupons were electroplated with 24 carat gold to imitate gilded silver objects.</td><td>Coupons were immersed in a 0.01 M basic thiourea solution (pH = 10) for 3 h.</td><td>Coupons showed a brownish-black tarnishing appearance as the result of the reaction of sulphide ions with Ag and Cu.</td><td>Unspecified</td><td>SEM-EDX, IR, XRF, micro-balance, OM, AAS</td><td>[34]</td></tr></tbody></table>
<figure><img src="image_43.png" /></figure>

<table><caption>Table 2. continued</caption><thead><tr><th>Sample type</th><th>Sample composition</th><th>Sample dimension (length×width×thickness)</th><th>Surface treatment</th><th>Aging method and conditions</th><th>Tarnish appearance</th><th>Tarnish composition</th><th>Analytical techniques<sup>[a]</sup></th><th>Reference</th></tr></thead><tbody><tr><td>Silver alloy</td><td>Out-of-circulation Brazilian coins made from alloys of 90% Ag and 10% Cu</td><td>Unspecified</td><td>Coins were washed with distilled water and dried on a cotton cloth, polished with Massa Polir 2 – Base água – ACS, washed in ultra-sound with a 1% neutral Tween 80, followed by washing with mineral white spirit, and then finally washed with ethanol.</td><td>Coins were immersed in a 0.01 M basic thiourea solution (pH = 10) for 5 h</td><td>Coupons showed a brown tarnish layer after aging.</td><td>Oxides and sulfides with unspecified composition.</td><td>SEM-EDX</td><td>[35]</td></tr><tr><td>Silver alloy</td><td>Rolled annealed sterling silver (Ag925)</td><td>Special shape with a protrude for clamping, thickness of 0.3 mm</td><td>Not described</td><td>Samples were immersed in a 0.05% potassium sulphide solution (0.1 g potassium sulphide in 200 ml de-ionized water) for 10 min at room temperature.</td><td>Unspecified</td><td>A silver sulphide layer with varying thicknesses from 0.5 to 1.0 µm.</td><td>OM, SEM</td><td>[31]</td></tr><tr><td>Silver alloy</td><td>18-gauge (1.05 mm) cold-rolled sterling silver sheet</td><td>discs of 19 mm diameter</td><td>Silver discs were embedded in black phenolic resin, mounted in the polishing device, and then wet-ground with 600 grit silicon carbide paper; the silver surfaces were then hand-polished on a Buehler Ecomet V using: Fibermet, 3 µm silicon carbide discs; Metadi, 1 µm cerium oxide on Mastertex cloth. Some samples were given a final polish with Metpolish (C<sub>2</sub>O<sub>3</sub>). After polishing, samples were cleaned ultrasonically, rinsed with acetone, air dried, and stored in a desiccator.</td><td>Samples were artificially tarnished in a stirred, 0.05% potassium sulfide (K<sub>2</sub>S) solution for 10 min at room temperature.</td><td>Unspecified</td><td>Sulfides.</td><td>Silver loss measured with Knoop indentations, SEM</td><td>[23]</td></tr><tr><td>Silver alloy</td><td>Rolled sterling silver Ag925 sheet</td><td>15×15×0.5 mm</td><td>Ag925 samples were hand polished, machine polished with a rotating fabric disc and polishing paste containing waxes and greases, and</td><td>Samples were subjected to mild indoor atmospheric aging with humidity and temperature fluctuations (bathroom conditions)</td><td>Unspecified</td><td>The surface treatments or (metalworks) were critical for corrosion phenomena: machine polished samples received the strongest</td><td>Colorimetry, OM, SEM-EDX</td><td>[7]</td></tr></tbody></table>
<figure><img src="image_44.png" /></figure>

<table><caption>Table 2. continued</caption><thead><tr><th>Sample type</th><th>Sample composition</th><th>Sample dimension (length×width×thickness)</th><th>Surface treatment</th><th>Aging method and conditions</th><th>Tarnish appearance</th><th>Tarnish composition</th><th>Analytical techniques<sup>[a]</sup></th><th>Reference</th></tr></thead><tbody><tr><td></td><td></td><td></td><td>as solid constituent mainly alumina grains, and manually hammered.</td><td>lying flat with the exposure surface facing upwards for 133 days.</td><td></td><td>corrosion attack, followed by hand polished samples, while natural aging caused a much lower corrosive effect on sterling Ag in its original state or on hand-hammered sterling Ag.</td><td></td><td></td></tr><tr><td>Silver alloy</td><td>Rolled sterling silver Ag925 sheet</td><td>15×15×0.5 mm</td><td>One Ag925 sample was kept as received, other Ag925 samples were hand polished, machine polished with a rotating fabric disc and polishing paste containing waxes and greases, and as solid constituent mainly alumina grains, and manually hammered.</td><td>Mild Aging with TAA for 6 days. Harsh aging via immersion in a 0.1 mM Na<sub>2</sub>S solution. Each cycle lasted 1 min and consisted of immersion for 12 s followed by aeration for 48 s. A total of 60 cycles were conducted. After aging, samples were rinsed with deionized water and acetone.</td><td>Corrosion attack happened in ca. 5–10 min in the solution, showing a red shift in the first 2 min and followed by a yellow shift after ca. 6 min. Such phenomena were more profound on the original and hand-hammered samples than the machine-polished and hand-polished samples.</td><td>The original and machine-hammered samples received stronger corrosion attack than the hand-polished and machine-polished samples. The immersion method caused a double corrosion rate than in the TAA method.</td><td>Colorimetry, OM, SEM-EDX</td><td>[7]</td></tr><tr><td>Silver alloy</td><td>80% Ag, 20% Cu</td><td>25×25×0.3 mm</td><td>Intentionally not polished, precleaned with acetone.</td><td>The aging chamber was constructed using elemental sulfur that limited in a sealed container at 22 °C and 65% RH. The coupons were hung inside of the chamber for 15 weeks.</td><td>Unspecified</td><td>An even, dull brown-grey cast over the surface, with almost black edges; patterns in the corrosion followed the surface characteristics of the worked metal. Metal gloss was maintained even after corrosion.</td><td>Oddy test, SEM-EDX, FTIR</td><td>[32]</td></tr></tbody></table>

[a] AAS: atomic absorption spectroscopy; AES: atomic emission spectroscopy; EDX: energy dispersive X-ray analysis; EPMA: electron probe microanalyzer; EQCM: electrochemical quartz crystal microbalance; ESEM: environmental scanning electron microscopy; GAXRD: glancing Angle X-ray Diffractometry; IRS: infrared spectroscopy; OM: optical microscopy; Raman: Raman spectroscopy; SE: spectroscopic ellipsometry; SEM: scanning electron microscopy; SIMS: secondary ion mass spectrometry; SLV: scanning linear voltammetry; SR-XPS: synchrotron-based XPS; TEM: transmission electron microscopy; XANES: X-ray absorption near edge structure spectroscopy; XPS: X-ray photoelectron spectroscopy; XRD: X-ray diffraction spectroscopy; XRF: X-ray fluorescence analysis; UV-VIS-IR: UV-VIS-IR spectrometry; UV-VIS: UV-VIS spectrometry.
<figure><img src="image_45.png" /></figure>

hard-boiled egg method, which was used by a few research
groups,[20,42] as the tarnishing effects were not always
satisfactory.[42] Lack of standard aging protocols and optimal
tarnishing effects may be the reasons why such type of
methods is not commonly used.

A further issue with chemical aging methods is the fact that
they sometimes require complicated settings, such as aging
chambers, and they need to be handled with proper equipment
(e.g., chemical hood), which are not always accessible for
conservators.

Table 2 presents an overview of silver samples and aging
methods employed in recent studies. Information regarding the
sample type and composition, sample preparation protocol,
aging protocol, obtained tarnish appearance and composition,
as well as the analysis techniques used for the tarnish character-
isation, is included in the table.

**3. Summary and Outlook**

In the past few decades, chemical aging methods have been
widely used in the field of conservation and heritage study, due
to their fast-tarnishing, easily controllable and reproducible
nature. The formation and morphology of the tarnish layers
created by such methods are strongly dependent on the aging
method and chemicals employed. However, some issues caused
by the chemical aging methods, as shown by the example of
the void formation and easy detachment of the tarnish layer,
resulting from the use of TAA and Na₂S immersion, can be
problematic for the evaluation of conservation treatments such
as cleaning. More importantly, some chemicals employed are
toxic, which can cause severe issues to environments and
operators. Therefore, the above review clearly demonstrates
that there is room for improvement when it comes to the
employment and development of green aging methods.

Following the global trend of sustainability, developing
aging protocols that are safe for the operators and environ-
mentally friendly becomes an increasing need in the field of
conservation. In the EU Horizon project GoGreen: Green
strategies to conserve the past and preserve the future of
cultural heritage, a complete set of green parameters have
been defined, with key factors into four impact areas including
hazard impacts on human and environment, impacts on climate
change, impacts on resources, and artwork specific/professional
parameters.[43] Such parameters, together with the twelve
principles of Green Chemistry,[44] can be used as guidelines to
achieve the design of new sustainable silver aging methods in
future or further development of existing aging protocols (e.g.,
the boiled egg white method). A sustainable aging protocol
should include the use of bio-degradable or at least bio-
originated materials as the tarnishing reagents, while still
providing homogeneous and reproducible tarnishing effects on
silver mock-ups that highly resemble the real situations of
historical silver artefacts in the aspects of tarnish appearance,
composition and stratigraphy. At the same time, a life cycle
analysis (LCA) methodology should be adopted for the assess-
ment of environmental impacts, which can be conducted under

the guidelines defined by European norms ISO 14040 and
14044.[45,46]

Acknowledgments

HESSO is an associated partner in the framework of the GoGreen Strategies to Conserve the Past and Preserve the Future of Cultural Heritage, Horizon-RIA project 2022–2026 [grant number 101060768]. The participation is funded by the State Secretary for Education, Research and Innovation (SERI) [grant number 22-00121] as transitional measures program for Horizon 2021–2027.

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available
from the corresponding author upon reasonable request.

Keywords: silver · aging · conservation · heritage

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Manuscript received: October 11, 2024

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