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Evaluation procedure for leaching studies on naturally weathered potash-lime-silica glasses with medieval composition by scanning electron microscopy
Journal of Non-Crystalline Solids 351 (2005) 1210–1225 www.elsevier.com/locate/jnoncrysol
Evaluation procedure for leaching studies on naturally weathered potash-lime-silica glasses with medieval composition by scanning electron microscopy M. Melcher a
a,b,*
, M. Schreiner
a,b
Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria b Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, 1060 Vienna, Austria Received 26 November 2004; received in revised form 2 February 2005 Available online 14 April 2005
Abstract In order to study the weathering mechanism and quantify the influence of the various air pollutants on the degradation process, potash-lime-silica (PLS) glasses with a chemical composition similar to medieval stained glass were exposed at more than 20 test sites in Europe and North America to the local environmental conditions within the International Co-operative Programme on Effects on Materials Including Historic and Cultural Monuments of the United Nations Economic Commission for Europe (ICPMaterials, UN/ECE). After exposure periods of up to 6 years the specimens were investigated in the scanning electron microscope in combination with energy dispersive X-ray microanalysis (SEM/EDX) in order to characterize the surface layer (leached layer) of the weathered samples. Cross-sections of the samples were prepared for depth profile analyses using linescan measurements in order to calculate the leaching depths of the network modifier ions. The typical diffusion profiles expected for these elements were observed. The evaluation is based on a numerical integration of the background corrected linescan data. This method proved to be suitable for the high amount of samples (more than 130) and yields acceptable errors of less than 10 rel.%. The first results show leaching depths for K+ and Ca2+ between 0.42 and 1.87 lm (average 1.21 lm for K and 1.11 lm for Ca) for exposure times of up to six years. So both network modifier ions present in the glass were leached out, with K+ showing slightly, but significantly higher leaching depths than the bivalent cation Ca2+. As no continuous increase of the leaching depths with time could be observed, network dissolution must have also occurred during the weathering process. Comparing these results to former leaching studies, the rate of leaching on glasses of this composition seems to decrease significantly after about three or four years of exposure. 2005 Elsevier B.V. All rights reserved. PACS: 81.65.K; 68.35.B
1. Introduction Compared to modern or ancient Roman glasses, which can be classified as soda-lime-silica (SLS) glasses [1], medieval stained glass objects produced as window panes for Romanesque and Gothic cathedrals or
*
Corresponding author. Tel.: +43 58816 206; fax: +43 58816 121. E-mail address: [email protected] (M. Melcher).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.02.020
churches often exhibit two characteristic differences: (a) the preferred use of potassium instead of sodium containing compounds for the glass production and (b) a high content of network modifier ions such as calcium, potassium and magnesium and therefore a low silica content [2]. Bezborodov [3] reports the results of more than 700 analyses of ancient and medieval glasses (Table 1). He clearly emphasizes that potash-lime-silica glasses (with more or less high contents of MgO and Al2O3) were probably not produced before the Middle
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Table 1 Average concentrations of the main glass components SiO2, K2O, Na2O, CaO, MgO, Al2O3, Fe2O3 in ancient and medieval potash containing glasses, calculated from data in Ref. [3] Type of Glass
Dating
Number of observations
Potash-lime Soda-potash-lime Soda-potash-lime
Medieval Medieval Ancient
n = 67 n = 138 n = 20
Average concentrations (mean ± standard deviation, in wt%) SiO2
K2O
Na2O
CaO
MgO
Al2O3
Fe2O3
53.6 ± 6.6 61,9 ± 5.7 64.5 ± 5.9
16.8 ± 5.8 5.4 ± 2.9 6.4 ± 4.2
1.3 ± 0.9 13.0 ± 3.9 12.2 ± 4.9
18.6 ± 4.7 8.9 ± 4.1 7.1 ± 2.7
3.4 ± 1.5 4.3 ± 1.7 2.5 ± 1.4
2.7 ± 1.8 3.6 ± 2.0 2.9 ± 1.5
1.0 ± 1.1 1.0 ± 0.6 1.3 ± 1.2
Ages and soda-potash-lime-silica glasses were mainly produced in medieval times. The reason for this change of the raw materials might be a growing demand for window glass for churches and cathedrals and a resulting shortage in marine plants, which have been the source of alkali by then. Therefore, the ash of beech wood or farn, both rich in potassium, was presumably used as flux [4]. The question of the chemical stability of glasses [5] came up as early as the first useful models of the structure of glass were set up [6]. Up to the 1930s it was still believed that the PLS glasses mentioned above are supe-
CaO–SiO2 glasses towards acidic solutions was studied. The results showed a preferential leaching of sodium for low lime glasses (<10 mol% CaO), but an approximately uniform dissolution of Na and Ca for glasses with higher CaO contents. However, several studies [10,11] showed that the glass corrosion process is a two-stage reaction. At first, leaching of the mono- or bivalent network modifier ions (M+ or M2+ in the Eqs. (1a) and (1b), respectively) by neutral or acidic solutions takes place, following the pffi t-law, as it is characteristic for diffusion controlled processes [12].
ð1aÞ
ð1bÞ
rior to SLS glasses regarding their stability [7], but today it is well known that a high content of network modifiers (especially K) is a great disadvantage for the stability and durability of glasses. El-Shamy [8] studied the chemical resistance of seven glasses in the system K2O–CaO– MgO–SiO2 towards water and acidic solutions and found a considerable dependence on the ratio of K2O to other constituents. Furthermore, a clear tendency of the glass samples to form a crusted layer on the surface could be determined on samples containing less than 66 mol% SiO2. In another work [9] the resistance of Na2O–
The true chemical nature of the hydrogen bearing species (simply H+ or H+ accompanied by one or more H2O-molecules) and the rate determining step in alkali leaching seem not to be clarified so far [13]. Perhaps a general answer to this question is not possible. In contact with alkaline solutions, another reaction mechanism is observed (Eq. (2)). Hydroxyl ions break up the Si–O bonds forming silanol-groups (Si–OH). H2O-molecules may form through a condensation reaction of the latter (Eq. (3)).
ð2Þ
ð3Þ
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In any case, an alkali- and earth alkali-depleted and hydrogen-enriched layer is formed in the near surface region of the glass. Typically the thickness of this surface layer is in the sub-lm range to some microns depending on the nature of the leaching solution, the leaching time and the type of glass used. While a huge amount of data on the chemical stability of glasses towards liquid media such as water or acids and bases of all pH-values is available in the literature [14–16], the natural weathering (i.e. the attack of various air pollutants including climatic factors such as the relative humidity) of glass and its resulting deterioration has not been studied so extensively [17–19]. Quantitative investigations on naturally or artificially weathered glass specimens are very rare [20–24]. The reason for this might be the long weathering times needed to achieve observable results and the high complexity of the system environment/glass due to the high number of factors involved (e.g. the formation of a water film on the sample surface, the absorption of acidifying gases of the ambient atmosphere in this water film and the resulting change of its pH). In the International Co-operative Programme on Effects on Materials including Historic and Cultural Monuments founded by the United Nations Economic Commission for Europe (ICP-Material, UN/ECE [25]) various materials, among them steel, bronze, copper, zinc, some sorts of stone and modern as well as historic glass are exposed since 1987 at about 25 test sites in Europe and North America to the local environmental conditions. The objectives of this large-scaled field exposure programme are (a) qualitative and quantitative estimations of the damage towards objects of our cultural heritage through many kinds of air pollutants such as NOx, SO2, O3 and (b) the development of dose-response functions describing the long-term costs of this destructive impact. These functions are also used for assessing acceptable corrosion rates and pollution levels and for mapping areas with elevated risk of corrosion damage. Since the early 1990s the Institute of Science and Technology in Art of the Academy of Fine Arts has been exposing glasses with compositions similar to that of medieval stained glass in the framework of the ICP-Materials project. In order to characterize the weathering process, scanning electron microscopy in combination with energy dispersive X-ray microanalysis has been used. By that combination the morphology as well as the chemical (elemental) composition of the weathering products formed on the glass surfaces could be obtained. If the exposure is carried out under sheltered conditions (i.e. the samples are protected from rain and sun radiation), the amount of products formed could be used for describing the kinetics of the weathering process [22,24,26]. This work aims to enable the study of glass specimens weathered under unsheltered conditions by measuring
the thickness of the leached layer formed also during the weathering process. The determination of the surface layer depleted in K and Ca and its correlation with time should give a hint to the kinetics involved. Cross-sections of the exposed glass samples with subsequent linescan measurements in the SEM were performed leading to depth profiles of the mono- and bivalent network modifier ions of the glass. Specimens from the UN/ECE-exposure within two periods (1993–95 and 1997–2001) could be used therefore. For reasons of comparison, former results on depth profiles using nuclear reaction analysis (NRA) obtained by Woisetschla¨ger and Schreiner [20,21] are also considered.
2. Experimental 2.1. Glass samples Within the UN/ECE project glass samples of the following composition (similar to medieval stained glass) were exposed (wt%): 60 SiO2, 25 CaO, 15 K2O. The glasses were prepared at the Fraunhofer-Institut fuer Silicatforschung in Wuerzburg/Germany by mixing oxides and carbonates of specific elements in certain stoichiometric quantities and melting in a Pt crucible at approximately 1720 K. After refining the glass at a temperature of 900 K for 5 h and cooling, the frits were ground and melted a second time in order to achieve homogeneity. Small plates of approximately 10 · 10 · 2 mm3 were cut from these glass ingots with a low-speed diamond saw (Buehler Isomet II-1180) and embedded in epoxy resin for easier handling in the subsequent polishing process using SiC paper of 500–4000 mesh. In the last polishing step isopropanol instead of water was used as lubricant in order to prevent leaching of glass components already during the polishing process. The polished glass samples were mounted on glass fiber reinforced plastic plates (sized 15 cm · 10 cm · 3 mm, Fig. 1) with approximately 10 cm pieces of shielded wire, covered with silicon caps and adhesive tapes, packed in polystyrene foam filled paper boxes and sent to the exposure sites. After the fixed exposure time the glass samples were sent back from the sites, again covered with silicon caps and packed in paper boxes to prevent any damage to the samples during shipping. The samples were stored in plastic boxes together with drying mats in order to guarantee a low relative humidity in the box and to avoid further glass corrosion prior to the investigations by SEM. The SEM measurements yielded the corrosion (weathering) products formed, such as gypsum (CaSO4 Æ 2H2O) and syngenite (K2SO4 Æ CaSO4 Æ H2O). Afterwards, the samples were covered with a thin layer (3 mm) of epoxy resin and cut with a low-speed
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environmental and climatic data such as the concentration of the acidifying gases SO2, NO2 and O3, the temperature T and the relative humidity RH. For a statistical evaluation a wide spread of these data is necessary in order to achieve reliable results. The test sites can be grouped in three categories: (a) low polluted sites: c(SO2) and c(NO2) < 15 lg/m3; (b) moderately polluted sites: c(SO2) or c(NO2) > 15 lg/m3; (c) heavily polluted sites: c(SO2) > 15 lg/m3 and c(NO2) > 15 lg/m3.
Fig. 1. Four glass specimens (visible glass surface 1 cm2 per sample) mounted on a glass-fiber reinforced plastic plate (sized 15 cm · 10 cm · 3 mm). The labels refer to the number of the exposure site (Kopisty, Czech Republic) and the end of the exposure time (September 2001).
diamond saw in order to produce cross-sections of the specimens (Fig. 2(a) and (b)), which were also investigated in the SEM using the linescan mode. 2.2. Exposure conditions The glass samples were exposed at up to 24 test sites. The positions of the sites were chosen according to their
Table 2 presents a short description of these exposure sites including their classification (r = rural, u = urban and i = industrial) and environmental parameters for the three year exposure. It must be annotated that these conditions only refer to the vicinity of the test site and not to the whole city or region. The environmental data are collected and reported by the Norwegian Institute for Air Research (NILU) [27,28]. Table 3 presents an overview of the exposure times and the number of samples exposed. In the case of the 3- and 4-year exposure, always two samples were exposed under the same conditions for the same time. It seems important to note that the 5- and 6-year exposures were performed with a two or three year intermission, where the samples were stored at low humidity (10%) in a conditioned box. The exposure could be carried out in two ways: sheltered (i.e. the samples were exposed vertically in a box in order to avoid a direct contact with rain or sun radiation) or unsheltered (on top of a wooden rack with an inclination of 45). The results of the sheltered exposure are described in [22,24].
Fig. 2. (a) and (b) Original glass sample in an epoxy resin matrix with a schematic drawing of the cutting plane (left) and the cut specimen with an additional layer of resin (right).
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Table 2 List of the exposure sites including their classification in rural (r), urban (u) and industrial (i) sites Site
Name (Country)
T [C]
RH [%]
c(SO2) [lg/m3]
c(NO2) [lg/m3]
c(O3) [lg/m3]
mm [mm]
pH
1 (u) 3 (i) 5 (r) 7 (r) 9 (r) 10 (i) 15 (u) 21 (u) 23 (r) 24 (u) 26 (r) 27 (u) 31 (u) 33 (r) 34 (u) 35 (r) 36 (u) 40 (u) 41 (u) 43 (u) 44 (r) 45 (r) 46 (u) 49 (u)
Prague-Letnany (CZ) Kopisty (CZ) ¨ htari (FIN) A Waldhof–Langenbru¨gge (GER) Langenfeld–Reusrath (GER) Bottrop (GER) Milan (ITA) Oslo (NOR) Birkenes (NOR) Stockholm South (SWE) Aspvreten (SWE) Lincoln Cathedral (GB) Madrid (SPA) Toledo (SPA) Moscow (RUS) Lahemaa (EST) Lisbon-Jeronimos Monastery (POR) Paris (FRA) Berlin (GER) Tel Aviv (ISR) Svanvik (NOR) Chaumont (CH) London (GB) Antwerp (BEL)
9.8 9.7 3.0 9.5 11.1 11.5 14.3 6.7 6.1 7.3 6.0 10.0 14.3 14.0 5.2 5.7 17.4 13.3 10.5 24.2 0.8 6.5 12.3 11.7
74.6 76.4 78.9 79.9 79.6 81.1 69.7 76.3 81.0 76.3 84.3 80.5 57.5 58.0 71.3 80.2 65.9 68.5 75.8 80.5 78.7 78.7 70.6 75.7
13.9 17.4 0.7 1.5 7.6 21.3 15.0 4.2 0.2 2.7 0.6 8.9 7.4 1.9 25.4 1.3 19.0 12.2 9.6 35.2 7.1 1.2 6.8 18.7
23.2 30.5 2.9 8.6 32.4 36.0 79.1 30.8 1.4 19.6 2.6 21.1 21.5 10.5 24.4 1.5 36.0 57.9 41.1 34.3 1.4 7.8 47.6 50.0
48.9 55.0 61.5 52.4 33.6 31.2 38.1 38.1 56.6 47.5 57.7 50.4 54.3 84.9 42.5 59.8 13.4 32.0 24.6 38.4 56.9 84.4 35.4 29.1
1407 1323 1998 1854 2635 2834 2923 2229 5032 1346 1264 2149 1443 1563 2136 2140 618 1827 1407 1254 1257 3242 1938 2841
4.8 4.5 4.7 5.1 4.9 4.9 – 4.7 4.5 4.7 4.6 – 6.2 5.9 6.6 4.9 6.1 5.7 – – 4.8 5.0 – 5.0
The environmental data for the temperature (T, [C]), the relative humidity (RH, [%]), the concentration of SO2, NO2 and O3 (c(SO2), c(NO2) and c(O3), [lg/m3]) and the amount (mm, [mm]) and pH of the precipitation are mean values and refer to the exposure period from October 1997 to September 2000. – can either mean that this parameter is not measured at this site or that not enough monthly mean values were available to calculate a reliable average for the whole period.
Table 3 Time of exposure and number of glass samples exposed in sheltered and unsheltered modes within the UN/ECE project Sheltered 1 year (10/93–09/94) 2 years (10/93–09/95) 3 years (10/97–09/00) 4 years (10/97–09/01) 5 years (10/93–09/94 and 10/97–09/01) 6 years (10/93–09/95 and 10/97–09/01)
69 69 48 44 10 13
samples samples samples samples samples samples
Unsheltered 23 23 24 24 10 13
sites sites sites sites sites sites
69 69 48 44 16 18
samples samples samples samples samples samples
23 23 24 22 16 18
sites sites sites sites sites sites
It is important to note that the exposure was not carried out continuously in case of the 5- and 6-year samples.
2.3. SEM parameters and method developed for the evaluation of the linescans The surfaces and cross-sections of all glass specimens were investigated by SEM/EDX (JEOL 6400 in combination with EDAX Phoenix). The measurements were performed with an acceleration voltage of 15 kV and a beam current of approximately 1 nA. For the linescan measurements the following parameters were chosen: • • • •
length of the line between 3 and 10 lm; number of measurement points: 300–500; dwell time: between 300 and 500 ms; total counts per second: 6000;
• dead time: between 25% and 40% (adjusted with amplification time); • linescan evaluation method: net-intensities (including a peak deconvolution and a background correction of the raw data). Prior to the analyses of the surface and cross-sections in the SEM the samples were coated with a thin layer of carbon in order to avoid charging during the electron bombardment. Long sputtering times (up to 20 s) were selected as smaller areas (10 · 10 lm2) were investigated. Stable conditions are absolutely necessary if analyses are carried out in these microdomains. As a first result of the investigations in all cases the leached
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layers, which are depleted in modifier cations, could be found (Fig. 3) according to Eq. (1). The boundary between the bulk glass and the leached layer is not sharp. Fig. 4 shows a scheme of the distance/intensity-linescan curves usually obtained with the intensities of Si as well as K and Ca. The upper image depicts the cross-section of a glass specimen with the bulk glass, the leached layer, the resin and the position of the linescan. The circles represent the two-dimensional projection of the excitation volume caused by the electron beam in the specimen. 8 significant points (I–VIII) are marked on that line, which are described with respect to the distribution of the intensities of the elements Si, K and Ca (middle and lower image). • Point I (bulk glass): represents the starting point of the linescan (xstart). • Point II (bulk glass): approximately constant intensity distributions of the elements Si, K and Ca: I Si;bulk , I K;bulk and I Ca;bulk . • Point III (bulk glass): last point with full K-intensity, as the finite diameter of the electron beam and/or information volume in the sample have to be taken into account (xK1). • Point IV (leached layer for K, bulk glass for Ca): first point providing information only from the K-leached layer. The intensity of K is steadily decreasing. The intensity of Ca is still constant. • Point V (leached layer for K, bulk glass for Ca): last point with full Ca-intensity (xCa1), still decreasing Kintensity. • Point VI (leached layer for both K and Ca): last point providing information only from the sample. The intensities of the network modifier cations are in the order of their background intensities I K;back and I Ca;back .
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• Point VII (resin): first point in the resin providing no information about the sample. • Point VIII (resin): end point of the linescan measurement. The distance data of the fundamental points xeog (xvalue of the end of glass), xll,K and xll, Ca (beginning of the leached layer of K or Ca, respectively) are not accessible directly because of the finite diameter of the excitation volume and/or of the electron beam diameter and have to be calculated first. The values for xSi1, xSi2, xK1 and xCa1, all determined by sudden changes in the corresponding intensity distributions, were identified visually for each linescan, as statistical methods, such as confidence intervals, did not show a sufficient accuracy. The diameter (D) of the excitation volume and the xvalue of the end of the glass (xeog) are given by the Eqs. (4) and (5): D ¼ xSi2 xSi1 ; xeog ¼
1 ðxSi1 þ xSi2 Þ: 2
ð4Þ ð5Þ
Assuming a constant diameter of the excitation volume for all elements in the glass and in the leached layer, the x-value for the starting point of the leached layer for K (xll,K, lower graph in Fig. 4) is given by Eq. (6). Similar equations can also be obtained for calcium. xll;K ¼ xK1 þ
D 1 ¼ xK1 þ ðxSi2 xSi1 Þ: 2 2
ð6Þ
The observed leaching depth (LDK) can then be calculated according to Eq. (7). LDK represents the thickness of the leached layer. LDK ¼ xeog xll;K ¼
1 1 1 ðxSi1 þ xSi2 Þ xK1 þ xSi2 xSi1 2 2 2
¼ xSi1 xK1 :
Fig. 3. Backscattered electron-image of sample no. 132 showing the bulk glass (bright grey, left), the leached layer (dark grey, middle) and the resin (black, right). Furthermore, the white line indicates the position where the linescan was performed.
ð7Þ
The amounts of K and Ca still present in the glass are calculated by numerical integration of the linescan curves. A linear approximation (trapezoidal rule) seems sufficient because of the small distance between the measuring points (approximately 10 nm). As the automatic background correction implemented in the EDAX system was not complete, a second correction was also performed. The total area has to be split up into 3 parts: AK,corr (from xs,K to xe,K), A0K;corr (from xll,K to xs,K) and A00K;corr (from xe,K to xeog). xs,K and xe,K represent start- and end-points in the leached layer, between which the corresponding intensity data show no admixtures of the bulk glass or the resin (Eqs. (8) and (9), Dx
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Fig. 4. Scheme of the elemental distributions obtained for Si, K and Ca by linescan measurements (distance/intensity-linescan curves).
"
i¼n X
!
#
in Eq. (10) represents the distance between two measuring points). In the regions xll,K 6 x 6 xs,K (xll,Ca 6 x 6 xs,Ca) and xe,K 6 x 6 xeog (xe,Ca 6 x 6 xeog) approximations (Eqs. (11) and (13)) have to be made. At least 40 points were used for the determination of the average bulk and background intensities.
I Kðx¼xll;K Þ;corr I K;bulk I K;back ;
ð11aÞ
1 xs;K ¼ xll;K þ D ¼ xK1 þ xSi2 xSi1 ; 2
ð8Þ
I Kðx¼xs;K Þ;corr I Kðx¼xs;K Þ I K;back ;
ð11bÞ
1 xe;K ¼ xll;K D ¼ xSi1 ; 2
ð9Þ
A0K;corr ¼
AK;corr
Dx ¼ 2
2
I K;i
I K;0 I K;n
i¼0
I K;back ½xe;K xs;K :
1 D ½I Kðx¼xll;K Þ;corr þ I Kðx¼xs;K Þ;corr ; 2 2
ð10Þ
ð12Þ
M. Melcher, M. Schreiner / Journal of Non-Crystalline Solids 351 (2005) 1210–1225
I Kðx¼xeog Þ;corr I K;back I K;back ¼ 0;
ð13aÞ
I Kðx¼xe Þ;corr ¼ I Kðx¼xe Þ I K;back ;
ð13bÞ
A00K;corr ¼
1 D ½I Kðx¼xe Þ I K;back : 2 2
ð14Þ
If no leaching (index nl) effects were observed on the cross-section of the specimen, the expected area (AK,nl,corr) is given by the following term: AK;nl;corr ¼ ½xeog xll;K ½I K;bulk I K;back ¼ ½xSi1 xK1 ½I K;bulk I K;back :
ð15Þ
The total, measured area from x = xll,K to x = xeog is given by Eq. (16): AK;corr;tot ¼ AK;corr þ A0K;corr þ A00K;corr :
ð16Þ
The degree of leaching (DLK) from xll to xeog for K can be calculated using Eq. (17): AK;corr;tot DLK ¼ 100 1 ½% 0 6 DLK 6 100: AK;nl;corr ð17Þ With this DL-value the hypothetic leaching depth dK can be calculated, assuming a totally alkali and/or earth-alkali depleted near-surface layer (Fig. 5). xtd,K (td for theoretic/hypothetic depletion) is calculated by area compensation and characterizes the fictive borderline between the non-depleted and the completely depleted surface layer. Furthermore, dK and dCa can be compared to results obtained earlier by NRA analysis [20,21] on the leaching depths of these model glasses by weathering for periods of 12 and 24 months. xtd;K ¼ xll;K þ ðxeog xll;K Þ ð1 DLK Þ
the
hypothetic
ð20aÞ
LMCa ¼ 0:179 qglass d Ca :
ð20bÞ
A further possibility to calculate these hypothetic leaching depths and the resulting leached masses from the data obtained by linescans in the SEM is given below. This method also considers slowly decreasing bulk-intensities by extending the numerical integration from xstart to xe and yields therefore slightly higher values (9.8% in average) for the hypothetic leaching depths (d2,K and d2,Ca) and the leached masses (LM2,K and LM2,Ca) than the first method described. The area below the distance–intensity curve, the no leaching area and the corresponding values for d2,K, LM2,K and LM2,Ca are given by Eqs. (21)–(24). A comparison of the two calculation methods and the parameters involved is shown in Fig. 6. " # i¼n X Dx 2 AK;corr2 ¼ I K;i I K;bulk I Kðx¼xe Þ 2 i¼0 I K;back ðxe;K xstart Þ D þ ½I Kðx¼xe Þ I K;back ; 4
ð19Þ
leaching
depth
dK
ð21Þ ð22Þ
AK;corr2 ¼ xeog 1 ; AK;nl;corr2
ð23Þ
LM2;K ¼ 0:125 qglass d 2;K ;
ð24aÞ
LM2;Ca ¼ 0:179 qglass d 2;Ca :
ð24bÞ
d 2;K
As the density of the original glass (qglass = 2643 kg m3) is known, the leached masses of K or Ca, respectively
of
LMK ¼ 0:125 qglass d K ;
ð18Þ
d K ¼ xeog xtd;K ¼ ðxSi1 xK1 Þ DLK :
Fig. 5. Explanation (=xeog xtd,K).
(LMK and LMCa in mg K or Ca/m2 glass surface) are given by the Eqs. (20a) and (20b) (dK and dCa in lm). The factors 0.125 and 0.179 in these equations consider the chemical composition of the glass (12.5 wt% K and 17.9 wt% Ca). It seems to be important to note that the calculated values of the leached masses correspond to the observed leached layer. The effective amounts of leached network modifier masses may be higher due to network dissolution.
AK;nl;corr2 ¼ ðxeog xstart Þ ½I K;bulk I K;back ;
1 ¼ ðxSi2 þ xSi1 Þ DLK ðxSi1 xK1 Þ; 2
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Thus 6 independent values characterize the weathering attack on the glasses exposed: LDK, LDCa, dK, dCa, d2,K and d2,Ca. On each of the 136 glass samples between 3 and 6 linescan measurements were performed, leading to a total number of 563 linescans (corresponding to approximately 4 linescans per specimen). As the 3- and 4-year samples were exposed in duplicate, 8 linescans originating from these two samples describe the weathering state of the material exposed at a particular test site. The 5and 6-year samples were only exposed singly, which has to be taken into account when discussing the statistical significance of any results.
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Fig. 6. Comparison of the two different evaluation methods of the linescan measurements. The upper image displays the splitting of the total area AK,corr,tot into A0K;corr , AK,corr and A00K;corr . In the image below the numerical integration is started at x = xstart yielding the measure AK,corr2.
As many of the data needed for the calculation of leaching depths (such as the bulk and background intensities of the network modifier elements, the values for xK1 and xCa1 and so on) have to be read off from the distance–intensity-linescan curves visually, the possible errors have to be checked. Table 4 gives the changes in
Table 4 Effect of an error in the determination of the input parameters xSi1, xSi2, xK1, xCa1, IK,bulk, ICa,bulk, IK, back or ICa,back on the results for the hypothetic leaching depths and the leached network modifier masses Input parameter
Change of result for dK, dCa, LMK or LMCa (%)
Variable
True value
Error
xSi1 (xSi2) xK1 (xCa1) IK,bulk (ICa,bulk) IK,back (ICa,back)
2.67 (2.96) lm 1.47 (1.73) lm 355 (444) counts 10 (23) counts
±0.03 lm ±0.03 lm ±1% ±10%
63.3% 65.0% 65.5% 60.3%
A false determination of the bulk intensities for the network modifier elements K and Ca has the greatest effects and produces deviations of about 6%.
the results for the leaching depths dK and dCa and for the leached masses LMK and LMCa, if a (realistic) error of y % or y lm in measuring xSi1, xSi2, xK1, xCa1, IK,bulk, ICa,bulk, IK,back or ICa, back is made (data from the sample no. 19). The resulting relative error should therefore not exceed 6%.
3. Results Due to the huge amount of data, a complete discussion is not possible in this context. Consequently, only the results obtained for the 3-year exposure are presented in more detail and listed in Table 5. Furthermore, the questions of the time dependent progression of the leaching process for selected exposure sites using the data of all exposure periods and nuclear reaction analysis (NRA)-data of earlier investigations and of a preferential leaching of one of the two network modifier ions present in the model glass, K+ and Ca2+, are discussed.
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Table 5 Leaching data obtained by linescan measurements for the 3-year exposure Site no.
LD(K+) LD(Ca2+) [lm]
d(K+) d(Ca2+) [lm]
LM(K+) LM(Ca2+) [mg/m2]
LMðKþ Þ LMðCa2þ Þ
d2(K+) d2(Ca2+) [lm]
LM2(K+) LM2(Ca2+) [mg/m2]
LM2 ðKþ Þ LM2 ðCa2þ Þ
1
1.19 1.07 0.80 0.71 0.93 0.82 1.05 0.99 0.95 0.87 0.86 0.86 1.05 1.04 1.30 1.27 1.87 1.71 1.55 1.47 1.59 1.48 1.70 1.46 1.23 1.22 1.36 1.20 0.98 0.82 1.37 1.35 1.07 0.92 0.89 0.74 1.05 0.96 1.13 0.90 1.58 1.51 1.29 1.24 1.12 1.03 1.02 0.92
0.57 0.50 0.38 0.33 0.54 0.48 0.50 0.43 0.49 0.43 0.34 0.32 0.42 0.40 0.66 0.58 1.02 1.01 0.78 0.67 0.96 0.88 0.79 0.67 0.36 0.37 0.56 0.52 0.39 0.34 0.76 0.73 0.38 0.31 0.30 0.24 0.56 0.47 0.47 0.38 0.56 0.53 0.69 0.65 0.39 0.32 0.53 0.47
188 236 124 157 176 229 163 203 161 204 110 151 138 187 218 276 336 477 255 315 316 415 260 314 118 176 185 244 129 159 250 346 124 146 97 114 185 223 156 179 186 250 226 307 129 149 176 220
0.79
0.60 0.53 0.44 0.41 0.62 0.55 0.55 0.45 0.53 0.48 0.40 0.35 0.44 0.42 0.71 0.62 1.18 1.15 0.81 0.70 1.00 0.92 0.83 0.70 0.46 0.46 0.60 0.57 0.44 0.38 0.80 0.78 0.41 0.35 0.32 0.27 0.59 0.51 0.51 0.45 0.65 0.56 0.76 0.71 0.44 0.35 0.58 0.53
199 249 145 193 203 261 183 212 174 228 130 163 145 201 233 293 389 543 268 329 330 433 272 332 151 216 197 268 145 178 264 366 134 164 105 128 194 239 167 212 212 265 251 337 145 167 192 248
0.80
1.20 1.11
0.56 0.50
184 237
0.78
0.61 0.55
201 259
0.78
3 5 7 9 10 15 21 23 24 26 27 31 33 34 35 36 40 41 43 44 45 46 49 Mean
0.79 0.77 0.80 0.79 0.73 0.74 0.79 0.71 0.81 0.76 0.83 0.67 0.76 0.81 0.72 0.84 0.85 0.83 0.87 0.74 0.74 0.87 0.80
0.75 0.78 0.86 0.76 0.80 0.72 0.80 0.72 0.81 0.76 0.82 0.70 0.74 0.82 0.72 0.82 0.83 0.81 0.79 0.80 0.75 0.87 0.78
Data written in plain text represent the data for K+, italic values below for Ca2+.
3.1. Linescan measurements Table 5 and Fig. 7 depict the leaching characteristics measured on the cross-sections of the 3-year samples. The average leaching depths observed on those 48 samples were 1.20 and 1.11 lm for potassium and calcium, respectively, indicating a preferential leaching of the uni-
valent network modifier cation K+. As the samples were exposed duplicate in the case of the 3-year exposure, these 48 samples yield 24 data points. The average hypothetic leaching depths (d2,K and d2,Ca, respectively) were almost exactly half of the corresponding LD-values: 0.61 and 0.55 lm. The values for dK and dCa are about 10% lower. All histograms in Fig. 7 show strong
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M. Melcher, M. Schreiner / Journal of Non-Crystalline Solids 351 (2005) 1210–1225
Fig. 7. Histograms of the leaching characteristics of the three-year samples: leaching depths (LDs) of K+ (upper left) and Ca2+ (upper right), hypothetic leaching depths d2,K (middle left) and d2,Ca (middle right) and the ratio of the leached masses LM2,K/LM2,Ca. The data indicate a preferential leaching of K+.
deviations from the corresponding normal distributions. Smaller values for LD, LM, d and d2 (for both network modifier cations) occur more often than higher ones. Contrary, the ratio LM2,K/LM2,Ca is approximately nor-
mally distributed with a mean value (l) of 0.78. If the network modifier cations K+ and Ca2+ were evenly leached, this ratio would yield 0.70, which is the ratio of the two elements in the bulk glass.
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High values for the (hypothetic) leaching depth are observed especially for site 23, which is Birkenes (Norway). This site shows values of 1.87 for LDK and 1.71 for LDCa. Consequently, also the leached masses for K and Ca are above average: 316 and 415 mg alkali/m2 glass surface. These values are even more astonishing, as site 23 is one of the least polluted sites (Table 3). Only the relative humidity and the concentration of ozone show high levels, whereas there is hardly any SO2 and NO2 pollution. As will be shown later on, similar results were obtained by NRA-analysis in previous investigations. Contrary, the samples exposed at the test site no. 3 (Kopisty, Czech Republic) exhibit notably low values for all calculated leaching parameters: 124 and 157 mg K and Ca, respectively, were leached out of 1 m2 of the glass surface. As Kopisty is an industrial site, it shows very high pollution data for SO2, NO2 and O3 and also above-average values for the climatic parameters T and RH (Table 2). Two conclusions might be drawn from these observations: (a) the relative humidity and the concentration of ozone play an important role in glass weathering and (b) the degree of weathering of glass (of this composition) is not simply describable in terms of a general pollution parameter including SO2, NO2 and O3 with the same weight. 3.2. Time-dependence of the leaching process At 10 test sites the exposure was carried out for all exposure times (i.e. 3–6 years). Together with the 1and 2-year leaching data obtained by Woisetschla¨ger and Schreiner [20,21] by NRA-measurements for the same type of glass, a time series is set up. Their leaching depths (dH) are directly comparable to the dK and dCa values. Fig. 8 depicts the mean values ±1 standard deviation of the hypothetic leaching depth dK and the hydrogen penetration depth dH for each exposure time. It becomes apparent that dK does not steadily increase with time. After 4 years of exposure dK decreases from approximately 650 nm to 600 nm in average, which is out of the error range of the evaluation method. Two facts must be kept in mind: (a) In contrast to the 3and 4-year exposure, the 5- and 6-year samples were only exposed singly. Therefore, the statistical error of the calculated leaching depths is higher than in the case of all other exposures, when two or three samples were exposed under the same conditions and for the same time (b) The 5- and 6-year samples were not exposed continuously, but with breaks of 2 and 3 years, respectively. Although the samples were stored under low humidity conditions, this storage might have caused this irregular progress of the leaching process shown below. However, it can be shown that not only alkali and earth alkali diffusion, but also network dissolution must be
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Fig. 8. Plot of the mean hypothetic leaching depths for K. The results obtained from the exposures at the sites 3, 10, 15, 21, 23, 24, 26, 27 and 33 are considered in this diagram. As no 2-year dH value was available for site 7, this site was excluded. The bars mark the range l ± r.
considered in the weathering of glass under natural conditions. Fig. 9 depicts the graph of the measured hypothetic leaching depths dK and the hydrogen-penetration depths for three selected exposure sites showing different levels of pollution: no. 3 (Kopisty, highly polluted), no. 33 (Toledo, moderately polluted) and no. 23 (Birkenes, very low polluted). Remarkable is the fact that the leaching depths are contrary to this classification. The results of the test site nos. 3 and 23 show one outlier each (too high leaching data in both cases), but no error could be identified in the measurement or calculation. The experimental data can be fitted using power-curves of the form dK = a Æ tb. The corresponding coefficients of determination (R2) show values of 0.28 (site 23), 0.66 (site 3) and 0.88 (site 33). Two main conclusions must be drawn considering these data: (a) The observed hypothetic leaching depths dK and dCa cannot be directly related to pollution data. (b) The leaching depths of samples increase with time, but the rate decreases. 3.3. Congruent or selective leaching? Table 5 shows higher (hypothetic) leaching depths of potassium for nearly all samples. However, the initial question might best be answered statistically. In total, 563 linescan measurements have been performed, each yielding one value for LDK, LDCa, dK, dCa, d2,K, d2,Ca, LMK/LMCa and LM2,K/LM2,Ca. The following null
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Fig. 9. Comparison of the progression of the hypothetic leaching depths for a low polluted (site no. 23), a moderately polluted (site no. 33) and a highly polluted exposure site (no. 3). The observed leaching depths are contrary regarding the degree of pollution. The data for the one and two year exposure are taken from NRA-results in [20,21].
hypotheses must be tested (the value 0.70 originates from the mass ratio of the two network modifier elements in the bulk glass): (1) (2) (3) (4)
H0: LDK = LDCa. H0: dK = dCa. H0: d2,K = d2,Ca. LMK H0: LM ¼ 0:70. Ca
(5) H0:
LM2;K LM2;Ca
at a high statistical significance of 99.9% and a preferential leaching of potassium is proven. The bivalent ions seem to be stronger bond to the NBO-sites of the silica network and are harder to remove by the ion exchange reaction. 3.4. Surface investigations
¼ 0:70.
In all cases modifications of a t-test for the comparison of 2 means were applied: the paired t-test for testing (1)–(3) and the t-test for the comparison of a mean with a target value for (4) and (5) using the Eqs. (25) and (26) for the calculation of the test statistic q. q¼
m pffiffiffi n; s
ð25Þ
q¼
mx m0 pffiffiffi n; s0
ð26Þ
where P m is the mean of the differences Di (m ¼ 1n 563 s is the standard deviation of 1 Di ), rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi! P563 ðDi mÞ2 1 Di s ¼ , mx the mean of the original data, n1 m0 the target value, s 0 the standard deviation of the original data and n the number of observation pairs (n = 563). The number of degrees of freedom (f = n 1) is in all cases 562. Table 6 depicts the results of the statistical test. All null hypotheses can be rejected
In addition to the investigations carried out on the cross-sections of the specimens, also the glass surfaces were studied, in order to characterize the surfaces of the leached layers to the ambient atmosphere. The Fig. 10(b)–(d) depict SE-images of glass surfaces of specimens exposed under unsheltered conditions. Besides very few deposits (mainly of organic origin) no weathering products are visible. Obviously all weathering products formed during the exposure have been washed away by the rain. The leached layers of most samples are still intact (Fig. 10(b)), but on a few specimens (about 10 out of 136) it is totally cracked (Fig. 10(c) and (d)). There are two feasible explanations for the destruction of the leached layer: (a) the resulting change in the density of the outmost glass layers together with variations of temperature and humidity of the ambient atmosphere or (b) the outgassing of H2O-molecules by applying a highvacuum during the SEM investigations. For reasons of comparison also a SE-image of a sample which was exposed under sheltered conditions (Fig. 10(a)) is shown. Many weathering products, mainly syngenite (K2SO4 Æ CaSO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O), can be identified.
M. Melcher, M. Schreiner / Journal of Non-Crystalline Solids 351 (2005) 1210–1225
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Table 6 Results of the statistical t-tests Parameter
H0 LDK = LDCa
m mx m0 s s0 q tcrit (f = 500; p = 99.9) Result
dK = dCa
0.10 0.05 – – – – 0.16 0.09 – – 14.31 13.82 3.31 (2-sided), 3.11 (1-sided) LDK > LDCa dK > dCa
d2,K = d2,Ca
LMK ¼ 0:6969 LMCa
LM2;K ¼ 0:6969 LM2;Ca
0.06 – – 0.08 – 19.24
– 0.78 0.70 – 0.09 23.09
– 0.79 0.70 – 0.10 20.54
d2,K > d2,Ca
LMK > 0:70 LMCa
LM2;K > 0:70 LM2;Ca
All null hypotheses can be rejected, the (hypothetic) leaching depths and leached masses of potassium are significantly higher than those of calcium. tcrit represents the critical t-value at a statistical significance of 99.9% and 500 degrees of freedom.
Fig. 10. (a)–(d) Secondary electron images of sample surfaces exposed under sheltered (a) and unsheltered (b)–(d) conditions. (a) Many weathering products, mainly consisting of syngenite (K2SO4 Æ CaSO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O), can be observed on this sample exposed under sheltered conditions; exposure time: 4 years, test site 26 (Aspvreten), magnification: 150·. (b) Leached layer with some depositions, surface is still intact, exposure time: 5 years, test site 37 (Dorset), magnification: 100·. (c) Numerous cracks of the leached layer all over the sample surface can be observed, exposure time: 4 years, test site 21 (Oslo), magnification: 90·. (d) Many cracks and pits of the leached layer are visible, some pieces of glass have already fallen off, exposure time: 3 years, test site 14 (Casaccia), magnification: 1800·.
4. Discussion and conclusion The major results of the SEM-investigations of the surfaces and cross-sections of more than 130 glass samples of medieval composition, which have been exposed for up to 6 years at more than 20 test sites, may be summarized and interpreted as follows:
(a) Even after six years of unsheltered exposure the leached layer of most samples shows no pits or cracks. Contrary to the sheltered exposed specimens, on which various crystalline weathering products such as syngenite and gypsum were found, hardly any weathering products could be identified on the surface of the unsheltered
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samples. Mainly carbon-rich (and therefore probably organic) depositions could be observed. Cracked glass surfaces are rare exceptions and it is uncertain, if these phenomena originate from the weathering process or from the high-vacuum conditions in the SEM. (b) The semi-automatic evaluation procedure, which is based on a numerical integration of the distance–intensity data, proved to be suitable especially for such a high amount of data. The input parameters for this model are average bulk- and background-intensities of K and Ca and characteristic points of the intensity distributions. The main disadvantage of this procedure is the time-consuming and error-prone visual detection. However, as realistic uncertainties result only in minor changes of the leaching data, the use of this method seems justifiable. (c) It was shown that leached layers of up to 1.87 lm can occur on glasses of this composition during weathering periods of up to 6 years. The average leaching depths were found to be slightly above 1 lm. Considering the time dependence of the weathering process, power-functions of the general form y = a Æ tb, where t stands for time, were found to fit the experimental data in the best way. The achieved coefficients of correlation for these curve estimations are rather good (between 0.24 and 0.88). As only six data points are available for each test site, the results will have to be confirmed by even longer exposure times. The leaching depths of most samples were found to increase with time, but the rate significantly decreases in many cases after about 3 years. As also clearly decreasing thickness of the leached layers were observed on some samples, network dissolution (SE-image in Fig. 10(d)) may have also occurred and the true amount of leached network modifier masses of Ca or K may be higher. The actual position of the linescan measurement along the glass cross-section may also result in (not quantifiable) errors of the final result, as the boundary between the unweathered bulk glass and the leached layer is not constant. The results of former NRA-measurements on glass samples of the same compositions weathered for 12 and 24 months fit also very well into the actual data. (d) The monovalent network modifier ion K+ is leached out more easily than the bivalent cation Ca2+, which is probably due to the stronger bond of the bivalent Ca2+ to the non-bridging oxygen sites in the glass. The leaching depths for K+ are in average 10% higher than those of Ca2+. A detailed statistical evaluation (paired t-test) proved
the significance of the differences of the weathering data between K+ and Ca2+. This result may not be transferable to glasses of other compositions. (e) The weathering data such as the (hypothetic) leaching depths cannot be directly related to the environmental or climatic data measured at the test sites. The correlation between the pollution and weathering data has to be inspected in more detail in future works.
Acknowledgement The authors gratefully thank the United Nations Economic Commission for Europe for the possibility to participate in this project and for the financial support. Special thanks also go to all partners within this and the succeeding project (MULTI-ASSESS, contract no. EVK4-CT-2001-00044) for the friendly and fruitful cooperation.
References [1] T. Rahren, J. Archaeol. Sci. 27 (2000) 1225. [2] K.H. Wedepohl, Die Herstellung Mittelalterlicher Und Antiker Gla¨ser, Franz Steiner Verlag, Stuttgart, 1993. [3] M.A. Bezborodev, Chemie Und Technologie Der Antiken Und Mittelalterlichen Gla¨ser, Verlag Philipp von Zabern, Mainz, 1975. [4] S. Frank, Glass and Archaeology, Academic, London, 1982. [5] G.W. Morey, Ind. Eng. Chem. 17 (4) (1925) 389. [6] W.H. Zachariasen, J. Am. Chem. Soc. 54 (1932) 3841. [7] A.N. Finn, Ind. Eng. Chem. 30 (8) (1938) 891. [8] T.M. El-Shamy, Phys. Chem. Glasses 14 (1) (1973) 1. [9] T.M El-Shamy, S.E. Morsi, H.D. Taki-Eldin, A.A. Ahmed, J. Non-Cryst. Solids 19 (1975) 241. [10] R.W. Douglas, T.M.M. El-Shamy, J. Am. Ceram. Soc. 50 (1) (1967) 1. [11] R.W. Douglas, J.O. Isard, J. Soc. Glass Technol. 33 (1949) 289. [12] R.H. Doremus, J. Non-Cryst. Solids 19 (1975) 137. [13] W.A. Lanford, K. Davis, P. Lamarche, T. Laursen, R. Groleau, R.H. Doremus, J. Non- Cryst. Solids 33 (1975) 249. [14] J. Scha¨fer, H.A. Schaeffer, Rivista della Staz. Sper. Vetro 5 (1984) 79. [15] G.I. Cooper, G.A. Cox, Appl. Geochem. 11 (1996) 511. [16] T. Richter, G.H. Frischat, G. Borchardt, S. Scherrer, Glastechn. Ber. 63 (10) (1990) 300. [17] G. Woisetschla¨ger, M. Dutz, S. Paul, M. Schreiner, Mikrochim. Acta 135 (2000) 121. [18] M. Schreiner, G. Woisetschla¨ger, I. Schmitz, M. Wadsak, J. Anal. At. Spektrom. 14 (1999) 395. [19] I. Schmitz, M. Schreiner, G. Friedbacher, M. Grasserbauer, Anal. Chem. 69 (1997) 1012. [20] G. Woisetschla¨ger, M. Schreiner, ICP on effects on materials including historic and cultural monuments, Report no. 21, Evaluation of decay to glass samples after 1 and 2 years of exposure, Institute of Chemistry, Academy of Fine Arts, Vienna, 1998. [21] G. Woisetschla¨ger, Weathering of Medieval Potash-Lime-Silica Glass in the International Co-operative Programme on Assessment and Monitoring or Air Pollution Effects of the Economic
M. Melcher, M. Schreiner / Journal of Non-Crystalline Solids 351 (2005) 1210–1225 Commission for Europe within the United Nations, PhD thesis, Vienna University of Technology, 1998. [22] M. Melcher, M. Schreiner, Results from the multipollutant programme: Evaluation of the decay to glass samples of medieval composition after 3, 4, 5 and 6 years of exposure, Part A: Results of the sheltered exposure. ICP on effects on materials including historic and cultural monuments. Report no. 48, Institute of Sciences and Technologies in Art, Academy of Fine Arts, Vienna, 2003. [23] M. Melcher, M. Schreiner, Results from the multipollutant programme: Evaluation of the decay to glass samples of medieval composition after 3, 4, 5 and 6 years of exposure, Part B: Results of the unsheltered exposure. ICP on effects on materials including historic and cultural monuments. Report no. 49, Institute of Sciences and Technologies in Art, Academy of Fine Arts, Vienna, 2004.
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[24] M. Melcher, M. Schreiner, Anal. Bioanal. Chem. 379 (2004) 628. [25] See the projects webpage avaiable from
Evaluation procedure for leaching studies on naturally weathered potash-lime-silica glasses with medieval composition by scanning electron microscopy M. Melcher a
a,b,*
, M. Schreiner
a,b
Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria b Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, 1060 Vienna, Austria Received 26 November 2004; received in revised form 2 February 2005 Available online 14 April 2005
Abstract In order to study the weathering mechanism and quantify the influence of the various air pollutants on the degradation process, potash-lime-silica (PLS) glasses with a chemical composition similar to medieval stained glass were exposed at more than 20 test sites in Europe and North America to the local environmental conditions within the International Co-operative Programme on Effects on Materials Including Historic and Cultural Monuments of the United Nations Economic Commission for Europe (ICPMaterials, UN/ECE). After exposure periods of up to 6 years the specimens were investigated in the scanning electron microscope in combination with energy dispersive X-ray microanalysis (SEM/EDX) in order to characterize the surface layer (leached layer) of the weathered samples. Cross-sections of the samples were prepared for depth profile analyses using linescan measurements in order to calculate the leaching depths of the network modifier ions. The typical diffusion profiles expected for these elements were observed. The evaluation is based on a numerical integration of the background corrected linescan data. This method proved to be suitable for the high amount of samples (more than 130) and yields acceptable errors of less than 10 rel.%. The first results show leaching depths for K+ and Ca2+ between 0.42 and 1.87 lm (average 1.21 lm for K and 1.11 lm for Ca) for exposure times of up to six years. So both network modifier ions present in the glass were leached out, with K+ showing slightly, but significantly higher leaching depths than the bivalent cation Ca2+. As no continuous increase of the leaching depths with time could be observed, network dissolution must have also occurred during the weathering process. Comparing these results to former leaching studies, the rate of leaching on glasses of this composition seems to decrease significantly after about three or four years of exposure. 2005 Elsevier B.V. All rights reserved. PACS: 81.65.K; 68.35.B
1. Introduction Compared to modern or ancient Roman glasses, which can be classified as soda-lime-silica (SLS) glasses [1], medieval stained glass objects produced as window panes for Romanesque and Gothic cathedrals or
*
Corresponding author. Tel.: +43 58816 206; fax: +43 58816 121. E-mail address: [email protected] (M. Melcher).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.02.020
churches often exhibit two characteristic differences: (a) the preferred use of potassium instead of sodium containing compounds for the glass production and (b) a high content of network modifier ions such as calcium, potassium and magnesium and therefore a low silica content [2]. Bezborodov [3] reports the results of more than 700 analyses of ancient and medieval glasses (Table 1). He clearly emphasizes that potash-lime-silica glasses (with more or less high contents of MgO and Al2O3) were probably not produced before the Middle
M. Melcher, M. Schreiner / Journal of Non-Crystalline Solids 351 (2005) 1210–1225
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Table 1 Average concentrations of the main glass components SiO2, K2O, Na2O, CaO, MgO, Al2O3, Fe2O3 in ancient and medieval potash containing glasses, calculated from data in Ref. [3] Type of Glass
Dating
Number of observations
Potash-lime Soda-potash-lime Soda-potash-lime
Medieval Medieval Ancient
n = 67 n = 138 n = 20
Average concentrations (mean ± standard deviation, in wt%) SiO2
K2O
Na2O
CaO
MgO
Al2O3
Fe2O3
53.6 ± 6.6 61,9 ± 5.7 64.5 ± 5.9
16.8 ± 5.8 5.4 ± 2.9 6.4 ± 4.2
1.3 ± 0.9 13.0 ± 3.9 12.2 ± 4.9
18.6 ± 4.7 8.9 ± 4.1 7.1 ± 2.7
3.4 ± 1.5 4.3 ± 1.7 2.5 ± 1.4
2.7 ± 1.8 3.6 ± 2.0 2.9 ± 1.5
1.0 ± 1.1 1.0 ± 0.6 1.3 ± 1.2
Ages and soda-potash-lime-silica glasses were mainly produced in medieval times. The reason for this change of the raw materials might be a growing demand for window glass for churches and cathedrals and a resulting shortage in marine plants, which have been the source of alkali by then. Therefore, the ash of beech wood or farn, both rich in potassium, was presumably used as flux [4]. The question of the chemical stability of glasses [5] came up as early as the first useful models of the structure of glass were set up [6]. Up to the 1930s it was still believed that the PLS glasses mentioned above are supe-
CaO–SiO2 glasses towards acidic solutions was studied. The results showed a preferential leaching of sodium for low lime glasses (<10 mol% CaO), but an approximately uniform dissolution of Na and Ca for glasses with higher CaO contents. However, several studies [10,11] showed that the glass corrosion process is a two-stage reaction. At first, leaching of the mono- or bivalent network modifier ions (M+ or M2+ in the Eqs. (1a) and (1b), respectively) by neutral or acidic solutions takes place, following the pffi t-law, as it is characteristic for diffusion controlled processes [12].
ð1aÞ
ð1bÞ
rior to SLS glasses regarding their stability [7], but today it is well known that a high content of network modifiers (especially K) is a great disadvantage for the stability and durability of glasses. El-Shamy [8] studied the chemical resistance of seven glasses in the system K2O–CaO– MgO–SiO2 towards water and acidic solutions and found a considerable dependence on the ratio of K2O to other constituents. Furthermore, a clear tendency of the glass samples to form a crusted layer on the surface could be determined on samples containing less than 66 mol% SiO2. In another work [9] the resistance of Na2O–
The true chemical nature of the hydrogen bearing species (simply H+ or H+ accompanied by one or more H2O-molecules) and the rate determining step in alkali leaching seem not to be clarified so far [13]. Perhaps a general answer to this question is not possible. In contact with alkaline solutions, another reaction mechanism is observed (Eq. (2)). Hydroxyl ions break up the Si–O bonds forming silanol-groups (Si–OH). H2O-molecules may form through a condensation reaction of the latter (Eq. (3)).
ð2Þ
ð3Þ
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In any case, an alkali- and earth alkali-depleted and hydrogen-enriched layer is formed in the near surface region of the glass. Typically the thickness of this surface layer is in the sub-lm range to some microns depending on the nature of the leaching solution, the leaching time and the type of glass used. While a huge amount of data on the chemical stability of glasses towards liquid media such as water or acids and bases of all pH-values is available in the literature [14–16], the natural weathering (i.e. the attack of various air pollutants including climatic factors such as the relative humidity) of glass and its resulting deterioration has not been studied so extensively [17–19]. Quantitative investigations on naturally or artificially weathered glass specimens are very rare [20–24]. The reason for this might be the long weathering times needed to achieve observable results and the high complexity of the system environment/glass due to the high number of factors involved (e.g. the formation of a water film on the sample surface, the absorption of acidifying gases of the ambient atmosphere in this water film and the resulting change of its pH). In the International Co-operative Programme on Effects on Materials including Historic and Cultural Monuments founded by the United Nations Economic Commission for Europe (ICP-Material, UN/ECE [25]) various materials, among them steel, bronze, copper, zinc, some sorts of stone and modern as well as historic glass are exposed since 1987 at about 25 test sites in Europe and North America to the local environmental conditions. The objectives of this large-scaled field exposure programme are (a) qualitative and quantitative estimations of the damage towards objects of our cultural heritage through many kinds of air pollutants such as NOx, SO2, O3 and (b) the development of dose-response functions describing the long-term costs of this destructive impact. These functions are also used for assessing acceptable corrosion rates and pollution levels and for mapping areas with elevated risk of corrosion damage. Since the early 1990s the Institute of Science and Technology in Art of the Academy of Fine Arts has been exposing glasses with compositions similar to that of medieval stained glass in the framework of the ICP-Materials project. In order to characterize the weathering process, scanning electron microscopy in combination with energy dispersive X-ray microanalysis has been used. By that combination the morphology as well as the chemical (elemental) composition of the weathering products formed on the glass surfaces could be obtained. If the exposure is carried out under sheltered conditions (i.e. the samples are protected from rain and sun radiation), the amount of products formed could be used for describing the kinetics of the weathering process [22,24,26]. This work aims to enable the study of glass specimens weathered under unsheltered conditions by measuring
the thickness of the leached layer formed also during the weathering process. The determination of the surface layer depleted in K and Ca and its correlation with time should give a hint to the kinetics involved. Cross-sections of the exposed glass samples with subsequent linescan measurements in the SEM were performed leading to depth profiles of the mono- and bivalent network modifier ions of the glass. Specimens from the UN/ECE-exposure within two periods (1993–95 and 1997–2001) could be used therefore. For reasons of comparison, former results on depth profiles using nuclear reaction analysis (NRA) obtained by Woisetschla¨ger and Schreiner [20,21] are also considered.
2. Experimental 2.1. Glass samples Within the UN/ECE project glass samples of the following composition (similar to medieval stained glass) were exposed (wt%): 60 SiO2, 25 CaO, 15 K2O. The glasses were prepared at the Fraunhofer-Institut fuer Silicatforschung in Wuerzburg/Germany by mixing oxides and carbonates of specific elements in certain stoichiometric quantities and melting in a Pt crucible at approximately 1720 K. After refining the glass at a temperature of 900 K for 5 h and cooling, the frits were ground and melted a second time in order to achieve homogeneity. Small plates of approximately 10 · 10 · 2 mm3 were cut from these glass ingots with a low-speed diamond saw (Buehler Isomet II-1180) and embedded in epoxy resin for easier handling in the subsequent polishing process using SiC paper of 500–4000 mesh. In the last polishing step isopropanol instead of water was used as lubricant in order to prevent leaching of glass components already during the polishing process. The polished glass samples were mounted on glass fiber reinforced plastic plates (sized 15 cm · 10 cm · 3 mm, Fig. 1) with approximately 10 cm pieces of shielded wire, covered with silicon caps and adhesive tapes, packed in polystyrene foam filled paper boxes and sent to the exposure sites. After the fixed exposure time the glass samples were sent back from the sites, again covered with silicon caps and packed in paper boxes to prevent any damage to the samples during shipping. The samples were stored in plastic boxes together with drying mats in order to guarantee a low relative humidity in the box and to avoid further glass corrosion prior to the investigations by SEM. The SEM measurements yielded the corrosion (weathering) products formed, such as gypsum (CaSO4 Æ 2H2O) and syngenite (K2SO4 Æ CaSO4 Æ H2O). Afterwards, the samples were covered with a thin layer (3 mm) of epoxy resin and cut with a low-speed
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environmental and climatic data such as the concentration of the acidifying gases SO2, NO2 and O3, the temperature T and the relative humidity RH. For a statistical evaluation a wide spread of these data is necessary in order to achieve reliable results. The test sites can be grouped in three categories: (a) low polluted sites: c(SO2) and c(NO2) < 15 lg/m3; (b) moderately polluted sites: c(SO2) or c(NO2) > 15 lg/m3; (c) heavily polluted sites: c(SO2) > 15 lg/m3 and c(NO2) > 15 lg/m3.
Fig. 1. Four glass specimens (visible glass surface 1 cm2 per sample) mounted on a glass-fiber reinforced plastic plate (sized 15 cm · 10 cm · 3 mm). The labels refer to the number of the exposure site (Kopisty, Czech Republic) and the end of the exposure time (September 2001).
diamond saw in order to produce cross-sections of the specimens (Fig. 2(a) and (b)), which were also investigated in the SEM using the linescan mode. 2.2. Exposure conditions The glass samples were exposed at up to 24 test sites. The positions of the sites were chosen according to their
Table 2 presents a short description of these exposure sites including their classification (r = rural, u = urban and i = industrial) and environmental parameters for the three year exposure. It must be annotated that these conditions only refer to the vicinity of the test site and not to the whole city or region. The environmental data are collected and reported by the Norwegian Institute for Air Research (NILU) [27,28]. Table 3 presents an overview of the exposure times and the number of samples exposed. In the case of the 3- and 4-year exposure, always two samples were exposed under the same conditions for the same time. It seems important to note that the 5- and 6-year exposures were performed with a two or three year intermission, where the samples were stored at low humidity (10%) in a conditioned box. The exposure could be carried out in two ways: sheltered (i.e. the samples were exposed vertically in a box in order to avoid a direct contact with rain or sun radiation) or unsheltered (on top of a wooden rack with an inclination of 45). The results of the sheltered exposure are described in [22,24].
Fig. 2. (a) and (b) Original glass sample in an epoxy resin matrix with a schematic drawing of the cutting plane (left) and the cut specimen with an additional layer of resin (right).
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Table 2 List of the exposure sites including their classification in rural (r), urban (u) and industrial (i) sites Site
Name (Country)
T [C]
RH [%]
c(SO2) [lg/m3]
c(NO2) [lg/m3]
c(O3) [lg/m3]
mm [mm]
pH
1 (u) 3 (i) 5 (r) 7 (r) 9 (r) 10 (i) 15 (u) 21 (u) 23 (r) 24 (u) 26 (r) 27 (u) 31 (u) 33 (r) 34 (u) 35 (r) 36 (u) 40 (u) 41 (u) 43 (u) 44 (r) 45 (r) 46 (u) 49 (u)
Prague-Letnany (CZ) Kopisty (CZ) ¨ htari (FIN) A Waldhof–Langenbru¨gge (GER) Langenfeld–Reusrath (GER) Bottrop (GER) Milan (ITA) Oslo (NOR) Birkenes (NOR) Stockholm South (SWE) Aspvreten (SWE) Lincoln Cathedral (GB) Madrid (SPA) Toledo (SPA) Moscow (RUS) Lahemaa (EST) Lisbon-Jeronimos Monastery (POR) Paris (FRA) Berlin (GER) Tel Aviv (ISR) Svanvik (NOR) Chaumont (CH) London (GB) Antwerp (BEL)
9.8 9.7 3.0 9.5 11.1 11.5 14.3 6.7 6.1 7.3 6.0 10.0 14.3 14.0 5.2 5.7 17.4 13.3 10.5 24.2 0.8 6.5 12.3 11.7
74.6 76.4 78.9 79.9 79.6 81.1 69.7 76.3 81.0 76.3 84.3 80.5 57.5 58.0 71.3 80.2 65.9 68.5 75.8 80.5 78.7 78.7 70.6 75.7
13.9 17.4 0.7 1.5 7.6 21.3 15.0 4.2 0.2 2.7 0.6 8.9 7.4 1.9 25.4 1.3 19.0 12.2 9.6 35.2 7.1 1.2 6.8 18.7
23.2 30.5 2.9 8.6 32.4 36.0 79.1 30.8 1.4 19.6 2.6 21.1 21.5 10.5 24.4 1.5 36.0 57.9 41.1 34.3 1.4 7.8 47.6 50.0
48.9 55.0 61.5 52.4 33.6 31.2 38.1 38.1 56.6 47.5 57.7 50.4 54.3 84.9 42.5 59.8 13.4 32.0 24.6 38.4 56.9 84.4 35.4 29.1
1407 1323 1998 1854 2635 2834 2923 2229 5032 1346 1264 2149 1443 1563 2136 2140 618 1827 1407 1254 1257 3242 1938 2841
4.8 4.5 4.7 5.1 4.9 4.9 – 4.7 4.5 4.7 4.6 – 6.2 5.9 6.6 4.9 6.1 5.7 – – 4.8 5.0 – 5.0
The environmental data for the temperature (T, [C]), the relative humidity (RH, [%]), the concentration of SO2, NO2 and O3 (c(SO2), c(NO2) and c(O3), [lg/m3]) and the amount (mm, [mm]) and pH of the precipitation are mean values and refer to the exposure period from October 1997 to September 2000. – can either mean that this parameter is not measured at this site or that not enough monthly mean values were available to calculate a reliable average for the whole period.
Table 3 Time of exposure and number of glass samples exposed in sheltered and unsheltered modes within the UN/ECE project Sheltered 1 year (10/93–09/94) 2 years (10/93–09/95) 3 years (10/97–09/00) 4 years (10/97–09/01) 5 years (10/93–09/94 and 10/97–09/01) 6 years (10/93–09/95 and 10/97–09/01)
69 69 48 44 10 13
samples samples samples samples samples samples
Unsheltered 23 23 24 24 10 13
sites sites sites sites sites sites
69 69 48 44 16 18
samples samples samples samples samples samples
23 23 24 22 16 18
sites sites sites sites sites sites
It is important to note that the exposure was not carried out continuously in case of the 5- and 6-year samples.
2.3. SEM parameters and method developed for the evaluation of the linescans The surfaces and cross-sections of all glass specimens were investigated by SEM/EDX (JEOL 6400 in combination with EDAX Phoenix). The measurements were performed with an acceleration voltage of 15 kV and a beam current of approximately 1 nA. For the linescan measurements the following parameters were chosen: • • • •
length of the line between 3 and 10 lm; number of measurement points: 300–500; dwell time: between 300 and 500 ms; total counts per second: 6000;
• dead time: between 25% and 40% (adjusted with amplification time); • linescan evaluation method: net-intensities (including a peak deconvolution and a background correction of the raw data). Prior to the analyses of the surface and cross-sections in the SEM the samples were coated with a thin layer of carbon in order to avoid charging during the electron bombardment. Long sputtering times (up to 20 s) were selected as smaller areas (10 · 10 lm2) were investigated. Stable conditions are absolutely necessary if analyses are carried out in these microdomains. As a first result of the investigations in all cases the leached
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layers, which are depleted in modifier cations, could be found (Fig. 3) according to Eq. (1). The boundary between the bulk glass and the leached layer is not sharp. Fig. 4 shows a scheme of the distance/intensity-linescan curves usually obtained with the intensities of Si as well as K and Ca. The upper image depicts the cross-section of a glass specimen with the bulk glass, the leached layer, the resin and the position of the linescan. The circles represent the two-dimensional projection of the excitation volume caused by the electron beam in the specimen. 8 significant points (I–VIII) are marked on that line, which are described with respect to the distribution of the intensities of the elements Si, K and Ca (middle and lower image). • Point I (bulk glass): represents the starting point of the linescan (xstart). • Point II (bulk glass): approximately constant intensity distributions of the elements Si, K and Ca: I Si;bulk , I K;bulk and I Ca;bulk . • Point III (bulk glass): last point with full K-intensity, as the finite diameter of the electron beam and/or information volume in the sample have to be taken into account (xK1). • Point IV (leached layer for K, bulk glass for Ca): first point providing information only from the K-leached layer. The intensity of K is steadily decreasing. The intensity of Ca is still constant. • Point V (leached layer for K, bulk glass for Ca): last point with full Ca-intensity (xCa1), still decreasing Kintensity. • Point VI (leached layer for both K and Ca): last point providing information only from the sample. The intensities of the network modifier cations are in the order of their background intensities I K;back and I Ca;back .
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• Point VII (resin): first point in the resin providing no information about the sample. • Point VIII (resin): end point of the linescan measurement. The distance data of the fundamental points xeog (xvalue of the end of glass), xll,K and xll, Ca (beginning of the leached layer of K or Ca, respectively) are not accessible directly because of the finite diameter of the excitation volume and/or of the electron beam diameter and have to be calculated first. The values for xSi1, xSi2, xK1 and xCa1, all determined by sudden changes in the corresponding intensity distributions, were identified visually for each linescan, as statistical methods, such as confidence intervals, did not show a sufficient accuracy. The diameter (D) of the excitation volume and the xvalue of the end of the glass (xeog) are given by the Eqs. (4) and (5): D ¼ xSi2 xSi1 ; xeog ¼
1 ðxSi1 þ xSi2 Þ: 2
ð4Þ ð5Þ
Assuming a constant diameter of the excitation volume for all elements in the glass and in the leached layer, the x-value for the starting point of the leached layer for K (xll,K, lower graph in Fig. 4) is given by Eq. (6). Similar equations can also be obtained for calcium. xll;K ¼ xK1 þ
D 1 ¼ xK1 þ ðxSi2 xSi1 Þ: 2 2
ð6Þ
The observed leaching depth (LDK) can then be calculated according to Eq. (7). LDK represents the thickness of the leached layer. LDK ¼ xeog xll;K ¼
1 1 1 ðxSi1 þ xSi2 Þ xK1 þ xSi2 xSi1 2 2 2
¼ xSi1 xK1 :
Fig. 3. Backscattered electron-image of sample no. 132 showing the bulk glass (bright grey, left), the leached layer (dark grey, middle) and the resin (black, right). Furthermore, the white line indicates the position where the linescan was performed.
ð7Þ
The amounts of K and Ca still present in the glass are calculated by numerical integration of the linescan curves. A linear approximation (trapezoidal rule) seems sufficient because of the small distance between the measuring points (approximately 10 nm). As the automatic background correction implemented in the EDAX system was not complete, a second correction was also performed. The total area has to be split up into 3 parts: AK,corr (from xs,K to xe,K), A0K;corr (from xll,K to xs,K) and A00K;corr (from xe,K to xeog). xs,K and xe,K represent start- and end-points in the leached layer, between which the corresponding intensity data show no admixtures of the bulk glass or the resin (Eqs. (8) and (9), Dx
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Fig. 4. Scheme of the elemental distributions obtained for Si, K and Ca by linescan measurements (distance/intensity-linescan curves).
"
i¼n X
!
#
in Eq. (10) represents the distance between two measuring points). In the regions xll,K 6 x 6 xs,K (xll,Ca 6 x 6 xs,Ca) and xe,K 6 x 6 xeog (xe,Ca 6 x 6 xeog) approximations (Eqs. (11) and (13)) have to be made. At least 40 points were used for the determination of the average bulk and background intensities.
I Kðx¼xll;K Þ;corr I K;bulk I K;back ;
ð11aÞ
1 xs;K ¼ xll;K þ D ¼ xK1 þ xSi2 xSi1 ; 2
ð8Þ
I Kðx¼xs;K Þ;corr I Kðx¼xs;K Þ I K;back ;
ð11bÞ
1 xe;K ¼ xll;K D ¼ xSi1 ; 2
ð9Þ
A0K;corr ¼
AK;corr
Dx ¼ 2
2
I K;i
I K;0 I K;n
i¼0
I K;back ½xe;K xs;K :
1 D ½I Kðx¼xll;K Þ;corr þ I Kðx¼xs;K Þ;corr ; 2 2
ð10Þ
ð12Þ
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I Kðx¼xeog Þ;corr I K;back I K;back ¼ 0;
ð13aÞ
I Kðx¼xe Þ;corr ¼ I Kðx¼xe Þ I K;back ;
ð13bÞ
A00K;corr ¼
1 D ½I Kðx¼xe Þ I K;back : 2 2
ð14Þ
If no leaching (index nl) effects were observed on the cross-section of the specimen, the expected area (AK,nl,corr) is given by the following term: AK;nl;corr ¼ ½xeog xll;K ½I K;bulk I K;back ¼ ½xSi1 xK1 ½I K;bulk I K;back :
ð15Þ
The total, measured area from x = xll,K to x = xeog is given by Eq. (16): AK;corr;tot ¼ AK;corr þ A0K;corr þ A00K;corr :
ð16Þ
The degree of leaching (DLK) from xll to xeog for K can be calculated using Eq. (17): AK;corr;tot DLK ¼ 100 1 ½% 0 6 DLK 6 100: AK;nl;corr ð17Þ With this DL-value the hypothetic leaching depth dK can be calculated, assuming a totally alkali and/or earth-alkali depleted near-surface layer (Fig. 5). xtd,K (td for theoretic/hypothetic depletion) is calculated by area compensation and characterizes the fictive borderline between the non-depleted and the completely depleted surface layer. Furthermore, dK and dCa can be compared to results obtained earlier by NRA analysis [20,21] on the leaching depths of these model glasses by weathering for periods of 12 and 24 months. xtd;K ¼ xll;K þ ðxeog xll;K Þ ð1 DLK Þ
the
hypothetic
ð20aÞ
LMCa ¼ 0:179 qglass d Ca :
ð20bÞ
A further possibility to calculate these hypothetic leaching depths and the resulting leached masses from the data obtained by linescans in the SEM is given below. This method also considers slowly decreasing bulk-intensities by extending the numerical integration from xstart to xe and yields therefore slightly higher values (9.8% in average) for the hypothetic leaching depths (d2,K and d2,Ca) and the leached masses (LM2,K and LM2,Ca) than the first method described. The area below the distance–intensity curve, the no leaching area and the corresponding values for d2,K, LM2,K and LM2,Ca are given by Eqs. (21)–(24). A comparison of the two calculation methods and the parameters involved is shown in Fig. 6. " # i¼n X Dx 2 AK;corr2 ¼ I K;i I K;bulk I Kðx¼xe Þ 2 i¼0 I K;back ðxe;K xstart Þ D þ ½I Kðx¼xe Þ I K;back ; 4
ð19Þ
leaching
depth
dK
ð21Þ ð22Þ
AK;corr2 ¼ xeog 1 ; AK;nl;corr2
ð23Þ
LM2;K ¼ 0:125 qglass d 2;K ;
ð24aÞ
LM2;Ca ¼ 0:179 qglass d 2;Ca :
ð24bÞ
d 2;K
As the density of the original glass (qglass = 2643 kg m3) is known, the leached masses of K or Ca, respectively
of
LMK ¼ 0:125 qglass d K ;
ð18Þ
d K ¼ xeog xtd;K ¼ ðxSi1 xK1 Þ DLK :
Fig. 5. Explanation (=xeog xtd,K).
(LMK and LMCa in mg K or Ca/m2 glass surface) are given by the Eqs. (20a) and (20b) (dK and dCa in lm). The factors 0.125 and 0.179 in these equations consider the chemical composition of the glass (12.5 wt% K and 17.9 wt% Ca). It seems to be important to note that the calculated values of the leached masses correspond to the observed leached layer. The effective amounts of leached network modifier masses may be higher due to network dissolution.
AK;nl;corr2 ¼ ðxeog xstart Þ ½I K;bulk I K;back ;
1 ¼ ðxSi2 þ xSi1 Þ DLK ðxSi1 xK1 Þ; 2
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Thus 6 independent values characterize the weathering attack on the glasses exposed: LDK, LDCa, dK, dCa, d2,K and d2,Ca. On each of the 136 glass samples between 3 and 6 linescan measurements were performed, leading to a total number of 563 linescans (corresponding to approximately 4 linescans per specimen). As the 3- and 4-year samples were exposed in duplicate, 8 linescans originating from these two samples describe the weathering state of the material exposed at a particular test site. The 5and 6-year samples were only exposed singly, which has to be taken into account when discussing the statistical significance of any results.
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Fig. 6. Comparison of the two different evaluation methods of the linescan measurements. The upper image displays the splitting of the total area AK,corr,tot into A0K;corr , AK,corr and A00K;corr . In the image below the numerical integration is started at x = xstart yielding the measure AK,corr2.
As many of the data needed for the calculation of leaching depths (such as the bulk and background intensities of the network modifier elements, the values for xK1 and xCa1 and so on) have to be read off from the distance–intensity-linescan curves visually, the possible errors have to be checked. Table 4 gives the changes in
Table 4 Effect of an error in the determination of the input parameters xSi1, xSi2, xK1, xCa1, IK,bulk, ICa,bulk, IK, back or ICa,back on the results for the hypothetic leaching depths and the leached network modifier masses Input parameter
Change of result for dK, dCa, LMK or LMCa (%)
Variable
True value
Error
xSi1 (xSi2) xK1 (xCa1) IK,bulk (ICa,bulk) IK,back (ICa,back)
2.67 (2.96) lm 1.47 (1.73) lm 355 (444) counts 10 (23) counts
±0.03 lm ±0.03 lm ±1% ±10%
63.3% 65.0% 65.5% 60.3%
A false determination of the bulk intensities for the network modifier elements K and Ca has the greatest effects and produces deviations of about 6%.
the results for the leaching depths dK and dCa and for the leached masses LMK and LMCa, if a (realistic) error of y % or y lm in measuring xSi1, xSi2, xK1, xCa1, IK,bulk, ICa,bulk, IK,back or ICa, back is made (data from the sample no. 19). The resulting relative error should therefore not exceed 6%.
3. Results Due to the huge amount of data, a complete discussion is not possible in this context. Consequently, only the results obtained for the 3-year exposure are presented in more detail and listed in Table 5. Furthermore, the questions of the time dependent progression of the leaching process for selected exposure sites using the data of all exposure periods and nuclear reaction analysis (NRA)-data of earlier investigations and of a preferential leaching of one of the two network modifier ions present in the model glass, K+ and Ca2+, are discussed.
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Table 5 Leaching data obtained by linescan measurements for the 3-year exposure Site no.
LD(K+) LD(Ca2+) [lm]
d(K+) d(Ca2+) [lm]
LM(K+) LM(Ca2+) [mg/m2]
LMðKþ Þ LMðCa2þ Þ
d2(K+) d2(Ca2+) [lm]
LM2(K+) LM2(Ca2+) [mg/m2]
LM2 ðKþ Þ LM2 ðCa2þ Þ
1
1.19 1.07 0.80 0.71 0.93 0.82 1.05 0.99 0.95 0.87 0.86 0.86 1.05 1.04 1.30 1.27 1.87 1.71 1.55 1.47 1.59 1.48 1.70 1.46 1.23 1.22 1.36 1.20 0.98 0.82 1.37 1.35 1.07 0.92 0.89 0.74 1.05 0.96 1.13 0.90 1.58 1.51 1.29 1.24 1.12 1.03 1.02 0.92
0.57 0.50 0.38 0.33 0.54 0.48 0.50 0.43 0.49 0.43 0.34 0.32 0.42 0.40 0.66 0.58 1.02 1.01 0.78 0.67 0.96 0.88 0.79 0.67 0.36 0.37 0.56 0.52 0.39 0.34 0.76 0.73 0.38 0.31 0.30 0.24 0.56 0.47 0.47 0.38 0.56 0.53 0.69 0.65 0.39 0.32 0.53 0.47
188 236 124 157 176 229 163 203 161 204 110 151 138 187 218 276 336 477 255 315 316 415 260 314 118 176 185 244 129 159 250 346 124 146 97 114 185 223 156 179 186 250 226 307 129 149 176 220
0.79
0.60 0.53 0.44 0.41 0.62 0.55 0.55 0.45 0.53 0.48 0.40 0.35 0.44 0.42 0.71 0.62 1.18 1.15 0.81 0.70 1.00 0.92 0.83 0.70 0.46 0.46 0.60 0.57 0.44 0.38 0.80 0.78 0.41 0.35 0.32 0.27 0.59 0.51 0.51 0.45 0.65 0.56 0.76 0.71 0.44 0.35 0.58 0.53
199 249 145 193 203 261 183 212 174 228 130 163 145 201 233 293 389 543 268 329 330 433 272 332 151 216 197 268 145 178 264 366 134 164 105 128 194 239 167 212 212 265 251 337 145 167 192 248
0.80
1.20 1.11
0.56 0.50
184 237
0.78
0.61 0.55
201 259
0.78
3 5 7 9 10 15 21 23 24 26 27 31 33 34 35 36 40 41 43 44 45 46 49 Mean
0.79 0.77 0.80 0.79 0.73 0.74 0.79 0.71 0.81 0.76 0.83 0.67 0.76 0.81 0.72 0.84 0.85 0.83 0.87 0.74 0.74 0.87 0.80
0.75 0.78 0.86 0.76 0.80 0.72 0.80 0.72 0.81 0.76 0.82 0.70 0.74 0.82 0.72 0.82 0.83 0.81 0.79 0.80 0.75 0.87 0.78
Data written in plain text represent the data for K+, italic values below for Ca2+.
3.1. Linescan measurements Table 5 and Fig. 7 depict the leaching characteristics measured on the cross-sections of the 3-year samples. The average leaching depths observed on those 48 samples were 1.20 and 1.11 lm for potassium and calcium, respectively, indicating a preferential leaching of the uni-
valent network modifier cation K+. As the samples were exposed duplicate in the case of the 3-year exposure, these 48 samples yield 24 data points. The average hypothetic leaching depths (d2,K and d2,Ca, respectively) were almost exactly half of the corresponding LD-values: 0.61 and 0.55 lm. The values for dK and dCa are about 10% lower. All histograms in Fig. 7 show strong
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Fig. 7. Histograms of the leaching characteristics of the three-year samples: leaching depths (LDs) of K+ (upper left) and Ca2+ (upper right), hypothetic leaching depths d2,K (middle left) and d2,Ca (middle right) and the ratio of the leached masses LM2,K/LM2,Ca. The data indicate a preferential leaching of K+.
deviations from the corresponding normal distributions. Smaller values for LD, LM, d and d2 (for both network modifier cations) occur more often than higher ones. Contrary, the ratio LM2,K/LM2,Ca is approximately nor-
mally distributed with a mean value (l) of 0.78. If the network modifier cations K+ and Ca2+ were evenly leached, this ratio would yield 0.70, which is the ratio of the two elements in the bulk glass.
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High values for the (hypothetic) leaching depth are observed especially for site 23, which is Birkenes (Norway). This site shows values of 1.87 for LDK and 1.71 for LDCa. Consequently, also the leached masses for K and Ca are above average: 316 and 415 mg alkali/m2 glass surface. These values are even more astonishing, as site 23 is one of the least polluted sites (Table 3). Only the relative humidity and the concentration of ozone show high levels, whereas there is hardly any SO2 and NO2 pollution. As will be shown later on, similar results were obtained by NRA-analysis in previous investigations. Contrary, the samples exposed at the test site no. 3 (Kopisty, Czech Republic) exhibit notably low values for all calculated leaching parameters: 124 and 157 mg K and Ca, respectively, were leached out of 1 m2 of the glass surface. As Kopisty is an industrial site, it shows very high pollution data for SO2, NO2 and O3 and also above-average values for the climatic parameters T and RH (Table 2). Two conclusions might be drawn from these observations: (a) the relative humidity and the concentration of ozone play an important role in glass weathering and (b) the degree of weathering of glass (of this composition) is not simply describable in terms of a general pollution parameter including SO2, NO2 and O3 with the same weight. 3.2. Time-dependence of the leaching process At 10 test sites the exposure was carried out for all exposure times (i.e. 3–6 years). Together with the 1and 2-year leaching data obtained by Woisetschla¨ger and Schreiner [20,21] by NRA-measurements for the same type of glass, a time series is set up. Their leaching depths (dH) are directly comparable to the dK and dCa values. Fig. 8 depicts the mean values ±1 standard deviation of the hypothetic leaching depth dK and the hydrogen penetration depth dH for each exposure time. It becomes apparent that dK does not steadily increase with time. After 4 years of exposure dK decreases from approximately 650 nm to 600 nm in average, which is out of the error range of the evaluation method. Two facts must be kept in mind: (a) In contrast to the 3and 4-year exposure, the 5- and 6-year samples were only exposed singly. Therefore, the statistical error of the calculated leaching depths is higher than in the case of all other exposures, when two or three samples were exposed under the same conditions and for the same time (b) The 5- and 6-year samples were not exposed continuously, but with breaks of 2 and 3 years, respectively. Although the samples were stored under low humidity conditions, this storage might have caused this irregular progress of the leaching process shown below. However, it can be shown that not only alkali and earth alkali diffusion, but also network dissolution must be
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Fig. 8. Plot of the mean hypothetic leaching depths for K. The results obtained from the exposures at the sites 3, 10, 15, 21, 23, 24, 26, 27 and 33 are considered in this diagram. As no 2-year dH value was available for site 7, this site was excluded. The bars mark the range l ± r.
considered in the weathering of glass under natural conditions. Fig. 9 depicts the graph of the measured hypothetic leaching depths dK and the hydrogen-penetration depths for three selected exposure sites showing different levels of pollution: no. 3 (Kopisty, highly polluted), no. 33 (Toledo, moderately polluted) and no. 23 (Birkenes, very low polluted). Remarkable is the fact that the leaching depths are contrary to this classification. The results of the test site nos. 3 and 23 show one outlier each (too high leaching data in both cases), but no error could be identified in the measurement or calculation. The experimental data can be fitted using power-curves of the form dK = a Æ tb. The corresponding coefficients of determination (R2) show values of 0.28 (site 23), 0.66 (site 3) and 0.88 (site 33). Two main conclusions must be drawn considering these data: (a) The observed hypothetic leaching depths dK and dCa cannot be directly related to pollution data. (b) The leaching depths of samples increase with time, but the rate decreases. 3.3. Congruent or selective leaching? Table 5 shows higher (hypothetic) leaching depths of potassium for nearly all samples. However, the initial question might best be answered statistically. In total, 563 linescan measurements have been performed, each yielding one value for LDK, LDCa, dK, dCa, d2,K, d2,Ca, LMK/LMCa and LM2,K/LM2,Ca. The following null
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Fig. 9. Comparison of the progression of the hypothetic leaching depths for a low polluted (site no. 23), a moderately polluted (site no. 33) and a highly polluted exposure site (no. 3). The observed leaching depths are contrary regarding the degree of pollution. The data for the one and two year exposure are taken from NRA-results in [20,21].
hypotheses must be tested (the value 0.70 originates from the mass ratio of the two network modifier elements in the bulk glass): (1) (2) (3) (4)
H0: LDK = LDCa. H0: dK = dCa. H0: d2,K = d2,Ca. LMK H0: LM ¼ 0:70. Ca
(5) H0:
LM2;K LM2;Ca
at a high statistical significance of 99.9% and a preferential leaching of potassium is proven. The bivalent ions seem to be stronger bond to the NBO-sites of the silica network and are harder to remove by the ion exchange reaction. 3.4. Surface investigations
¼ 0:70.
In all cases modifications of a t-test for the comparison of 2 means were applied: the paired t-test for testing (1)–(3) and the t-test for the comparison of a mean with a target value for (4) and (5) using the Eqs. (25) and (26) for the calculation of the test statistic q. q¼
m pffiffiffi n; s
ð25Þ
q¼
mx m0 pffiffiffi n; s0
ð26Þ
where P m is the mean of the differences Di (m ¼ 1n 563 s is the standard deviation of 1 Di ), rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi! P563 ðDi mÞ2 1 Di s ¼ , mx the mean of the original data, n1 m0 the target value, s 0 the standard deviation of the original data and n the number of observation pairs (n = 563). The number of degrees of freedom (f = n 1) is in all cases 562. Table 6 depicts the results of the statistical test. All null hypotheses can be rejected
In addition to the investigations carried out on the cross-sections of the specimens, also the glass surfaces were studied, in order to characterize the surfaces of the leached layers to the ambient atmosphere. The Fig. 10(b)–(d) depict SE-images of glass surfaces of specimens exposed under unsheltered conditions. Besides very few deposits (mainly of organic origin) no weathering products are visible. Obviously all weathering products formed during the exposure have been washed away by the rain. The leached layers of most samples are still intact (Fig. 10(b)), but on a few specimens (about 10 out of 136) it is totally cracked (Fig. 10(c) and (d)). There are two feasible explanations for the destruction of the leached layer: (a) the resulting change in the density of the outmost glass layers together with variations of temperature and humidity of the ambient atmosphere or (b) the outgassing of H2O-molecules by applying a highvacuum during the SEM investigations. For reasons of comparison also a SE-image of a sample which was exposed under sheltered conditions (Fig. 10(a)) is shown. Many weathering products, mainly syngenite (K2SO4 Æ CaSO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O), can be identified.
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Table 6 Results of the statistical t-tests Parameter
H0 LDK = LDCa
m mx m0 s s0 q tcrit (f = 500; p = 99.9) Result
dK = dCa
0.10 0.05 – – – – 0.16 0.09 – – 14.31 13.82 3.31 (2-sided), 3.11 (1-sided) LDK > LDCa dK > dCa
d2,K = d2,Ca
LMK ¼ 0:6969 LMCa
LM2;K ¼ 0:6969 LM2;Ca
0.06 – – 0.08 – 19.24
– 0.78 0.70 – 0.09 23.09
– 0.79 0.70 – 0.10 20.54
d2,K > d2,Ca
LMK > 0:70 LMCa
LM2;K > 0:70 LM2;Ca
All null hypotheses can be rejected, the (hypothetic) leaching depths and leached masses of potassium are significantly higher than those of calcium. tcrit represents the critical t-value at a statistical significance of 99.9% and 500 degrees of freedom.
Fig. 10. (a)–(d) Secondary electron images of sample surfaces exposed under sheltered (a) and unsheltered (b)–(d) conditions. (a) Many weathering products, mainly consisting of syngenite (K2SO4 Æ CaSO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O), can be observed on this sample exposed under sheltered conditions; exposure time: 4 years, test site 26 (Aspvreten), magnification: 150·. (b) Leached layer with some depositions, surface is still intact, exposure time: 5 years, test site 37 (Dorset), magnification: 100·. (c) Numerous cracks of the leached layer all over the sample surface can be observed, exposure time: 4 years, test site 21 (Oslo), magnification: 90·. (d) Many cracks and pits of the leached layer are visible, some pieces of glass have already fallen off, exposure time: 3 years, test site 14 (Casaccia), magnification: 1800·.
4. Discussion and conclusion The major results of the SEM-investigations of the surfaces and cross-sections of more than 130 glass samples of medieval composition, which have been exposed for up to 6 years at more than 20 test sites, may be summarized and interpreted as follows:
(a) Even after six years of unsheltered exposure the leached layer of most samples shows no pits or cracks. Contrary to the sheltered exposed specimens, on which various crystalline weathering products such as syngenite and gypsum were found, hardly any weathering products could be identified on the surface of the unsheltered
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samples. Mainly carbon-rich (and therefore probably organic) depositions could be observed. Cracked glass surfaces are rare exceptions and it is uncertain, if these phenomena originate from the weathering process or from the high-vacuum conditions in the SEM. (b) The semi-automatic evaluation procedure, which is based on a numerical integration of the distance–intensity data, proved to be suitable especially for such a high amount of data. The input parameters for this model are average bulk- and background-intensities of K and Ca and characteristic points of the intensity distributions. The main disadvantage of this procedure is the time-consuming and error-prone visual detection. However, as realistic uncertainties result only in minor changes of the leaching data, the use of this method seems justifiable. (c) It was shown that leached layers of up to 1.87 lm can occur on glasses of this composition during weathering periods of up to 6 years. The average leaching depths were found to be slightly above 1 lm. Considering the time dependence of the weathering process, power-functions of the general form y = a Æ tb, where t stands for time, were found to fit the experimental data in the best way. The achieved coefficients of correlation for these curve estimations are rather good (between 0.24 and 0.88). As only six data points are available for each test site, the results will have to be confirmed by even longer exposure times. The leaching depths of most samples were found to increase with time, but the rate significantly decreases in many cases after about 3 years. As also clearly decreasing thickness of the leached layers were observed on some samples, network dissolution (SE-image in Fig. 10(d)) may have also occurred and the true amount of leached network modifier masses of Ca or K may be higher. The actual position of the linescan measurement along the glass cross-section may also result in (not quantifiable) errors of the final result, as the boundary between the unweathered bulk glass and the leached layer is not constant. The results of former NRA-measurements on glass samples of the same compositions weathered for 12 and 24 months fit also very well into the actual data. (d) The monovalent network modifier ion K+ is leached out more easily than the bivalent cation Ca2+, which is probably due to the stronger bond of the bivalent Ca2+ to the non-bridging oxygen sites in the glass. The leaching depths for K+ are in average 10% higher than those of Ca2+. A detailed statistical evaluation (paired t-test) proved
the significance of the differences of the weathering data between K+ and Ca2+. This result may not be transferable to glasses of other compositions. (e) The weathering data such as the (hypothetic) leaching depths cannot be directly related to the environmental or climatic data measured at the test sites. The correlation between the pollution and weathering data has to be inspected in more detail in future works.
Acknowledgement The authors gratefully thank the United Nations Economic Commission for Europe for the possibility to participate in this project and for the financial support. Special thanks also go to all partners within this and the succeeding project (MULTI-ASSESS, contract no. EVK4-CT-2001-00044) for the friendly and fruitful cooperation.
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