Before any quantitative analyses were conducted using SEM-EDX, a method of standardisation was devised and a number of standard reference materials and certified reference materials were selected and used to calibrate the instrument. Preliminary qualitative and quantitative analyses of a large number of archaeological slag samples were conducted to identify the major and minor elemental constituents. This information was used to aid in the selection of the most suitable standard reference materials. From this preliminary study it was determined that a total of thirteen individual elements would be standardised for (Table.9.). The standardisation procedure was conducted for elements rather than simple oxides, and all data was later converted to simple oxides for comparative purposes.
Table.9. The elements determined and their simple oxides.
A matrix-matched standard reference material was selected in the form of a Viking Age bloomery slag from a site at Gryssen in Dalecarlia, Sweden. This standard, W-25:R, was supplied by AGL National Heritage Board, Geoarchaeological Laboratory and was used during the element
standardisation procedure and was also used during the daily standardisation check procedure. The reference material was provided as a fine homogeneous powder which was pressed in to a pellet for analysis by SEM-EDX.
W-25:R was used to provide the standardisation of aluminium, silicon, calcium, iron and oxygen.
Micro-Analysis Consultants Ltd. (MAC), Reference Standard for X-ray Microanalysis, Registered Standard No. 3918 was used to standardise sodium, magnesium, phosphorous, sulphur, chlorine, titanium and manganese. Natural orthoclase ORT10 SRM, on the University of Liverpool, Department of Archaeology's own internal standard reference material was used to standardise potassium. Oxygen standardisation was verified using artificial orthoclase (KAlSi3O8), artificial wollastonite (CaSiO3), artificial anorthite (CaAl2Si2O8) and artificial periclase (MgO) on Johnson, Matthey & Co. Ltd. Mineral Certified Reference Material for Microprobe MSS S.4002/J340. It was found that the best oxygen correlation was with both the artificial anorthite mineral and the artificial orthoclase mineral, when compared to the W-25:R reference material. Figures obtained for the elemental composition of W-25:R were also cross-checked against both MAC 3918 and MSS S.4002/J340. All figures are presented as weight percentage. After successful standardisation, each of the standard reference materials was analysed in triplicate, with a typical total variance of less than 1.3 wt% being observed.
Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO FeO Total Av. 0.62 0.43 7.72 25.59 0.30 0.09 1.06 1.46 0.35 3.21 59.63 100.19 SD 0.10 0.06 0.36 0.83 0.10 0.02 0.04 0.23 0.04 0.11 1.39 ΣSD 3.28 Table.10. The average results of seven published concordant analytical datasets for W-25:R using various analytical techniques (Kresten and Hjärthner-Holdar 2001).
Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO FeO Total Av. 1.35 0.42 7.90 22.54 0.24 0.27 1.18 1.61 0.35 3.31 60.83 100.00 SD 0.15 0.08 0.18 0.74 0.07 0.11 0.07 0.05 0.03 0.11 0.79 ΣSD 2.38 Table.11. The average normalised results of six repeat EDX analyses for W-25:R (Paynter 2006).
Na2O MgO Al2O3 SiO2 P2O5 S K2O CaO TiO2 MnO FeO Total 0.61 0.38 7.14 24.73 0.26 0.04 1.02 1.42 0.32 3.01 57.10 96.03 Table.12. The analytical results for W-25:R provided along with the standard reference material by AGL National Heritage Board, Geoarchaeological Laboratory. No standard deviation figures were provided with these results.
Na2O MgO Al2O3 SiO2 P2O5 S K2O CaO TiO2 MnO FeO Total Av. 0.75 0.26 7.33 24.53 0.33 nd 1.56 1.39 nd 3.05 60.80 100.00 SD 0.09 0.08 0.09 0.15 0.08 ~ 0.06 0.04 ~ 0.16 1.28 ΣSD 2.03 Table.13. The average results of ten repeat EDX analyses for W-25:R using the SEM-EDX internal standardless analysis algorithm, this algorithm provides only normalised results. Small peaks for both sulphur and titanium were resolved above the continuum bremsstrahlung baseline, however, the software was unable to provide a numerical value for these peaks.
Na Mg Al Si P S K Ca Ti Mn Fe O Total
Av. 0.50 0.23 3.93 11.32 0.10 0.18 0.91 1.05 0.20 2.41 46.33 31.45 98.61 SD 0.13 0.07 0.08 0.11 0.10 0.06 0.08 0.03 0.04 0.14 1.08 0.80 ΣSD 2.72 Table.14. The average results of ten repeat EDX analyses for W-25:R after the Certified Reference Material calibration had been completed. This data was collected using the modified method determining all elements rather than simple oxides.
Na2O MgO Al2O3 SiO2 P2O5 S K2O CaO TiO2 MnO FeO Total Av. 0.68 0.38 7.42 24.22 0.24 0.18 1.10 1.47 0.33 3.11 59.60 98.73 SD 0.13 0.07 0.08 0.11 0.10 0.06 0.08 0.03 0.04 0.14 1.08 ΣSD 1.92 Table.15. This table displays the simple oxide results for W-25:R generated from the above elemental dataset in Table.14. The conversion of elements to stoichiometric simple oxides was achieved using published compound ratios according to Potts et al (1992).
As you can see from the above results in Table.14. and Table.15., the totals for both the directly measured elemental percentages (98.61 wt%) and the calculated simple oxides (98.73 wt%) are almost identical. This result is a particularly good indicator that the simple oxide stoichiometric composition of the W-25:R reference material is close to ideal.
Simple Oxide Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO FeO Fe2O3
Compound Ratio 1.3480 1.6582 1.8895 2.1392 2.2916 1.2046 1.3992 1.1174 1.2912 1.2865 1.4297
Table.16. Compound ratios according to Potts et al (1992). These ratio values were used to convert elements to oxides.
Even though the elements sodium, chlorine, sulphur, titanium and manganese were calibrated for using the relevant standard reference materials, these elements were deliberately ignored in the final results. The reason these elements have been omitted is due to them not being detected in the bulk slag component of any of the Tell Dhiban or Armenian Garden samples. A small number of samples did indicate detection limit quantities of sulphur, however this was only present in very few of the samples or analysis zones within samples, and typically in concentrations of <0.5 wt%. Chlorine was also detected in numerous samples in very minor quantities, this has been entirely omitted as the epoxy resin used to encapsulate the samples displayed a high chlorine content. A carbon peak was also observed on many of the EDX spectra, which was not unexpected as the samples are coated with carbon, this peak was also ignored due to both the coating as well as having no suitable carbon standard reference materials.