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Leaching studies on naturally weathered potash-lime–silica glasses
Journal of Non-Crystalline Solids 352 (2006) 368–379 www.elsevier.com/locate/jnoncrysol
Leaching studies on naturally weathered potash-lime–silica glasses M. Melcher *, M. Schreiner Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, 1060 Vienna, Austria Received 27 May 2005; received in revised form 28 December 2005 Available online 9 March 2006
Abstract Within the international, EU-supported research project MULTI-ASSESS more than 150 glass samples with a chemical composition similar to medieval stained glasses were exposed in six European cities (Athens, Krakow, London, Prague, Rome and Riga) to the local environmental and climatic conditions. After exposure periods of 6 or 12 months the samples were investigated in the scanning electron microscope with energy dispersive X-ray microanalysis (SEM/EDX). Surface analyses indicate that the main weathering products were sulphates such as syngenite (CaSO4 Æ K2SO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O). Chlorides, organic compounds and/or carbonates or nitrates of the elements K, Ca and Na were predominantly detected on the 6-months samples, whereas Si-containing compounds appeared particularly on the 12-months samples. Linescan measurements performed on the cross-sectioned glass samples allowed for the determination of leaching depths of the network modifier ions according to an evaluation procedure recently presented. The average leaching depths after 6 months of exposure were d(K) = 0.88 ± 0.43 lm, d(Ca) = 0.62 ± 0.37 lm, d(Na) = 0.34 ± 0.32 lm and d(Mg) = 0.16 ± 0.18 lm. After 12 months an average increase of the leaching depths between 38% (K) and 63% (Mg) is observed. No leaching could be determined for the network former elements Al and P. A clear influence of the environmental conditions at the atmospheric test sites on the degree of weathering was observed. In a first approximation, the leaching depths exhibit a linear dependence on a general pollution factor comprising the concentrations of the acidifying gases SO2, NO2 and O3. 2006 Elsevier B.V. All rights reserved. PACS: 81.65.K; 68.35.B Keywords: Chemical durability; Ion exchange; Silicates; Surfaces and interfaces
1. Introduction Glasses are known to be attacked by various chemicals in a two-stage process: at first, an ion-exchange between the network modifier ions in the glass (Na+, K+, Ca2+ and Mg2+) and H+, H3O+ and/or H2O species from the solution can be observed, which is displayed for a monoand bivalent network modifier in Eqs. (1) and (2). The ions are diffusing in opposite directions, leading to the formation of a so-called leached or gel layer in the glass, which
is depleted in alkali and alkaline earth elements and enriched in hydrogen bearing species [1–5]. The ratio c(M)/c(Si) measured in the solution is significantly higher than in the bulk glass, indicating a selective (preferential) leaching of the modifiers M. Hence, the effective surface area of the glass exposed to the attacking medium is increased as micro-pores are produced by the leaching process, which may cause an accelerated rate of glass dissolution. Moreover, the pH of the solution increases, as hydroxyl ions remain in the solution [6]. ð1Þ
*
Corresponding author. Address: Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria. Tel.: +43 1 58816 203; fax: +43 1 58816 121. E-mail address: [email protected] (M. Melcher). 0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.017
ð2Þ
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In the second stage of the corrosion process, occurring predominantly in alkaline media at a pH > 9, the silicate network breaks down completely (Eq. (3)). The concentration ratio c(M)/c(Si) in the solution approximately equals that in the bulk glass, as congruent rather than selective dissolution is observed. Various authors also suggest a dehydration reaction in the leached layer according to Eq. (4), causing an increase of the water content in the outmost glass layers [6]. ð3Þ ð4Þ The amount of leached network modifier masses, often denoted as Q, are found to be proportional to the squareroot of time at the early stage of corrosion (when the selective leaching of network modifiers is observed), which is typical for diffusion controlled processes. As the reaction proceeds, the dissolution of the pffi silicate network prevails and the kinetics change from a t- to a linear t-dependence (i.e. the reaction rate remains constant with time) [7–10]. Less data are available for the weathering of glass, which is often defined as the ‘degradation of a glass surface due to interaction with the atmosphere’ [11–16]. It is assumed that similar reactions occur like in the case of glass corrosion. A water film may form on the surface of the glass by rain, fog or condensation of air moisture, in which gases from the ambient atmosphere such as CO2, SO2 or NOx can be absorbed. Consequently, the pH is decreased, favouring the ion-exchange according to Eqs. (1) and (2) and hence the deterioration of the glass. When the temperature increases and the water film evaporates (or the solubility products are exceeded), crystalline weathering products such as sulphates, carbonates, hydro-carbonates, chlorides or nitrates of the network modifiers are left behind. Compared to modern or ancient glasses, medieval glasses produced north of the alps exhibit two main differences: the use of potassium instead of sodium as a flux and generally a higher content of network modifiers and hence a lower silica concentration [17]. A large collection of results of chemical analyses of glasses of different ancient and medieval periods can be found in [18]. Fig. 1 shows 559 of these results (only the lead glasses and those with unusual high contents of certain elements, corresponding to Bezborodov’s groups XVIII to XXIV are omitted) after statistical treatment by principal component analysis (PCA). Although only 40.6% of the initial variance of the data is reproduced by the first two principal components (PCs), a satisfactory separation is achieved. Ancient glasses, displayed with squares, cluster on the right of the plot, indicating relative high concentrations of Na2O and SiO2 and/or low concentrations of K2O and CaO. Furthermore it can be seen that the medieval glasses (circles) appear in two – not separable – clusters. Glasses high in K2O and CaO are arranged on the far left,
Fig. 1. Score–score plot of the first two PCs. PC1 mainly covers the concentrations of the compounds SiO2, Na2O (positive correlation), K2O and CaO (negative correlation), PC2 contains Mn2O3, CuO and SO3 (positive), Al2O3, Fe2O3 and TiO2 (negative).
while the second medieval group significantly overlaps with the ancient glasses. On the basis of these data, it can be concluded that ancient glasses are of the soda-lime–silica (SLS) type, while medieval glasses may also contain significant amounts of potassium and belong to the potash-lime–silica (PLS) glass type. Although both SLS- and PLS-glasses are subjected to the same weathering mechanisms in principle, SLS-glasses proved to be much more durable. Schreiner et al. [19,20] measured the concentration profiles of leached glass constituents as well as hydrogen in medieval PLS-glasses and found leaching depths in the lm-range. Melcher and Schreiner [21] investigated PLS model glasses after periods of 3–6 years of natural weathering in the ambient atmosphere of low, moderately and highly polluted sites. Different leaching depths of approximately 1.2 and 1.1 lm were found for K and Ca, respectively. On the other hand, Chabas and Lefe`vre [22] examined samples of durable soda-lime–silica float-glass, which were exposed within a field experiment to the polluted atmosphere in the center of Paris for a period of two years. The authors report dealkalization phenomena for Na and Ca only in the first 40 nm of the samples, irrespective of the exposure mode (sheltered or unsheltered). These examples illustrate the much higher vulnerability of PLS-glasses when exposed to urban or industrial atmospheres. The present exposure was carried out in the framework of the MULTI-ASSESS project [23], which main objectives are qualitative and quantitative investigations of the weathering attack on various materials such as stone, steel, zinc, copper, bronze as well as modern and medieval glass. Special attention is paid to the relative new multi-pollutant situation, which is dominated by elevated levels of nitrogen compounds (NOx, HNO3, etc.), ozone and particulate matter caused by the increased car traffic and by decreasing sulphur dioxide levels, which was the main pollutant throughout the last decades.
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2. Experimental 2.1. Sample preparation As the exposure times were only 6 and 12 months, a sensitive glass with high concentrations of the network modifier elements was chosen (Table 1) in order to achieve measurable weathering effects. The glasses were prepared at the Fraunhofer-Institut fuer Silicatforschung in Wuerzburg (Germany) by mixing oxides and carbonates in certain stoichiometric quantities, melting in a Pt crucible (approximately 1720 K), refining at a temperature of 720 K for 5 h and cooling. Subsequently the frits were ground and melted a second time in order to achieve homogeneity. Small plates (10 · 10 · 2 mm3) were cut from these glass ingots with a low-speed diamond saw (Buehler Isomet II-1180), embedded in epoxy resin and polished using SiC paper of 500–4000 mesh. In the last polishing step isopropanol instead of water was used as lubricant in order to decrease the leaching of glass components already during the polishing process. The polished glass samples were mounted on glass fibre reinforced plastic plates (sized approximately 5 · 5 cm2) with approximately 10 cm pieces of shielded wire and covered with silicon caps and adhesive tapes for protection during shipping. Prior to the investigations in the SEM, 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. After the surface investigations of the weathered glass specimens, the samples were covered with a thin layer (approximately 3 mm) of epoxy resin and cut with a low-speed diamond saw perpendicular to their surfaces in order to produce cross-sections of the specimens, which were also investigated in the SEM. In order to avoid charging during the SEM-investigations, the surfaces were sputtered with a thin layer of carbon. For all measurements an acceleration voltage of 15 kV and a beam current of approximately 1 nA were used. Linescans were performed with in average 400 measurement points and a length of 9 lm. A JEOL JSM-6400 electron microscope with the energy dispersive system EDAX Phoenix, equipped with an ultra-thin window detector allowing also the detection of low-Z elements such as C and O, was used. 2.2. Site description The exposure was carried out in six European cities: Athens, Krakow, London, Prague, Rome and Riga. In each of the first five cities, a so-called main and up to six Table 1 Chemical composition (in wt%) of the model glasses exposed within the MULTI-ASSESS project for periods of 6 and 12 months Glass
SiO2
K2O
CaO
P2O5
Na2O
MgO
Al2O3
M1
48.0
25.5
15.0
4.0
3.0
3.0
1.5
smaller sub-racks were available for exposing the glass samples. In Riga, there were two main racks in different parts of the city. All exposures were performed under sheltered conditions, i.e. the samples were protected from rain, direct sun-radiation and for the most part also from wind. In Athens (sites T60 to T66) the main and sub-racks were installed on the building of the ministry of health, which is located at the inner circle of the city. The surrounding area can be classified as urban and residential. The racks were mounted up the building on the ground, first, second, fourth and fifth floor as well as on the roof of the seven-floor building. Environmental and climatic data were reported from the main rack on the roof. Hence, the influence of the vertical position of a sample on its weathering could be investigated. The exposure sites in Krakow were located in different distances from local sources and regional large industry sources: site 70 in a residential area with a park, sites 71– 73 (near congested roads), site 74 (main square of the city without traffic), site 75 (close to the main rack 70) and site 76 (in a residential area with streets and park). The target sites in London were located in different distances from traffic pollution. The main site 80 has the same pollution and environmental level as sub-site 81 (both racks are placed on a roof at the campus of Middlesex University). The sub-racks 82, 83 and 85 were located in a residential area with light industry. It is, more or less, equivalent to an urban background site. Site 84 was near a major road with a large municipal incinerator close by it, site 86 was placed near a busy road (80 000 vehicles per day). The target sites in Prague were designed to study the change in the environmental impact around a building (National Museum at the Wenceslav square). The surrounding area is commercial and residential. A heavy influence by transport pollution sources (highway and crossing with 97 000 vehicles per day) could be expected. The main site 90 and the sub-site 91 were located close to each other (top floor under the cupola of the building in direction to Wenceslav square). The sub-sites 92 (top floor, opposite to Wenceslav square), 93 (balcony on the 1st floor in direction to Wenceslav square), 94 (balcony on the 1st floor in direction right to Wenceslav square) and 95 (balcony on the 1st floor in direction left to Wenceslav square) were located on different parts outside the building, site 96 was in the yard of the National Museum on ground level. The environmental parameters were measured at the sites 90 and 96. The 6 target sites (main rack and 5 sub-racks) in Rome were located in areas with moderate traffic influence. The main rack (100) and the two sub-sites 101 and 102 were situated at Montelibretti in the outskirt of Rome (30 km). The surrounding area there is semirural. All of those sites have the same environmental data. Sites 103 and 104 were located on a building (National Health Institute in a central area of Rome with intense traffic of both cars and public transport busses), and also have nearly the same environmental data. Sub-site 105 was in a park in the north eastern area of Rome, inside a background monitoring
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station. Close to it there is an important state road with a high volume of traffic. The two Riga sites were located within a fairly short distance with comparable climatic and precipitation data but different concentrations of gases such as SO2, NOx, etc. Table 2 lists the average environmental and climatic data (temperature T, relative humidity RH and the concentrations of the acidifying gases SO2, NO2 and O3) of the main exposure sites for the 6- and 12-months exposure periods. All data were collected and reported to the Norwegian Institute for Air Research (NILU). The 6-months exposure was performed from November 2002 to April 2003, the 12-months period continued until November 2003. During that period in Athens the highest SO2- and NO2-concentrations were measured, whereas the ozone concentration showed high values in Krakow, Rome and Riga. Besides Athens (RH around 60%), the relative humidities at all other exposure sites are comparable (about 75%), Athens, Rome and London had the highest average temperatures, whereas for Krakow, Prague and Riga similar temperature means between 4 and 6 C are reported. It must be noted, that the exposure sites chosen within this project are by no means typical for the whole city. Unfortunately, many environmental data are missing for the London test sites.
The evaluation procedure is mainly based on a numerical integration of these linescans and described in detail in a previous work [21]. Hence, two measures can be obtained by these calculations: the leaching depths LD(Y) and the hypothetic leaching depths d(Y) for an element Y (Y = K, Ca, Na, Mg, Al and P), both explained in Figs. 2 and 3, where typical linescan profiles (background corrected intensities versus distance from starting point xstart) of the leached layer are shown. The leaching depth LD of an element Y is the distance (in microns) from the beginning of the decrease of the intensity at xY to the end of the glass xeog (Eq. (5)). For the calculation of the hypothetic leaching depth d(Y) a completely leached glass surface layer is assumed. The (hypothetic) boundary between the unleached and totally leached glass layer (xtd for total depletion) is calculated by area compensation (Eq. (6), Fig. 3). While LD can be considered as the maximum depth at which ion exchange has occurred, the advantage of d is its higher robustness. The determination of xY is performed visually for each linescan, as statistical methods (such as confidence intervals around the bulk intensity of the network modifiers) did not yield satisfying results. Moreover, d(Y) can easily be converted into leached masses (LM) in mg leached alkalis per m2 glass surface (Eq. (7) for Y = K, the density of the glass is qglass = 2618 kg/m3).
2.3. Evaluation method and error estimation
LDðYÞ ¼ xeog xY ;
ð5Þ
In order to quantify the weathering effects on the specimens, linescan measurements were performed on the crosssectioned samples. These measurements were only carried out in domains, where no weathering products, which might have formed on the glass surface, could be observed.
Table 2 Average environmental and climatic data at the main exposure sites City
Site
T (C)
RH (%)
c(SO2) (lg/m3)
c(NO2) (lg/m3)
c(O3) (lg/m3)
Athens
T60
19.0 20.1
57.6 59.9
42.5 43.2
89.7 77.2
23.0 19.7
Krakow
T70
4.3 7.9
74.0 74.7
22.9 19.9
30.9 30.4
57.4 60.2
London
T80
9.8 12.0
–
–
–
–
Prague
T90
6.4 8.6
72.4 72.5
9.8 8.7
54.2 49.5
32.9 33.2
Rome
T100
11.0 15.8
76.3 72.1
1.2 1.3
16.5 14.4
45.4 52.8
Riga I
T110
4.47 6.63
74.2 80.2
6.7 5.8
29.2 27.0
70.2 59.4
Riga II
T111
4.17 6.30
76.7 82.6
8.7 6.8
19.2 19.0
65.5 53.6
In all columns the first value indicates the 6-months mean, the second the 12-months average. Data are calculated from [24], where also precipitation data such as the amount, the pH and ionic concentrations of the rain are listed.
Fig. 2. Scheme of a typical linescan profile depicting the plot of the background corrected X-ray intensities of element Y (IY,corr) versus the distance x (in lm) and the measure LD(Y).
Fig. 3. The hypothetic leaching depth d(Y) calculated by area compensation (AI = AII).
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dðYÞ ¼ xeog xtd;Y ;
ð6Þ
LMðKÞ ¼ 0:212 qglass dðKÞ.
ð7Þ
As described in [21], the inaccurate determination of the background intensities IY,back and of the points xY are the main sources of error causing deviations from the ‘true value’ (d(K) or LD(K)) of approximately 5%. Repeated measurements on the same position on the same cross-section produce errors of the same size and were <10% in all cases. Hence, the results of d(Y) and LD(Y), which will be discussed in the next chapters, should not be affected by errors larger than 10%. As no other analytical method was available in order to estimate the systematic errors of these measurements, only a plausibility consideration of the results can be done by comparing the calculated leaching depths and the mass of weathering products formed on the glass surface. Under the simplified assumption that all WPs consist of syngenite and if the average height of the weathering crust (h) and the percentage of surface coverage with weathering products (c) were known, the hypothetic leaching depth d would be given by Eq. (8) d¼
A h c qsyn xK;syn ¼ 1:11 h c; A qglass xK;glass
Irrespective of their chemical composition, the WPs occur in different morphologies: shapeless, flat, cubic, needle-shaped, round, plate-like, elliptic or hexagonal. Only in the case of chlorides (cubic and hexagonal) and K-organic (round) WPs preferred crystal-forms could be determined. The size of the WPs is typically in the range of 10–20 lm in diameter (Fig. 4(a)), sometimes up to 100 lm, only chlorides and organic K-compounds are significantly smaller (1–5 lm, Fig. 4(c)). The WPs can appear as single crystals or in large agglomerations, forming weathering crusts (Fig. 4(b)), which can cover large parts of the glass surface. Weathering crusts were mainly found on samples exposed in Krakow, Athens and Prague. The leached layer seems to be intact in most cases, cracks and rough surface regions (Fig. 4(d)) were only observed in about 20% of all samples (especially on those, which were exposed in Krakow). These cracks may be due to the weathering process itself or due to the vacuum conditions in the SEM (approximately 108 bar), which may also cause the evaporation of the water in the leached layer and the cracking of the glass surface by this dehydration. 3.2. Leaching measurements
ð8Þ
where A (m2) is the exposed surface area of the glass, qsyn and qglass stand for the densities of syngenite (2580 kg/m3) and the glass (2618 kg/m3) and xK,syn and xK,glass for the mass fraction of K in syngenite and in the original glass. h and c cannot be determined directly, but can be estimated on the basis of SEM-images of the weathered glass surfaces. Hence, the average height of the WPs is probably between 3 and 10 lm, whereas c lies between 0.1 and 0.4, resulting in dmin = 0.3 and dmax = 4.4 lm, which is in very good agreement with the actually measured leaching depths for K (between 0.2 and 4.0 lm for single measurements and between 0.3 and 2.6 lm for sample means). 3. Results 3.1. Surface investigations The main weathering products (WPs) on the surfaces of the specimens were syngenite (CaSO4 Æ K2SO4 Æ H2O), which could be detected on nearly all samples, and gypsum (CaSO4 Æ 2H2O). Chlorides of Na and K, organic Kand Ca-compounds and/or (hydrogen)carbonates and/or nitrates could be observed predominantly on the 6-months samples, which suggests a conversion of these compounds into sulphates during longer exposure times. Si-containing crystals appear more frequently on the 12-months specimens, which is probably due to the breakdown of the silicate network. The chemical characterization of the WPs is based on energy-dispersive point- or area-measurements of the crystals. Generally, no correlation between the environmental or climatic conditions at the individual test sites and the chemical composition of the WPs could be determined.
On each glass cross-section about three to four linescans were performed on different positions of the leached layer (LL). Hence, the uniformity of the thickness of the LL for a single sample can be assessed by a modified coefficient of variation (CV 0 , Eqs. (9a) and (9b)), which is normally defined as the ratio of the standard deviation r and the mean l for n measurements. As only between two and five results of measurements of LD and d are available for each sample, half of the range R (RLD = LDmax LDmin, Rd = dmax dmin) is taken rather than the standard deviation r. Fig. 5(a) and (b) shows the distribution of these CV 0 -values of the leaching depth LD and the hypothetic leaching depth d, both for the element K. The histograms for both measures look similar and suggest a relative uniform thickness of the LL on the majority of the samples, as CV 0 < 0.3 in most cases. For instance, the three measurements performed on sample no. 195 (exposed at the test site T82, t = 6 months) yield the results LD(K) = 1.91, 1.93 and 1.99 lm corresponding to a CV 0 as low as 0.02, whereas for the sample no. 165 (site T63, t = 12 months) LD(K) = 1.27, 1.96, 2.06 and 3.75 lm (CV 0 = 0.55) were obtained CV0 ½LDðKÞ ¼
0:5 RLDðKÞ
LDðKÞ 0:5 RdðKÞ CV0 ½dðKÞ ¼ . dðKÞ
;
ð9aÞ ð9bÞ
Although LD and d are determined independently for the most part, they are of course correlated to a high extent, as both represent some kind of leaching depths (if the shape of the diffusion profiles were the same for all measurements and if xY could be determined precisely in all cases, there
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Fig. 4. SE-images of weathering products (WPs) and sample surfaces: (a) elliptically shaped WPs, length some ten microns, consisting mainly of syngenite, (b) weathering crust covering large parts of the specimen surface, (c) extraordinary small (<5 lm) and round WPs, consisting of K and C, therefore probably organic or carbonates or hydrogencarbonates and (d) cracked leached layer and some detached parts of glass.
Fig. 5. Histograms of the modified coefficients of variation CV 0 for the leaching depths LD(K) and d(K) for all 501 linescan measurements performed on 155 samples. Besides a few outliers, the data indicate a relative narrow distribution of the results for each sample.
would be a perfect correlation of d and LD). Consequently, one of these measures can be estimated by the other with a sufficient accuracy (Eqs. (10a)–(10d)) and only one of them has to be discussed. At least 86%, mostly above 90% of the
variance in the data is explained by these regression equations before the elimination of potential outliers. The hypothetic leaching depths d are chosen for further discussion, as they are also capable of considering different shapes or
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curvatures of the linescan curves and are directly proportional to the leached masses of the corresponding elements dðKÞ ¼ 0:62 LDðKÞ 0:09 dðCaÞ ¼ 0:65 LDðCaÞ 0:13
R2 ¼ 0:86;
ð10aÞ
2
R ¼ 0:92;
ð10bÞ
2
dðMgÞ ¼ 0:36 LDðMgÞ þ 0:05 R ¼ 0:90; dðNaÞ ¼ 0:47 LDðNaÞ þ 0:03
ð10cÞ
2
R ¼ 0:91.
ð10dÞ
In general, different (hypothetic) leaching depths were measured for different elements. The selective leaching of certain elements can be assessed by comparing the ratio of the leached masses LM (or the hypothetic leaching depths d) for two elements A and B (LM(A)/LM(B)) to their ratio in the bulk glass. As d(K), LM(K) and LD(K) can be determined with the highest accuracy due to the high potassium concentrations in the original glass and the resulting high X-ray intensities, potassium leaching data were taken as reference points LM(B). Fig. 6 displays the histogram of the leached mass ratio LM(Ca)/LM(K) of all 501 measurements. The majority of the data can be found in the interval between 0.2 and 0.5 with a mean of 0.37 and a standard deviation of 0.11. As the ratio LM(Ca)/LM(K) in the original glass is 0.51, a selective leaching of potassium compared to calcium can be determined in 94.4% (473 out of 501) of all samples. The data in Table 3 prove that potassium is leached preferentially compared to all other elements. Furthermore it becomes clear that calcium is leached preferentially compared to the other bivalent net-
Fig. 6. Histogram of the leached mass ratios LM(Ca)/LM(K) for the two main network modifiers in the glass. Most values are between 0.2 and 0.5, clearly indicating a preferential leaching of K.
work modifier magnesium. The low values for the P- and Al-ratios suggest that these two elements are not leached, which was expected due to their role as network formers. The histograms of their hypothetic leaching depths d(P) and d(Al) are shown in Fig. 7. The agreement of the results for the leaching depths obtained for two samples A and B (dA(K) and dB(K)), each calculated as the mean of nA or nB linescan measurements, can be assessed with the ratio r given in Eq. (11). It can be interpreted as the ratio of half of the difference between the (hypothetic) leaching depths of the two samples, normalized with the weighted mean. It has a similar meaning as CV 0 and can be used to compare the results for samples, which were exposed under the same climatic and environmental conditions for the same period. If specimens were exposed in triplicate, the enumerator should contain the range of all three results and the denominator the weighted mean of those three samples. rdðKÞ ¼
1 nA þnB
0:5 jd A ðKÞ d B ðKÞj . ðnA d A ðKÞ þ nB d B ðKÞÞ
ð11Þ
The distribution of the ratio rd(K) for all duplicate and triplicate exposures is shown in Fig. 8. Most pairs/triples of samples exhibit r-values below 0.16. Thus the normalized difference between such duplicate or triplicate samples is less than 16% of the weighted mean in 74% of all cases, the arithmetic mean of these ratios is 0.127, the minimum value is achieved for the sample pair at site T91 with d(K) = 1.65 and 1.63 lm corresponding to a ratio r = 0.006, the maximum relative difference is calculated for site T83 (d(K) = 0.71 and 1.52 lm, r = 0.37). A compilation of all results of the 12-months exposure for d(K) is presented in Table 4 and Fig. 9. The samples exposed in Krakow exhibit the highest hypothetic leaching depths of up to 2.16 lm at site T73 and 2.10 lm at site T74. While site T73 is situated near a congested road and high leaching depths are expected, site T74 is located at the main square of the city without traffic (lower NO2-pollution) and the result is somehow astonishing. The other sub-sites in Krakow also show relative high d(K)-values between 1.53 and 1.83 lm. On the other hand, very low levels of glass deterioration were measured for the samples exposed in Rome. Four of the six sites (T100–T102 and T105) can be classified as ‘background sites’ with low pollution levels. The samples exposed at the sites T103 and T104 (central area of Rome with intense traffic) exhibit much higher d(K)-values.
Table 3 Leached mass ratios for n = 501 linescan measurements
Measured ratio (mean ± SD) Ratio in bulk glass Selective leaching of K in . . . cases
LMðCaÞ LMðKÞ
LMðMgÞ LMðKÞ
LMðNaÞ LMðKÞ
LMðPÞ LMðKÞ
LMðAlÞ LMðKÞ
LMðMgÞ LMðCaÞ
0.37 ± 0.11 0.51 94.4%
0.02 ± 0.02 0.09 99.6%
0.04 ± 0.03 0.11 98.0%
0.004 ± 0.010 0.08 100%
0.001 ± 0.002 0.04 100%
0.05 ± 0.01 0.17 99.0%
Potassium is leached preferentially compared to all other elements. The last row indicates that this selective K-leaching is observed for at least 94.4% of all linescan measurements.
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Fig. 7. Histogram of the hypothetic leaching depths d(P) and d(Al). As by far most measurements exhibit values below 0.1 (P) or even below 0.05 (Al), the leaching of these elements can be neglected.
Fig. 8. Distribution of the rd(K) – ratio measuring the agreement of results obtained for d(K) of duplicate or triplicate samples exposed under the same conditions.
At the sites in Athens two clearly different levels of glass deterioration were determined. Samples exposed at the sites T60 and T64, which are close to each other on the roof of the building, show about 50% higher hypothetic leaching depths than all other samples, which were exposed on a lower height level on the fac¸ade of the building. Similar results were also obtained for the exposure in Prague. The samples exposed at the sites T90, T91 and T92, which are all located on the top floor of the National Museum under the cupola, show d(K)-values of 1.45 lm in average, while for samples at the sites T93, T94, T95 (all exposed on the first floor level) and T96 (ground level in the yard) only 0.85 lm were measured. The orientation of the exposure rack and hence of the samples also has an influence on the degree of weathering, but a much smaller one than the exposure height.
Table 4 Hypothetical leaching depths d(K) and d(Ca) in microns Test site
Athens T60
Krakow T70
London T80
Prague T90
Rome T100
Riga T110
Riga T111
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
Main rack
0.98 1.55
0.72 1.29
1.45 1.83
1.04 1.44
0.87 1.23
0.63 0.97
0.79 1.40
0.62 1.17
0.36 0.51
0.21 0.33
1.06 1.50
0.67 0.97
0.89 0.84
0.51 0.52
Al 1
0.41 0.90
0.29 0.55
1.66 1.70
1.33 1.60
1.10 1.57
0.64 1.10
0.96 1.64
0.65 1.33
0.43 0.56
0.28 0.39
–
–
–
–
Al 2
0.46 1.01
0.38 0.83
1.41 1.73
0.92 1.53
0.97 1.12
0.54 0.76
0.98 1.27
0.77 0.85
0.56 0.39
0.29 0.29
–
–
–
–
Al 3
0.46 1.01
0.30 0.78
1.83 2.16
1.54 1.68
0.71 1.05
0.40 0.69
0.49 0.74
0.36 0.56
0.77 0.80
0.49 0.58
–
–
–
–
Al 4
0.80 1.57
0.46 1.31
1.74 2.10
1.51 1.80
0.68 0.99
0.47 0.60
0.64 0.98
0.34 0.78
0.63 1.09
0.45 0.81
–
–
–
–
Al 5
0.33 0.85
0.21 0.69
1.68 1.52
1.32 1.27
1.03 1.10
0.60 0.80
0.64 0.73
0.45 0.62
0.46 0.45
0.26 0.30
–
–
–
–
Al 6
0.56 1.14
0.32 0.88
1.13 1.65
0.99 1.30
1.27 1.57
0.87 1.27
0.65 0.96
0.42 0.75
–
–
–
–
–
–
All values displayed are average values of the duplicate or triplicate exposure of glass samples. The upper value in each cell is the 6-months value, the lower the 12-months value. Uncertainties due to random errors are 610%.
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Fig. 9. Leaching depths d(K) after 12 months of exposure. Samples exposed in Krakow exhibit the most weathered surface layer, Rome the least. The first column for each city represents d(K) of samples exposed at the main rack, the 2nd column of the first sub-rack, and so on. There are only five sub-sites in Rome and only main racks at the two Latvian sites.
In London two sites (T81 at the university campus and T86 near a congested road) show outstanding high values of depletion depths of potassium (d(K)): both 1.57 lm compared to 1.10 lm in average for the other sites. Surprising is the fact that samples of the sites T80 and T81, which are very close to each other, show very different results (1.23 compared to 1.57 lm). The two exposure sites in Riga also exhibit very different levels of weathering (d(K) = 1.50 and 0.84 lm). Fig. 10 displays the change of d(K) with the exposure time (6–12 months) in percent. Generally, an increase of the thickness of the leached layer can be determined in all but four cases: sites T75, T83, T105 and T111, which all show a decrease between 5% and 9%. Fig. 11 depicts the bivariate scatter plots of the hypothetic leaching depths d(K) after 12 months of exposure
Fig. 10. Change of d(K) with time (in percent). All but the Athens samples exhibit similar growth rates of up to 70%. Samples exposed in Athens show a doubling of d(K) in average after 12 months of exposure.
(denoted as d(K)12) versus T, RH, c(SO2), c(NO2) and c(O3). Obviously the temperature has no linear influence on d(K), more a parabolic one with a minimum somewhere between 13 and 17 C. On the one hand, a positive correlation between the leaching depth (or leaching rate) and T is expected, but too high temperatures might cause an advanced evaporation of the water film on the glass surface on the other hand. Probably a combination of these two effects produces this parabolic temperature-dependence. Also the relative humidity and the concentration of O3 do not show a (linear) correlation with d(K), not even a trend can be figured out. Contrary to these results an increasing concentration of SO2 and NO2 causes an increased leaching depth d(K), even though the relationship does not seem to be linear. A power-relationship of the general form d(K) = a Æ c(SO2)b or d(K) = a Æ c(NO2)b exhibits satisfying fits of R2 = 0.68 and 0.38, respectively. 4. Discussion and conclusion The surface analyses of more than 150 weathered glass samples of the potash-lime–silica type in the scanning electron microscope showed that the prevailing weathering products were sulphates such as syngenite (K2SO4 Æ CaSO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O), chlorides (NaCl and KCl) and organic K- and Ca-containing compounds. Especially after the longer weathering period (12 months), Si-containing WPs occurred more frequently. The surfaces of those samples, which were exposed in more polluted regions (e.g. Krakow), exhibited definitely more WPs and thicker and larger weathering crusts than those specimens, which were exposed in so-called background areas (e.g. most of the Roman sites) with a lower pollutant concentration. Linescan measurements and application of the previously introduced evaluation method yield two measures for the thickness of the leached layer: LD and d. At first, multiple measurements at different positions of the leached layer of a single sample show a relative uniform thickness of the leached layer (Fig. 5). Assuming a typical hypothetic leaching depth d(K) of 1 lm, measurements on different positions on the leached layer are likely to fall in the interval [0.8; 1.2]. Secondly, a comparison of the results obtained for samples exposed duplicate or triplicate under the same conditions exhibits a good agreement in most cases (Fig. 8), even though larger deviations are also observed in approximately 10% of all cases. So far no satisfactory explanation could be found for that phenomenon. The observed selective leaching of the main network modifier cation K+ can probably be explained with differences in the size of the cations present in the glass and hence in their mobilities. According to Zachariasen’s model of the structure of silicate glasses [25], bivalent network modifiers such as Ca2+ or Mg2+ are bond to two nonbridging oxygen (NBO)-sites and are stronger retained in the glass compared to the monovalent ions K+ or Na+.
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Fig. 11. Scatter plots of d(K) versus the climatic parameters T and RH and the pollution parameters c(SO2), c(NO2) and c(O3). Particularly the influence of c(SO2) and c(NO2) on the leaching depth and hence on the glass degradation is visible.
Alkaline earth oxides are known to decrease the electrical conductivities as well as the diffusivities of alkali ions, which is often explained by their blocking effect: the immobile bivalent ions occupy interstices, which are then no longer available for alkali ion migration [26]. The preferential leaching of potassium was statistically demonstrated in an previous work [21] for another potash-lime–silica glass (composition M3: 60 wt% SiO2, 25CaO and 15K2O). The amount of K leached was approximately 10% higher than expected for congruent leaching. The selective leaching is
much more evident for this glass (M1). The hypothetic leaching depths for potassium are about 35% higher compared to those of Ca, 5 times higher than those of Mg and 2.5 times those of Na. El-Shamy et al. [27] investigated the durability of soda-lime–silica glasses in acid solutions and found that the amount of Ca extracted strongly depends on the lime content of the glass. Above a concentration of about 10–15 mol% lime, the sodium and calcium were dissolved in the same proportion in which they are present in the bulk glass. A similar trend can be observed
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for the weathering of the potash-lime–silica glasses M1 (17.8 mol% CaO) and M3 (27.8 mol% CaO). A preferential leaching of potassium compared to sodium cannot be explained in terms of this simple model. Regarding the size of these two cations, a selective Na-leaching would be more conclusive due to its higher mobility. A satisfactory answer to this phenomenon has not been found so far. Moreover, due to the low concentration of Na (2.2 wt%), Mg (1.8%), P (1.7%) and Al (0.8%) in glass M1, much lower intensities (about one order of magnitude) were detected for the characteristic X-ray lines of these elements, leading to a significantly higher error for their leaching depths. Generally, high hypothetic leaching depths of K imply high values for the corresponding d(Ca)-values, which is shown by the high coefficient of determination of the regression function (Eq. (12)) for n = 501 measurements dðCaÞ ¼ 0:828 dðKÞ 0:091
R2 ¼ 0:87.
ð12Þ
Regarding the deterioration of medieval glasses of this composition, d(K) and/or LD(K) seem to be the most appropriate measures, as the highest (hypothetic) leaching depths are measured for potassium. As expected, the (hypothetic) leaching depths increase with time (average increase of d(K): ca. 38%). Nevertheless, in four cases slightly decreasing values were observed. There are two possible explanations for this astonishing result: (a) the choice of the measuring positions on the crosssections: as discussed earlier, the thickness of the LL over the whole glass cross-section is not totally uniform and small inhomogeneities are possible, or (b) by the prevailing network dissolution compared to the ion-exchange reaction, which might cause a reduction of the thickness of the LL. The first explanation seems to be much more feasible, and no reason for (b) can be found. The highest rate of growth is measured for the samples exposed in Athens (Fig. 10): in average the 12-months value of d(K) is more than twice as high as the 6-months value. For the sites T60 and T65 an increase of 58% and 162%, respectively, was determined. All other sites showed much lower values. Compared to Krakow, London, Rome and Riga, the growth rates for the Prague samples are relative high (between 15% and 76%, in average 49%). The strong increase of the hypothetic leaching depths of the Athens samples must be due to a climatic or environmental particularity at this site, which does not occur at all other sites (or at least to a much lower extent). Furthermore, this effect should only be present during the second exposure period of six months. As can be seen from Table 2, the temperature and the SO2-concentration remain quite constant, while both c(NO2) and c(O3) even decrease by about 14% during the second exposure interval. Hence, the strong increase in d(K) can only be explained by the increasing
relative humidity. Munier et al. [28] exposed low-durability potash-lime and soda–potash glasses to the polluted urban atmosphere and found a remarkable dependence of the formation of sulphate on the glass surfaces from the relative humidity. Below about 65% relative humidity the formation of sulphates occurred to a much lower extent. Their explanation for this phenomenon was that above 65% RH the water film on the glass surface behaves like an electrolyte and has a sufficient thickness for the acidifying gases to dissolve. In the present case, the exposure sites in Athens are the only ones to show RH-values below the 65% threshold. Even in the more humid period, this threshold is not exceeded, but RH = 59.9% is rather close to it. Unfortunately, more detailed interpretations are not possible, as the environmental and climatic data of the exposure site in Athens were only measured on the roof of the building (corresponding to the sites T60 and T64), and not at each sub-site. From the graphs in Fig. 11 (bivariate scatter plot involving d(K) after 12 months of exposure and T, RH, c(SO2), c(NO2) and c(O3)) a dependence of the degree of weathering on the environmental and climatic parameters can be assumed. If c(SO2), c(NO2) and c(O3) are mathematically combined to some kind of ‘general pollution’ (GP) using Eq. (13), the influence of the pollution parameters becomes even more clearer. GP ¼
cðSO2 Þ cðSO2 Þ cðNO2 Þ cðNO2 Þ þ rðSO2 Þ rðNO2 Þ cðO3 Þ cðO3 Þ ; þ rðO3 Þ
ð13Þ
where cðX Þ stands for the average gas concentration and r(X) for the standard deviation of the distribution of the values of X. A scatter plot of d(K)12 versus this measure (for all 39 specimens, for which all three gas concentrations were available) exhibits a clear and perhaps linear correlation between d(K) and GP (corresponding equation: d(K) = 1.25 Æ GP + 0.25, R2 = 0.58). Even if this relationship has no relevance in an absolute sense because of the scaling procedure in Eq. (13), the correlation between the degree of weathering d(K) and the environmental pollution is significant. Another possibility for the interpretation of Fig. 12 could be that only a low weathering of the glass samples takes place below GP 1.5, which is the case for the sites 100, 101, 102 and 105 (Rome). This degree of weathering could be classified as ‘background weathering’. Above this critical GP-value of 1.5, the weathering increases strongly and in a non-linear way. Further calculations will have to follow. The present work is an attempt to quantify the weathering attack of polluted urban atmospheres on the surface of low-durability potash-lime–silica glasses. The application of a previously introduced evaluation method based on linescan measurements lead to reasonable results, as thorough statistical investigations showed. The correlation between the environmental pollution at a specific test site
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Fig. 12. Scatter plot of the hypothetic leaching depth d(K) after 12 months of weathering versus the ‘general pollution’ parameter defined by Eq. (13).
and the observed and measured weathering attack on the glasses becomes obvious due to the introduction of a simple ‘general pollution’ parameter GP. In a first approximation this dependence is approximately linear. Future work will therefore focus on the calculation of dose–response functions (DRFs), relating the weathering data to the individual environmental and climatic parameters. The resulting equations could for example be very helpful for conservators or restorers for choosing optimal conditions for the storage of medieval glass objects of our cultural heritage. In addition, these DRFs could also be used for assessing threshold levels for pollutants in order to keep the risk for cultural assets by environmental pollution as low as possible. Acknowledgments The authors gratefully thank all partners within the MULTI-ASSESS project (contract number EVK4-CT2001-00044) for the possibility of participating and for the friendly and fruitful cooperation. References [1] R.H. Doremus, J. Non-Cryst. Solids 19 (1975) 137. [2] R.H. Doremus, J. Non-Cryst. Solids 48 (1982) 431.
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[3] W.A. Lanford, K. Davis, P. Lamarche, T. Laursen, R. Groleau, R.H. Doremus, J. Non-Cryst. Solids 33 (1979) 249. [4] B.C. Bunker, G.W. Arnold, E.K. Beauchamp, D.E. Day, J. NonCryst. Solids 58 (1983) 295. [5] R.H. Doremus, Glass Science, 2nd Ed., John Wiley, New York, 1994. [6] D.E. Clark, C.G. Pantano, L.L. Hench, Corrosion of Glass, Books for Industry, 1979. [7] R.W. Douglas, T.M.M. El-Shamy, J. Am. Ceram. Soc. 50 (1) (1967) 1. [8] R.W. Douglas, J.O. Isard, J. Glass Tech. 33 (1949) 289. [9] M.A. Rana, R.W. Douglas, Phys. Chem. Glasses 2 (6) (1961) 179. [10] M.A. Rana, R.W. Douglas, Phys. Chem. Glasses 2 (6) (1961) 196. [11] Z. Boksay, G. Bouquet, S. Dobos, Phys. Chem. Glasses 8 (4) (1967) 140. [12] G. Woisetschla¨ger, M. Schreiner, Evaluation of decay to glass samples after 1 and 2 years of exposure, Report No. 21, Institute of Chemistry, Academy of Fine Arts, Vienna, 1998. [13] M. Melcher, M. Schreiner, 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. [14] M. Melcher, M. Schreiner, 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. [15] M. Schreiner, H. Scholze, Forschungsber. No. 10608104, Umweltbundesamt Berlin, 1985. [16] M. Melcher, M. Schreiner, Statistical evaluation of potash-lime–silica glass weathering, Anal. Bioanal. Chem. 379 (2004) 628. [17] S. Frank, Glass and Archaeology, Academic, London, 1982. [18] M.A. Bezborodev, Chemie und Technologie der antiken und mittelalterlichen Gla¨ser, Verlag Philipp von Zabern, Mainz, 1975. [19] M. Schreiner, M. Grasserbauer, P. March, Fresen. Z. Anal. Chem. 331 (1988) 428. [20] M. Schreiner, Glastechn. Ber. 61 (8) (1988) 223. [21] M. Melcher, M. Schreiner, J. Non-Cryst. Solids 351 (2005) 1210. [22] A. Chabas, R.A. Lefe`vre, Glass Technol. 43C (2002) 79. [23] MULTI-ASSESS: Model for multi-pollutant impact and assessment of threshold levels for cultural heritage. A research project supported by the European Commission under the Fifth Framework Programme Key Action City of Tomorrow and Cultural Heritage within the Energy, Environment and Sustainable Development. Contract number: EVK4-CT-2001-00044. Available from:
Leaching studies on naturally weathered potash-lime–silica glasses M. Melcher *, M. Schreiner Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, 1060 Vienna, Austria Received 27 May 2005; received in revised form 28 December 2005 Available online 9 March 2006
Abstract Within the international, EU-supported research project MULTI-ASSESS more than 150 glass samples with a chemical composition similar to medieval stained glasses were exposed in six European cities (Athens, Krakow, London, Prague, Rome and Riga) to the local environmental and climatic conditions. After exposure periods of 6 or 12 months the samples were investigated in the scanning electron microscope with energy dispersive X-ray microanalysis (SEM/EDX). Surface analyses indicate that the main weathering products were sulphates such as syngenite (CaSO4 Æ K2SO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O). Chlorides, organic compounds and/or carbonates or nitrates of the elements K, Ca and Na were predominantly detected on the 6-months samples, whereas Si-containing compounds appeared particularly on the 12-months samples. Linescan measurements performed on the cross-sectioned glass samples allowed for the determination of leaching depths of the network modifier ions according to an evaluation procedure recently presented. The average leaching depths after 6 months of exposure were d(K) = 0.88 ± 0.43 lm, d(Ca) = 0.62 ± 0.37 lm, d(Na) = 0.34 ± 0.32 lm and d(Mg) = 0.16 ± 0.18 lm. After 12 months an average increase of the leaching depths between 38% (K) and 63% (Mg) is observed. No leaching could be determined for the network former elements Al and P. A clear influence of the environmental conditions at the atmospheric test sites on the degree of weathering was observed. In a first approximation, the leaching depths exhibit a linear dependence on a general pollution factor comprising the concentrations of the acidifying gases SO2, NO2 and O3. 2006 Elsevier B.V. All rights reserved. PACS: 81.65.K; 68.35.B Keywords: Chemical durability; Ion exchange; Silicates; Surfaces and interfaces
1. Introduction Glasses are known to be attacked by various chemicals in a two-stage process: at first, an ion-exchange between the network modifier ions in the glass (Na+, K+, Ca2+ and Mg2+) and H+, H3O+ and/or H2O species from the solution can be observed, which is displayed for a monoand bivalent network modifier in Eqs. (1) and (2). The ions are diffusing in opposite directions, leading to the formation of a so-called leached or gel layer in the glass, which
is depleted in alkali and alkaline earth elements and enriched in hydrogen bearing species [1–5]. The ratio c(M)/c(Si) measured in the solution is significantly higher than in the bulk glass, indicating a selective (preferential) leaching of the modifiers M. Hence, the effective surface area of the glass exposed to the attacking medium is increased as micro-pores are produced by the leaching process, which may cause an accelerated rate of glass dissolution. Moreover, the pH of the solution increases, as hydroxyl ions remain in the solution [6]. ð1Þ
*
Corresponding author. Address: Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria. Tel.: +43 1 58816 203; fax: +43 1 58816 121. E-mail address: [email protected] (M. Melcher). 0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.017
ð2Þ
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369
In the second stage of the corrosion process, occurring predominantly in alkaline media at a pH > 9, the silicate network breaks down completely (Eq. (3)). The concentration ratio c(M)/c(Si) in the solution approximately equals that in the bulk glass, as congruent rather than selective dissolution is observed. Various authors also suggest a dehydration reaction in the leached layer according to Eq. (4), causing an increase of the water content in the outmost glass layers [6]. ð3Þ ð4Þ The amount of leached network modifier masses, often denoted as Q, are found to be proportional to the squareroot of time at the early stage of corrosion (when the selective leaching of network modifiers is observed), which is typical for diffusion controlled processes. As the reaction proceeds, the dissolution of the pffi silicate network prevails and the kinetics change from a t- to a linear t-dependence (i.e. the reaction rate remains constant with time) [7–10]. Less data are available for the weathering of glass, which is often defined as the ‘degradation of a glass surface due to interaction with the atmosphere’ [11–16]. It is assumed that similar reactions occur like in the case of glass corrosion. A water film may form on the surface of the glass by rain, fog or condensation of air moisture, in which gases from the ambient atmosphere such as CO2, SO2 or NOx can be absorbed. Consequently, the pH is decreased, favouring the ion-exchange according to Eqs. (1) and (2) and hence the deterioration of the glass. When the temperature increases and the water film evaporates (or the solubility products are exceeded), crystalline weathering products such as sulphates, carbonates, hydro-carbonates, chlorides or nitrates of the network modifiers are left behind. Compared to modern or ancient glasses, medieval glasses produced north of the alps exhibit two main differences: the use of potassium instead of sodium as a flux and generally a higher content of network modifiers and hence a lower silica concentration [17]. A large collection of results of chemical analyses of glasses of different ancient and medieval periods can be found in [18]. Fig. 1 shows 559 of these results (only the lead glasses and those with unusual high contents of certain elements, corresponding to Bezborodov’s groups XVIII to XXIV are omitted) after statistical treatment by principal component analysis (PCA). Although only 40.6% of the initial variance of the data is reproduced by the first two principal components (PCs), a satisfactory separation is achieved. Ancient glasses, displayed with squares, cluster on the right of the plot, indicating relative high concentrations of Na2O and SiO2 and/or low concentrations of K2O and CaO. Furthermore it can be seen that the medieval glasses (circles) appear in two – not separable – clusters. Glasses high in K2O and CaO are arranged on the far left,
Fig. 1. Score–score plot of the first two PCs. PC1 mainly covers the concentrations of the compounds SiO2, Na2O (positive correlation), K2O and CaO (negative correlation), PC2 contains Mn2O3, CuO and SO3 (positive), Al2O3, Fe2O3 and TiO2 (negative).
while the second medieval group significantly overlaps with the ancient glasses. On the basis of these data, it can be concluded that ancient glasses are of the soda-lime–silica (SLS) type, while medieval glasses may also contain significant amounts of potassium and belong to the potash-lime–silica (PLS) glass type. Although both SLS- and PLS-glasses are subjected to the same weathering mechanisms in principle, SLS-glasses proved to be much more durable. Schreiner et al. [19,20] measured the concentration profiles of leached glass constituents as well as hydrogen in medieval PLS-glasses and found leaching depths in the lm-range. Melcher and Schreiner [21] investigated PLS model glasses after periods of 3–6 years of natural weathering in the ambient atmosphere of low, moderately and highly polluted sites. Different leaching depths of approximately 1.2 and 1.1 lm were found for K and Ca, respectively. On the other hand, Chabas and Lefe`vre [22] examined samples of durable soda-lime–silica float-glass, which were exposed within a field experiment to the polluted atmosphere in the center of Paris for a period of two years. The authors report dealkalization phenomena for Na and Ca only in the first 40 nm of the samples, irrespective of the exposure mode (sheltered or unsheltered). These examples illustrate the much higher vulnerability of PLS-glasses when exposed to urban or industrial atmospheres. The present exposure was carried out in the framework of the MULTI-ASSESS project [23], which main objectives are qualitative and quantitative investigations of the weathering attack on various materials such as stone, steel, zinc, copper, bronze as well as modern and medieval glass. Special attention is paid to the relative new multi-pollutant situation, which is dominated by elevated levels of nitrogen compounds (NOx, HNO3, etc.), ozone and particulate matter caused by the increased car traffic and by decreasing sulphur dioxide levels, which was the main pollutant throughout the last decades.
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2. Experimental 2.1. Sample preparation As the exposure times were only 6 and 12 months, a sensitive glass with high concentrations of the network modifier elements was chosen (Table 1) in order to achieve measurable weathering effects. The glasses were prepared at the Fraunhofer-Institut fuer Silicatforschung in Wuerzburg (Germany) by mixing oxides and carbonates in certain stoichiometric quantities, melting in a Pt crucible (approximately 1720 K), refining at a temperature of 720 K for 5 h and cooling. Subsequently the frits were ground and melted a second time in order to achieve homogeneity. Small plates (10 · 10 · 2 mm3) were cut from these glass ingots with a low-speed diamond saw (Buehler Isomet II-1180), embedded in epoxy resin and polished using SiC paper of 500–4000 mesh. In the last polishing step isopropanol instead of water was used as lubricant in order to decrease the leaching of glass components already during the polishing process. The polished glass samples were mounted on glass fibre reinforced plastic plates (sized approximately 5 · 5 cm2) with approximately 10 cm pieces of shielded wire and covered with silicon caps and adhesive tapes for protection during shipping. Prior to the investigations in the SEM, 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. After the surface investigations of the weathered glass specimens, the samples were covered with a thin layer (approximately 3 mm) of epoxy resin and cut with a low-speed diamond saw perpendicular to their surfaces in order to produce cross-sections of the specimens, which were also investigated in the SEM. In order to avoid charging during the SEM-investigations, the surfaces were sputtered with a thin layer of carbon. For all measurements an acceleration voltage of 15 kV and a beam current of approximately 1 nA were used. Linescans were performed with in average 400 measurement points and a length of 9 lm. A JEOL JSM-6400 electron microscope with the energy dispersive system EDAX Phoenix, equipped with an ultra-thin window detector allowing also the detection of low-Z elements such as C and O, was used. 2.2. Site description The exposure was carried out in six European cities: Athens, Krakow, London, Prague, Rome and Riga. In each of the first five cities, a so-called main and up to six Table 1 Chemical composition (in wt%) of the model glasses exposed within the MULTI-ASSESS project for periods of 6 and 12 months Glass
SiO2
K2O
CaO
P2O5
Na2O
MgO
Al2O3
M1
48.0
25.5
15.0
4.0
3.0
3.0
1.5
smaller sub-racks were available for exposing the glass samples. In Riga, there were two main racks in different parts of the city. All exposures were performed under sheltered conditions, i.e. the samples were protected from rain, direct sun-radiation and for the most part also from wind. In Athens (sites T60 to T66) the main and sub-racks were installed on the building of the ministry of health, which is located at the inner circle of the city. The surrounding area can be classified as urban and residential. The racks were mounted up the building on the ground, first, second, fourth and fifth floor as well as on the roof of the seven-floor building. Environmental and climatic data were reported from the main rack on the roof. Hence, the influence of the vertical position of a sample on its weathering could be investigated. The exposure sites in Krakow were located in different distances from local sources and regional large industry sources: site 70 in a residential area with a park, sites 71– 73 (near congested roads), site 74 (main square of the city without traffic), site 75 (close to the main rack 70) and site 76 (in a residential area with streets and park). The target sites in London were located in different distances from traffic pollution. The main site 80 has the same pollution and environmental level as sub-site 81 (both racks are placed on a roof at the campus of Middlesex University). The sub-racks 82, 83 and 85 were located in a residential area with light industry. It is, more or less, equivalent to an urban background site. Site 84 was near a major road with a large municipal incinerator close by it, site 86 was placed near a busy road (80 000 vehicles per day). The target sites in Prague were designed to study the change in the environmental impact around a building (National Museum at the Wenceslav square). The surrounding area is commercial and residential. A heavy influence by transport pollution sources (highway and crossing with 97 000 vehicles per day) could be expected. The main site 90 and the sub-site 91 were located close to each other (top floor under the cupola of the building in direction to Wenceslav square). The sub-sites 92 (top floor, opposite to Wenceslav square), 93 (balcony on the 1st floor in direction to Wenceslav square), 94 (balcony on the 1st floor in direction right to Wenceslav square) and 95 (balcony on the 1st floor in direction left to Wenceslav square) were located on different parts outside the building, site 96 was in the yard of the National Museum on ground level. The environmental parameters were measured at the sites 90 and 96. The 6 target sites (main rack and 5 sub-racks) in Rome were located in areas with moderate traffic influence. The main rack (100) and the two sub-sites 101 and 102 were situated at Montelibretti in the outskirt of Rome (30 km). The surrounding area there is semirural. All of those sites have the same environmental data. Sites 103 and 104 were located on a building (National Health Institute in a central area of Rome with intense traffic of both cars and public transport busses), and also have nearly the same environmental data. Sub-site 105 was in a park in the north eastern area of Rome, inside a background monitoring
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station. Close to it there is an important state road with a high volume of traffic. The two Riga sites were located within a fairly short distance with comparable climatic and precipitation data but different concentrations of gases such as SO2, NOx, etc. Table 2 lists the average environmental and climatic data (temperature T, relative humidity RH and the concentrations of the acidifying gases SO2, NO2 and O3) of the main exposure sites for the 6- and 12-months exposure periods. All data were collected and reported to the Norwegian Institute for Air Research (NILU). The 6-months exposure was performed from November 2002 to April 2003, the 12-months period continued until November 2003. During that period in Athens the highest SO2- and NO2-concentrations were measured, whereas the ozone concentration showed high values in Krakow, Rome and Riga. Besides Athens (RH around 60%), the relative humidities at all other exposure sites are comparable (about 75%), Athens, Rome and London had the highest average temperatures, whereas for Krakow, Prague and Riga similar temperature means between 4 and 6 C are reported. It must be noted, that the exposure sites chosen within this project are by no means typical for the whole city. Unfortunately, many environmental data are missing for the London test sites.
The evaluation procedure is mainly based on a numerical integration of these linescans and described in detail in a previous work [21]. Hence, two measures can be obtained by these calculations: the leaching depths LD(Y) and the hypothetic leaching depths d(Y) for an element Y (Y = K, Ca, Na, Mg, Al and P), both explained in Figs. 2 and 3, where typical linescan profiles (background corrected intensities versus distance from starting point xstart) of the leached layer are shown. The leaching depth LD of an element Y is the distance (in microns) from the beginning of the decrease of the intensity at xY to the end of the glass xeog (Eq. (5)). For the calculation of the hypothetic leaching depth d(Y) a completely leached glass surface layer is assumed. The (hypothetic) boundary between the unleached and totally leached glass layer (xtd for total depletion) is calculated by area compensation (Eq. (6), Fig. 3). While LD can be considered as the maximum depth at which ion exchange has occurred, the advantage of d is its higher robustness. The determination of xY is performed visually for each linescan, as statistical methods (such as confidence intervals around the bulk intensity of the network modifiers) did not yield satisfying results. Moreover, d(Y) can easily be converted into leached masses (LM) in mg leached alkalis per m2 glass surface (Eq. (7) for Y = K, the density of the glass is qglass = 2618 kg/m3).
2.3. Evaluation method and error estimation
LDðYÞ ¼ xeog xY ;
ð5Þ
In order to quantify the weathering effects on the specimens, linescan measurements were performed on the crosssectioned samples. These measurements were only carried out in domains, where no weathering products, which might have formed on the glass surface, could be observed.
Table 2 Average environmental and climatic data at the main exposure sites City
Site
T (C)
RH (%)
c(SO2) (lg/m3)
c(NO2) (lg/m3)
c(O3) (lg/m3)
Athens
T60
19.0 20.1
57.6 59.9
42.5 43.2
89.7 77.2
23.0 19.7
Krakow
T70
4.3 7.9
74.0 74.7
22.9 19.9
30.9 30.4
57.4 60.2
London
T80
9.8 12.0
–
–
–
–
Prague
T90
6.4 8.6
72.4 72.5
9.8 8.7
54.2 49.5
32.9 33.2
Rome
T100
11.0 15.8
76.3 72.1
1.2 1.3
16.5 14.4
45.4 52.8
Riga I
T110
4.47 6.63
74.2 80.2
6.7 5.8
29.2 27.0
70.2 59.4
Riga II
T111
4.17 6.30
76.7 82.6
8.7 6.8
19.2 19.0
65.5 53.6
In all columns the first value indicates the 6-months mean, the second the 12-months average. Data are calculated from [24], where also precipitation data such as the amount, the pH and ionic concentrations of the rain are listed.
Fig. 2. Scheme of a typical linescan profile depicting the plot of the background corrected X-ray intensities of element Y (IY,corr) versus the distance x (in lm) and the measure LD(Y).
Fig. 3. The hypothetic leaching depth d(Y) calculated by area compensation (AI = AII).
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dðYÞ ¼ xeog xtd;Y ;
ð6Þ
LMðKÞ ¼ 0:212 qglass dðKÞ.
ð7Þ
As described in [21], the inaccurate determination of the background intensities IY,back and of the points xY are the main sources of error causing deviations from the ‘true value’ (d(K) or LD(K)) of approximately 5%. Repeated measurements on the same position on the same cross-section produce errors of the same size and were <10% in all cases. Hence, the results of d(Y) and LD(Y), which will be discussed in the next chapters, should not be affected by errors larger than 10%. As no other analytical method was available in order to estimate the systematic errors of these measurements, only a plausibility consideration of the results can be done by comparing the calculated leaching depths and the mass of weathering products formed on the glass surface. Under the simplified assumption that all WPs consist of syngenite and if the average height of the weathering crust (h) and the percentage of surface coverage with weathering products (c) were known, the hypothetic leaching depth d would be given by Eq. (8) d¼
A h c qsyn xK;syn ¼ 1:11 h c; A qglass xK;glass
Irrespective of their chemical composition, the WPs occur in different morphologies: shapeless, flat, cubic, needle-shaped, round, plate-like, elliptic or hexagonal. Only in the case of chlorides (cubic and hexagonal) and K-organic (round) WPs preferred crystal-forms could be determined. The size of the WPs is typically in the range of 10–20 lm in diameter (Fig. 4(a)), sometimes up to 100 lm, only chlorides and organic K-compounds are significantly smaller (1–5 lm, Fig. 4(c)). The WPs can appear as single crystals or in large agglomerations, forming weathering crusts (Fig. 4(b)), which can cover large parts of the glass surface. Weathering crusts were mainly found on samples exposed in Krakow, Athens and Prague. The leached layer seems to be intact in most cases, cracks and rough surface regions (Fig. 4(d)) were only observed in about 20% of all samples (especially on those, which were exposed in Krakow). These cracks may be due to the weathering process itself or due to the vacuum conditions in the SEM (approximately 108 bar), which may also cause the evaporation of the water in the leached layer and the cracking of the glass surface by this dehydration. 3.2. Leaching measurements
ð8Þ
where A (m2) is the exposed surface area of the glass, qsyn and qglass stand for the densities of syngenite (2580 kg/m3) and the glass (2618 kg/m3) and xK,syn and xK,glass for the mass fraction of K in syngenite and in the original glass. h and c cannot be determined directly, but can be estimated on the basis of SEM-images of the weathered glass surfaces. Hence, the average height of the WPs is probably between 3 and 10 lm, whereas c lies between 0.1 and 0.4, resulting in dmin = 0.3 and dmax = 4.4 lm, which is in very good agreement with the actually measured leaching depths for K (between 0.2 and 4.0 lm for single measurements and between 0.3 and 2.6 lm for sample means). 3. Results 3.1. Surface investigations The main weathering products (WPs) on the surfaces of the specimens were syngenite (CaSO4 Æ K2SO4 Æ H2O), which could be detected on nearly all samples, and gypsum (CaSO4 Æ 2H2O). Chlorides of Na and K, organic Kand Ca-compounds and/or (hydrogen)carbonates and/or nitrates could be observed predominantly on the 6-months samples, which suggests a conversion of these compounds into sulphates during longer exposure times. Si-containing crystals appear more frequently on the 12-months specimens, which is probably due to the breakdown of the silicate network. The chemical characterization of the WPs is based on energy-dispersive point- or area-measurements of the crystals. Generally, no correlation between the environmental or climatic conditions at the individual test sites and the chemical composition of the WPs could be determined.
On each glass cross-section about three to four linescans were performed on different positions of the leached layer (LL). Hence, the uniformity of the thickness of the LL for a single sample can be assessed by a modified coefficient of variation (CV 0 , Eqs. (9a) and (9b)), which is normally defined as the ratio of the standard deviation r and the mean l for n measurements. As only between two and five results of measurements of LD and d are available for each sample, half of the range R (RLD = LDmax LDmin, Rd = dmax dmin) is taken rather than the standard deviation r. Fig. 5(a) and (b) shows the distribution of these CV 0 -values of the leaching depth LD and the hypothetic leaching depth d, both for the element K. The histograms for both measures look similar and suggest a relative uniform thickness of the LL on the majority of the samples, as CV 0 < 0.3 in most cases. For instance, the three measurements performed on sample no. 195 (exposed at the test site T82, t = 6 months) yield the results LD(K) = 1.91, 1.93 and 1.99 lm corresponding to a CV 0 as low as 0.02, whereas for the sample no. 165 (site T63, t = 12 months) LD(K) = 1.27, 1.96, 2.06 and 3.75 lm (CV 0 = 0.55) were obtained CV0 ½LDðKÞ ¼
0:5 RLDðKÞ
LDðKÞ 0:5 RdðKÞ CV0 ½dðKÞ ¼ . dðKÞ
;
ð9aÞ ð9bÞ
Although LD and d are determined independently for the most part, they are of course correlated to a high extent, as both represent some kind of leaching depths (if the shape of the diffusion profiles were the same for all measurements and if xY could be determined precisely in all cases, there
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Fig. 4. SE-images of weathering products (WPs) and sample surfaces: (a) elliptically shaped WPs, length some ten microns, consisting mainly of syngenite, (b) weathering crust covering large parts of the specimen surface, (c) extraordinary small (<5 lm) and round WPs, consisting of K and C, therefore probably organic or carbonates or hydrogencarbonates and (d) cracked leached layer and some detached parts of glass.
Fig. 5. Histograms of the modified coefficients of variation CV 0 for the leaching depths LD(K) and d(K) for all 501 linescan measurements performed on 155 samples. Besides a few outliers, the data indicate a relative narrow distribution of the results for each sample.
would be a perfect correlation of d and LD). Consequently, one of these measures can be estimated by the other with a sufficient accuracy (Eqs. (10a)–(10d)) and only one of them has to be discussed. At least 86%, mostly above 90% of the
variance in the data is explained by these regression equations before the elimination of potential outliers. The hypothetic leaching depths d are chosen for further discussion, as they are also capable of considering different shapes or
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curvatures of the linescan curves and are directly proportional to the leached masses of the corresponding elements dðKÞ ¼ 0:62 LDðKÞ 0:09 dðCaÞ ¼ 0:65 LDðCaÞ 0:13
R2 ¼ 0:86;
ð10aÞ
2
R ¼ 0:92;
ð10bÞ
2
dðMgÞ ¼ 0:36 LDðMgÞ þ 0:05 R ¼ 0:90; dðNaÞ ¼ 0:47 LDðNaÞ þ 0:03
ð10cÞ
2
R ¼ 0:91.
ð10dÞ
In general, different (hypothetic) leaching depths were measured for different elements. The selective leaching of certain elements can be assessed by comparing the ratio of the leached masses LM (or the hypothetic leaching depths d) for two elements A and B (LM(A)/LM(B)) to their ratio in the bulk glass. As d(K), LM(K) and LD(K) can be determined with the highest accuracy due to the high potassium concentrations in the original glass and the resulting high X-ray intensities, potassium leaching data were taken as reference points LM(B). Fig. 6 displays the histogram of the leached mass ratio LM(Ca)/LM(K) of all 501 measurements. The majority of the data can be found in the interval between 0.2 and 0.5 with a mean of 0.37 and a standard deviation of 0.11. As the ratio LM(Ca)/LM(K) in the original glass is 0.51, a selective leaching of potassium compared to calcium can be determined in 94.4% (473 out of 501) of all samples. The data in Table 3 prove that potassium is leached preferentially compared to all other elements. Furthermore it becomes clear that calcium is leached preferentially compared to the other bivalent net-
Fig. 6. Histogram of the leached mass ratios LM(Ca)/LM(K) for the two main network modifiers in the glass. Most values are between 0.2 and 0.5, clearly indicating a preferential leaching of K.
work modifier magnesium. The low values for the P- and Al-ratios suggest that these two elements are not leached, which was expected due to their role as network formers. The histograms of their hypothetic leaching depths d(P) and d(Al) are shown in Fig. 7. The agreement of the results for the leaching depths obtained for two samples A and B (dA(K) and dB(K)), each calculated as the mean of nA or nB linescan measurements, can be assessed with the ratio r given in Eq. (11). It can be interpreted as the ratio of half of the difference between the (hypothetic) leaching depths of the two samples, normalized with the weighted mean. It has a similar meaning as CV 0 and can be used to compare the results for samples, which were exposed under the same climatic and environmental conditions for the same period. If specimens were exposed in triplicate, the enumerator should contain the range of all three results and the denominator the weighted mean of those three samples. rdðKÞ ¼
1 nA þnB
0:5 jd A ðKÞ d B ðKÞj . ðnA d A ðKÞ þ nB d B ðKÞÞ
ð11Þ
The distribution of the ratio rd(K) for all duplicate and triplicate exposures is shown in Fig. 8. Most pairs/triples of samples exhibit r-values below 0.16. Thus the normalized difference between such duplicate or triplicate samples is less than 16% of the weighted mean in 74% of all cases, the arithmetic mean of these ratios is 0.127, the minimum value is achieved for the sample pair at site T91 with d(K) = 1.65 and 1.63 lm corresponding to a ratio r = 0.006, the maximum relative difference is calculated for site T83 (d(K) = 0.71 and 1.52 lm, r = 0.37). A compilation of all results of the 12-months exposure for d(K) is presented in Table 4 and Fig. 9. The samples exposed in Krakow exhibit the highest hypothetic leaching depths of up to 2.16 lm at site T73 and 2.10 lm at site T74. While site T73 is situated near a congested road and high leaching depths are expected, site T74 is located at the main square of the city without traffic (lower NO2-pollution) and the result is somehow astonishing. The other sub-sites in Krakow also show relative high d(K)-values between 1.53 and 1.83 lm. On the other hand, very low levels of glass deterioration were measured for the samples exposed in Rome. Four of the six sites (T100–T102 and T105) can be classified as ‘background sites’ with low pollution levels. The samples exposed at the sites T103 and T104 (central area of Rome with intense traffic) exhibit much higher d(K)-values.
Table 3 Leached mass ratios for n = 501 linescan measurements
Measured ratio (mean ± SD) Ratio in bulk glass Selective leaching of K in . . . cases
LMðCaÞ LMðKÞ
LMðMgÞ LMðKÞ
LMðNaÞ LMðKÞ
LMðPÞ LMðKÞ
LMðAlÞ LMðKÞ
LMðMgÞ LMðCaÞ
0.37 ± 0.11 0.51 94.4%
0.02 ± 0.02 0.09 99.6%
0.04 ± 0.03 0.11 98.0%
0.004 ± 0.010 0.08 100%
0.001 ± 0.002 0.04 100%
0.05 ± 0.01 0.17 99.0%
Potassium is leached preferentially compared to all other elements. The last row indicates that this selective K-leaching is observed for at least 94.4% of all linescan measurements.
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Fig. 7. Histogram of the hypothetic leaching depths d(P) and d(Al). As by far most measurements exhibit values below 0.1 (P) or even below 0.05 (Al), the leaching of these elements can be neglected.
Fig. 8. Distribution of the rd(K) – ratio measuring the agreement of results obtained for d(K) of duplicate or triplicate samples exposed under the same conditions.
At the sites in Athens two clearly different levels of glass deterioration were determined. Samples exposed at the sites T60 and T64, which are close to each other on the roof of the building, show about 50% higher hypothetic leaching depths than all other samples, which were exposed on a lower height level on the fac¸ade of the building. Similar results were also obtained for the exposure in Prague. The samples exposed at the sites T90, T91 and T92, which are all located on the top floor of the National Museum under the cupola, show d(K)-values of 1.45 lm in average, while for samples at the sites T93, T94, T95 (all exposed on the first floor level) and T96 (ground level in the yard) only 0.85 lm were measured. The orientation of the exposure rack and hence of the samples also has an influence on the degree of weathering, but a much smaller one than the exposure height.
Table 4 Hypothetical leaching depths d(K) and d(Ca) in microns Test site
Athens T60
Krakow T70
London T80
Prague T90
Rome T100
Riga T110
Riga T111
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
d(K)
d(Ca)
Main rack
0.98 1.55
0.72 1.29
1.45 1.83
1.04 1.44
0.87 1.23
0.63 0.97
0.79 1.40
0.62 1.17
0.36 0.51
0.21 0.33
1.06 1.50
0.67 0.97
0.89 0.84
0.51 0.52
Al 1
0.41 0.90
0.29 0.55
1.66 1.70
1.33 1.60
1.10 1.57
0.64 1.10
0.96 1.64
0.65 1.33
0.43 0.56
0.28 0.39
–
–
–
–
Al 2
0.46 1.01
0.38 0.83
1.41 1.73
0.92 1.53
0.97 1.12
0.54 0.76
0.98 1.27
0.77 0.85
0.56 0.39
0.29 0.29
–
–
–
–
Al 3
0.46 1.01
0.30 0.78
1.83 2.16
1.54 1.68
0.71 1.05
0.40 0.69
0.49 0.74
0.36 0.56
0.77 0.80
0.49 0.58
–
–
–
–
Al 4
0.80 1.57
0.46 1.31
1.74 2.10
1.51 1.80
0.68 0.99
0.47 0.60
0.64 0.98
0.34 0.78
0.63 1.09
0.45 0.81
–
–
–
–
Al 5
0.33 0.85
0.21 0.69
1.68 1.52
1.32 1.27
1.03 1.10
0.60 0.80
0.64 0.73
0.45 0.62
0.46 0.45
0.26 0.30
–
–
–
–
Al 6
0.56 1.14
0.32 0.88
1.13 1.65
0.99 1.30
1.27 1.57
0.87 1.27
0.65 0.96
0.42 0.75
–
–
–
–
–
–
All values displayed are average values of the duplicate or triplicate exposure of glass samples. The upper value in each cell is the 6-months value, the lower the 12-months value. Uncertainties due to random errors are 610%.
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Fig. 9. Leaching depths d(K) after 12 months of exposure. Samples exposed in Krakow exhibit the most weathered surface layer, Rome the least. The first column for each city represents d(K) of samples exposed at the main rack, the 2nd column of the first sub-rack, and so on. There are only five sub-sites in Rome and only main racks at the two Latvian sites.
In London two sites (T81 at the university campus and T86 near a congested road) show outstanding high values of depletion depths of potassium (d(K)): both 1.57 lm compared to 1.10 lm in average for the other sites. Surprising is the fact that samples of the sites T80 and T81, which are very close to each other, show very different results (1.23 compared to 1.57 lm). The two exposure sites in Riga also exhibit very different levels of weathering (d(K) = 1.50 and 0.84 lm). Fig. 10 displays the change of d(K) with the exposure time (6–12 months) in percent. Generally, an increase of the thickness of the leached layer can be determined in all but four cases: sites T75, T83, T105 and T111, which all show a decrease between 5% and 9%. Fig. 11 depicts the bivariate scatter plots of the hypothetic leaching depths d(K) after 12 months of exposure
Fig. 10. Change of d(K) with time (in percent). All but the Athens samples exhibit similar growth rates of up to 70%. Samples exposed in Athens show a doubling of d(K) in average after 12 months of exposure.
(denoted as d(K)12) versus T, RH, c(SO2), c(NO2) and c(O3). Obviously the temperature has no linear influence on d(K), more a parabolic one with a minimum somewhere between 13 and 17 C. On the one hand, a positive correlation between the leaching depth (or leaching rate) and T is expected, but too high temperatures might cause an advanced evaporation of the water film on the glass surface on the other hand. Probably a combination of these two effects produces this parabolic temperature-dependence. Also the relative humidity and the concentration of O3 do not show a (linear) correlation with d(K), not even a trend can be figured out. Contrary to these results an increasing concentration of SO2 and NO2 causes an increased leaching depth d(K), even though the relationship does not seem to be linear. A power-relationship of the general form d(K) = a Æ c(SO2)b or d(K) = a Æ c(NO2)b exhibits satisfying fits of R2 = 0.68 and 0.38, respectively. 4. Discussion and conclusion The surface analyses of more than 150 weathered glass samples of the potash-lime–silica type in the scanning electron microscope showed that the prevailing weathering products were sulphates such as syngenite (K2SO4 Æ CaSO4 Æ H2O) and gypsum (CaSO4 Æ 2H2O), chlorides (NaCl and KCl) and organic K- and Ca-containing compounds. Especially after the longer weathering period (12 months), Si-containing WPs occurred more frequently. The surfaces of those samples, which were exposed in more polluted regions (e.g. Krakow), exhibited definitely more WPs and thicker and larger weathering crusts than those specimens, which were exposed in so-called background areas (e.g. most of the Roman sites) with a lower pollutant concentration. Linescan measurements and application of the previously introduced evaluation method yield two measures for the thickness of the leached layer: LD and d. At first, multiple measurements at different positions of the leached layer of a single sample show a relative uniform thickness of the leached layer (Fig. 5). Assuming a typical hypothetic leaching depth d(K) of 1 lm, measurements on different positions on the leached layer are likely to fall in the interval [0.8; 1.2]. Secondly, a comparison of the results obtained for samples exposed duplicate or triplicate under the same conditions exhibits a good agreement in most cases (Fig. 8), even though larger deviations are also observed in approximately 10% of all cases. So far no satisfactory explanation could be found for that phenomenon. The observed selective leaching of the main network modifier cation K+ can probably be explained with differences in the size of the cations present in the glass and hence in their mobilities. According to Zachariasen’s model of the structure of silicate glasses [25], bivalent network modifiers such as Ca2+ or Mg2+ are bond to two nonbridging oxygen (NBO)-sites and are stronger retained in the glass compared to the monovalent ions K+ or Na+.
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Fig. 11. Scatter plots of d(K) versus the climatic parameters T and RH and the pollution parameters c(SO2), c(NO2) and c(O3). Particularly the influence of c(SO2) and c(NO2) on the leaching depth and hence on the glass degradation is visible.
Alkaline earth oxides are known to decrease the electrical conductivities as well as the diffusivities of alkali ions, which is often explained by their blocking effect: the immobile bivalent ions occupy interstices, which are then no longer available for alkali ion migration [26]. The preferential leaching of potassium was statistically demonstrated in an previous work [21] for another potash-lime–silica glass (composition M3: 60 wt% SiO2, 25CaO and 15K2O). The amount of K leached was approximately 10% higher than expected for congruent leaching. The selective leaching is
much more evident for this glass (M1). The hypothetic leaching depths for potassium are about 35% higher compared to those of Ca, 5 times higher than those of Mg and 2.5 times those of Na. El-Shamy et al. [27] investigated the durability of soda-lime–silica glasses in acid solutions and found that the amount of Ca extracted strongly depends on the lime content of the glass. Above a concentration of about 10–15 mol% lime, the sodium and calcium were dissolved in the same proportion in which they are present in the bulk glass. A similar trend can be observed
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for the weathering of the potash-lime–silica glasses M1 (17.8 mol% CaO) and M3 (27.8 mol% CaO). A preferential leaching of potassium compared to sodium cannot be explained in terms of this simple model. Regarding the size of these two cations, a selective Na-leaching would be more conclusive due to its higher mobility. A satisfactory answer to this phenomenon has not been found so far. Moreover, due to the low concentration of Na (2.2 wt%), Mg (1.8%), P (1.7%) and Al (0.8%) in glass M1, much lower intensities (about one order of magnitude) were detected for the characteristic X-ray lines of these elements, leading to a significantly higher error for their leaching depths. Generally, high hypothetic leaching depths of K imply high values for the corresponding d(Ca)-values, which is shown by the high coefficient of determination of the regression function (Eq. (12)) for n = 501 measurements dðCaÞ ¼ 0:828 dðKÞ 0:091
R2 ¼ 0:87.
ð12Þ
Regarding the deterioration of medieval glasses of this composition, d(K) and/or LD(K) seem to be the most appropriate measures, as the highest (hypothetic) leaching depths are measured for potassium. As expected, the (hypothetic) leaching depths increase with time (average increase of d(K): ca. 38%). Nevertheless, in four cases slightly decreasing values were observed. There are two possible explanations for this astonishing result: (a) the choice of the measuring positions on the crosssections: as discussed earlier, the thickness of the LL over the whole glass cross-section is not totally uniform and small inhomogeneities are possible, or (b) by the prevailing network dissolution compared to the ion-exchange reaction, which might cause a reduction of the thickness of the LL. The first explanation seems to be much more feasible, and no reason for (b) can be found. The highest rate of growth is measured for the samples exposed in Athens (Fig. 10): in average the 12-months value of d(K) is more than twice as high as the 6-months value. For the sites T60 and T65 an increase of 58% and 162%, respectively, was determined. All other sites showed much lower values. Compared to Krakow, London, Rome and Riga, the growth rates for the Prague samples are relative high (between 15% and 76%, in average 49%). The strong increase of the hypothetic leaching depths of the Athens samples must be due to a climatic or environmental particularity at this site, which does not occur at all other sites (or at least to a much lower extent). Furthermore, this effect should only be present during the second exposure period of six months. As can be seen from Table 2, the temperature and the SO2-concentration remain quite constant, while both c(NO2) and c(O3) even decrease by about 14% during the second exposure interval. Hence, the strong increase in d(K) can only be explained by the increasing
relative humidity. Munier et al. [28] exposed low-durability potash-lime and soda–potash glasses to the polluted urban atmosphere and found a remarkable dependence of the formation of sulphate on the glass surfaces from the relative humidity. Below about 65% relative humidity the formation of sulphates occurred to a much lower extent. Their explanation for this phenomenon was that above 65% RH the water film on the glass surface behaves like an electrolyte and has a sufficient thickness for the acidifying gases to dissolve. In the present case, the exposure sites in Athens are the only ones to show RH-values below the 65% threshold. Even in the more humid period, this threshold is not exceeded, but RH = 59.9% is rather close to it. Unfortunately, more detailed interpretations are not possible, as the environmental and climatic data of the exposure site in Athens were only measured on the roof of the building (corresponding to the sites T60 and T64), and not at each sub-site. From the graphs in Fig. 11 (bivariate scatter plot involving d(K) after 12 months of exposure and T, RH, c(SO2), c(NO2) and c(O3)) a dependence of the degree of weathering on the environmental and climatic parameters can be assumed. If c(SO2), c(NO2) and c(O3) are mathematically combined to some kind of ‘general pollution’ (GP) using Eq. (13), the influence of the pollution parameters becomes even more clearer. GP ¼
cðSO2 Þ cðSO2 Þ cðNO2 Þ cðNO2 Þ þ rðSO2 Þ rðNO2 Þ cðO3 Þ cðO3 Þ ; þ rðO3 Þ
ð13Þ
where cðX Þ stands for the average gas concentration and r(X) for the standard deviation of the distribution of the values of X. A scatter plot of d(K)12 versus this measure (for all 39 specimens, for which all three gas concentrations were available) exhibits a clear and perhaps linear correlation between d(K) and GP (corresponding equation: d(K) = 1.25 Æ GP + 0.25, R2 = 0.58). Even if this relationship has no relevance in an absolute sense because of the scaling procedure in Eq. (13), the correlation between the degree of weathering d(K) and the environmental pollution is significant. Another possibility for the interpretation of Fig. 12 could be that only a low weathering of the glass samples takes place below GP 1.5, which is the case for the sites 100, 101, 102 and 105 (Rome). This degree of weathering could be classified as ‘background weathering’. Above this critical GP-value of 1.5, the weathering increases strongly and in a non-linear way. Further calculations will have to follow. The present work is an attempt to quantify the weathering attack of polluted urban atmospheres on the surface of low-durability potash-lime–silica glasses. The application of a previously introduced evaluation method based on linescan measurements lead to reasonable results, as thorough statistical investigations showed. The correlation between the environmental pollution at a specific test site
M. Melcher, M. Schreiner / Journal of Non-Crystalline Solids 352 (2006) 368–379
Fig. 12. Scatter plot of the hypothetic leaching depth d(K) after 12 months of weathering versus the ‘general pollution’ parameter defined by Eq. (13).
and the observed and measured weathering attack on the glasses becomes obvious due to the introduction of a simple ‘general pollution’ parameter GP. In a first approximation this dependence is approximately linear. Future work will therefore focus on the calculation of dose–response functions (DRFs), relating the weathering data to the individual environmental and climatic parameters. The resulting equations could for example be very helpful for conservators or restorers for choosing optimal conditions for the storage of medieval glass objects of our cultural heritage. In addition, these DRFs could also be used for assessing threshold levels for pollutants in order to keep the risk for cultural assets by environmental pollution as low as possible. Acknowledgments The authors gratefully thank all partners within the MULTI-ASSESS project (contract number EVK4-CT2001-00044) for the possibility of participating and for the friendly and fruitful cooperation. References [1] R.H. Doremus, J. Non-Cryst. Solids 19 (1975) 137. [2] R.H. Doremus, J. Non-Cryst. Solids 48 (1982) 431.
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