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Alteration and corrosion phenomena in Roman submerged glass fragments
Journal of Non-Crystalline Solids 337 (2004) 136–141 www.elsevier.com/locate/jnoncrysol
Alteration and corrosion phenomena in Roman submerged glass fragments Fabio Barbana b
a,b
, Renzo Bertoncello
a,b,*
, Laura Milanese a, Cinzia Sada
c
a Dipartimento di Scienze Chimiche, University of Padova, Via Loredan n. 4, 35131 Padova, Italy Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Via Varchi n. 59, 50132 Florence, Italy c Dipartimento di Fisica ‘Galileo Galilei’, University of Padova, Via Marzolo n. 8, 35131 Padova, Italy
Received 31 July 2003; received in revised form 17 March 2004 Available online 14 May 2004
Abstract In the present paper the surface analytical techniques of XPS (X-ray photoelectron spectroscopy) and SIMS (secondary ions mass spectrometry) have allowed an understanding of the alteration and corrosion phenomena occurring in Roman glass fragments. The samples come from the seawater archaeological site of the Roman ship Iulia Felix, which sank near Grado (Gorizia, Italy) in the second century A.D. Through XPS the surface concentration of the main chemical elements and their depth profiles have been obtained. Attention has been focused on the oxidation states of the elements, oxygen/silicon ratio, peculiar presence of carbonate salts and concentration trends of some elements in the outer atomic layers. Depth trends of aluminium, calcium, magnesium and carbon were studied also through SIMS depth profiles. The whole set of XPS and SIMS data allows the segregation of particular chemical species and the evolution of the original glass to a multilayer system. 2004 Elsevier B.V. All rights reserved. PACS: 79.60; 68.49.Sf; 82.80.Ms; 61.43.Fs; 68.35.Dv
1. Introduction The knowledge of surface degradation of historical glasses, exposed for centuries to weathering, is very important. It is really interesting to understand the type and the features of chemical–physical phenomena which occur on these materials, like change in surface composition, chemical stability, different ways of leaching and alteration products. In this paper the alteration processes that occur on glasses immersed in seawater are investigated. Seawater glass alteration and/or corrosion involve many different processes like surface adsorption of water, leaching reactions [1,2], localized increasing in pH [2], dissolution reactions, crystallization of insoluble carbonates and sulphates [3], biological deterioration [4]. Hence there
* Corresponding author. Address: Dipartimento di Scienze Chimiche, University of Padova, Via Loredan n. 4, 35131 Padova, Italy. Tel.: +39-049 827 5204; fax: +39-049 827 5161. E-mail address: [email protected] (R. Bertoncello).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.118
are a number of physical and chemical processes that occur in the interface region ranging from the effect of contacting atmosphere up to the aqueous layer coating the glass fragment itself. The theme is tackled by a chemical approach, turning our attention to the investigation of glass surfaces because the main alteration phenomena occur at the interface between the glass fragments and the external environment [5]. The most important reactions that can take place when silicate glasses are in contact with an aqueous environment at pH < 9 are ionic exchange reactions between alkali ions of glass and protons of the solution [6], while aqueous solutions at pH > 9 induce the destruction of the bridging oxygen bond of the silica network [7]. The interaction of gaseous CO2 and SO2 species with the vitreous surface can involve the formation and co-precipitation of many alteration products like calcium carbonates and calcium sulphates. These phenomena favour the decrease of alkaline-earth ions concentrations in the glass [3,8,9]. The substitution of alkaline or alkaline-earth ions with protons also causes a volume contraction of the vitreous matrix that induces fracture,
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
collapse of the structure and pit formation. In pits crystallization causes a break-up action similar to the one of ice inside rock fractures [10,11].
2. Experimental 2.1. The samples The examined glass fragments come from the Roman ship Iulia Felix, which sank near Grado in the second century A.D. [12]. The glass composition is typical of the Roman era, with a large use of ‘natron’ (‘natron’ is a melting agent, Na2 CO3 Æ NaHCO3 Æ 2H2 O) and with small quantities of Mediterranean and Continental plant ashes [12–14]. The glass fragments are identified with the following abbreviations: IF2, IF3, IF4, IF8. In Fig. 1, a picture of the fragment IF2 is represented. All the analyzed samples present visible thin exfoliation layers. They are very fragile, easily broken and removable. In some samples, like IF2, IF3 and IF8, the thin layers are iridescent while in others, like IF4, they present a white coloration. 2.1.1. Thin sections preparation The thin sections were obtained by cutting the glass fragments at Dept. Mineralogia and Petrologia of Padua University with a water-cooled rotating saw. The glass fragments were pasted on a slide using an epoxy resin and the slides were cut to obtain a section with thickness no larger than 350 lm. The sections were then ground by SiC powder and then polished with diamond paste (1 lm).
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2.2. Sample characterization 2.2.1. Optical microscopy The samples were preliminarily examined with optical microscopy to obtain morphological information, especially for the sample IF8. We used an optical reflection microscope equipped by a digital camera to observe the surface structure of all the samples. 2.2.2. XPS measures XPS spectrometer was used and it was equipped with Al/Mg anodes working with AlKa (1486.6 eV) and MgKa (1253.6 eV) sources at 20 mA and 14 kV. Also an Al monochromatic source (1486.6 eV) with the same parameters, and a charge neutralizer were used. The working pressure was in the range of 107 Pa. Survey scans were obtained in the 0–1350 eV range (using AlKa source) or 0–1150 eV (using MgKa source) by steps of 0.8 eV. The pass energy was 187.86 eV. The depth profiles were obtained by sputtering with Arþ ions, operating at 3 keV, at a pressure of 5 · 106 Pa. A raster size of 2 · 2 mm was used. The analyzed areas were circles 0.8 mm in diameter. Operating at the same conditions, by the same instrumental apparatus, in flat glass samples of known thickness, the sputtering rate was about 1 nm/min. A direct measure of the sputtering rate and its extensive error analysis was not possible. 2.2.3. SIMS measures The instrument used for SIMS analyses is an ionic microscope Cameca ims 4f. The compositional analysis was recorded with several primary beam current values (10, 20 and 40 nA respectively) to optimize the measurement conditions and charge compensation. In the present paper the samples were bombarded with primary ions Csþ at 1–20 keV of power, an optimal current of 10 nA and the secondary ions emitted were analyzed by a mass analyzer made of magnetic and electrostatic sectors. A variable raster size of 125 · 125 lm was used. The working pressure was 107 Pa. A flood gun of electrons at 500 eV was used because the samples were insulators (better results was obtained by a 10 nA beam secondary current). The sputtering rate, estimated for flat glass samples, was about 60 nm/min. A direct measure of the sputtering rate was not possible, because the roughness of the samples was too large. So it is not possible to estimate the errors.
3. Results
Fig. 1. Iulia Felix, fragment IF2.
All the samples (sample surfaces and thin sections) described were analyzed by optical microscopy and XPS. Samples were analyzed also through SIMS and XPS depth profiles to point out the element concentration trends in depth.
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F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
Sample IF8: SIMS depth profile C O Na Mg Al Si Ca
Yield (Counts/s)
100000 O Si C
10000
Al Ca
Mg 1000
Na
200 nm
100 0
200
400
600
800
1000
1200
Sputtering time (s)
Fig. 2. Multilayer system in sample IF8.
Fig. 3. SIMS depth profile of the fragment IF8. The sputtering rate was not measured directly, because the samples were not homogeneous and too much rough. However in the same experimental conditions, with other glass samples, the sputtering rate was about 60 nm/min.
The image obtained by optical reflection microscopy on IF8 sample (Fig. 2) shows a layered structure and the areas not focused indicate layers at a different depth. 3.1. SIMS results Through secondary ion mass spectrometry three Iulia Felix samples were analyzed: IF2, IF3, IF8. The roughness of the samples does not allow quantitative analyses. The boron signal is close to the detection limit and it was omitted from the graph (Fig. 3). To evaluate its dynamics one should use Oþ for the primary beam and detect Bþ , but this was not possible because the electrical insulating characteristics of the samples require use of a flood gun for charge compensation and hence allows the detection of only secondary negative ions. As can be seen in the SIMS depth profile of sample IF8 (Fig. 3) the sodium and aluminium signals change in yield in a synchronous mode and the trend is not attributable to mass interferences; even carbon, magnesium and calcium yields show a sinusoidal synchronous profile. The SIMS analysis suggests the presence, in this sample, of an outer portion characterized by a multilayered structure, with maxima and minima concentration sequence (Fig. 3). The distance, which separates two adjacent maxima corresponds to about 200 nm in depth. It is also easy to observe that the trend of sodium and aluminium signals is just out of phase with the carbon, calcium and magnesium one. These phenomena confirm a multilayered structure with alternating shells. 3.2. XPS results 3.2.1. IF8 depth profile A very interesting trend is present in the IF8 sample depth profile (Fig. 4). After 90 min of sputtering the carbon amount (exclusively hydrocarbon contamina-
Fig. 4. XPS depth profile of the fragment IF8. The sputtering rate was not measured directly. However in the same experimental conditions, with other glass samples, the sputtering rate was about 1 nm/min.
tion) shrinks from 18 to less than 1% (atomic abundance). To reach this value, however, the carbon concentration follows a sinusoidal depth profile with alternated maxima and minima of carbon concentration below 5% of atomic abundance, which suggests the presence of a multilayered sample where hydrocarbon contamination piles up at the interface between two succeeding layers. These maxima are separated from 20 min of sputtering, and hence 20 nm in depth. 3.2.2. Samples surfaces In Table 1 the average values of surface composition of Iulia Felix ship samples are reported. Some elements like boron, iron and potassium, whose concentrations are close to the instrumental detection limit (0.1–0.2%), are not reported. The surface of the easy peeling layers was analyzed before and after the removal of the outer layer. For two samples, IF2 and IF3, some little unpeeling glass fragments (Table 1), which belong to the
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
139
Table 1 Average composition data obtained by XPS analyses of the sample surfaces Sample
%C
%O
% Si
% Na
% Ca
% Al
%N
% Mg
IF2
Sample surface without removal Peeling layer after removal Unpeeling sample surface
19.0 16.8 20.0
55.6 57.8 52.4
17.6 20.2 21.0
2.2 2.2 2.5
0.8 0.4 0.8
2.1 1.9 2.6
0.8 0.6 0.0
1.9 0.1 0.7
IF3
Sample surface without removal Peeling layer after removal Unpeeling sample surface
16.3 15.1 14.9
59.7 58.4 56.7
20.1 20.0 22.0
2.2 2.3 2.4
0.2 0.0 0.0
0.2 1.7 3.0
1.1 2.3 1.0
0.2 0.2 0.0
IF4
Sample surface without removal Peeling layer after removal
29.5 15.8
44.6 59.8
17.9 20.1
2.6 1.9
0.6 0.2
3.9 1.9
0.0 0.1
0.9 0.2
IF8
Sample surface without removal Peeling layer after removal
18.7 14.1
57.7 59.2
18.5 20.3
1.1 2.1
1.8 1.2
1.1 2.1
0.5 0.9
0.6 0.1
All the reported data are expressed in chemical element atomic percentage. The error in all the concentration values is ±0.1%.
Table 2 Oa /Si ratio in Iulia Felix samples before and after sputtering Sample Oa /Si
Before sputtering After sputtering
IF2
IF2b
IF3
IF3b
IF4
IF8
2.8 2.3
2.1 1.9
2.4 1.7
2.3 1.9
2.0 2.2
2.8 2.6
Oa Indicates the amount of oxygen subtracting the oxygen contained in sodium, calcium, magnesium and aluminium oxides. The values have been obtained by XPS analyses. The error in all the values is about ±0.1. b The values are referred to the unpeeling sample surface.
same primary sample, were observed, too. In these samples aluminium concentration increases with sputtering. This increase cannot be due to the decrease of others cations (relative increase) because even the XPS peaks area, in the spectra, increases. An interesting datum recorded by XPS analyses is the O/Si ratio. As shown in Table 2 most depth profiles indicate a decrease of the O/Si ratio toward the value of 2.0 (the stoichiometric value for pure silica) as the sputtering time increases. The trend is due to nonbridging oxygen atoms decrease inside [15]. In Table 2 the O/Si ratio is calculated by removing, from total oxygen percentage, the oxygen of all others oxides (Na2 O, CaO, MgO and Al2 O3 ).
carbon but there are zones where the C1s peak is split in two components for the presence of carbonatic carbon [16,17]. In these two samples (for IF2 only IF2-1 follows this trend), in correspondence of the carbonated area, the calcium concentration increases up to larger values than the silicon one. Even the concentration of magnesium increases in these zones. These evidences suggest the presence of calcium and magnesium carbonates in several layers of the sample. The XPS analyses of the thin sections were planned as linear surface profiles (Fig. 5) to report the various elements concentration trends. It was examined the direction along the thickness of the original sample. It was analyzed one point each 0.25 mm with a spot size of 0.8 mm. The total large size of the thin section analyzed (which corresponds to the thickness of the original sample) is 3 mm. Even if the spot size is too large to permit a good spatial resolution, however the presence of calcium and magnesium carbonates in several layers of the sample is confirmed.
3.2.3. Thin sections In the thin section of sample IF2 there were two fragments, hence the abbreviations IF2-1 and IF2-2 utilized in Table 3. In the thin sections of samples IF2 and IF3 the carbon is not only due to adventitious
Table 3 Average composition data obtained by XPS analyses at the surface of Iulia Felix thin sections Sample
%C
%O
% Si
% Zn
% Cl
%N
% Na
% Ca
% Mg
IF2-1 IF2-2 IF3
58.1 72.9 49.9
33.9 22.7 37.0
2.8 1.7 5.6
0.1 0.0 0.6
0.1 0.0 0.1
0.9 1.1 1.2
0.8 0.3 1.4
2.3 0.8 3.8
1.0 0.5 0.4
All the reported data are expressed in chemical element atomic percentage. The error in all the concentration values is ±0.1%.
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F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
Fig. 5. XPS linear profile of the sample IF3 thin section. It was examined the side to side direction along the thickness of the sample. The size of the analyzed thin section (which corresponds to the thickness of the original sample) is 3.25 mm.
4. Discussion By a comparison of the XPS and SIMS depth profiles (Figs. 3 and 4) it is easy to observe that the samples (especially the sample IF8) present a multilayered structure. The thickness of the single layer, detectable in the SIMS depth profile (Fig. 3) by measuring the periodicity lengths, is really different from the one detectable in the XPS depth profile (Fig. 4). This phenomenon takes place because the two analytical techniques detect different layers. By SIMS it is not possible to observe the first 20 nm of depth, because the signal is not completely stable. By XPS it was observed that there are thin adventitious hydrocarbon accumulating layers (about 20 nm thick) at the surface. These layers are not detectable by SIMS. They cannot be related to the roughness or to the lack of homogeneity of the sample surface, because the layer thickness is lower than the average roughness of the sample and the other elements, in the XPS depth profile (Fig. 4), present still the same atomic percentage. It was not possible to measure the porosity of the samples because they are too fragile and easily broken. Each layer detectable by SIMS depth profile (Fig. 3) is about 200 nm thick. It is easy to observe that most elements present a sinusoidal trend. The variations of carbon, calcium and magnesium concentrations are out of phase with that of sodium and aluminium. Mass interferences between Mg and C2 H molecule and between Ca and SiC may be excluded. This behavior is confirmed also by the analysis of the various isotopes. The sinusoidal element trends may be related to the presence of carbonates. The oxygen trend, in the SIMS depth profile (Fig. 3), does not show large variations because its sputtering yield may be really different from
different surrounding substance and the large amounts of oxygen in both, carbonates and silicates layers, does not permit one to enhance the variations. As found in precedent works [18] calcium, and a little also magnesium, may form thin accumulation layers. Carbon dioxide or hydrocarbonatic ions may be adsorbed by the sample. So thin layers of calcium, and in certain cases also magnesium, carbonates may appear also near the surface sample, at less than 1 lm depth. The growth of carbonates crystals inside the glass may be responsible, or may accelerate, the exfoliation of the outer glass layers. The carbonates presence is confirmed by XPS analyses of the thin sections (Fig. 5). Some samples, like IF2, present quite large amounts of aluminium (about 2% atomic), which increases after sputtering. It was suggested that aluminium is involved in a leaching process in the first atomic layers where its atomic concentration increases, but this hypothesis is in contrast with the Al3þ cation behavior, because this ion is not very mobile (it presents large charge to radius ratio). Enrichment in aluminium can be ascribed to crucible composition, which favours Al2 O3 migration during glass fusion [3,14]. About the O/Si ratio, as it can be found in Table 2, there are not many differences between the samples before and after sputtering. In most cases the differences are in the error uncertainty. In the samples IF2 and IF8 the ratio is larger than 2, even after sputtering. This may be ascribed to the presence of internal hydrated zones. There are Si–OH groups present even at the internal layers, covered by thin films of leached glass. When the sample is sputtered the Si–OH groups become exposed and a larger O/Si ratio is recorded. Sodium is present at the surface mainly as an impurity and after sputtering it decreases up to the XPS instrumental detection limit. The amount of sodium and
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
potassium found is very low because the alkaline depletion, due to leaching phenomena [1,2], is really marked.
5. Conclusions Detailed XPS and SIMS analyses on Roman glass fragments from a seawater archaeological site have shown that the evolution of the glass submerged in marine water is similar to the one found in glasses from other terrestrial archaeological sites. The alteration processes, like the ion migrations, the formation of calcium accumulation layers and carbonates near the surface, are mostly similar to the ones observed in other Roman and Early-medieval glasses. The only difference is that when the glass fragments remain for several centuries submerged in marine water, all the processes are faster and the final detectable results are more evident. So the presence of carbonates or hydrated layers inside the glass is very relevant and it produces a strongly visible exfoliation layer.
Acknowledgements The authors thank A. Cagnana of Museo Archeologico Nazionale di Cividale del Friuli for providing samples. These samples are also investigated through a mineralogical approach by A. Silvestri, G. Molin and G. Salviulo at Dipartimento di Mineralogia e Petrologia of Padua University, the authors thank the research group for helpful discussion.
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References [1] H. Scholze, Glass: Nature, Structure and Properties, Springer, USA, 1990. [2] M. Verita, in: Le Materiau Vitreux: Verre et Vitraux, Actes du Cours Intensif Europeen, Ravello, 28–30 April 1995, Edipuglia, Bari, 1998, pp. 53–73. [3] R. Newton, S. Davison, Conservation of Glass, ButterworthHeinemann, London, 1989. [4] E.W. Krumbein, A.A. Gorbushina, A.K. Palinska, K. Sterflinger, in: Le Materiau Vitreux: Verre et Vitraux, Actes du Cours Intensif Europeen, Ravello, 28–30 April 1995, Edipuglia, Bari, 1998, pp. 107–124. [5] M. Schreiner, G. Woisetschl€ager, I. Schmitz, M. Wadsak, J. Anal. Atomic Spectrom. 14 (1999) 395. [6] B.C. Bunker, J. Non-Cryst. Solids 179 (1994) 300. [7] R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. [8] S. Lorusso, C. Fiori, M. Vandini, Riv. Staz. Speriment. Vetro 6 (2000) 73. [9] M. Verita, P. Santopadre, M. Marabelli, Bollettino ICR-Nuova Serie 1 (2000) 64. [10] G. Moraitou, A. Kontogeorgakos, G. Kordas, Riv. Staz. Speriment. Vetro 6 (2000) 69. [11] F. Branda, G. Laudisio, G. Luciani, A. Costantini, C. Piccioli, Riv. Staz. Speriment. Vetro 6 (2000) 23. [12] A.A.V.V., Operazione Iulia Felix, dal Mare al Museo, Edizioni della Laguna, 1999. [13] F. Geotti Bianchini, G. Formenton, M. Placidi, Riv. Staz. Speriment. Vetro 5 (2000) 277. [14] J. Henderson, The Science and Archaeology of Materials––An Investigation of Inorganic Materials, Routledge, London, 2000. [15] V.Y.A. Davydov, A.V. Kiselev, L.T. Zhuravlev, Trans. Faraday Soc. 60 (1964) 2254. [16] J. Moulder, W. Stickle, P. Sobol, K. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer, USA, 1992. [17] D. Briggs, M.P. Seah, Practical Surface Analysis, vol. 1: Auger and X-Ray Photoelectron Spectroscopy, 2nd Ed., Wiley, Chichester, 1986. [18] R. Bertoncello, L. Milanese, U. Russo, P. Guerriero, S. Barison, J. Non-Cryst. Solids 306/3 (2002) 249.
Alteration and corrosion phenomena in Roman submerged glass fragments Fabio Barbana b
a,b
, Renzo Bertoncello
a,b,*
, Laura Milanese a, Cinzia Sada
c
a Dipartimento di Scienze Chimiche, University of Padova, Via Loredan n. 4, 35131 Padova, Italy Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Via Varchi n. 59, 50132 Florence, Italy c Dipartimento di Fisica ‘Galileo Galilei’, University of Padova, Via Marzolo n. 8, 35131 Padova, Italy
Received 31 July 2003; received in revised form 17 March 2004 Available online 14 May 2004
Abstract In the present paper the surface analytical techniques of XPS (X-ray photoelectron spectroscopy) and SIMS (secondary ions mass spectrometry) have allowed an understanding of the alteration and corrosion phenomena occurring in Roman glass fragments. The samples come from the seawater archaeological site of the Roman ship Iulia Felix, which sank near Grado (Gorizia, Italy) in the second century A.D. Through XPS the surface concentration of the main chemical elements and their depth profiles have been obtained. Attention has been focused on the oxidation states of the elements, oxygen/silicon ratio, peculiar presence of carbonate salts and concentration trends of some elements in the outer atomic layers. Depth trends of aluminium, calcium, magnesium and carbon were studied also through SIMS depth profiles. The whole set of XPS and SIMS data allows the segregation of particular chemical species and the evolution of the original glass to a multilayer system. 2004 Elsevier B.V. All rights reserved. PACS: 79.60; 68.49.Sf; 82.80.Ms; 61.43.Fs; 68.35.Dv
1. Introduction The knowledge of surface degradation of historical glasses, exposed for centuries to weathering, is very important. It is really interesting to understand the type and the features of chemical–physical phenomena which occur on these materials, like change in surface composition, chemical stability, different ways of leaching and alteration products. In this paper the alteration processes that occur on glasses immersed in seawater are investigated. Seawater glass alteration and/or corrosion involve many different processes like surface adsorption of water, leaching reactions [1,2], localized increasing in pH [2], dissolution reactions, crystallization of insoluble carbonates and sulphates [3], biological deterioration [4]. Hence there
* Corresponding author. Address: Dipartimento di Scienze Chimiche, University of Padova, Via Loredan n. 4, 35131 Padova, Italy. Tel.: +39-049 827 5204; fax: +39-049 827 5161. E-mail address: [email protected] (R. Bertoncello).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.118
are a number of physical and chemical processes that occur in the interface region ranging from the effect of contacting atmosphere up to the aqueous layer coating the glass fragment itself. The theme is tackled by a chemical approach, turning our attention to the investigation of glass surfaces because the main alteration phenomena occur at the interface between the glass fragments and the external environment [5]. The most important reactions that can take place when silicate glasses are in contact with an aqueous environment at pH < 9 are ionic exchange reactions between alkali ions of glass and protons of the solution [6], while aqueous solutions at pH > 9 induce the destruction of the bridging oxygen bond of the silica network [7]. The interaction of gaseous CO2 and SO2 species with the vitreous surface can involve the formation and co-precipitation of many alteration products like calcium carbonates and calcium sulphates. These phenomena favour the decrease of alkaline-earth ions concentrations in the glass [3,8,9]. The substitution of alkaline or alkaline-earth ions with protons also causes a volume contraction of the vitreous matrix that induces fracture,
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
collapse of the structure and pit formation. In pits crystallization causes a break-up action similar to the one of ice inside rock fractures [10,11].
2. Experimental 2.1. The samples The examined glass fragments come from the Roman ship Iulia Felix, which sank near Grado in the second century A.D. [12]. The glass composition is typical of the Roman era, with a large use of ‘natron’ (‘natron’ is a melting agent, Na2 CO3 Æ NaHCO3 Æ 2H2 O) and with small quantities of Mediterranean and Continental plant ashes [12–14]. The glass fragments are identified with the following abbreviations: IF2, IF3, IF4, IF8. In Fig. 1, a picture of the fragment IF2 is represented. All the analyzed samples present visible thin exfoliation layers. They are very fragile, easily broken and removable. In some samples, like IF2, IF3 and IF8, the thin layers are iridescent while in others, like IF4, they present a white coloration. 2.1.1. Thin sections preparation The thin sections were obtained by cutting the glass fragments at Dept. Mineralogia and Petrologia of Padua University with a water-cooled rotating saw. The glass fragments were pasted on a slide using an epoxy resin and the slides were cut to obtain a section with thickness no larger than 350 lm. The sections were then ground by SiC powder and then polished with diamond paste (1 lm).
137
2.2. Sample characterization 2.2.1. Optical microscopy The samples were preliminarily examined with optical microscopy to obtain morphological information, especially for the sample IF8. We used an optical reflection microscope equipped by a digital camera to observe the surface structure of all the samples. 2.2.2. XPS measures XPS spectrometer was used and it was equipped with Al/Mg anodes working with AlKa (1486.6 eV) and MgKa (1253.6 eV) sources at 20 mA and 14 kV. Also an Al monochromatic source (1486.6 eV) with the same parameters, and a charge neutralizer were used. The working pressure was in the range of 107 Pa. Survey scans were obtained in the 0–1350 eV range (using AlKa source) or 0–1150 eV (using MgKa source) by steps of 0.8 eV. The pass energy was 187.86 eV. The depth profiles were obtained by sputtering with Arþ ions, operating at 3 keV, at a pressure of 5 · 106 Pa. A raster size of 2 · 2 mm was used. The analyzed areas were circles 0.8 mm in diameter. Operating at the same conditions, by the same instrumental apparatus, in flat glass samples of known thickness, the sputtering rate was about 1 nm/min. A direct measure of the sputtering rate and its extensive error analysis was not possible. 2.2.3. SIMS measures The instrument used for SIMS analyses is an ionic microscope Cameca ims 4f. The compositional analysis was recorded with several primary beam current values (10, 20 and 40 nA respectively) to optimize the measurement conditions and charge compensation. In the present paper the samples were bombarded with primary ions Csþ at 1–20 keV of power, an optimal current of 10 nA and the secondary ions emitted were analyzed by a mass analyzer made of magnetic and electrostatic sectors. A variable raster size of 125 · 125 lm was used. The working pressure was 107 Pa. A flood gun of electrons at 500 eV was used because the samples were insulators (better results was obtained by a 10 nA beam secondary current). The sputtering rate, estimated for flat glass samples, was about 60 nm/min. A direct measure of the sputtering rate was not possible, because the roughness of the samples was too large. So it is not possible to estimate the errors.
3. Results
Fig. 1. Iulia Felix, fragment IF2.
All the samples (sample surfaces and thin sections) described were analyzed by optical microscopy and XPS. Samples were analyzed also through SIMS and XPS depth profiles to point out the element concentration trends in depth.
138
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
Sample IF8: SIMS depth profile C O Na Mg Al Si Ca
Yield (Counts/s)
100000 O Si C
10000
Al Ca
Mg 1000
Na
200 nm
100 0
200
400
600
800
1000
1200
Sputtering time (s)
Fig. 2. Multilayer system in sample IF8.
Fig. 3. SIMS depth profile of the fragment IF8. The sputtering rate was not measured directly, because the samples were not homogeneous and too much rough. However in the same experimental conditions, with other glass samples, the sputtering rate was about 60 nm/min.
The image obtained by optical reflection microscopy on IF8 sample (Fig. 2) shows a layered structure and the areas not focused indicate layers at a different depth. 3.1. SIMS results Through secondary ion mass spectrometry three Iulia Felix samples were analyzed: IF2, IF3, IF8. The roughness of the samples does not allow quantitative analyses. The boron signal is close to the detection limit and it was omitted from the graph (Fig. 3). To evaluate its dynamics one should use Oþ for the primary beam and detect Bþ , but this was not possible because the electrical insulating characteristics of the samples require use of a flood gun for charge compensation and hence allows the detection of only secondary negative ions. As can be seen in the SIMS depth profile of sample IF8 (Fig. 3) the sodium and aluminium signals change in yield in a synchronous mode and the trend is not attributable to mass interferences; even carbon, magnesium and calcium yields show a sinusoidal synchronous profile. The SIMS analysis suggests the presence, in this sample, of an outer portion characterized by a multilayered structure, with maxima and minima concentration sequence (Fig. 3). The distance, which separates two adjacent maxima corresponds to about 200 nm in depth. It is also easy to observe that the trend of sodium and aluminium signals is just out of phase with the carbon, calcium and magnesium one. These phenomena confirm a multilayered structure with alternating shells. 3.2. XPS results 3.2.1. IF8 depth profile A very interesting trend is present in the IF8 sample depth profile (Fig. 4). After 90 min of sputtering the carbon amount (exclusively hydrocarbon contamina-
Fig. 4. XPS depth profile of the fragment IF8. The sputtering rate was not measured directly. However in the same experimental conditions, with other glass samples, the sputtering rate was about 1 nm/min.
tion) shrinks from 18 to less than 1% (atomic abundance). To reach this value, however, the carbon concentration follows a sinusoidal depth profile with alternated maxima and minima of carbon concentration below 5% of atomic abundance, which suggests the presence of a multilayered sample where hydrocarbon contamination piles up at the interface between two succeeding layers. These maxima are separated from 20 min of sputtering, and hence 20 nm in depth. 3.2.2. Samples surfaces In Table 1 the average values of surface composition of Iulia Felix ship samples are reported. Some elements like boron, iron and potassium, whose concentrations are close to the instrumental detection limit (0.1–0.2%), are not reported. The surface of the easy peeling layers was analyzed before and after the removal of the outer layer. For two samples, IF2 and IF3, some little unpeeling glass fragments (Table 1), which belong to the
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
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Table 1 Average composition data obtained by XPS analyses of the sample surfaces Sample
%C
%O
% Si
% Na
% Ca
% Al
%N
% Mg
IF2
Sample surface without removal Peeling layer after removal Unpeeling sample surface
19.0 16.8 20.0
55.6 57.8 52.4
17.6 20.2 21.0
2.2 2.2 2.5
0.8 0.4 0.8
2.1 1.9 2.6
0.8 0.6 0.0
1.9 0.1 0.7
IF3
Sample surface without removal Peeling layer after removal Unpeeling sample surface
16.3 15.1 14.9
59.7 58.4 56.7
20.1 20.0 22.0
2.2 2.3 2.4
0.2 0.0 0.0
0.2 1.7 3.0
1.1 2.3 1.0
0.2 0.2 0.0
IF4
Sample surface without removal Peeling layer after removal
29.5 15.8
44.6 59.8
17.9 20.1
2.6 1.9
0.6 0.2
3.9 1.9
0.0 0.1
0.9 0.2
IF8
Sample surface without removal Peeling layer after removal
18.7 14.1
57.7 59.2
18.5 20.3
1.1 2.1
1.8 1.2
1.1 2.1
0.5 0.9
0.6 0.1
All the reported data are expressed in chemical element atomic percentage. The error in all the concentration values is ±0.1%.
Table 2 Oa /Si ratio in Iulia Felix samples before and after sputtering Sample Oa /Si
Before sputtering After sputtering
IF2
IF2b
IF3
IF3b
IF4
IF8
2.8 2.3
2.1 1.9
2.4 1.7
2.3 1.9
2.0 2.2
2.8 2.6
Oa Indicates the amount of oxygen subtracting the oxygen contained in sodium, calcium, magnesium and aluminium oxides. The values have been obtained by XPS analyses. The error in all the values is about ±0.1. b The values are referred to the unpeeling sample surface.
same primary sample, were observed, too. In these samples aluminium concentration increases with sputtering. This increase cannot be due to the decrease of others cations (relative increase) because even the XPS peaks area, in the spectra, increases. An interesting datum recorded by XPS analyses is the O/Si ratio. As shown in Table 2 most depth profiles indicate a decrease of the O/Si ratio toward the value of 2.0 (the stoichiometric value for pure silica) as the sputtering time increases. The trend is due to nonbridging oxygen atoms decrease inside [15]. In Table 2 the O/Si ratio is calculated by removing, from total oxygen percentage, the oxygen of all others oxides (Na2 O, CaO, MgO and Al2 O3 ).
carbon but there are zones where the C1s peak is split in two components for the presence of carbonatic carbon [16,17]. In these two samples (for IF2 only IF2-1 follows this trend), in correspondence of the carbonated area, the calcium concentration increases up to larger values than the silicon one. Even the concentration of magnesium increases in these zones. These evidences suggest the presence of calcium and magnesium carbonates in several layers of the sample. The XPS analyses of the thin sections were planned as linear surface profiles (Fig. 5) to report the various elements concentration trends. It was examined the direction along the thickness of the original sample. It was analyzed one point each 0.25 mm with a spot size of 0.8 mm. The total large size of the thin section analyzed (which corresponds to the thickness of the original sample) is 3 mm. Even if the spot size is too large to permit a good spatial resolution, however the presence of calcium and magnesium carbonates in several layers of the sample is confirmed.
3.2.3. Thin sections In the thin section of sample IF2 there were two fragments, hence the abbreviations IF2-1 and IF2-2 utilized in Table 3. In the thin sections of samples IF2 and IF3 the carbon is not only due to adventitious
Table 3 Average composition data obtained by XPS analyses at the surface of Iulia Felix thin sections Sample
%C
%O
% Si
% Zn
% Cl
%N
% Na
% Ca
% Mg
IF2-1 IF2-2 IF3
58.1 72.9 49.9
33.9 22.7 37.0
2.8 1.7 5.6
0.1 0.0 0.6
0.1 0.0 0.1
0.9 1.1 1.2
0.8 0.3 1.4
2.3 0.8 3.8
1.0 0.5 0.4
All the reported data are expressed in chemical element atomic percentage. The error in all the concentration values is ±0.1%.
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F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
Fig. 5. XPS linear profile of the sample IF3 thin section. It was examined the side to side direction along the thickness of the sample. The size of the analyzed thin section (which corresponds to the thickness of the original sample) is 3.25 mm.
4. Discussion By a comparison of the XPS and SIMS depth profiles (Figs. 3 and 4) it is easy to observe that the samples (especially the sample IF8) present a multilayered structure. The thickness of the single layer, detectable in the SIMS depth profile (Fig. 3) by measuring the periodicity lengths, is really different from the one detectable in the XPS depth profile (Fig. 4). This phenomenon takes place because the two analytical techniques detect different layers. By SIMS it is not possible to observe the first 20 nm of depth, because the signal is not completely stable. By XPS it was observed that there are thin adventitious hydrocarbon accumulating layers (about 20 nm thick) at the surface. These layers are not detectable by SIMS. They cannot be related to the roughness or to the lack of homogeneity of the sample surface, because the layer thickness is lower than the average roughness of the sample and the other elements, in the XPS depth profile (Fig. 4), present still the same atomic percentage. It was not possible to measure the porosity of the samples because they are too fragile and easily broken. Each layer detectable by SIMS depth profile (Fig. 3) is about 200 nm thick. It is easy to observe that most elements present a sinusoidal trend. The variations of carbon, calcium and magnesium concentrations are out of phase with that of sodium and aluminium. Mass interferences between Mg and C2 H molecule and between Ca and SiC may be excluded. This behavior is confirmed also by the analysis of the various isotopes. The sinusoidal element trends may be related to the presence of carbonates. The oxygen trend, in the SIMS depth profile (Fig. 3), does not show large variations because its sputtering yield may be really different from
different surrounding substance and the large amounts of oxygen in both, carbonates and silicates layers, does not permit one to enhance the variations. As found in precedent works [18] calcium, and a little also magnesium, may form thin accumulation layers. Carbon dioxide or hydrocarbonatic ions may be adsorbed by the sample. So thin layers of calcium, and in certain cases also magnesium, carbonates may appear also near the surface sample, at less than 1 lm depth. The growth of carbonates crystals inside the glass may be responsible, or may accelerate, the exfoliation of the outer glass layers. The carbonates presence is confirmed by XPS analyses of the thin sections (Fig. 5). Some samples, like IF2, present quite large amounts of aluminium (about 2% atomic), which increases after sputtering. It was suggested that aluminium is involved in a leaching process in the first atomic layers where its atomic concentration increases, but this hypothesis is in contrast with the Al3þ cation behavior, because this ion is not very mobile (it presents large charge to radius ratio). Enrichment in aluminium can be ascribed to crucible composition, which favours Al2 O3 migration during glass fusion [3,14]. About the O/Si ratio, as it can be found in Table 2, there are not many differences between the samples before and after sputtering. In most cases the differences are in the error uncertainty. In the samples IF2 and IF8 the ratio is larger than 2, even after sputtering. This may be ascribed to the presence of internal hydrated zones. There are Si–OH groups present even at the internal layers, covered by thin films of leached glass. When the sample is sputtered the Si–OH groups become exposed and a larger O/Si ratio is recorded. Sodium is present at the surface mainly as an impurity and after sputtering it decreases up to the XPS instrumental detection limit. The amount of sodium and
F. Barbana et al. / Journal of Non-Crystalline Solids 337 (2004) 136–141
potassium found is very low because the alkaline depletion, due to leaching phenomena [1,2], is really marked.
5. Conclusions Detailed XPS and SIMS analyses on Roman glass fragments from a seawater archaeological site have shown that the evolution of the glass submerged in marine water is similar to the one found in glasses from other terrestrial archaeological sites. The alteration processes, like the ion migrations, the formation of calcium accumulation layers and carbonates near the surface, are mostly similar to the ones observed in other Roman and Early-medieval glasses. The only difference is that when the glass fragments remain for several centuries submerged in marine water, all the processes are faster and the final detectable results are more evident. So the presence of carbonates or hydrated layers inside the glass is very relevant and it produces a strongly visible exfoliation layer.
Acknowledgements The authors thank A. Cagnana of Museo Archeologico Nazionale di Cividale del Friuli for providing samples. These samples are also investigated through a mineralogical approach by A. Silvestri, G. Molin and G. Salviulo at Dipartimento di Mineralogia e Petrologia of Padua University, the authors thank the research group for helpful discussion.
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