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Room temperature corrosion of museum glass: an investigation using low-energy SIMS
Applied Surface Science 231–232 (2004) 510–514
Room temperature corrosion of museum glass: an investigation using low-energy SIMS Sarah Fearna,*, David S. McPhaila, Victoria Oakleyb a
b
Department of Materials, Imperial College, London SW7 2AZ, UK Ceramics and Glass Conservation Section, Victoria and Albert Museum, London SW7 2RL, UK Available online 24 May 2004
Abstract Glass is often regarded as a stable durable material and the wide range of contemporary applications of glass reinforces this belief. There is nothing inherent in the glassy state, however, that confers stability, and the problem of glass corrosion has been well documented since the 17th century. Glass corrosion still affects commercial float glass production and glasses used to contain high level nuclear waste, but one area in particular where glass corrosion is very common is in the museum environment. In order to conserve these artefacts it is essential to understand fully both the composition of the corroded glass and the corrosion mechanism. In this study, the application of low-energy SIMS for the depth profiling of corroded glass is studied with the aim of finding a suitable environment for the safe storage of glass objects in a museum. # 2004 Elsevier B.V. All rights reserved. Keywords: Low-energy SIMS; Depth profiling; Glass corrosion; Conservation
1. Introduction It has been well documented over many years that certain types of glassware displayed within the glass collection at the Victoria and Albert museum in London, are susceptible to deterioration over time. Venetian glassware of the 17th and 18th centuries is particularly prone to corrode and the instability of glass from this period has been found to be due to the purification of the raw materials such as plant ashes which were used to produce the well-known clear ‘cristallo’ glass [1]. This purification of the raw materials depleted the glass of the essential network-former calcium oxide, and increased the weight percentage of sodium and potassium oxides. * Corresponding author. E-mail address: [email protected] (S. Fearn).
In all glasses, sodium and potassium oxides are hygroscopic and the surface of the glass thus readily absorbs moisture from the atmosphere. This interaction between the glass surface and atmospheric moisture starts the deterioration of the glass, which manifests itself in a number of ways. Initially the glass artefact may appear foggy and dull, under humid conditions droplets of moisture appear on the surface as the hygroscopic alkali salts deliquesce—a condition referred to as ‘weeping’ [2]. As the deterioration progresses a series of fine micro-cracks start to appear. This stage is known as ‘crizzling’ and can eventually lead to the formation of flakes on the surface of the glass. With continued leaching of the alkali metal ions, the glass surface becomes increasingly alkaline. Once the pH reaches 9 or above, dissolution of the silica network occurs and the glass object will lose much of its inherent strength and ultimately collapse [3].
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.205
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514
SIMS has been increasingly used to measure the corrosion of glass, or rather the leaching of the alkali metal ions over time [4,5]. In the SIMS depth profiling mode the leaching alkali ions can be monitored easily as a function of depth but, in most cases the glasses analysed have either been buried samples with large corrosion depths or in the case of Ryan’s work on atmospheric glass corrosion, glass which was aged at elevated temperatures and humidities in order to obtain sodium depletion depths that could be measured by SIMS. The depth resolution of the SIMS tool used during this study was of the order of 10 nm. Ageing glass at elevated temperatures is not however an accurate representation of the long-term corrosion that occurs under the ambient conditions of a museum. With advances in SIMS instrumentation over the past few years, it is now possible to carry out depth profiling routinely using sub-keV beam energies and nanometer depth resolution. This means that glass corrosion can now be performed at room temperature, over reasonable time periods, and accurate depth concentration profiles of the leaching alkali ions can be obtained of the top 50–80 nm. Improved depth resolution will allow a more accurate picture of room temperature corrosion to be determined, and the work presented here marks the very beginning of this research. The final long-term aim is to obtain a more precise description of the mechanism of glass corrosion at room temperature, and help museum conservators to select suitable environmental conditions for the safe storage of vulnerable glass objects.
2. Experimental procedure After compositional analysis (EPMA) of an original artefact from the V&A museum, a replica glass, RG1 was fabricated according to the composition of 72.72 wt.% SiO2, 17.95 wt.% Na2O, 3.27 wt.% K2O, 2.17 wt.% CaO, 0.74 wt.% MgO, 1.21 wt.% Al2O3, 0.23 wt.% Fe2O3 and 0.37 wt.% Mn2O3. The batch was then melted at 1200 8C and blown into circular flat plates. EPMA was then used again to ensure that the glass produced was homogenous over the nine samples produced. In order to age the glass, pieces from the plates were cut and placed in environmental chambers set to known humidities which were representative of a museum environment. The
511
glass was then left for a period of time to age at room temperature. After ageing the glass surfaces were depth profiled using an Atomika quadrupole SIMS tool [6] equipped with a floating low energy primary ion gun [7,8]. A nitrogen ion beam at 500 eVand normal incidence was used to perform the analyses, with a current of 90 nA and scan size of 375 mm. Charge compensation was used in all the analyses, using an electron energy of 70 eV. SIMS crater depths were measured using a Zygo white light optical interferometer.
3. Results Fig. 1 shows typical concentration depth profiles for the main elemental species of the aged replica glass, RG1. The concentration scale has been quantified by setting the bulk glass signals to the bulk concentration levels of the glass obtained via EPMA. In Fig. 1, the glass has been aged at 55% RH for 91 h at room temperature. At the start of the SIMS profile both the Na and K signals drop to a minimum, of 0.40 and 0.05%, compared with bulk concentrations of 11.92 and 1.43 at.%, respectively. This first region represents the removal of the leached corrosion products from the surface of the glass. After reaching a minimum, both signals then gradually increase to flat stable signals indicating that the bulk glass state has been reached. The depletion of these cations near the surface is used as a measure of how corroded the glass has become. In contrast, the relative concentrations of Al and Si near to the surface are higher then their respective bulk values which is largely due to a concentration effect, whereby their apparent concentration increases as the alkali metal cations are leached from the glass surface. The Si 28 signal has also been enhanced by a mass interference from the COþ signal, however, mass interferences will occur at all of the Si isotopes, and their corresponding molecular signals. Fig. 2 shows the Na concentration depth profiles from the RG1 glass aged at humidities of 37 and 55% RH for 91 h, and the as-produced, ‘non-aged glass’, as a function of relative humidity. (It should be noted here that the ‘non-aged’ glass does show a slight depletion of Na up to a depth of 22 nm, because the sample was actually analysed 1 day after it had been removed from the annealing oven.) The effect of
512
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514 Region I – corroded glass
Region II – bulk glass
100
Conc. (at % cations)
10
1
Na - 23 0.1
Al - 27 Si - 28 K - 39 Ca -40
0.01 0
10
20
30
40
50 60 70 Depth / nm
80
90
100 110 120
Fig. 1. Concentration depth profile for the main elemental species of the aged replica glass, RG1, aged at 55% RH for 91 h at room temperature. At the start of the profile the Si signal is greatly enhanced due to mass interference.
Conc. (at% cations)
100
10
1 Na - as made Na - 37%RH - 91 hours Na- 55%RH - 91 hours 0.1 0
10
20
30
40
50
60
70
80
90
100
110
Depth / nm
Fig. 2. Naþ concentration depth profiles from the RG1 glass un-aged glass and aged at humidities of 37 and 55% RH for 91 h.
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514
increased atmospheric water concentration is an increased depletion in terms of both concentration and depth of the Na cations within the near the surface of the glass. As the humidity increases from 37% RH, the depletion depth after 91 h is 76 nm, whereas at 55% RH after the same ageing time the depletion depth is 86 nm. The shapes of the two profiles are also different as can be seen by changes in slope of the profiles. It is not yet fully understood what is causing the differences between these two profiles, but it is thought to be due to migration of the Na caused by the SIMS beam, or an effect of surface roughening. Although the ageing was performed at room temperature, both profiles indicate that near the surface the glass has very rapidly become depleted in Na cations compared to the non-aged glass. Similarly, the K cations (not shown on the profiles) show the same correlation between depletion depth and increasing humidity. By measuring the depletion depths of the Na cations, it is possible to get an estimate of the depletion rate in this very early stage of glass corrosion. These leaching rates at room temperature have been plotted in Fig. 3, along with data previously attained by Ryan at 40 8C and similar humidities [9]. It is clear that the slopes of the two plots clearly differ but, unfortunately there is insufficient data so far to identify the corrosion mechanism occurring at room temperature. It is predicted that the initial, fast depletion of Na will decrease with the square root of time,
Leaching Rate of Na / nm/hour
0.75 0.7 0.65 0.6 0.55 0.5
22C
0.45
40C
0.4 30
40
50
60
Relative Humidity / %RH
Fig. 3. Leaching rates of Na at different temperatures.
513
until a steady state is reached [10], but if the leaching of Na were to proceed at the rates estimated from these initial results over 1 year, then Na would be depleted to approximate depths of 5 mm at 37% RH and 6 mm at 55% RH. These values differ considerably from the leaching rate at room temperature estimated by Ryan on the same glass composition aged at 40% RH. By extrapolation from the leaching rates obtained from RG1 aged at elevated temperatures, Ryan obtained a Na leach rate of approximately 1.5 mm per year. This value suggests that the glass is corroding at a much slower rate, and is therefore in a ‘safer’ environment than appears to be the case from the Na leach rates actually measured at room temperature by low-energy SIMS. From these very early SIMS results it appears that Na and K cations are still leaching out of the replica glass at humidities that have been suggested as safe for storage of museum glass objects [11].
4. Conclusion Continuing work on ageing replica glass at a wide range of humidities has produced promising initial SIMS results. It has been shown that depth profiling performed with a nitrogen beam at 500 eV can usefully be applied to analyse accurately the concentration profile of the main elemental species in the top surface region of a glass that has been corroded at known humidities and, most important, at room temperature. From the SIMS depth profiles it is possible to measure accurately the depth to which the Na and K cations have leached within the top 100 nm of the glass surface. It is essential that more humidities are studied for a greater range of time periods in order to identify accurately the room temperature corrosion mechanism for this glass. The glass must also be aged for a wider range of times.
Acknowledgements The author would like to thank Ian Hankey who produced the blown glass samples and assisted with glass melting, and Kevin Reeves of the Wolfson Archaeological Science Laboratories at the Institute
514
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514
of Archaeology UCL for performing the EPMA analysis. References [1] A. Neri, L’arte Vittaria, O. Pulleyn, London, 1662 (reprinted by the Society of Glass Technology). [2] V. Oakley, Fighting the inevitable: the continuing search for a solution to glass decay at the V&A, Glass Technol. 42 (3) (2001) 65–69. [3] R.K. Iler, The Chemistry of Silica, Wiley/Interscience, New York, 1979. [4] J.L. Ryan, D.S. McPhail, P.S. Rogers, in: Proceedings of the 11th Triennial Meeting of ICOM, 1996, pp. 839–844.
[5] G.G. Wicks, J. Nucl. Mater. 298 (2001) 78–85. [6] Atomika 6500 SIMS, FEI Deutschland GmbH, D-85764 Oberschleissheim/Munich, Germany. [7] M.G. Dowsett, N.S. Smith, D.R. Bridgeland, A.C. Lovejoy, P. Pendrick, in: A. Benninghoven, B. Hagenhoff, H.W. Werner (Eds.), Proceedings of the Secondary Ion and Mass Spectrometry, SIMS XI, Wiley, 1997, pp. 367–370. [8] Ionoptika FLIG. [9] J.L. Ryan, PhD Thesis, The Atmospheric Deterioration of Glass: Studies of Decay Mechanisms and Conservation Techniques, University of London, 1996. [10] B. Grambow, R. Muller, J. Nucl. Mater. 298 (2001) 112–124. [11] R.H. Brill, Crizzling–A problem in Glass Conservation, Conservation in Arcaeology and the Applied Arts, Stockholm Congress, London. The International Institute for Conservation of Historic and Artistic Works, 1975, pp. 121–134.
Room temperature corrosion of museum glass: an investigation using low-energy SIMS Sarah Fearna,*, David S. McPhaila, Victoria Oakleyb a
b
Department of Materials, Imperial College, London SW7 2AZ, UK Ceramics and Glass Conservation Section, Victoria and Albert Museum, London SW7 2RL, UK Available online 24 May 2004
Abstract Glass is often regarded as a stable durable material and the wide range of contemporary applications of glass reinforces this belief. There is nothing inherent in the glassy state, however, that confers stability, and the problem of glass corrosion has been well documented since the 17th century. Glass corrosion still affects commercial float glass production and glasses used to contain high level nuclear waste, but one area in particular where glass corrosion is very common is in the museum environment. In order to conserve these artefacts it is essential to understand fully both the composition of the corroded glass and the corrosion mechanism. In this study, the application of low-energy SIMS for the depth profiling of corroded glass is studied with the aim of finding a suitable environment for the safe storage of glass objects in a museum. # 2004 Elsevier B.V. All rights reserved. Keywords: Low-energy SIMS; Depth profiling; Glass corrosion; Conservation
1. Introduction It has been well documented over many years that certain types of glassware displayed within the glass collection at the Victoria and Albert museum in London, are susceptible to deterioration over time. Venetian glassware of the 17th and 18th centuries is particularly prone to corrode and the instability of glass from this period has been found to be due to the purification of the raw materials such as plant ashes which were used to produce the well-known clear ‘cristallo’ glass [1]. This purification of the raw materials depleted the glass of the essential network-former calcium oxide, and increased the weight percentage of sodium and potassium oxides. * Corresponding author. E-mail address: [email protected] (S. Fearn).
In all glasses, sodium and potassium oxides are hygroscopic and the surface of the glass thus readily absorbs moisture from the atmosphere. This interaction between the glass surface and atmospheric moisture starts the deterioration of the glass, which manifests itself in a number of ways. Initially the glass artefact may appear foggy and dull, under humid conditions droplets of moisture appear on the surface as the hygroscopic alkali salts deliquesce—a condition referred to as ‘weeping’ [2]. As the deterioration progresses a series of fine micro-cracks start to appear. This stage is known as ‘crizzling’ and can eventually lead to the formation of flakes on the surface of the glass. With continued leaching of the alkali metal ions, the glass surface becomes increasingly alkaline. Once the pH reaches 9 or above, dissolution of the silica network occurs and the glass object will lose much of its inherent strength and ultimately collapse [3].
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.205
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514
SIMS has been increasingly used to measure the corrosion of glass, or rather the leaching of the alkali metal ions over time [4,5]. In the SIMS depth profiling mode the leaching alkali ions can be monitored easily as a function of depth but, in most cases the glasses analysed have either been buried samples with large corrosion depths or in the case of Ryan’s work on atmospheric glass corrosion, glass which was aged at elevated temperatures and humidities in order to obtain sodium depletion depths that could be measured by SIMS. The depth resolution of the SIMS tool used during this study was of the order of 10 nm. Ageing glass at elevated temperatures is not however an accurate representation of the long-term corrosion that occurs under the ambient conditions of a museum. With advances in SIMS instrumentation over the past few years, it is now possible to carry out depth profiling routinely using sub-keV beam energies and nanometer depth resolution. This means that glass corrosion can now be performed at room temperature, over reasonable time periods, and accurate depth concentration profiles of the leaching alkali ions can be obtained of the top 50–80 nm. Improved depth resolution will allow a more accurate picture of room temperature corrosion to be determined, and the work presented here marks the very beginning of this research. The final long-term aim is to obtain a more precise description of the mechanism of glass corrosion at room temperature, and help museum conservators to select suitable environmental conditions for the safe storage of vulnerable glass objects.
2. Experimental procedure After compositional analysis (EPMA) of an original artefact from the V&A museum, a replica glass, RG1 was fabricated according to the composition of 72.72 wt.% SiO2, 17.95 wt.% Na2O, 3.27 wt.% K2O, 2.17 wt.% CaO, 0.74 wt.% MgO, 1.21 wt.% Al2O3, 0.23 wt.% Fe2O3 and 0.37 wt.% Mn2O3. The batch was then melted at 1200 8C and blown into circular flat plates. EPMA was then used again to ensure that the glass produced was homogenous over the nine samples produced. In order to age the glass, pieces from the plates were cut and placed in environmental chambers set to known humidities which were representative of a museum environment. The
511
glass was then left for a period of time to age at room temperature. After ageing the glass surfaces were depth profiled using an Atomika quadrupole SIMS tool [6] equipped with a floating low energy primary ion gun [7,8]. A nitrogen ion beam at 500 eVand normal incidence was used to perform the analyses, with a current of 90 nA and scan size of 375 mm. Charge compensation was used in all the analyses, using an electron energy of 70 eV. SIMS crater depths were measured using a Zygo white light optical interferometer.
3. Results Fig. 1 shows typical concentration depth profiles for the main elemental species of the aged replica glass, RG1. The concentration scale has been quantified by setting the bulk glass signals to the bulk concentration levels of the glass obtained via EPMA. In Fig. 1, the glass has been aged at 55% RH for 91 h at room temperature. At the start of the SIMS profile both the Na and K signals drop to a minimum, of 0.40 and 0.05%, compared with bulk concentrations of 11.92 and 1.43 at.%, respectively. This first region represents the removal of the leached corrosion products from the surface of the glass. After reaching a minimum, both signals then gradually increase to flat stable signals indicating that the bulk glass state has been reached. The depletion of these cations near the surface is used as a measure of how corroded the glass has become. In contrast, the relative concentrations of Al and Si near to the surface are higher then their respective bulk values which is largely due to a concentration effect, whereby their apparent concentration increases as the alkali metal cations are leached from the glass surface. The Si 28 signal has also been enhanced by a mass interference from the COþ signal, however, mass interferences will occur at all of the Si isotopes, and their corresponding molecular signals. Fig. 2 shows the Na concentration depth profiles from the RG1 glass aged at humidities of 37 and 55% RH for 91 h, and the as-produced, ‘non-aged glass’, as a function of relative humidity. (It should be noted here that the ‘non-aged’ glass does show a slight depletion of Na up to a depth of 22 nm, because the sample was actually analysed 1 day after it had been removed from the annealing oven.) The effect of
512
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514 Region I – corroded glass
Region II – bulk glass
100
Conc. (at % cations)
10
1
Na - 23 0.1
Al - 27 Si - 28 K - 39 Ca -40
0.01 0
10
20
30
40
50 60 70 Depth / nm
80
90
100 110 120
Fig. 1. Concentration depth profile for the main elemental species of the aged replica glass, RG1, aged at 55% RH for 91 h at room temperature. At the start of the profile the Si signal is greatly enhanced due to mass interference.
Conc. (at% cations)
100
10
1 Na - as made Na - 37%RH - 91 hours Na- 55%RH - 91 hours 0.1 0
10
20
30
40
50
60
70
80
90
100
110
Depth / nm
Fig. 2. Naþ concentration depth profiles from the RG1 glass un-aged glass and aged at humidities of 37 and 55% RH for 91 h.
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514
increased atmospheric water concentration is an increased depletion in terms of both concentration and depth of the Na cations within the near the surface of the glass. As the humidity increases from 37% RH, the depletion depth after 91 h is 76 nm, whereas at 55% RH after the same ageing time the depletion depth is 86 nm. The shapes of the two profiles are also different as can be seen by changes in slope of the profiles. It is not yet fully understood what is causing the differences between these two profiles, but it is thought to be due to migration of the Na caused by the SIMS beam, or an effect of surface roughening. Although the ageing was performed at room temperature, both profiles indicate that near the surface the glass has very rapidly become depleted in Na cations compared to the non-aged glass. Similarly, the K cations (not shown on the profiles) show the same correlation between depletion depth and increasing humidity. By measuring the depletion depths of the Na cations, it is possible to get an estimate of the depletion rate in this very early stage of glass corrosion. These leaching rates at room temperature have been plotted in Fig. 3, along with data previously attained by Ryan at 40 8C and similar humidities [9]. It is clear that the slopes of the two plots clearly differ but, unfortunately there is insufficient data so far to identify the corrosion mechanism occurring at room temperature. It is predicted that the initial, fast depletion of Na will decrease with the square root of time,
Leaching Rate of Na / nm/hour
0.75 0.7 0.65 0.6 0.55 0.5
22C
0.45
40C
0.4 30
40
50
60
Relative Humidity / %RH
Fig. 3. Leaching rates of Na at different temperatures.
513
until a steady state is reached [10], but if the leaching of Na were to proceed at the rates estimated from these initial results over 1 year, then Na would be depleted to approximate depths of 5 mm at 37% RH and 6 mm at 55% RH. These values differ considerably from the leaching rate at room temperature estimated by Ryan on the same glass composition aged at 40% RH. By extrapolation from the leaching rates obtained from RG1 aged at elevated temperatures, Ryan obtained a Na leach rate of approximately 1.5 mm per year. This value suggests that the glass is corroding at a much slower rate, and is therefore in a ‘safer’ environment than appears to be the case from the Na leach rates actually measured at room temperature by low-energy SIMS. From these very early SIMS results it appears that Na and K cations are still leaching out of the replica glass at humidities that have been suggested as safe for storage of museum glass objects [11].
4. Conclusion Continuing work on ageing replica glass at a wide range of humidities has produced promising initial SIMS results. It has been shown that depth profiling performed with a nitrogen beam at 500 eV can usefully be applied to analyse accurately the concentration profile of the main elemental species in the top surface region of a glass that has been corroded at known humidities and, most important, at room temperature. From the SIMS depth profiles it is possible to measure accurately the depth to which the Na and K cations have leached within the top 100 nm of the glass surface. It is essential that more humidities are studied for a greater range of time periods in order to identify accurately the room temperature corrosion mechanism for this glass. The glass must also be aged for a wider range of times.
Acknowledgements The author would like to thank Ian Hankey who produced the blown glass samples and assisted with glass melting, and Kevin Reeves of the Wolfson Archaeological Science Laboratories at the Institute
514
S. Fearn et al. / Applied Surface Science 231–232 (2004) 510–514
of Archaeology UCL for performing the EPMA analysis. References [1] A. Neri, L’arte Vittaria, O. Pulleyn, London, 1662 (reprinted by the Society of Glass Technology). [2] V. Oakley, Fighting the inevitable: the continuing search for a solution to glass decay at the V&A, Glass Technol. 42 (3) (2001) 65–69. [3] R.K. Iler, The Chemistry of Silica, Wiley/Interscience, New York, 1979. [4] J.L. Ryan, D.S. McPhail, P.S. Rogers, in: Proceedings of the 11th Triennial Meeting of ICOM, 1996, pp. 839–844.
[5] G.G. Wicks, J. Nucl. Mater. 298 (2001) 78–85. [6] Atomika 6500 SIMS, FEI Deutschland GmbH, D-85764 Oberschleissheim/Munich, Germany. [7] M.G. Dowsett, N.S. Smith, D.R. Bridgeland, A.C. Lovejoy, P. Pendrick, in: A. Benninghoven, B. Hagenhoff, H.W. Werner (Eds.), Proceedings of the Secondary Ion and Mass Spectrometry, SIMS XI, Wiley, 1997, pp. 367–370. [8] Ionoptika FLIG. [9] J.L. Ryan, PhD Thesis, The Atmospheric Deterioration of Glass: Studies of Decay Mechanisms and Conservation Techniques, University of London, 1996. [10] B. Grambow, R. Muller, J. Nucl. Mater. 298 (2001) 112–124. [11] R.H. Brill, Crizzling–A problem in Glass Conservation, Conservation in Arcaeology and the Applied Arts, Stockholm Congress, London. The International Institute for Conservation of Historic and Artistic Works, 1975, pp. 121–134.