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Appendix 2a: SEM-EDS analysis of residues from pottery from Wade Street, Bristol

Kamal Badreshany

Cite this as: Badreshany, K. SEM-EDS analysis of residues from pottery from Wade Street, Bristol in N. Corcos et al. Excavations in 2014 at Wade Street, Bristol - a documentary and archaeological analysis, Internet Archaeology 45. https://doi.org/10.11141/ia.45.3.3

Introduction

The analysis of five samples of residues was undertaken by SEM-EDS at the Durham Archaeomaterials Research Centre. The aim of the analysis was to gain a better understanding of the function of these pots by determining the composition of these residues. The five samples were collected from two different contexts in Area A, 1145 and 1181.

Samples and description

Analysis was undertaken on the following samples: AG100, AG101, AG102, AG103 (context 1145), and AG104 (context 1181) as follows:

SEM-EDS Bulk Analysis: Analytical procedure and sample preparation

The samples were examined at the Durham Archaeomaterials Research Centre (DARC) using a Hitachi TM3000 SEM fitted with a SwiftED3000 EDS. The accelerating voltage was set to 15 kV and the probe current was set to 700 pA. The bulk compositional analysis was generated by the SwiftED software using standardless matrix corrections and is semi-quantitative. The residues of AG100 and AG101 were broken in half, exposing fresh sections for analysis. The cross-section of both residues was analysed to investigate whether they were affected by the weathering of the vessel's interior glazing. Contamination would be indicated by a greater amount of metals and other components commonly found in glazes towards the margins of the cross-section, where they would have accumulated to a greater extent after deposition as the pot surfaces weathered. Conversely, a uniform composition throughout the section would likely indicate that weathering of the vessel glazes had little impact on the results of the analysis. The residues from samples AG102, AG103 and AG104 were composed of a fine powder; no additional preparation steps were required for analysis.

Results

AG100

The Back-scattered electron (BSE) images of AG100 show a heterogeneous, but uniform, composition throughout the section. The EDS analysis indicates an inorganic composition, mostly of lead and iron, with lesser amounts of aluminium, silicon, sulphur, and calcium.

Figure 57
Figure 57: Back-Scattered Electron Image of AG100 showing a heterogeneous, but uniform inorganic composition. The brighter spots in the image are mostly composed of lead and iron and the darker areas are a mixture of sulphur, aluminium, silicon, and calcium
Figure 58
Figure 58: EDS spectrum AG100

Summary results

Table 9: Results from the EDS analysis of AG100
ElementWeight %Weight % σAtomic %
Oxygen28.1621.21561.841
Aluminium1.7750.2782.312
Silicon4.7320.3595.920
Sulphur1.9710.5192.160
Calcium2.1390.3381.875
Iron33.7591.23021.237
Lead27.4601.3904.656

AG101

The Back-scattered electron (BSE) images of AG101 similarly indicate a heterogeneous, but uniform, composition. The analysis of AG101 indicated a similar inorganic composition to that of AG100, containing roughly the same concentration of lead, aluminium, silicon, sulphur, and calcium. The composition of AG101 varies from that of AG100, as it contains high levels of copper and lower levels of iron.

Figure 59
Figure 59: Back-Scattered Electron Image of AG101 showing a heterogeneous, but uniform, inorganic composition. The brighter spots are mostly composed of lead and copper and the darker areas are a mixture of sulphur, aluminium, silicon, phosphorus, calcium and iron
Figure 60
Figure 60: EDS spectrum of AG101

Summary results

Table 10: Results from the EDS analysis of AG101
ElementWeight %Weight % σAtomic %
Oxygen38.5390.75874.553
Aluminium2.3210.1612.662
Silicon2.8840.1753.178
Phosphorus0.5470.1490.547
Sulphur1.3110.2901.265
Calcium1.9220.1861.484
Iron1.8780.3741.041
Copper22.8380.70211.124
Lead27.7610.7894.147

As mentioned above, the residues AG100 and AG101 probably derive from a liquid. The SEM-EDS analysis confirmed that the original material was inorganic and composed largely of metals. The BSE images show a uniform composition throughout the section of the residues, indicating no contamination from the vessel surfaces. Positively identifying the original materials from which the residues were derived would require further analysis. SEM-EDS analyses of pre-modern lead-based paints (Zagora 2013; Higgitt et al. 2003; Galván Josa et al. 2010) present a similar bulk chemistry to that of samples AG100 and AG101. AG100 and AG101 could thus represent lead-based paint residues. Interestingly, both AG100 and AG101 contain a similar concentration of lead, but vary in the composition of other elements like iron and copper, which are both well-known colourants. The differences in the composition of these elements could indicate that the residues represent different pigments.

AG102

AG102, the white residue from the interior surface of a vessel, was analysed to investigate whether it represented a lead residue. The SEM-EDS analysis confirmed it is mostly composed of lead. The amount of lead found in AG102 is somewhat higher than that found in AG100 or AG101, but the residue is otherwise of broadly similar composition to these two samples, suggesting it might also represent a paint residue.

Figure 61
Figure 61: Back-Scattered Electron Image of AG102 showing a heterogeneous, but uniform, inorganic composition. The brighter spots are mostly composed of lead. The darker areas are a mixture of aluminium, silicon, phosphorus, potassium, calcium and iron
Figure 62
Figure 62: EDS spectrum of AG102

Summary results

Table 11: Results from the EDS analysis of AG102
ElementWeight %Weight % σAtomic %
Oxygen27.5450.43375.182
Aluminium2.1480.0863.477
Silicon2.7330.0954.249
Phosphorus0.5040.0820.711
Potassium0.3420.0920.382
Calcium1.6550.1081.803
Iron0.8430.2130.659
Lead64.2300.45713.537

AG103

AG103 was tested to investigate whether the residue contained lead or resulted from post-depositional concretions. The EDS analysis showed a high calcareous content.

AG104

AG104, the greenish residue found on the inside of a dark brown glazed pottery fragment that may have been part of a crucible or a drainage pipe, was analysed to determine if the colour was due to a high copper composition. A small sample of the residue was scraped from the interior of the fragment. The results of the EDS analysis confirmed the residue was composed mostly of copper oxide and some aluminium.

Figure 63
Figure 63: Back-Scattered Electron image of a fragment of the residue from AG104. The fragment is composed mainly of Copper Oxide
Figure 64
Figure 64: EDS spectrum of AG104

Summary results

Table 12: Results from the EDS analysis of AG104
ElementWeight %Weight % σAtomic %
Oxygen49.3633.23172.321
Aluminium18.0051.79415.642
Copper32.6323.55512.038

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