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Study on the formation of heterogeneous structures in leached layers during the corrosion process of glass

Willemien Anaf

Abstracts

Glass that corrodes under natural conditions often shows heterogeneities in the leached layer, such as a lamellar structure or inclusions of MnO2 or Ca3(PO4)2. The formation of these heterogeneities is still not well understood. By means of experiments under laboratory conditions, our aim was to artificially generate specific structures. Therefore, glass samples were immersed in metal-rich solutions. The experimental results were compared with theories describing glass corrosion from a molecular point of view.

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Editor's notes

Artesis Hogeschool Antwerpen - Universiteit Antwerpen – Contact: Eva Annys

Full text

In the first place, I want to thank my promotor Olivier Schalm and my co-promotors Marc David, Kristel De Vis and Joost Caen for the constructive comments and help. A lot of gratefulness goes to Koen Janssens for the use of the Scanning Electron Microscope. Finally, a lot of gratitude goes to all the other people that have supported me during my master thesis.

Introduction

1One of the most striking phenomena in glass corrosion under natural conditions is the transformation of a material that is homogeneous to a scale down to 0.5 µm into a leached layer that contains heterogeneities of about 1 to 5 µm or even larger. Examples of heterogeneities such as lamellar structures (Brill 1961; Newton 1971; Cox and Ford 1989; Römich 1999; Doménech-Carbó et al. 2001; Sterpenich and Libourel 2001; Salviulo et al. 2004; Dal Bianco et al. 2005; Silvestri et al. 2005; Doménech-Carbó et al. 2006), phosphate precipitations (Freestone 1985) or oxidized manganese inclusions (Müller et al. 1986; Newton and Davison 1989; Perez y Jorba and Bettembourg 1991; Pinto 1991; Schreiner 1991; Cox and Ford 1993; Knight 1996; Barbey et al. 1997; Krawczyk-Bärsch et al. 1997; Römich 1999; Janssens et al. 2000; Doménech-Carbó et al. 2001; Watkinson et al. 2005) have been reported in the past. However, almost none of these publications mention that such phenomena cannot be explained with the currently accepted theories describing glass corrosion (Douglas and El-Shamy 1967; Doremus 1979; Scholze 1988; Bunker 1994). Therefore, the objective of the present study is to elucidate the formation of heterogeneous structures in (historical) glass in relation to the currently accepted theories describing corrosion processes. Experiments in which heterogeneous structures were generated under laboratory circumstances will be presented. The experimental results will be discussed and included in a new approach of the corrosion process that starts with the molecular structure of glass.

Background information

2The currently accepted corrosion theories assume that glass is a homogenous matrix in which mobile cations are present. As a consequence of the corrosion process, the cations in the glass matrix are leached out while protons from the environment penetrate into the glass. The mobility of these cations is related to the chemical composition of the glass matrix and is described by the diffusion coefficient. This corrosion model is based on the continuous random network (CRN) of Zachariasen and Warren (Zachariasen 1932; Warren and Loring 1935; Vogel 1971). In the field of conservation science it is probably the most referred glass structure. However, presently the most accepted description of the glass structure is the modified random network (MRN) of Greaves (Greaves 1991). The MRN-model describes glass as a mixture of two networks: a covalent immobile SiO2-rich network and a network of channels formed by network modifiers such as alkali and alkaline earth ions. The channels can be considered as percolation pathways with high ion mobility. Therefore, glass is not homogeneous at the nanometre scale. Fig. 1 gives a two-dimensional overview of a glass matrix according to the MRN-model. The model has been successfully applied to describe several physical properties of glass such as electrical conductivity, glass fracture, thermal expansion coefficient, etc. (Greaves 1991; Baker et al. 1995; Célarié et al. 2007). According to the authors’ knowledge, the MRN-model has never been used to explain glass corrosion. The present study uses this model as a starting point.

Fig. 1: Two-dimensional representation of the Modified Random Network

Fig. 1: Two-dimensional representation of the Modified Random Network

©Robinet 2006

3It is well known that glass composition has a significant influence on the corrosion sensitivity of glass. This can to some extent be explained using the MRN-model. The amount of network modifiers determines if a glass matrix contains less or more channels. The interconnection of the channels increases with the amount of network modifiers. Therefore, a higher amount of network modifiers causes a denser network of finite clusters of interconnected channels, resulting in a significant decrease in the average activation energy for alkali-hopping (Gedeon et al. 2008). This implies an increase in ion mobility and thus a higher corrosion sensitivity. Some of the channels are in contact with the glass surface and are able to interact directly with the ambient environment. Other channels are isolated and have no contact with the surface.

Experiments

4Several experiments were designed in order to obtain specific features that cannot be explained by the classic glass corrosion theories. In the past, several attempts were performed to artificially produce leached layers with lamellar structures (Aerts 1998; Römich et al. 2003) or to introduce external ions into the leached layer (Aerts 1998; Watkinson et al. 2005). The present study is based on similar experiments. Glass samples were immersed in a metal-rich solution. As a result of a concentration gradient between glass and environment, a migration of the metal ions into the glass is expected. A selection of the performed experiments will be explained in the following paragraph.

Samples

5Two different glass samples were used in the experiments. The first one was a corrosion sensitive model glass (type M1.0, Fraunhofer Institut für Silicatforschung, Würzburg) with a composition of 59.0% SiO2, 21.6% K2O and 19.3% CaO. From one sheet of model glass, small pieces of around 1 cm x 0.5 cm were cut by means of a diamond saw. The thickness of the samples was 0.7 mm. Afterwards, the samples were fire-polished and saved in a box with silica gel until the beginning of the experiments. The second glass type was an historical piece of 17th century window glass from a private collection. The historical glass had a more complex composition (60.5% SiO2, 19.4% CaO, 8.0% K2O, 3.0% MgO, 2.7% Al2O3, 2.0% P2O5, 1.6% Fe2O3, 1.5% Na2O, 0.6% MnO, 0.4%TiO2, 0.2% Cl) and showed clear signs of pit corrosion. Pieces of 1 cm x 1 cm were cut by means of a glass cutter. The thickness of the sample was 2 mm.

Description of the experiments

6Table I gives an overview of the different solutions in which the glass samples were immersed. The solutions were made by dissolving the proper amounts of metal chloride in deionised water. The initial pH was adjusted by addition of HCl. The beakers were sealed off in order to prevent evaporation. Due to ion exchange, an increase in the pH of the solution close to the surface was expected. Thiswas reduced in two different ways. On one hand, the solutions in the 20 ml beakers were continuously homogenized by placing the beakers on a shaking platform. To prevent the samples from moving, they were glued with acid-free silicon to the bottom of the beaker. The solution in the 200 ml beaker was manually stirred with a clean glass bar (1 to 2 times a day). After immersion, all glass samples were taken out of the beakers and left to dry in ambient air.

Table I: Overview of the experiments to generate features in leached layers that cannot be explained with the currently accepted theories describing glass corrosion.

Table I: Overview of the experiments to generate features in leached layers that cannot be explained with the currently accepted theories describing glass corrosion.

Analytical techniques

7The mass of the samples (dry condition) was measured before and after the corrosion experiments with a balance with a precision of 0.001 gram (A&D HF-300G). The increase in acidity was empirically measured with a pH meter (Sentron Argus), which was calibrated before each measurement. After the experiments the surface structure of the samples was observed with a binocular optical microscope with reflected and transmitted light (Olympus BX41). To study the cross-sections, several glass splinters were embedded in a block of resin (Technovit® 2000 LC), oriented perpendicular to the original glass surface. The surface of the resin was polished with corundum papers and fine diamond pastes down to 1 µm. The cross-sections were examined with an optical microscope. Finally, the embedded samples were analysed by means of a JEOL 6300 Electron Microprobe system equipped with a digital, thin-window energy dispersive Si(Li) X‑ray detector of Princeton Gamma Tech (PGT).

Results

Penetration of copper ions in the leached layer

8Experiment 1 (see Table I) resulted in a leached layer of about 20 µm thickness in which small amounts of copper and chlorine were present. In the near-surface zone, the leached layer showed cracks perpendicular to the glass surface. On the surface a blue copper precipitation was present, probably due to a fast but local increase of the pH of the solution (Fig. 2). At the initial pH it was indeed possible to completely dissolve all the metal salts used in the experiments. However, when corrosion proceeded, the pH of the solution increased, especially in cracks and close to the surface. This resulted in the precipitation of compounds of the type Cu(Cl,OH)2.nH2O. As a consequence, the amount of mobile Cu2+ cations in solution dropped as well. It can be concluded that even when shaking the solution, local pH-increases have to be taken into account. Although the precipitation was already visible after one week, a small amount of copper had migrated into the leached layer as can be seen in the X-ray spectra (measured after 3 months of immersion). Remarkable was also the migration of Cl--ions (Fig. 3). The incorporation of Cl--ions has already been mentioned in other publications (e.g.,Mäder et al. 1998). To maintain the electrical neutrality in the glass after incorporation of anions, two possible mechanisms can be considered: (1) a cation migrates together with the anion into the glass; (2) ion exchange takes place with an anion present in the glass. The last mechanism is less plausible as there are normally no mobile anions present in the glass. As a consequence of the formation of a leached layer, a decrease in mass was expected. However, in this case the mass of the glass sample increased by 1 wt%, probably due to the copper compound precipitation on the glass surface.

Fig. 2: Backscattered image of the model glass after experiment 1.

Fig. 2: Backscattered image of the model glass after experiment 1.

Fig. 3: X-ray spectra: bulk glass (black), model glass after experiment 1 (grey).

Fig. 3: X-ray spectra: bulk glass (black), model glass after experiment 1 (grey).

Penetration of manganese ions into the leached layer

9The immersion of glass samples in manganese solutions received special attention because several historical fragments were reported to contain inclusions of Mn4+-oxy-hydroxides (Newton and Davison 1989; Doménech-Carbó et al. 2001; Watkinson et al. 2005). Both in experiment 2 and 3, a brown manganese-rich deposition appeared on the glass surface after a few days of immersion. Analogous to the Cu-experiment, this was probably caused by a local increase of the pH. After 3 months of immersion, the cross-sections of the model glass samples showed irregular leached layers. Within the leached layers a heterogeneous network of manganese- and calcium-rich cracks was formed  (Fig. 4-5). The source of the calcium could only be the glass itself since there was no Ca2+ present in the original solution. The higher concentration of MnCl2 in experiment 3 had caused a more explicit pattern of Mn-rich cracks with clear vertical fissures, interconnected by horizontal fissures (Fig. 5). With light microscopy the manganese precipitations appeared as dark brown, irregular inclusions in the glass.   

Fig. 4: Backscattered and X-ray image of the model glass after experiment 2.

Fig. 4: Backscattered and X-ray image of the model glass after experiment 2.

Fig. 5: Backscattered image of the model glass after experiment 3.

Fig. 5: Backscattered image of the model glass after experiment 3.

10After immersing a model glass in a strongly acidic MnCl2-solution (experiment 4), a thick leached layer was formed. The glass surface showed a cracked pattern. The cracks probably originated during the drying period (dehydration of the glass) (Römich 1999; Prochazka 2007; Robinet et al. 2007; Geisler et al. 2010). Remarkable was the formation of a heterogeneous lamellar structure in the leached layer. After 15 days however, no manganese had migrated into the structure, but some chlorine and calcium enrichments were visible. The enrichments followed the lamellar structure (Fig. 6). However, the high amount of too many uncontrollable experimental parameters did not allow for a selection of a possible cause for the formation of the lamellar structure. Nevertheless, it can be supposed that external ions are not necessary for the formation of a lamellar structure. As reported before (Aerts 1998), the formation of a lamellar structure is also not dependent on cyclic changes in temperature and humidity. It is probably caused by a reorganisation of the glass structure itself.

Fig. 6: Backscattered and X-ray image of the model glass after experiment 4.

Fig. 6: Backscattered and X-ray image of the model glass after experiment 4.

The experiment demonstrates that lamellar structures can be generated under laboratory conditions. (Prolonged exposure to the electron beam has led to additional cracks in the leached layer as can be seen in the X-ray images.)

Penetration of lead ions into the leached layer

11After experiment 6, the pH of the solution had slightly increased up to 3.5, probably due to an ongoing leaching process. A comparison of the sample mass before and after the experiment confirmed this assumption, as the mass had decreased by 1.2 w%. SEM-images of the cross-section clearly showed an increased amount of lead in the already existing corrosion pit, following a specific structure. Around cracks, the Pb-amount was remarkably larger. In the bulk glass, no significant amount of Pb was measured (Fig. 7). To investigate whether the Pb was already present in the corrosion pits before the artificial corrosion experiment, the spectrum of the pit after artificial corrosion was compared with a reference sample without artificial corrosion. This made clear that the Pb had indeed migrated into the leached layer of the corrosion pit during artificial alteration (Fig. 8).

12The results of experiment 5 were similar to those of experiment 6 (both historic glass samples). Therefore, only experiment 6 has been treated in the present article.

Fig. 7: Backscattered and X-ray image of the historical glass after experiment 6.

Fig. 7: Backscattered and X-ray image of the historical glass after experiment 6.

Fig. 8: X-ray spectra: reference sample (black), historical glass after experiment 6 (grey).

Fig. 8: X-ray spectra: reference sample (black), historical glass after experiment 6 (grey).

General remarks on the experimental results

13The experiments once more demonstrate that the migration of external ions in glass is possible. The migration is limited to the leached layer because of a higher ion mobility in this layer (Grambow and Müller 2001; De Rosa et al. 2003; Tournié et al. 2008; Conradt 2008). The circumstances determine to a significant extent the thickness and value of the leached layer. First of all, the mobility of the external ions is important: only mobile ions can migrate into the glass. As the mobility of the external ions can be related to the pH of the aqueous environment, this can be indicated as the reason why the migration of external ions was limited in the model glass experiments: the metal ions were quickly deposited on the glass surface and could not migrate into the leached layer anymore. Precipitation reactions thus reduce the mobility. The exposure time is also a possible parameter: the migration of external ions into the glass is likely to be much slower than the leaching process. However, cracks in the leached layer certainly promote the migration.

Discussion

14The experimental results, combined with an extensive literature study, could give an interesting input for a new approach of the corrosion process in which the MRN-model is considered as a basis.

15The chemical deterioration of glass can be described as any combination of three simultaneous partial primary processes: (1) ion exchange, (2) hydration and (3) hydrolysis (Fig. 9) (Bunker 1994, Silvestri et al. 2005; Pollard and Heron 2008). The primary corrosion processes are strongly coupled to each other and can take place in conditions where the glass is exposed to an aqueous environment. Besides, secondary corrosion processes such as the formation of precipitations, also have to be taken into account. Each process will be separately described in the following paragraphs. It should be noted that subtle differences in micro-environment, internal stress or the fraction of channels in contact with the surface can lead to completely different corrosion patterns, even when the glass originates from the same object and when they were buried close to each other.

Fig. 9: Overview of the different primary corrosion processes.

Fig. 9: Overview of the different primary corrosion processes.

Ion exchange

16In an acidic (aqueous) environment the concentration of H+ is higher than the OH--concentration. Therefore, the attack on the glass is dominated by the attack of H+-ions, causing a leaching process. The diffusion-directed ion exchange occurs between the protons of the aqueous environment and the mobile network modifiers (Na+, K+, Ca2+) in the channels that are in contact with the glass surface. Inside such a channel, ion exchange transforms a channel filled with network modifiers (i.e., M+-channels) into a channel filled with protons (i.e., H+-channels). Therefore, deteriorated glass contains M+-channels and H+-channels, with a much higher ion mobility in H+-channels than in M+-channels. From a macroscopic point of view, this inter-diffusion process causes the formation of a leached layer, poor in alkali and alkaline earth ions. The interface between the leached layer and the bulk glass is sharp and therefore clearly distinguishable, as can be seen in Fig. 2, 4, 5, 6 and 7. As the corrosion progresses, the interface moves further into the glass.

Hydration

17Hydration describes the diffusion of molecular water into the glass. The rate of water diffusion is mainly determined by the size of the voids present in the glass network. When the voids are large compared to the size of the water molecule, a rapid diffusion is possible. The smaller the openings in the structure, the slower the water diffusion will be. When the voids are too small for the penetration of water molecules, they can react with the network by breaking Si-O-Si-bonds (hydrolysis) and thus open the structure (Bunker 1994). According to Sterpenich and Libourel (2006) water diffusion is not restricted to the leached layer, forming a gradual transition between hydrated and non-hydrated glass.

18In the present study a new hypothesis is proposed in which water is considered as a predominant agent in the formation of a lamellar structure. In the experiments, it is already suggested that external ions probably do not play a role in the initial formation of a lamellar structure. Besides, it is known that diffusion processes coupled to precipitation reactions tend to create chaotic patterns. Of late, Scott (2005) interrelated heterogeneous morphologies in the corrosion layer of metal artefacts with self-organising systems. He mentioned fractal geometries and layered or banded structures. An example of such patterns are Liesegang bands. In the literature, a few studies shortly cite Liesegang bands in the perspective of the formation of lamellar structures during glass corrosion (Dal Bianco et al. 2005; Scott 2005; Weber 2005; Schreiter et al. 2007). However, none of the studies gives a clear explanation of the occurring process. The present study hypothesizes the following: water is able to diffuse into the glass, but once it reacts with the silicate network, it is trapped (formation of Si-OH-groups). This results in a depolymerisation of the silicate network and the channel structure probably disappears. By the combination of water diffusion and water entrapment, the conditions for reaction-diffusion are fulfilled and a certain pattern, such as a lamellar structure, can arise. It is assumed that the amount of molecular water varies in the different layers of the lamellar structure, which could be linked to variations in ion mobility. As a consequence of this, external ions will preferentially migrate into the layers with the highest ion mobility.

Hydrolysis

19The last primary corrosion process, hydrolysis, dominates in alkaline environments. OH--anions will, together with H2O, attack the SiO2-rich islands of the glass network, causing a depolymerisation of the glass structure. In an advanced stage of deterioration, hydrolysis can cause a dissolution of the silicate network whereby the original glass surface will finally disappear. A condensation reaction whereby the formed Si-OH groups polymerize again forming Si-O-Si bonds, can also take place. This reaction is accompanied by the release of molecular water. Successive hydrolysis and condensation reactions create a more open glass structure, enhancing the diffusion rates of water and other (external) ions.

Secondary corrosion processes

20The secondary corrosion processes can be divided into two main groups, i.e. (1) processes that take place on the glass surface, and (2) processes that take place in the leached layer. The first group contains processes in which leached cations that are still present on the glass surface interact with products from the environment to which the glass is exposed. A well known example is the interaction with gaseous pollutants such as CO2 and SO2, causing secondary corrosion products such as gypsum (CaCO3.2H2O) and syngenite (CaCO3.K2O.H2O) which destroy the transparency of the glass. Besides, solutes that do not originate from the glass can precipitate on the surface (e.g., soluble corrosion products of surrounding stones). The secondary corrosion processes in the leached layer itself can be described as diffusion processes in combination with precipitation reactions. External ions such as manganese and phosphor can migrate into the leached layer and subsequently form heterogeneous structures with different features. Observations have shown that manganese, for example, can give rise to planar shapes, tubular structures and dendrites (Schalm et al. 2010). These secondary precipitations are formed when saturation is reached. Furthermore, their formation is catalyzed by previous formed aggregates (Chopard et al. 1991; Schalm et al. 2010). In a lamellar structure, the diffusion of (external) ions will be preferential in low density lamellae, creating a specific heterogeneous pattern.

Conclusion

21The present study has shown the possibility of an artificial generation of heterogeneous structures by immersing corrosion-sensitive and historical glass samples in a metal-rich aqueous solution. The experiments have demonstrated that the migration of external ions in glass is possible. Under certain circumstances the external ions can create heterogeneities. In one experiment, a lamellar structure has been artificially obtained. Despite the fact that many parameters were uncontrolled in this particular experiment, some interesting information on the formation of the lamellar structure could be deduced. The lamellar structure presumably arose by a reorganisation of the glass structure itself as a result of strong hydration reactions. Therefore, (external) water plays an important role in the formation of a lamellar structure. External ions such as Mn2+ are probably not necessary in the initial formation of this structure. However, differences in ion mobility in the layers could stimulate the migration of (external) ions in a certain heterogeneous pattern.

22In order to conclude, the different corrosion processes were discussed starting from the modified random structure of glass. This new approach has the potential to clarify some observations that cannot be explained by the currently accepted corrosion theories. The presented preliminary ideas could give rise to the development of a more subtle model of glass corrosion.

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List of illustrations

Title Fig. 1: Two-dimensional representation of the Modified Random Network
Credits ©Robinet 2006
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-1.jpg
File image/jpeg, 60k
Title Table I: Overview of the experiments to generate features in leached layers that cannot be explained with the currently accepted theories describing glass corrosion.
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-2.jpg
File image/jpeg, 36k
Title Fig. 2: Backscattered image of the model glass after experiment 1.
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-3.jpg
File image/jpeg, 12k
Title Fig. 3: X-ray spectra: bulk glass (black), model glass after experiment 1 (grey).
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-4.jpg
File image/jpeg, 20k
Title Fig. 4: Backscattered and X-ray image of the model glass after experiment 2.
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-5.jpg
File image/jpeg, 20k
Title Fig. 5: Backscattered image of the model glass after experiment 3.
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-6.jpg
File image/jpeg, 12k
Title Fig. 6: Backscattered and X-ray image of the model glass after experiment 4.
Caption The experiment demonstrates that lamellar structures can be generated under laboratory conditions. (Prolonged exposure to the electron beam has led to additional cracks in the leached layer as can be seen in the X-ray images.)
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-7.jpg
File image/jpeg, 24k
Title Fig. 7: Backscattered and X-ray image of the historical glass after experiment 6.
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-8.jpg
File image/jpeg, 28k
Title Fig. 8: X-ray spectra: reference sample (black), historical glass after experiment 6 (grey).
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-9.jpg
File image/jpeg, 20k
Title Fig. 9: Overview of the different primary corrosion processes.
URL http://journals.openedition.org/ceroart/docannexe/image/1561/img-10.jpg
File image/jpeg, 11k
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References

Electronic reference

Willemien Anaf, “Study on the formation of heterogeneous structures in leached layers during the corrosion process of glass”CeROArt [Online], EGG 1 | 2010, Online since 21 November 2010, connection on 19 March 2024. URL: http://journals.openedition.org/ceroart/1561; DOI: https://doi.org/10.4000/ceroart.1561

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About the author

Willemien Anaf

Willemien Anaf achieved her Master in Conservation Studies (Artesis University of Antwerp, Belgium) in 2009. She specialized in the conservation of stained glass windows. During her studies, she took part in the Erasmus Exchange Program with the Academy of Fine Arts in Vienna (Akademie der bildenden Künste, Wien), where she worked for 5 months in the Institute of Science and Technology in Arts (Naturwissenschaften und Technologie in der Kunst) under the direction of Prof. Dr. M. Schreiner. Currently she is working as a PhD-student at the University of Antwerp, Department of Chemistry, in the research group ‘Environmental Analysis’ of Prof. R. Van Grieken. Her PhD-research emphasizes on the influence of particulate matter on the deterioration of cultural heritage.

By this author

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Copyright

CC-BY-NC-ND-4.0

The text only may be used under licence CC BY-NC-ND 4.0. All other elements (illustrations, imported files) are “All rights reserved”, unless otherwise stated.

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