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Some applications of SIMS in conservation science, archaeometry and cosmochemistry
Applied Surface Science 252 (2006) 7107–7112 www.elsevier.com/locate/apsusc
Some applications of SIMS in conservation science, archaeometry and cosmochemistry D.S. McPhail Department of Materials, Imperial College, London, SW7 2AZ, UK Available online 3 May 2006
Abstract Some applications of SIMS in conservation science, archaeometry and cosmochemistry are described. Ultra-low energy SIMS depth profiling and TOF-SIMS imaging are used to study the corrosion of low-lime glass vessels from the V&A museum. Static SIMS and focused ion beam (FIB) SIMS are used to study the effects of laser cleaning on museum artefacts. Archaeological glass from Raqqa, Syria is studied with FIB-SIMS and micrometeorite impacts on space vessels are studied with FIB and FIB-SIMS. The new analytical challenges provided to the SIMS community by these materials are presented and the ethical issues associated with sampling and destructive analysis discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Archaeometry; Conservation; Cosmochemistry; Depth Profiling; FIB; Imaging; SIMS
1. Introduction Developments in secondary ion mass spectrometry (SIMS) have been driven primarily by the needs of the semiconductor industry for which accurate concentration-depth profiles and high lateral resolution chemical maps are required. It is now becoming increasingly obvious, however, that the excellent lateral resolution, depth resolution and sensitivity of SIMS are relevant to many other areas of materials science so that in recent years the scope of the technique has expanded considerably [1]. Materials from the fields of archaeometry, conservation science and cosmochemistry, for example, present fascinating new challenges for the SIMS community [2]. The materials science of ‘ancient’ materials, both manmade and natural, is generally much more complex than 21st century materials such as silicon based devices. In the case of man-made materials, the details of the fabrication and processing of the object are usually obscure and the provenance of the raw materials uncertain and furthermore the raw materials used are typically based on readily available natural materials such as plant ash, sand, clay and salt, materials which have many components and are of variable composition. The primary criteria in fashioning the artefacts will have been aesthetic and economic and generally a large
E-mail address: [email protected]. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.132
degree of uncertainty surrounds the environmental history of the objects. Subsequent museum based conservation treatments, if they have been applied, are often un-documented and may have introduced, or removed, surface layers. The microstructure of ‘ancient’ materials is generally complex, and can involve a mixture of amorphous and crystalline phases, the surfaces of such materials tend to be rough and porous and generally the materials are rather poor conductors of electricity. All these features are in marked contrast to the analytical challenge presented by a silicon wafer! There is one more very important consideration, that of conservation ethics, that will inform the analytical protocol. Conservation science is a relatively young discipline and has two aspects to it, passive conservation in which the environment is controlled in such a way as to minimise damage and active conservation in which treatments are applied to arrest decay and stabilise the surface against further deterioration. Conservation ethics have developed rapidly and are designed to protect and preserve collections. The main guidelines as described by Ashley-Smith in 1999 [3] are that: All treatments should be adequately documented and there should be no structural and decorative falsification of the object. All conservation and restoration processes should, as far as it is practicable, be fully reversible even after a number of years.
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D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
Decayed parts of objects should, wherever possible, be conserved in situ. The natural consequences of ageing of the original material should not normally be disguised or removed. A more detailed set of guidelines were published recently in the V&A conservation journal [4] and may also be found on their web-site [5]. It is clear that sampling and application of techniques that consume the material are usually to be the last resort. The fundamental SIMS equation indicates some of the issues associated with the analysis of ‘ancient’ materials. The relationship between the secondary ion intensity I (counts per second) and the concentration of the element c being measured is given by:
nanometre per day or less, where in the past accelerated ageing would have been needed to produce a measurable effect in a reasonable time. A corrosion rate of 1 nm/day corresponds to 1 mm every 3000 years. Elemental mapping is essential in the study of most materials as lateral homogeneity is very rare, and processes such as corrosion and oxidation are typically initiated at flaws, defects and grain boundaries. The best lateral resolution possible is 5 nm, for example on our FEI FIB 200 SIMS system (FEI Eindhoven) but one should note that the small analytical volume associated with such a probe will in most cases preclude high sensitivity SIMS analysis. We have devised some special FIB-SIMS strategies to maximize the analytical volume in such cases [6]. 2. Experimental
0
I ¼ ðaTÞ ðAz Þc a is the ionization probability, T combines the secondary ion transmission and detector efficiency, their product (aT) is termed the useful ion yield, A is the analysed area and z0 the sputter rate. The consequences of this equation are that the useful ion yield must be measured for every element in every matrix in the sample, if the analysis is to be fully quantitative, and furthermore the sputter rate must be known for each component in a multi-layer structure. The SIMS equation is only true in the dilute regime which is typically taken to mean for concentrations less that a few atom percent. This equation and these caveats must be borne in mind when applying SIMS to all materials systems [1]. The key attributes that SIMS brings to the analysis of ‘ancient’ materials are the very high depth resolution, the good lateral resolution and the excellent sensitivity of the technique. Sub-nanometre depth resolution permits very accurate measurement of the concentration as a function of depth of the elements of interest. Moreover sub-nanometre depth resolution facilitates the measurement of slow changes in the surfaces of materials, i.e. processes with sub-nanometre/unit time kinetics. Thus we can use SIMS depth profiling to measure corrosion/ oxidation/diffusion processes taking place at the rate of a
The SIMS instruments used in these studies were (i) an Atomika 6500 depth profiling instrument that has been retrofitted with a floating low energy ion gun (O2+, N2+, Ar+) (Cameca Deutschland) and is capable of sub-nanometre depth resolution and ppm sensitivities, (ii) an FEI 200 Focused Ion Beam system with a Ga+ LMIS and a quadrupole SIMS attachment and (iii) a Millbrook mini-SIMS (Millbrook Instruments, Blackburn) for Static SIMS and mapping over areas of up to 5 mm, again using a gallium liquid metal ion source. Instruments also used were (iv) an Atomika 4600 system at FEI Atomika (Munich) (now Cameca Deutschland) and (v) an ION TOF V (ION-TOF Munster). Sample details and experimental procedures are given in the individual case studies. 2.1. Case study 1: vessel glass corrosion The first study to be described involves the conservation of glass artefacts from the collection at the V&A museum. Glass that is relatively low in lime and magnesia undergoes atmospheric attack due to moisture in the air and the corrosion rates can be very low. This instability is present in many different objects spanning different geographical regions and times. In 1990 the results of a survey of the Victoria and Albert
Fig. 1. Examples from the V&A glass gallery of three objects exhibiting varying degrees of glass corrosion: (a) 17th/18th century Venetian goblet (102-1853) of opaque white glass. (b) 18th century German (Potsdam) Roemer goblet. (c) Jug made by George Ravenscroft 1675–1680 (height 20.4 cm).
D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
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Table 1 The composition of the replica low-lime glass produced for ageing experiments Replica composition cations (at./cm3 1021) Si Na Ca Mg K Al Fe Mn
12.20 5.73 0.69 0.98 0.37 0.24 0.03 0.05
Museums’ glass collection were published [7]. The surfaces of the glass pieces were examined for deposits, moisture, crizzling, flaking and changes of colour to assess the degree of deterioration. From a total of 6500 objects, some 400 were found to be of an unstable nature to the extent that visual signs of deterioration had appeared. Fig. 1a shows a 17th/18th century Venetian goblet (102–1853) of opaque white glass with applied red decoration in which the corrosion rate is quite severe and has led to mechanical instability, Fig. 1b is of 18th century German (Potsdam) Roemer goblet made between 1701–1705 and Fig. 1c is of a 17th century jug made by George Ravenscroft. The compositions of many of these vulnerable silica-based glasses have been assessed using electron probe micro-beam analysis [8] and it has been established that they all have relatively low concentrations of the network stabilisers (CaO, MgO) and relatively high concentrations of the network modifiers (Na2O, K2O). It is believed that this may have been done to improve the working properties of the glass and its appearance [9]. In a programme of work that aims to determine appropriate methods of active and passive conservation we have analysed a fragment of glass from a facon de Venise goblet using electron probe micro analysis (EPMA) and we have then made up an analogue batch, as described in Ref. [10]. The low lime-high soda glass (Table 1) has been aged under various conditions of humidity, time and temperature and ultra-low energy SIMS depth profiling has been used to determine the rate of reaction of the glass surface with atmospheric moisture. The depth profile in Fig. 2 is of a sample RG1 exposed to the environment for 1 year. The analysis was conducted on an ION-TOF V instrument using 1 keV Cs+ primary ions for sputtering and 25 keV Bi3+ ions for analysis, with the caesium beam incident at 458 and the Bi3+ normal to the surface. The analysis has been depth calibrated assuming a uniform sputter-rate. This data confirms in general terms the model of glass corrosion due to exchange of alkali ions and protons, and the extent of the corrosion can be defined, for example, in terms of the thickness of the alkali depleted front (420 nm) or the areal dose of sodium lost from the glass (at./cm2). However alkali peaks in the first 100 nm of the surface vary in a complex fashion, a region in which there is a sudden drop in the calcium channel and a slight increase in the silicon channel. It is believed that the alkali ions that have migrated to the surface have formed a corrosive alkaline solution that has reacted with the glass. Thus the surface 100 nm is a complex reaction front
Fig. 2. A SIMS depth profile of the low-lime glass after a year of exposure to normal environmental conditions. Analysis on an ION TOF V using 25 keV Bi3+ primary ions for analysis and 1 keV Cs+ ions for sputtering. The analysis area was 300 mm 300 mm.
and the next 300 nm an alkali depleted gel-type layer. Imaging of the surface was conducted in the FIB instrument. In the FIBSEM mode it became obvious that the salt deposits were nonuniform and a local area FIB SIMS depth profile (Fig. 3) revealed a large excess of sodium in a salt crystal (as compared with the surrounding surface). These reaction products suggest (i) that the sodium transportation through the glass may be anisotropic with the presence of fast diffusion pathways (ii) that salt must be removed from the surface before attempting a depth profile to avoid inaccurate measurements and the effects of roughness in the crater base. Fearn [10] has recently shown the effects that the surface salt deposits can have on calculations of sodium transport. Two
Fig. 3. Local area FIB SIMS depth profiles confirming that the salt reaction products are rich in sodium.
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depth profiles were taken on our Atomika 6500 before and after the surface was cleaned (Fig. 4a). The pre-cleaned depth profile exhibited a large surface spike at mass 23, due it is believed to the sodium salt on the surface. The large surface peak in the silicon mass channel at mass 28 was due to the CO mass interference (from carbonate deposits on the surface). Fig. 4b. represents a 3D plot of the sodium distribution within the near surface of the glass, reconstructed from the 3D data-set after the analysis. This data was collected on an Atomika 4600 at FEI Atomika (now Cameca Deutschland) and clearly has great potential for following sodium diffusion pathways. 2.2. Case study 2: micrometeorite studies Micrometeorite impacts on space vehicles are a subject of considerable interest as is the capture of micrometeorites for fundamental studies. We have studied impact sites on solar cells recovered from the Hubble space telescope [11] and in some instances have found micrometeorites that are essentially intact. The FIB-SIMS was used to prepare a ‘double crosssection’ of one such particle, as shown in Fig. 5. The micrometeorite was then imaged in the FIB-SEM mode at two tilt angles and ion channeling contrast used to reveal the grain structure. Finally the particle was imaged in the SEM to reveal that it had a Fe–Ni composition. Stardust is a mission designed to capture micrometeorites using aerogel [12], an ultra-low density silica foam, and one potential challenge is particle extraction. Again FIB-SIMS has a role to play, both in materials processing and materials analysis. Fig. 6a. shows a test sample in which some aerogel has been deliberately implanted with a meteorite fragment, a polystyrene sphere and a tungsten sphere. The FIB is used to mill away the aerogel Fig. 6b. around these particles, which can then be imaged in the SEM or SIMS instrument. In this case SEM EDX analysis was then used to reveal the elements of interest (Fig. 6c). Fig. 4. (a): SIMS depth profiles of a corroded glass surface before and after removal of surface salts; (b) A 3D plot of the sodium distribution in a corroded glass (VG1).
2.3. Further applications of SIMS In another programme of work we have established that the very high sensitivity of the SIMS technique allows us to see subtle changes to the surface chemistry of museum objects, long before these changes become visible to the human eye.
Fig. 5. FIB sections of a micrometeorite.
D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
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Fig. 6. FIB processing and imaging of particles in aerogel.
Fig. 7. The FIB was used to prepare a bevel-section in the material, the sample rotated and the same ion beam then used to generate secondary ion or electron images.
This capability flows from the incredible sensitivity of SIMS which can be used, for example, to assess the efficiency of cleaning processes on works of art and for monitoring the rate of recontamination once the surfaces have been cleaned. The Millbrook mini-SIMS (Millbrook Instruments, Blackburn) allows us to perform static SIMS on such materials reducing the damage due to the analysis [13]. Often it is found that the surfaces of ancient materials are simply too rough for depth profiling. In such cases the milling capability of the focused ion beam (FIB) instrument has proven very useful for preparing cross-sections through the layers of interest. For example in one study archaeological samples from a site in Raqqa, Syria, were assessed [14]. The corrosion layers formed by the materials, which are fragments from the production site that have been buried in the soil, are incredibly uniform, and we used the focused ion beam FIB-SIMS to produce cross-sections that were then imaged with the same focused ion beam Fig. 7. One of the great strengths of FIB as a sample preparation technique is that the area to be crosssectioned can be defined with sub-micron precision. 3. Conclusions Ancient materials present the surface analyst with a unique set of challenges due to their complex microstructure and uncertain history; furthermore the surfaces are often rough and porous and the material is usually an electrical insulator. Conservation ethics impose further boundary conditions on sampling and on the use of destructive analytical techniques, so SIMS should be used as the final stage in any investigation. It follows that experimental protocols involving analogue materials are very important.
The high depth resolution of ultra-low energy SIMS depth profiling facilitates the measurement of surface processes with slow kinetics, i.e. sub-nanometre per unit time rate measurements and the high sensitivity of SIMS allows us to detect changes to the surface chemistry long before they become visible to the human eye, for example during laser cleaning. Surface reaction products can seriously affect SIMS depth profile measurements and should be removed prior to analysis. FIB is a powerful process both for analysis and for sample preparation and FIB milling can be used to ‘open-up’ for cross-sectional imaging structures too rough for conventional depth profiling. The outcome of the SIMS analyses of these materials are, unsurprisingly, complex and require detailed interpretation. These studies have shown that there is a considerable synergy in the study of materials from the fields of conservation science, archaeometry and cosmochemistry and that SIMS has an important role to play in that activity. Indeed the range of applications is limited only by our imaginations! Acknowledgements I am grateful to Dr. Birgit Hagenhoff at Tascan for the IONTOF analyses and to Dr. Hans-Ulrich Ehrke of Cameca Deutschland for the Atomika 4600 analyses. References [1] D.S. McPhail, Applications of secondary ion mass spectrometry (SIMS) in materials science, J. Mater. Sci. 41 (2006) 873 (40th Anniversary Issue). [2] M.G. Dowsett, A. Adrieans, The role of SIMS in cultural heritage studies, Nuclear Instrum. Methods Phys. Res. B, online, 2004.
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[3] J. Ashley-Smith, Risk Assessment for Object Conservation, ButterworthHeinemann, 1999. [4] A. Richmond, The ethics checklist-ten years on, V&A Conserv. J. 50 (2005) 11. [5] http://www/vam.ac.uk/res_con/conservation/advice/policies/index.html. [6] M.P. Ryan, D.E. Williams, R.J. Chater, B.M. Hutton, D.S. McPhail, Why stainless steel corrodes, Nature 70 (2002) 774. [7] V. Oakley, Vessel glass deterioration at the Victoria and Albert Museum: surveying the collection, The Conservator 14 (1990) 30. [8] S. Davison, Conservation and Restoration of Glass, Butterworth-Heinemann, 2003, ISBN: 0 7506 43412. [9] A. Neri, L’Arte Vittaria, London, O. Pulleyn, Reprints : http://www.societyofglasstechnology.org.uk/, 1662. [10] S. Fearn, D.S. McPhail, V. Oakley, Moisture attack on museum glass measured by SIMS, Phys. Chem. Glasses 46 (5) (2005) 505.
[11] S. Kettle, R.J. Chater, G.A. Graham, D.S. McPhail, A.T. Kearsley, FIBSIMS analysis of micro-particle impacts on spacecraft materials returned from low-Earth orbit, Appl. Surf. Sci. 231-232 (2004) 893. [12] G.A. Graham, P.G. Grant, R.J. Chater, A.J. Westphal, A.T. Kearsley, C. Snead, G. Dominguez, A.L. Butterworth, D.S. McPhail, G. Bench, J.P. Bradley, Investigation of ion beam techniques for the analysis and exposure of particles encapsulated by silica aerogel: applicability for stardust, Meteor. Planet. Sci. 39 (9) (2004) 1461. [13] D.S. McPhail, M. Sokhan, E.E. Rees, B. Cliff, A.J. Eccles, R.J. Chater, Rapid characterisation of surface modification and treatments using a bench top SIMS instrument, Appl. Surf. Sci. 231–232 (2004) 967. [14] J. Henderson, S.D. McLoughlin, D.S. McPhail, Radical changes in Islamic glass technology: evidence for conservatism and experimentation with new glass recipes, Archaeometry 46 (2004) 439.
Some applications of SIMS in conservation science, archaeometry and cosmochemistry D.S. McPhail Department of Materials, Imperial College, London, SW7 2AZ, UK Available online 3 May 2006
Abstract Some applications of SIMS in conservation science, archaeometry and cosmochemistry are described. Ultra-low energy SIMS depth profiling and TOF-SIMS imaging are used to study the corrosion of low-lime glass vessels from the V&A museum. Static SIMS and focused ion beam (FIB) SIMS are used to study the effects of laser cleaning on museum artefacts. Archaeological glass from Raqqa, Syria is studied with FIB-SIMS and micrometeorite impacts on space vessels are studied with FIB and FIB-SIMS. The new analytical challenges provided to the SIMS community by these materials are presented and the ethical issues associated with sampling and destructive analysis discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Archaeometry; Conservation; Cosmochemistry; Depth Profiling; FIB; Imaging; SIMS
1. Introduction Developments in secondary ion mass spectrometry (SIMS) have been driven primarily by the needs of the semiconductor industry for which accurate concentration-depth profiles and high lateral resolution chemical maps are required. It is now becoming increasingly obvious, however, that the excellent lateral resolution, depth resolution and sensitivity of SIMS are relevant to many other areas of materials science so that in recent years the scope of the technique has expanded considerably [1]. Materials from the fields of archaeometry, conservation science and cosmochemistry, for example, present fascinating new challenges for the SIMS community [2]. The materials science of ‘ancient’ materials, both manmade and natural, is generally much more complex than 21st century materials such as silicon based devices. In the case of man-made materials, the details of the fabrication and processing of the object are usually obscure and the provenance of the raw materials uncertain and furthermore the raw materials used are typically based on readily available natural materials such as plant ash, sand, clay and salt, materials which have many components and are of variable composition. The primary criteria in fashioning the artefacts will have been aesthetic and economic and generally a large
E-mail address: [email protected]. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.132
degree of uncertainty surrounds the environmental history of the objects. Subsequent museum based conservation treatments, if they have been applied, are often un-documented and may have introduced, or removed, surface layers. The microstructure of ‘ancient’ materials is generally complex, and can involve a mixture of amorphous and crystalline phases, the surfaces of such materials tend to be rough and porous and generally the materials are rather poor conductors of electricity. All these features are in marked contrast to the analytical challenge presented by a silicon wafer! There is one more very important consideration, that of conservation ethics, that will inform the analytical protocol. Conservation science is a relatively young discipline and has two aspects to it, passive conservation in which the environment is controlled in such a way as to minimise damage and active conservation in which treatments are applied to arrest decay and stabilise the surface against further deterioration. Conservation ethics have developed rapidly and are designed to protect and preserve collections. The main guidelines as described by Ashley-Smith in 1999 [3] are that: All treatments should be adequately documented and there should be no structural and decorative falsification of the object. All conservation and restoration processes should, as far as it is practicable, be fully reversible even after a number of years.
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D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
Decayed parts of objects should, wherever possible, be conserved in situ. The natural consequences of ageing of the original material should not normally be disguised or removed. A more detailed set of guidelines were published recently in the V&A conservation journal [4] and may also be found on their web-site [5]. It is clear that sampling and application of techniques that consume the material are usually to be the last resort. The fundamental SIMS equation indicates some of the issues associated with the analysis of ‘ancient’ materials. The relationship between the secondary ion intensity I (counts per second) and the concentration of the element c being measured is given by:
nanometre per day or less, where in the past accelerated ageing would have been needed to produce a measurable effect in a reasonable time. A corrosion rate of 1 nm/day corresponds to 1 mm every 3000 years. Elemental mapping is essential in the study of most materials as lateral homogeneity is very rare, and processes such as corrosion and oxidation are typically initiated at flaws, defects and grain boundaries. The best lateral resolution possible is 5 nm, for example on our FEI FIB 200 SIMS system (FEI Eindhoven) but one should note that the small analytical volume associated with such a probe will in most cases preclude high sensitivity SIMS analysis. We have devised some special FIB-SIMS strategies to maximize the analytical volume in such cases [6]. 2. Experimental
0
I ¼ ðaTÞ ðAz Þc a is the ionization probability, T combines the secondary ion transmission and detector efficiency, their product (aT) is termed the useful ion yield, A is the analysed area and z0 the sputter rate. The consequences of this equation are that the useful ion yield must be measured for every element in every matrix in the sample, if the analysis is to be fully quantitative, and furthermore the sputter rate must be known for each component in a multi-layer structure. The SIMS equation is only true in the dilute regime which is typically taken to mean for concentrations less that a few atom percent. This equation and these caveats must be borne in mind when applying SIMS to all materials systems [1]. The key attributes that SIMS brings to the analysis of ‘ancient’ materials are the very high depth resolution, the good lateral resolution and the excellent sensitivity of the technique. Sub-nanometre depth resolution permits very accurate measurement of the concentration as a function of depth of the elements of interest. Moreover sub-nanometre depth resolution facilitates the measurement of slow changes in the surfaces of materials, i.e. processes with sub-nanometre/unit time kinetics. Thus we can use SIMS depth profiling to measure corrosion/ oxidation/diffusion processes taking place at the rate of a
The SIMS instruments used in these studies were (i) an Atomika 6500 depth profiling instrument that has been retrofitted with a floating low energy ion gun (O2+, N2+, Ar+) (Cameca Deutschland) and is capable of sub-nanometre depth resolution and ppm sensitivities, (ii) an FEI 200 Focused Ion Beam system with a Ga+ LMIS and a quadrupole SIMS attachment and (iii) a Millbrook mini-SIMS (Millbrook Instruments, Blackburn) for Static SIMS and mapping over areas of up to 5 mm, again using a gallium liquid metal ion source. Instruments also used were (iv) an Atomika 4600 system at FEI Atomika (Munich) (now Cameca Deutschland) and (v) an ION TOF V (ION-TOF Munster). Sample details and experimental procedures are given in the individual case studies. 2.1. Case study 1: vessel glass corrosion The first study to be described involves the conservation of glass artefacts from the collection at the V&A museum. Glass that is relatively low in lime and magnesia undergoes atmospheric attack due to moisture in the air and the corrosion rates can be very low. This instability is present in many different objects spanning different geographical regions and times. In 1990 the results of a survey of the Victoria and Albert
Fig. 1. Examples from the V&A glass gallery of three objects exhibiting varying degrees of glass corrosion: (a) 17th/18th century Venetian goblet (102-1853) of opaque white glass. (b) 18th century German (Potsdam) Roemer goblet. (c) Jug made by George Ravenscroft 1675–1680 (height 20.4 cm).
D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
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Table 1 The composition of the replica low-lime glass produced for ageing experiments Replica composition cations (at./cm3 1021) Si Na Ca Mg K Al Fe Mn
12.20 5.73 0.69 0.98 0.37 0.24 0.03 0.05
Museums’ glass collection were published [7]. The surfaces of the glass pieces were examined for deposits, moisture, crizzling, flaking and changes of colour to assess the degree of deterioration. From a total of 6500 objects, some 400 were found to be of an unstable nature to the extent that visual signs of deterioration had appeared. Fig. 1a shows a 17th/18th century Venetian goblet (102–1853) of opaque white glass with applied red decoration in which the corrosion rate is quite severe and has led to mechanical instability, Fig. 1b is of 18th century German (Potsdam) Roemer goblet made between 1701–1705 and Fig. 1c is of a 17th century jug made by George Ravenscroft. The compositions of many of these vulnerable silica-based glasses have been assessed using electron probe micro-beam analysis [8] and it has been established that they all have relatively low concentrations of the network stabilisers (CaO, MgO) and relatively high concentrations of the network modifiers (Na2O, K2O). It is believed that this may have been done to improve the working properties of the glass and its appearance [9]. In a programme of work that aims to determine appropriate methods of active and passive conservation we have analysed a fragment of glass from a facon de Venise goblet using electron probe micro analysis (EPMA) and we have then made up an analogue batch, as described in Ref. [10]. The low lime-high soda glass (Table 1) has been aged under various conditions of humidity, time and temperature and ultra-low energy SIMS depth profiling has been used to determine the rate of reaction of the glass surface with atmospheric moisture. The depth profile in Fig. 2 is of a sample RG1 exposed to the environment for 1 year. The analysis was conducted on an ION-TOF V instrument using 1 keV Cs+ primary ions for sputtering and 25 keV Bi3+ ions for analysis, with the caesium beam incident at 458 and the Bi3+ normal to the surface. The analysis has been depth calibrated assuming a uniform sputter-rate. This data confirms in general terms the model of glass corrosion due to exchange of alkali ions and protons, and the extent of the corrosion can be defined, for example, in terms of the thickness of the alkali depleted front (420 nm) or the areal dose of sodium lost from the glass (at./cm2). However alkali peaks in the first 100 nm of the surface vary in a complex fashion, a region in which there is a sudden drop in the calcium channel and a slight increase in the silicon channel. It is believed that the alkali ions that have migrated to the surface have formed a corrosive alkaline solution that has reacted with the glass. Thus the surface 100 nm is a complex reaction front
Fig. 2. A SIMS depth profile of the low-lime glass after a year of exposure to normal environmental conditions. Analysis on an ION TOF V using 25 keV Bi3+ primary ions for analysis and 1 keV Cs+ ions for sputtering. The analysis area was 300 mm 300 mm.
and the next 300 nm an alkali depleted gel-type layer. Imaging of the surface was conducted in the FIB instrument. In the FIBSEM mode it became obvious that the salt deposits were nonuniform and a local area FIB SIMS depth profile (Fig. 3) revealed a large excess of sodium in a salt crystal (as compared with the surrounding surface). These reaction products suggest (i) that the sodium transportation through the glass may be anisotropic with the presence of fast diffusion pathways (ii) that salt must be removed from the surface before attempting a depth profile to avoid inaccurate measurements and the effects of roughness in the crater base. Fearn [10] has recently shown the effects that the surface salt deposits can have on calculations of sodium transport. Two
Fig. 3. Local area FIB SIMS depth profiles confirming that the salt reaction products are rich in sodium.
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D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
depth profiles were taken on our Atomika 6500 before and after the surface was cleaned (Fig. 4a). The pre-cleaned depth profile exhibited a large surface spike at mass 23, due it is believed to the sodium salt on the surface. The large surface peak in the silicon mass channel at mass 28 was due to the CO mass interference (from carbonate deposits on the surface). Fig. 4b. represents a 3D plot of the sodium distribution within the near surface of the glass, reconstructed from the 3D data-set after the analysis. This data was collected on an Atomika 4600 at FEI Atomika (now Cameca Deutschland) and clearly has great potential for following sodium diffusion pathways. 2.2. Case study 2: micrometeorite studies Micrometeorite impacts on space vehicles are a subject of considerable interest as is the capture of micrometeorites for fundamental studies. We have studied impact sites on solar cells recovered from the Hubble space telescope [11] and in some instances have found micrometeorites that are essentially intact. The FIB-SIMS was used to prepare a ‘double crosssection’ of one such particle, as shown in Fig. 5. The micrometeorite was then imaged in the FIB-SEM mode at two tilt angles and ion channeling contrast used to reveal the grain structure. Finally the particle was imaged in the SEM to reveal that it had a Fe–Ni composition. Stardust is a mission designed to capture micrometeorites using aerogel [12], an ultra-low density silica foam, and one potential challenge is particle extraction. Again FIB-SIMS has a role to play, both in materials processing and materials analysis. Fig. 6a. shows a test sample in which some aerogel has been deliberately implanted with a meteorite fragment, a polystyrene sphere and a tungsten sphere. The FIB is used to mill away the aerogel Fig. 6b. around these particles, which can then be imaged in the SEM or SIMS instrument. In this case SEM EDX analysis was then used to reveal the elements of interest (Fig. 6c). Fig. 4. (a): SIMS depth profiles of a corroded glass surface before and after removal of surface salts; (b) A 3D plot of the sodium distribution in a corroded glass (VG1).
2.3. Further applications of SIMS In another programme of work we have established that the very high sensitivity of the SIMS technique allows us to see subtle changes to the surface chemistry of museum objects, long before these changes become visible to the human eye.
Fig. 5. FIB sections of a micrometeorite.
D.S. McPhail / Applied Surface Science 252 (2006) 7107–7112
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Fig. 6. FIB processing and imaging of particles in aerogel.
Fig. 7. The FIB was used to prepare a bevel-section in the material, the sample rotated and the same ion beam then used to generate secondary ion or electron images.
This capability flows from the incredible sensitivity of SIMS which can be used, for example, to assess the efficiency of cleaning processes on works of art and for monitoring the rate of recontamination once the surfaces have been cleaned. The Millbrook mini-SIMS (Millbrook Instruments, Blackburn) allows us to perform static SIMS on such materials reducing the damage due to the analysis [13]. Often it is found that the surfaces of ancient materials are simply too rough for depth profiling. In such cases the milling capability of the focused ion beam (FIB) instrument has proven very useful for preparing cross-sections through the layers of interest. For example in one study archaeological samples from a site in Raqqa, Syria, were assessed [14]. The corrosion layers formed by the materials, which are fragments from the production site that have been buried in the soil, are incredibly uniform, and we used the focused ion beam FIB-SIMS to produce cross-sections that were then imaged with the same focused ion beam Fig. 7. One of the great strengths of FIB as a sample preparation technique is that the area to be crosssectioned can be defined with sub-micron precision. 3. Conclusions Ancient materials present the surface analyst with a unique set of challenges due to their complex microstructure and uncertain history; furthermore the surfaces are often rough and porous and the material is usually an electrical insulator. Conservation ethics impose further boundary conditions on sampling and on the use of destructive analytical techniques, so SIMS should be used as the final stage in any investigation. It follows that experimental protocols involving analogue materials are very important.
The high depth resolution of ultra-low energy SIMS depth profiling facilitates the measurement of surface processes with slow kinetics, i.e. sub-nanometre per unit time rate measurements and the high sensitivity of SIMS allows us to detect changes to the surface chemistry long before they become visible to the human eye, for example during laser cleaning. Surface reaction products can seriously affect SIMS depth profile measurements and should be removed prior to analysis. FIB is a powerful process both for analysis and for sample preparation and FIB milling can be used to ‘open-up’ for cross-sectional imaging structures too rough for conventional depth profiling. The outcome of the SIMS analyses of these materials are, unsurprisingly, complex and require detailed interpretation. These studies have shown that there is a considerable synergy in the study of materials from the fields of conservation science, archaeometry and cosmochemistry and that SIMS has an important role to play in that activity. Indeed the range of applications is limited only by our imaginations! Acknowledgements I am grateful to Dr. Birgit Hagenhoff at Tascan for the IONTOF analyses and to Dr. Hans-Ulrich Ehrke of Cameca Deutschland for the Atomika 4600 analyses. References [1] D.S. McPhail, Applications of secondary ion mass spectrometry (SIMS) in materials science, J. Mater. Sci. 41 (2006) 873 (40th Anniversary Issue). [2] M.G. Dowsett, A. Adrieans, The role of SIMS in cultural heritage studies, Nuclear Instrum. Methods Phys. Res. B, online, 2004.
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[11] S. Kettle, R.J. Chater, G.A. Graham, D.S. McPhail, A.T. Kearsley, FIBSIMS analysis of micro-particle impacts on spacecraft materials returned from low-Earth orbit, Appl. Surf. Sci. 231-232 (2004) 893. [12] G.A. Graham, P.G. Grant, R.J. Chater, A.J. Westphal, A.T. Kearsley, C. Snead, G. Dominguez, A.L. Butterworth, D.S. McPhail, G. Bench, J.P. Bradley, Investigation of ion beam techniques for the analysis and exposure of particles encapsulated by silica aerogel: applicability for stardust, Meteor. Planet. Sci. 39 (9) (2004) 1461. [13] D.S. McPhail, M. Sokhan, E.E. Rees, B. Cliff, A.J. Eccles, R.J. Chater, Rapid characterisation of surface modification and treatments using a bench top SIMS instrument, Appl. Surf. Sci. 231–232 (2004) 967. [14] J. Henderson, S.D. McLoughlin, D.S. McPhail, Radical changes in Islamic glass technology: evidence for conservatism and experimentation with new glass recipes, Archaeometry 46 (2004) 439.