In the Laboratory
Analyzing Lead Content in Ancient Bronze Coins by Flame Atomic Absorption Spectroscopy An Archaeometry Laboratory with Nonscience Majors Mary Kate Donais* Department of Chemistry, Saint Anselm College, Manchester, NH 03102; *
[email protected] Greg Whissel Merrimack, NH 03054 Ashley Dumas Kingston, RI 02881 Kathleen Golden Somerset, MA 02726
This article describes a laboratory experiment used in an Introduction to Archaeology course to teach students about archaeometry—the measurement of chemical and physical properties of samples of archaeological materials—and numismatics, the study of and collecting of coins. The students determined the lead content of ancient bronze coins by flame atomic absorption spectroscopy (FAAS) and then compared their lead data to literature values for lead in visually identified coins (1) to approximate the mint date and mint location. (See Figure 1.) This type of data matching by metal content in coins is possible because ancient mints each had unique procedures for making bronze and regional ore sources, thus leading to predictable differences in the major and minor metals in coins. As well, the coins used were severely corroded such that visual identification was not possible; provenance could therefore be determined only through chemical analysis. This Classics Department archaeology course has no science-based prerequisites, so the experiment has been performed by students with limited chemistry backgrounds. The experiment could easily be adapted for chemistry majors by either or both expanding the number of samples analyzed and the statistical evaluation of data. Advances in archaeometry have led to the use of methods such as laser-induced breakdown spectroscopy, X-ray fluorescence, and laser-ablation inductively coupled plasma–mass spectrometry (2–5) for analysis of major and minor metals in bronze. These techniques require minimal amounts of sample material, allow for direct analyses of solids, and produce multielemental results. However, the instrumentation is complex, ex-
Figure 1. An example coin shown prior to subsampling it.
pensive, and available mainly to graduate students and advanced researchers. Sample homogeneity and data precision are also of concern with these lesser- and nondestructive methods. Although it is a destructive method of analysis, acid digestion followed by FAAS analysis was chosen for our measurements for two reasons. First, some nondestructive techniques such as X-ray fluorescence are based on surface measurements and may not provide a true indication of the bulk metal content in coins (6–7). Secondly, many archaeological artifacts from excavations are plentiful enough to subject some samples that are nondiagnostic—that is not able to be identified by simple visual inspection—to destructive analysis in order to gain valuable chemical information. Recent reports on destructive analyses of ancient coins and other bronze artifacts (5, 7–8), pottery (9), and glass (10) demonstrate this. An experiment was developed that allowed undergraduate students to obtain lead data on an instrument available at most colleges and universities. Although other quantitative experiments for lead by atomic absorption spectroscopy have been reported in this Journal, only three were flame-based methods (11–13). More importantly, the interdisciplinary nature of this classics–chemistry lab makes it stand out even from other labs that integrate chemistry and environmental science (11), materials science (14), and art (15). In the first of two, seventy-five minute sessions, students learned about bulk subsampling, data precision, and data validation. Acid digestion of coin segments was then started. The principles of FAAS and external calibration were reviewed and discussed at the instrument while the digests heated. The session concluded with quantitative solution preparation. The second session started with completion of solution preparation followed by analyses of samples and standards. Students then calculated the percentage of lead by mass in their subsamples and compared these results to literature values (1). A list of possible mint dates and mint locations based on these comparisons was obtained. Students critically evaluated their results and discussed what further laboratory data could be collected to refine their lists. Students learned that precision played a significant role in data evaluation and that analyses of additional subsamples for each coin and for other metal concentrations could improve confidence in their conclusions.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 3 March 2009 • Journal of Chemical Education
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In the Laboratory List 1. Instrument Settings and Parameters Lamp current: Wavelength: Flame type: Fuel flow:
15 mA 217.0 nm air/acetylene 1.1/L min−1
Burner height:
7.0 mm
Bandpass:
0.5 nm
Nebuliser uptake:
4.6 mL min−1
Experimental Methods Chemicals and Other Materials Aqua regia, a 3:1 mixture of concentrated hydrochloric acid and concentrated nitric acid, was used for acid digestion of the coin subsamples. A 10,000 μg mL−1 certified lead standard (VHG Laboratories) was used to prepare calibration standards. A 2% aqueous nitric acid solution was prepared as a diluent for samples and standards. A reference standard from the National Institute of Standards and Technology, SRM 872 Phosphor Bronze, was used for method validation. Bronze coins were excavated by Saint Anselm College students on summer archaeological expeditions in Crete. Uncleaned ancient coins may also be obtained from various Internet vendors at a minimal cost (16–17). Equipment A rotary cutting tool (Robert Bosch Tool Corporation) and a small, table-mounted vise were used to subsample coins. A DigiPREP Jr. digestion system with 50-mL volumetric polypropylene digestion vessels (SCP Science) was used for digestion of the coin subsamples. Alternatively, small beakers and a water bath could be used. A Thermo Elemental S Series FAAS with a lead hollow cathode lamp (Thermo Scientific) was used to measure and collect all the data. Calipers and a digital camera were used to make a record of each coin prior to its destruction. List 1 provides details about equipment settings and variable parameters; additional information is discussed in the online supplement. Preparation Prior to Laboratory Sessions A handout describing the principles and basic operation of a FAAS was provided to students one week prior to the lab sessions. Discussions of coin excavation from archaeological sites and numismatics were conducted in class; this material is normally covered in the Introduction to Archaeology course and may not be necessary for a science course performing the experiment. Each coin was washed with a soft-textured brush and warm water and then rinsed in deionized water prior to analysis. A light sanding of the coin surface was performed using the rotary tool for coins with significant corrosion, followed by rewashing and rinsing. For each coin, dimensions were recorded, a digital picture was taken of the front and back, and a laboratory identification number was assigned. Each coin was then clamped along its thickness in the vise such that approximately two-thirds of the coin remained exposed above the clamp. Using the rotary tool, the coin was halved horizontally and each half placed into separate containers. The halves were then individually secured in the clamp and segmented into 0.2-g portions. Subsamples from each half were kept separate and labeled using a system such as “5A-1” and “5A-2” for two 344
subsamples of Coin 5 from Half A. To avoid cross-contamination of subsamples, a fresh rotary blade was used for each coin. The coin subsamples were weighed and put into labeled digestion vessels. Subsamples of SRM 872, each at 0.5 g because of the homogeneity of the reference material, were also prepared and labeled using the same “-1” and “-2” system. All washing, sanding, and documentation of coins was done by the instructor prior to lab to save time. All coin subsampling with the rotary tool was performed by the instructor as a safety precaution. Lead standards at 4.0 ppm, 6.0 ppm, 8.0 ppm, 10.0 ppm, and 12.0 ppm in 2% nitric acid and 0.8% aqua regia were prepared from the certified lead standard; the acid content in these solutions was matched to the diluted coin digests to be analyzed by FAAS during session two. Hazards The rotary tool is very sharp and can cause serious injury. (All cutting of coins with the rotary tool was done by the instructor.) Aqua regia is highly corrosive—to avoid contact with skin and eyes, safety glasses and gloves should be worn by the instructor and students at all times during the laboratory sessions. Toxic fumes are generated during coin digestion: all sample preparation must be done in a well-vented hood. The use of volumetric digestion vessels eliminated a quantitative transfer step in the experiment and reduced risks to students’ safety. Students were reminded throughout the sessions that the solutions contained both corrosive acids and toxic heavy metals. The FAAS was operated according to the manufacturer’s specifications with proper exhaust venting of the gases. Laboratory Session One To start, the instructor demonstrated using the rotary tool on one coin to obtain subsamples. During this demonstration the instructor also discussed sample homogeneity, data precision, and data validation with a certified reference material. Students were then assigned subsamples of the coins to analyze. Depending on the number of students and subsamples, each student can prepare his or her own sample or students can do this in small groups. In a well-vented hood, 20.0 mL aqua regia was added to each digestion vessel. The vessels were then loaded into the digestion system and heated at 85 °C for thirty minutes. While the coin solutions heated, students were shown the FAAS and its operation was described as a review of the handout. External calibration with lead standards was explained. After the coin solutions cooled they were brought to volume with deionized water. A 1:50 dilution of each coin digest solution—which consisted of 1 mL coin solution plus 400 μL aqua regia diluted to 50.00 mL with 2% nitric acid solution as the diluent—was prepared to end lab session one. This final step in student solution preparation can be done to start session two if time is short in session one. Laboratory Session Two Instructor-prepared, acid-matched lead standard solutions were provided to the students. Diluted coin solutions and standards were analyzed in triplicate by FAAS using the conditions specified in List 1. The instrument software was used to establish external calibration for lead and to calculate the concentration of lead in each diluted coin solution. A detection limit of 0.014% Pb by weight in bronze coins has been determined for
Journal of Chemical Education • Vol. 86 No. 3 March 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Table 1. Comparison of Measured Lead in Ancient Bronze Coins Coin Identification
Coin Subsample Mass/g
Measured Pb in Solution, ppm
Pb in Coin, Percentage Pb by Weight
Mean Pb in Coin, Percentage Pb by Weight
2A 2B
0.2476 0.2049
0.751 0.553
1.52 1.35
1.43
3A 3B
0.2028 0.2456
5.176 6.019
0.255 0.245
0.251
4A 4B
0.2575 0.2510
1.018 0.958
1.98 1.91
1.94
5A 5B
0.2708 0.2683
4.707 5.798
8.69 10.80
9.75
6A 6B
0.2347 0.2019
6.494 5.580
13.83 13.82
13.83
8A 8B
0.1810 0.2550
11.131 7.385
15.37 7.24
11.31
9A1 9B1
0.1810 0.1980
8.739 9.849
24.14 24.87
24.50
10A 10B-1 10B-2
0.1954 0.1618 0.1645
5.390 4.777 6.596
6.77 7.38 10.02
8.06
11A 11B
0.1435 0.1576
9.872 11.513
17.20 18.26
17.73
SRM 872-1 SRM 872-2
0.4958 0.50002
8.168 7.934
4.12 3.97
4.04
1For
the Coin 9 digest a 1:100 dilution was needed to keep the sample within the method calibration range.
the method. The detection limit was determined as three times the standard deviation of an acid-matched blank calculated back to a value of the percentage of lead by weight in the solid using a 1:50 dilution factor and 0.2-g sample mass. Results and Discussion Typical Data Data for nine coins and the standard SRM 872 are reported in Table 1. Using the equation provided in their handout (see the online supplement), students calculated the percentage of lead by weight for each subsample and the mean percentage of lead by weight for each coin. Note that only results from subsamples of the same coin were averaged, as the lead levels are expected to vary for different mint locations (1). Data Evaluation Students were given a packet of published data (1) for comparing with their results, and a spreadsheet onto which they recorded matches. Through guided inquiry, students determined that a mean value for each coin should be calculated and that data precision must be considered. General trends in the variability of lead content of one class of ancient bronze coins, imperial bronzes, are shown in Table 2 as an example of literature data that could be provided to students (1). Lead content in imperial bronze coins increases with mint date from very low amounts in coins from late B.C.E. to the 1st century C.E., a gradual increase throughout the 2nd century C.E., and finally a significant increase in lead content during the late 2nd century and 3rd century C.E. These trends are reflected
Table 2. Distribution of Lead Content Measured in Imperial Bronze Coins Mint Date of Coin
Mean Pb in Coin, Percentage Pb by Weight
Range of Pb in Coins, Percentage Pb by Weight
23 B.C.E to first century C.E.
0.89
0.00–1.67 (for most coins)
Early to midsecond century C.E.
2.21
0.11–4.41
Late second century to third century C.E.
11.95
5.59–22.43
in the mean lead content and ranges provided in Table 2. As well, high lead content can also be indicative of an imitation (counterfeit) ancient coin commonly referred to as a “fourrée” (1). Precision for the measured lead values in each coin was evaluated as the percentage of deviation from the mean for each coin subsample. Deviation values of 10% and less are acceptable and have been found by other researchers (1). Six of the nine coins (Coins 2, 3, 4, 6, 9, and 11) reported in Table 1 show deviations of 5.9% or less, and Coin 5 was just above this at 10.8%. Coins with more significant variability, such as Coins 8 and 10, allowed for discussion with students about sample homogeneity and experimental error. The minting techniques used to make ancient coins would occasionally cause pooling of elements such as lead within a coin, and could cause deviations greater than 10% (1).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 3 March 2009 • Journal of Chemical Education
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In the Laboratory Table 3. Coin 6 Sample List of Possible Matches to Coin Data in the Literature Student Data
Matches to Literature Data Location of Mint
Obverse Legenda
Reverse Legend or Imageb
Ruler Associated with the Coin
Mint Date (C.E.)
14.18
Rome
“IMP ALEXANDER PIVS AVG”
“P M TR P X COS III P P S C. Sol”
Severus Alexander
231
13.86
Imitation
—
—
Domitian
79–96
13.40
Imitation
“DIVO CLAVDIO”
“CONSECRATIO”; altar
Claudius II
—c
12.98
Alexandria
“ΑΚΜΑΝ V ΡΠΡΟ BOC CEB”
eagle pictured on right
Claudius
282
Pb in Coin, Pb in Coin, Percentage Pb Percentage Pb by Weight by Weight 13.83
aUsually
13.40
London
“IMP CONSTANTINVS AVG”
“SOLI INVICTO COMITI”
Claudius I
313–317
12.47
Arles
“IMP CONSTANTINVS P F AVG”
“SOLI INVICTO COMITI”
Claudius I
313–317
13.05
Rome
“IMP CONSTANTINVS P F AVG”
“SOLI INVICTO COMITI”
Claudius I
313–317
the “heads” side of a coin
bUsually
the “tails” side of a coin cNo mint date provided for this coin
Table 3 shows some possible sample matches for Coin 6. A 10% deviation from the mean percentage of Pb by weight for this coin generates a range of 12.45% Pb to 15.21% Pb. Coins with literature values within that range were located in the provided literature data and transferred to the match spreadsheet. Method validation was also discussed as demonstrated through analyses of the standard SRM 872 with a certified lead content of 4.13 ± 0.03% by weight. The result for SRM 872-1 agreed within 0.3% of the certified value while SRM 872-2 was 3.2% below the low value in the certified range. This deviation provided students with the opportunity to discuss method validity and the need for additional analyses.
Students gained experience with quantitative sample and solution preparation, and chemical data collection. The concepts of method validation and data precision were illustrated through evaluation of their data. The list of best matches generated at the end of the labs provided students with possible age and mint locations for the coins, although they concluded that additional data for other metals would aid in refining matches. This unique application of analytical chemistry to archaeology provided hands-on experience to students who would typically never be exposed to chemical instrumentation. The experiment demonstrated the interdisciplinary nature of the chemical sciences and how collaboration between two seemingly dissimilar disciplines (chemistry and classics) can lead to advances in both fields.
2. Burakov, V. S.; Raikov, S. N. Spectrochim. Acta B 2007, 62B (3), 217–223. 3. Quynh, N. T.; An, T. T.; Thiep, T. D.; Chien, N. D.; Cao, D. T.; Liem, N. Q. Comm. Phys. 2004, 14 (1), 50–56. 4. De Wannemacker, G.; Vanhaecke, F.; Moens, L.; Van Mele, A.; Thoen, H. J. Anal. Atom. Spectrosc. 2000, 15 (4), 323–327. 5. Talib, D.; Ma, R.; McLeod, C. W.; Green, D. Can. J. Anal. Sci. Spectrosc. 2004, 49 (3), 156–165. 6. Notis, M.; Shugar, A.; Herman, D. Ariel, D. T. Archaeological Chemisty, ACS Symposium Series 968, Glascock, M., Ed.; American Chemical Society: Washington DC, 2007; pp 258–274. 7. Bourgarit, D.; Mille, B. Meas. Sci. Tech. 2003, 14, 1538–1555. 8. Glumlia-Mair, A. Archaeometry 2005, 47, 275–292. 9. Bruno, P.; Caselli, M.; Curri, M.; Genga, A.; Striccoli, R.; Train, A. Analyt. Chim. Acta 2000, 410, 193–202. 10. Zachariadis, G.; Kimitrakoudi, E.; Anthemidis, A.; Stratis, J. Talanta 2006, 68, 1448–1456. 11. Weidenhamer, J. J. Chem. Educ. 2007, 84, 1165–1166. 12. Markow, P. G. J. Chem. Educ. 1996, 73, 178–179. 13. Choi, S.; Larrabee, J. A. J. Chem. Educ. 1989, 66, 864–865. 14. Van Hecke, G. R.; Karukstis, K. K.; Li, H.; Hendargo, H. C.; Cosand, A. J.; Fox, M. M. J. Chem. Educ. 2005, 82, 1349–1354. 15. Kafetzopoulos, C.; Spyrellis, N.; Lymperopoulou-Karaliota, A. J. Chem. Educ. 2006, 83, 1484–1488. 16. Dirty Old Coins Home Page. http://dirtyoldcoins.com (accessed Nov 2008). 17. Vcoins Online Coin Show Home Page. http://www.vcoins.com (accessed Nov 2008).
Acknowledgments
Supporting JCE Online Material
Conclusions
Many thanks to David George for providing the bronze coins and seeing the need to teach archaeology students a bit of chemistry. Partial funding for this research was supplied through a Sigma Xi Research Grants-in-Aid. Literature Cited 1. Cope, L. H.; King, C. E.; Northover, J. P.; Clay, T. Metals Analyses of Roman Coins Minted under the Empire, British Museum Occasional Paper Number 120; The British Museum: London, 1997. 346
http://www.jce.divched.org/Journal/Issues/2009/Mar/abs343.html Abstract and keywords Full text (PDF) Links to cited URLs and JCE articles Supplement Detailed instructor notes; student handout including prelaboratory questions, instructions for the experiment, and data recording sheets
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