8 Major, Minor, and Trace Element Analysis of Medieval Stained Glass by Flame Atomic Absorption Spectrometry Ν. H . T E N N E N T Glasgow Museums and Art Galleries, Kelvingrove, Glasgow G3 8AG, Scotland P. McKENNA, Κ. Κ. N. LO, G. McLEAN, and J. M. OTTAWAY University of Strathclyde, Department of Pure and Applied Chemistry, Glasgow G1 1XL, Scotland
Procedures are described for the analysis of 12 key elements (sodium, potassium, calcium, magnesium, aluminum, iron, manganese, lead, zinc, copper, cobalt, and nickel) in me dieval stained glass by flame atomic absorption spectrom etry (AAS). The method of choice involves dissolution of powdered glass samples (50-100 mg) in a hydrofluoric/ perchloric acid medium. Alternative dissolution methods are discussed. Analytical data for the 10 synthetic medieval glasses prepared for the European Science Foundation are reported, and the accuracy and precision of the AAS method are evaluated in terms of these glasses and Corning Stand ard D. The compositions of 12 medieval stained glass frag ments excavated from Scottish cathedral sites are presented and considered in terms of the source of plant ash and colorant materials. The results are consistent with the use of a mixture of fern and beech ash and the introduction of blue and green colors by means of colorants derived from copper-based alloys.
A
T O M I C A B S O R P T I O N S P E C T R O M E T R Y (AAS) has been extensively em ployed in the analysis of metallic and siliceous materials of archae
ological and art historical significance. T h e scope and practical aspects of the technique of relevance to archaeology were comprehensively re viewed in 1976 (I); a more recent report focused on the application of atomic absorption analysis to archaeological ceramics (2). T h e technique carries the potential for analysis of a wide range of elements with good 0065-2393/84/0205-0133$06.00/0 © 1984 American Chemical Society
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accuracy and high sensitivity. Sample atomization can be accomplished by means of a flame or furnace (3). Although detection limits are generally several orders of magnitude lower for furnace atomization (J, p. 20), flame A A S , primarily for reasons of cost, is the more routinely employed method of analysis. T h e analysis of modern glass is a well-established area of application of A A S (4). Analyses of archaeological glass include, for example, studies of Egyptian glass (5,6) and Venetian glass (7). Medieval stained glass compositions, determined by A A S , are also documented (8-10), but no published method developed specifically for the analysis of medieval stained glass was available when the program of analysis, reported i n part i n this chapter, was begun. Details of the analysis of silicates have since been reported by Hughes et al. (J), but well-evaluated procedures for the analysis of medieval stained glass by flame A A S remain undocumented. M a n y analytical studies of ancient glass have yielded results of archaeological significance (I J). Glass compositions have been interpreted primarily with a view to clarifying glass-making techniques (12), provenance (13), or corrosion phenomena (14). A n extensive series of analytical studies has been critically assessed (15), but, despite some early investigations of stained glass (16,17), it is only i n recent years that this field has become the subject of more intensive study. In a pioneering series of papers, Geilmann and his coworkers employed wet chemical techniques to determine specific elements of interest in understanding glassmaking technology (18-20); more recently, the same approach has been adopted using x-ray fluorescence (21). This spectrometric technique has also enabled a relationship between the weathering and composition of medieval stained glass to be established (22). T h e importance of the analysis of major and minor constituents has been stressed b y Brill (23) in a paper that suggests that data on five major constituents (the oxides of sodium, potassium, calcium, magnesium, and aluminum) are often sufficient to characterize a glass. Moreover, the levels of major, minor, and trace elements, determined by neutron activation analysis, have been shown to be consistent for glass from a single stained glass panel (and probably even from a particular workshop), despite the general heterogeneity of stained glass compositions (24). A n understanding of the colorant role of elements i n stained glass has also been advanced by several analytical studies (8,25,26), most recently by Mossbauer spectroscopy (27). T h e purpose of this chapter is to outline the development of a reliable procedure that can be used to determine the elements that are the most significant constituents of medieval stained glass. T h e scope,
accuracy,
and precision of the method are evaluated i n terms of standard glasses; the archaeological significance of the analyses, i n terms of the criteria
8.
TENNENT ET AL.
AAS Analysis of Medieval Stained Glass
135
outlined above, is assessed for medieval stained glass excavated from Scottish sites.
Experimental Glasses. The standard glasses employed in this study consisted of the series of 10 glasses with typical medieval composition prepared for the European Sci ence Foundation by Pilkington Brothers Ltd., with the aid of a grant from the Nuffield Foundation (28), and Standard D , prepared for the Corning Museum of Glass as part of an interlaboratory analytical investigation (29). Twelve frag ments of medieval stained glass excavated from the sites of St. Andrews Cathedral (sample StAl), Melrose Abbey (sample MAI), Holyrood Abbey (sample HA1), and, principally, Elgin Cathedral (samples E C 1-9) formed the basis of the in vestigation. The colors of these samples are described in Table IV and, in some cases, illustrated in Figure 1. The glass fragments had all undergone surface weathering due to burial, but the underlying glass remained in good condition. The weathering consisted of a fine corrosion film (EC1,2,9), deep pits (EC7, 8, and HA1), heavy encrustation (EC4-6), or a heavily pitted encrustation (EC3, StAl, and MAI), and was removed mechanically prior to analysis. In two in stances (StAl and HA1) there was evidence of painted designs on the fragments prior to treatment. Apparatus. Perkin-Elmer (PE) 360 and 272 atomic absorption spectrom eters, with an air-acetylene or a nitrous oxide-acetylene flame, were employed for the analyses. Standard hollow cathode lamps were used as light sources. In all cases, the instruments were operated as recommended by the manufacturer; gas pressures, gas flows, slits, wavelengths, and other controls were adjusted to the prescribed values. The readout was obtained directly using a 5-s integration in the concentration (PE 360) or absorbance (PE 272) mode. Reagents. Reagents of the highest purity (Spectrosol or Aristar grades, B D H ) were used throughout. Stock solutions of each element were prepared from an appropriate salt and stored in polythene bottles in 10" M hydrochloric acid. Cesium chloride (AnalaR grade, B D H ) was added as an ionization sup pressor to sodium and potassium standard solutions and glass sample solutions. Lanthanum oxide (Spectrosol grade, B D H ) was added as a releasing agent to magnesium and calcium solutions and glass sample solutions and as an ionization suppressor to the aluminium solutions. Acetylene and nitrous oxide gases were supplied by B O C . 2
Sample Preparation. The glass samples (—100 mg) were carefully ground in an agate mortar and passed through a 200 British Standard mesh sieve. The finely ground samples (^75 μιτι) were placed in an electric oven at 105 °C for 1 h, cooled in a desiccator, and weighed prior to dissolution. (Sample weights of 30-100 mg have been analyzed successfully.) Dissolution was accomplished by the method of Langmyhr and Paus (30). Each finely ground sample (~100 mg) was transferred to an open 250 m L P T F E beaker, and 5 m L of 40% (v/v) hydrofluoric acid added, followed by 0.5 m L of perchloric acid. This mixture was taken to dryness on an electric hotplate and cooled. A further 5 m L of 40% hydrofluoric acid and 0.5 m L of perchloric acid were added, and the mixture again taken to dryness. The beaker was cooled, and 10 m L of 0.1 M hydrochloric acid was added. The salts were then brought into solution by heating. The clear solution was cooled and made up to 100 m L by the addition of distilled water to a polythene volumetric flask. Lanthanum
136
ARCHAEOLOGICAL CHEMISTRY
and cesium (1000 ppm) were added, where appropriate (as indicated in the following section), to the solutions at the final dilution stage prior to analysis. Interferences. Each element under consideration was investigated for the effect of interference on the determination of all other elements. The effects of titanium, tin, and antimony, not analyzed in this study but known to be present in medieval stained glass, albeit at levels less than 1%, were also investigated. Silicon was not considered because it is removed by volatilization at the dissolution stage. A solution of the element of interest, 1 ppm (10 ppm and 20 ppm for lead and aluminium, respectively) in 10~ M hydrochloric acid was determined by AAS. A similar solution of this element was then prepared in the presence of 500 ppm of the "interfering" element and the atomic absorption signal compared with the blank solution. If the analytical signal did not change by more than ± 5 % , then the element was regarded as not having a significant interference effect on the analyte element. 2
Only chemical interferences were observed; sodium and potassium ionized in the air-acetylene flame, and aluminum ionized in the nitrous oxide-acetylene flame; magnesium and calcium exhibited evidence of interference by both phosphorus and aluminum. All the other elements were found to be interferencefree. The addition of 1000 ppm of cesium as an ionization suppressor effectively removed the ionization interference in the sodium and potassium solutions. Similarly, 1000 ppm of lanthanum removed the interference due to phosphorus and aluminum in the magnesium and calcium solutions and suppressed the ionization of aluminum.
Results and Discussion Development of the Technique.
T h e quality of analyses by A A S
is highly dependent on procedures for sample treatment. Consequently, recent reports have recognized the need to focus on techniques designed to optimize the stages from sample removal to sample dissolution (1,2,31). F o r stained glass, the removal of small chips, in a manner analogous to the grozing originally used to trim the glass to size, provides a straightforward means of sampling. F o r intact glass panels, it is usually possible to remove a sample unobtrusively from under the leading, most conveniently when the panel is undergoing conservation. In many cases, and virtually without exception for excavated glass, removal of the alteration products resulting from atmospheric weathering or burial is necessary. This removal can be accomplished by mechanical means using a variety of probes, scalpels, and fiberglass brushes. Vestiges of paint should also be removed. F o r the excavated glasses examined in this chapter, paint lines could be discerned in only two cases. T h e samples were ground manually and when possible were sieved. In cases where only a small amount (^50
mg) of glass was available, it
has been found acceptable to omit the sieving step, particularly if the analyst has gained considerable experience in sample preparation. T h e merits of mechanical grinding have been discussed elsewhere (2).
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137
AAS Analysis of Medieval Stained Glass
F o r sample dissolution the method of Langmyhr and Paus (30), described above, offers an attractive combination of ease of handling and good reliability. T h e method suffers from the disadvantage that, because open beakers are used, volatilization of silicon tetrafluoride occurs, precluding the determination of silicon by this means. This has not proved to be a severe handicap; because 12 of the principal medieval stained glass components are determined i n the reported method, silicon can be determined to a good approximation by difference. T h e removal of silicon interference in the determination of aluminum and magnesium (1) is an added virtue of this method. Scrutiny of the boiling points of the fluorides of the other elements of interest established that losses of these fluorides during dissolution would be insignificant. T w o additional hydrofluoric acid methods have been reported (1,2), and are similar to that described above. T h e method of Hughes et al. has also been the subject of two comparative studies relevant to the analysis of ceramics (2,31). Techniques that retain silicon have been discussed (1,2) and involve either fusion with lithium metaborate [or sodium carbonate (2)] or high pressure dissolution i n a P T F E bomb. A n alternative high pressure method, developed by Price and Whiteside (32), was evaluated i n the course of this investigation but was found to be unreliable for stained glass of medieval composition; in many experiments dissolution was incomplete. Attempts to modify the procedure by varying the prescribed dissolution parameters produced insufBciently consistent results although superior conditions were established (Table I). T h e importance of interferences in A A S has been stressed (2). W e have observed only chemical interferences for stained glass of medieval composition. T h e interference due to ionization of sodium, potassium,
Table I. Data for the Variation of Dissolution Parameters
Parameter Water (mL) Aqua regia (mL) 40% Hydrofluoric acid (mL) Time of first heating stage (min) 4% Boric acid (mL) Time of second heating stage (min)
Optimum Conditions
Prescribed Conditions"
Variation
2.5 1.0
0.5-3.0 0.5-3.0
0.5
0.5-4.0^
3.0
30 5
20 - 70 0.5-6.0*
60 2.5
20
10-40=
30
c
6
6
0.5 2.0
0.2 g samples were proposed (33). The reagents were scaled down for 0.1 g samples in this work, but the reaction times were kept constant. Varied by increments of 0.5 m L . Varied by increments of 10 min. a
b 0
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ARCHAEOLOGICAL CHEMISTRY
and aluminum has not been reported in the analysis of ancient glasses by A A S , but the ionization is effectively suppressed by the addition of an excess of cesium or lanthanum. In the analysis of calcium and magnesium, interferences were found to be due to the formation of compounds of aluminum and phosphorus with the analyte element i n the flame, thereby decreasing the rates of atomization compared with those for the analyte element alone. T h e interference is removed by the addition of lanthanum, which acts as a releasing agent and avoids the necessity of using the nitrous oxide-acetylene flame as proposed by Hughes (J). In the A A S determination of copper, cobalt, and nickel in blue glasses described by Bettembourg (S), calibration was accomplished in a complex medium containing five elements (aluminum, potassium, calcium, magnesium, and phosphorus) at levels corresponding to the average concentrations in the glasses under investigation. T h e absence of interferences for copper, cobalt, and nickel established in this study obviates the need for this approach. Standard Classes.
T h e accuracy of the A A S method has been as-
sessed by analysis of the 10 synthetic glasses prepared for the European Science Foundation ( E S F ) and of Corning Standard D , which also served to determine the precision of the results. These standards were selected in preference to glasses with better-evaluated specifications because their compositions are representative of medieval stained glass. T h e E S F standards were prepared primarily for the purpose of i n vestigating glass durability and conservation methods. Their role as analytical standards has been secondary, but is important in view of the wide range of medieval stained glass compositions that they typify. In addition to the analysis initially reported (28, quoted in part in Table II), expressed conventionally as weight percentage of the oxide, determinations have been carried out by at least one additional laboratory (33) and the results provide a reasonable gauge of the accuracy of new analytical procedures. T h e results in Table II not only attest to the accuracy of the present A A S method, but also provide a sounder basis for the use of these glasses as standards for future analytical investigations. T h e A A S results are, for the most part, the means of three instrumental readings from two duplicate analyses carried out by two of the authors. T h e agreement is good in almost all instances; in those cases where a significant discrepancy from the published figures was found, repeat analyses were performed. T h e sodium analysis of standard 7 6 - C - 1 5 1 ( N a 0 , 3.7%) 2
is,
for example, the mean of 10 samples, and the standard deviation (0.099) gives a good measure of confidence in the A A S result. T h e tentative recommended figures for C o r n i n g Standard D
(29,
quoted in part in Table III) are based on the results of an interlaboratory analytical investigation, but, as a consequence of the Corning M u s e u m flood in 1972,
confidence levels for these figures have not yet been
2
0.1(0.1) N.D. 0.1(0.1) 0.1(0.1) 0.1(0.1) 8.1(9.5) 3.7(5.0) N.D. N.D. 22.2(21.7)
76-C-144 76-C-145 76-C-147 76-C-148 76-C-149 76-C-150 76-C-151 76-C-158 76- C-159 77- C-33
30.0(29.5) 24.2(24.9) 15.2(14.6) 14.6(14.6) 14.6(14.3) 1.6(1.5) 6.9(7.2) 23.2(24.2) 13.9(14.5) N.D.
2
K0
20.6(20.6) 29.2(29.4) 30.2(30.1) 26.0(25.9) 21.2(21.5) 22.4(21.9) 19.9(19.0) 28.8(28.4) 34.0(34.9) 22.8(22.6)
CaO 0.04(0.04) 0.06(0.05) 0.06(0.05) 0.07(0.05) N.D. 6.5(6.6) 3.0(3.2) 0.10(0.06) 0.08(0.07) 0.06(0.05)
MgO 3
3.7(3.9) 3.9(3.8) 3.9(3.8) 4.0(3.9) 4.0(4.2) 4.3(4.3) 3.8(3.9) 3.8(3.8) 3.8(3.9) 4.0(4.1)
2
Al 0 3
2.4(2.1) N.D. N.D. 0.4(0.4) N.D. 0.7(0.3) 0.4(0.3) N.D. 0.1(0.1) 2.8(2.5)
2
Fe 0 0.03(0.05) 0.1(0.1) N.D. 0.2(0.2) 1.4(1.8) N.D. 0.3(0.5) 0.1(0.1) 0.6(1.0) 0.04(0.05)
N.D. 0.16(0.16) N.D. 1.0(0.9) N.D. N.D. 0.82(0.71) 0.16(0.14) 0.5(0.5) N.D.
PbO
(published)
MnO
Composition Determined N.D. 0.5(0.5) 0.08(0.07) N.D. N.D. 0.1(0.1) N.D. 0.5(0.5) N.D. N.D.
ZnO
CoO
NiO
N.D. N.D. N.D. N.D. N . D . 0.1(0.1) 1.8(1.8) N.D. N.D. 0.08(0.07) N . D . 0.08(0.08) 0.82(0.91) 0.25(0.24) N . D . N.D. 0.09(0.09) N . D . 0.1(0.1) N.D. N.D. N.D. N . D . 0.1(0.1) 0.5(0.5) N.D. N.D. N.D. N.D. N.D.
CuO
NOTE: Data were determined (published in Ref. 28) as weight percentage. N . D . , Not determined; these components were not present in the original batch constituents.
Na 0
No.
Sample
Table IL Analyses of Synthetic Medieval Stained Class Standards
140
ARCHAEOLOGICAL CHEMISTRY
Table III. Analysis of Corning Class Standard D
Mean Composition (x) Determined Standard Deviation (published) (σ) NaaO
κ ο 2
CaO MgO A1 0 Fe 0 MnO PbO ZnO CuO CoO NiO 2
3
2
3
1.36 (1.39) 11.01(11.56) 14.73(14.99) 4.89 (4.06) 5.00 (5.42) 0.46 (0.51) 0.40 (0.55) 0.21 (0.25) 0.10 (0.10) 0.37 (0.38) 0.03 (0.02) 0.06 (0.06)
Relative Standard Deviation [(100σ/ χ)]
0.021 0.110 0.138 0.101 0.125 0.021 0.022 0.011 0.002 0.011 0.002 0.001
1.5 1.0 0.9 2.1 2.5 4.5 5.5 4.9 2.0 3.0 6.7 2.1
published. T h e agreement of the A A S results with the published values is again good, although the discrepancies i n magnesium and manganese analyses are surprising in the face of good agreement found with the E S F standards. T h e precision of the results for Standard D , based on the analysis of eight samples, is given i n Table III. It was the initial aim of this investigation that all elements of interest should be determined by A A S . Phosphorus is known to be an important elemental component of medieval stained glass (18,21) at levels ranging from about 1 to 7%. [Figures extracted from 90 published analyses (34,35) gave a mean P 0 2
5
concentration of 3.6%. ] Although direct determination
of phosphorus by A A S has been reported, several practical difficulties are encountered i n achieving low detection limits. G o o d sensitivity can be achieved by ensuring an intense phosphorus line source and a vacuum or inert-gas purged monochromator (36) but direct determination of phos phorus remains a field of development. Because the levels of phosphorus in medieval stained glass are generally >1%, it is hoped that the direct determination by flame A A S , currently under investigation, may be pos sible. It is intended that A A S determination of phosphorus as part of a program of trace element analyses with furnace techniques will also be evaluated. Analyses of certain glasses from Table IV, by using a scanning electron microscope with an energy dispersive x-ray fluorescence spec trometer attachment (37), gave P 0 2
5
levels i n the range 2.4-3.5%.
T h e tin concentration i n the glasses under consideration lies close the detection limit (0.02%) for the flame A A S method described herein. T i n analyses were carried out but are not quoted because the proposed method does not offer sufficient accuracy for this element.
19.,7 26. 6 12. 6 21. 0 15. 8 15..8 14. 1 14. 9 25. 2 21. ,1 12. ,2 13.,2
12.,2 10.,2 18.,3 17..8 19. 4 18. 0 9. 6 11. 2 10. ,5 18.,8 20. ,9 18, .1
0.,6 0..6 0. 9 1. 3 1.,7 1.,7 2. 0 3. 1 0.,8 0.,3 2,,0 1..7
(sky blue, 1.7 mm) (sky blue, 1.8 mm) (deep blue, 5.0 mm) (deep blue, 3.0 mm) (amber, 2.1 mm) (amber, 2.3 mm) (emerald green, 2.9 mm) (emerald green, 2.7 mm) (light green, 2.3 mm) (light green, 3.7 mm) (purple, 5.1 mm) (emerald green, 5.0 mm)
4.5 3.7 6.7 6.9 6.3 6.3 6.2 9.1 3.8 5.3 6.3 6.2
1.,1 1.,0 1. 0 0.,8 0..6 0.,4 0,.5 0,,6 0,.8 0,.5 0,.6 0 .7
3
3.,4 3.,4 2.,4 3. 0 1. 9 1. 4 1.,3 1.,2 3..3 3..3 1..5 1, 6
2
Fe Q
3
2
MgO Al Q 0. 9 0. 6 0.Λ 0. 7 0. 9 1. 2 0. 9 0, 8 0. 8 0.,8 1. 4 1. 2
ZnO 0. 17 0. 31 0. 60 0. 20 0. 09 0. 08 0. 07 0. 07 0. 02 0.,03 0.,07 0.,21
PbO 0. 10 0. 14 0. 32 0. 22 0. 08 0. 16 1. 07 1.,23 0.,02 0.,01 0,,01 0, 18
percentage)
MnO
(weight
0.,07 0. 17 0. 25 0. 28 0. 05 0. 05 2. 40 3. 07 0. 02 0.,01 0.,01 2.,10
CuO
NiO
