Mode selection

Mining mode on the Niton/Thermo Scientific pXRF is used in similar research, such as Gauss et al. (2013) and Hayes (2013), because it contains all elements of interest to archaeological research. However, as stated in chapter 3, each mode assumes a set sample composition. This was tested using SRM NIST 2711a. As table 4.3 shows, lighter elements, in this case Ca, are closer to the certified value in soils mode. Strontium values are also closer to the certified value in soils mode, but with marginally higher instrument error (set to 2σ). The reverse is true of Cu, although here the differences are minimal. The heavier element Pb, is notably closer to the certified value in mining mode, with little difference in instrumental error.

y = 9947e^-0.001x R² = 0.7353

6000 7000 8000 9000 10000 11000 12000

0 20 40 60 80 100 120 140

ppm

TIme in seconds

P

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Table 4. 2. Table showing the effect of different in filter times on measured values and error. All figures in ppm, with filter times in seconds, for the filters main, low, high and light, in that order.

SAMPLE

Therefore, the selection of inbuilt filter settings is dependent upon the elements of interest and the sample matrix. In general, lighter elements such as Ca, K and Ti performed better under soils mode, however, this mode does not measure P or S, which are both historically and currently seen as central to archaeological geochemical interpretation.

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Table 4. 3. Analysis of SRM NIST 2711a in two modes: mining (Cu/Zn) and soils. Each mode was repeated three times and an average taken. The certified values for the selected elements are also shown. All values on ppm, all times in seconds. individual research questions, environmental and archaeological restraints. Within this was also a process of evolution as the method was refined

by experience. The methodological variation is expanded on a site-by-site basis in the following sections.

In the following section, the deviations from the standard laboratory and analytical procedures are given on a site-by-site basis.

Figure 4. 9. An opened core being directly analysed using pXRF. Photo: Marianne Hem Eriksen/author.

76 4.3.4 Avaldsnes

Process

As the majority of the Avaldsnes samples were small bagged samples, no direct pXRF measurements were undertaken, not even on the cores. The cores analysed were recorded as detailed above prior to sub-sampling at stratigraphic intervals. The aim of using the cores for the geochemical analysis was either as background samples to assess soil formation or to see if cores could add temporality to geochemical datasets. The data was intended to be comparable to the individual samples; therefore regular interval sampling on cores would have served no purpose, as the bagged samples were from a specifically identified pedological horizon. All samples were dried at 105⁰C for 24 hours, before being sieved to 1 mm in a stainless steel sieve, as detailed above. In many cases, as the sample was composed of fine silt, sieving made no difference to the sample composition.

Samples were then placed in purpose made sample cups, the base covered by a 4 µm polypropylene film, before being placed in the field-stand for analysis. Prior to each analytical

‘run’ and at an interval of ten samples, the SRMs were analysed using the same filter times and settings as the samples. In addition, a Si sample with c. 400 ppm Ca was analysed after the SRMs.

This was used as a blank to ensure the instrument window was clean and as a quick method to check the basic instrument response. This frequency of re-measuring the SRMs was necessary to calibrate for potential drift and to have a statistically significant and relevant data set for calibration purposes if required.

Time and variation

Sample time was 180 seconds, in mining (Cu/Zn) mode, with the filter times being 30 seconds for the main filter, 60 seconds for the low, 30 seconds for high and 60 seconds for the light. This was selected based upon other’s experience (Gauss et al., 2013, Doonan et al., 2014, Vos, 2016) and the test data presented above and in Appendix 1. The use of a helium purge was strongly considered to improve the detection of lighter elements. Access, however, was a problem, and therefore it was not used.

Initially, each sample was analysed three times and averaged. After evaluating the data, the trebling of the analytical time was deemed too time consuming, although it did highlight occasional instrumental errors when the results were viewed in spreadsheet or spectra format.

These were rare, and almost exclusively connected to P and S. As these errors were detectable by examining the data, and the analysis non-destructive and therefore ensuring repeats were possible, it was decided upon to pursue single analysis per sample.

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Through the evaluation of the data set, it was considered that 180 seconds was not sufficient without the use of a helium purge to sufficiently reduce error in the lighter element readings. A few samples were run on a longer filter time of 300 seconds (50 main, 100 low, 50 high, 100 light), and this reduced error. Therefore in the second case study, Heimdalsjordet, considering that helium was not available, longer filter times were employed.

Limitations of the data set

The variation in analytical time is a concern for the Avaldsnes data set. It is not possible to standardise the data after the analysis is complete nor, due to time restrictions, was it possible to repeat a large number of the samples. In total 493 readings were taken from 189 samples, the time taken to process and analyse the samples and check the data was approximately one month. The data was not empirically calibrated to test the internal fundamental parameters calibration of the instrument.

4.3.5 Heimdalsjordet

A different approach was employed with the Heimdalsjordet samples in response to the archaeological conditions, as detailed in chapter 6, section 6.4. The site was intended to be the central case study in this research, where the methodology of using pXRF directly on minimally prepared cores would be employed with the aim of gaining high-resolution, three dimensional geochemical data sets from secondary contexts. Therefore data from core analysis forms the bulk of the data for this case study.

Process

As stated in section 4.3.1, all cores were assessed and recorded prior to analysis. Each core was photographed, the soil structure, Munsell colour, inclusions and interfaces observed, and an interpretive archaeological stratigraphy created as an Excel spreadsheet (see Appendix 2). Prior to analysis, 6 µm polypropylene film was placed between the instrument and sample point to keep the instrument window clean. Unfortunately the finer 4 µm film was not available. The instrument window also has a 4µm polypropylene film. This was checked between each sample to ensure no stray material was on the pXRF detector window. The instrument was handheld for the direct core analysis, however it was controlled via a computer, and an automatic cut off at 300 seconds was used to ensure comparable times. The sample time was therefore 300 seconds for all but two cores, using mining (Cu/Zn) mode, as described in section 4.3.1. Cores were analysed at 2 cm intervals using pXRF, the distance measured from the top of the core using a manual ruler. Data was viewed in real time via the instrument screen and the connected laptop computer. Repeats were taken where readings suggested an error had occurred, and at random

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points to verify repeatability. Sub-samples from cores were taken from selected cores after direct analysis to verify differences in values and error as a result of sample processing. The selection of cores for sub-sampling was per parcel, which ensured variations in subsoil composition were represented in the sub-sample data. The sub-samples were dried at 40⁰C for at least 24 hours in single use containers. The lower temperature came from concern that organic content may be lost with the higher temperature of 105⁰C used on the Avaldsnes samples. On reflection, this is probably not the case, however this was pursued at the time. All data is presented in section 4.4 or Appendix 2.

Time and variation

A 30 cm core had potentially 15 sample points, although in practice this was 14 as the upmost and lowest 1 cm were avoided as these could be composed of material displaced by coring.

Therefore 2 cm was the first sample point, and 28 cm the final. Before any sampling commenced for the day, there was a system check on the instrument and then four SRMs were measured.

Sampling was begun at 2 cm, and preceded logically in 2 cm intervals, each sample point requiring a new polypropylene film. After ten or less samples, the SRMs would be reanalysed.

Depending upon the complexity of the stratigraphy, sampling problems and challenges and required sub-sampling, between one and three cores could be processed in an eight hour day.

Therefore, in theory, between 5 and 15 cores could be analysed per standard working week, and it was not difficult to achieve 10-15 per week. The bulk of the analysis was completed in a one month period, allowing for processing of sub-samples and checking the data. Additional analysis was done at a later stage, using the same methodology.

Some cores were not entirely composed of ‘archaeological’ sediments. The lowest centimetres in many of the cores represented undisturbed subsoil, and in cores taken from the topsoil, there was up to 26 cm of topsoil. Initially the ‘non-archaeological’ layers were sampled at 2 cm intervals, as it was thought it would be of interest to assess leaching and contamination. As time progressed, it became clear this was both time consuming and unnecessary for all cores.

Therefore, per parcel and plot, at least one core was analysed at 2 cm for all contexts, archaeological and non-archaeological. In the remaining cores, 4 cm intervals were employed on the non-archaeological contexts. The statistical treatment of the data from the topsoil, subsoil and archaeological layers is detailed in chapter 6, section 6.4.6.

Challenges specific to Heimdalsjordet

The instrument sample window is 8 mm, making a higher sampling resolution possible. However, once the instrument is placed, the bulk of the instrument nose makes millimetre precise sampling challenging, and higher resolution obviously significantly increases the time required

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per 30 cm core. Few identified contexts were thinner than 8 mm, ensuring that the sample window represented one context only per sample. Interfaces and very fine contexts were avoided, as were inclusions such as bone, charcoal or other archaeological objects. As these rarely dominated any context, this was simply a matter of turning the core and placing the instrument with great care. Avoiding ferrous and ochre mottles, a product of hydromorphic processes, was sometimes more challenging as they appear in dense clusters and in several contexts, and were often large and dominant. The data clearly showed when such a mottle had been analysed by the high proportion of Fe. As this would distort the archaeological information, these readings were not included in the statistical analysis. On rare occasions, a sample interval was left without analysis as gaining a representative 8 mm wide sample point was impossible.

In an interval between processing and analysing the Heimdalsjordet samples, the instrument was returned to the retailer for repair and calibration after it developed a fault. The consequences of this are covered in section 4.4.

The first two cores from Heimdalsjordet had analytical times of 30-60-30-60, giving a total of 180 seconds. This was selected as holding the instrument perfectly still for 5 minutes was taxing.

These are cores 7936 and 7945. After viewing the error for the lighter elements, longer filter times were chosen for all other cores, and a better positioning reduced movement and strain.

The readings from the two cores are not greatly dissimilar to the other cores, although error values are slightly higher. After the experience gained from analysing 40 cores, some with several sections, it was observed that the slightly shorter filter times of 40-80-40-80 would have been equally sufficient for the majority of elements, however, without the use of the helium purge, relatively long analytical times are necessary.

Limitations of the data set

In short, the major limitation is stratigraphical comparability. Depths are precise to 2 cm, however as each sample is isolated, no contextual continuality can be assumed. This subject is discussed further in chapter 6, section 6.8. As far as the data itself is concerned, there are unquantifiable sources of variation. These occur as the instrument is handheld, meaning that despite all efforts, there will be slight variations in the distance between the instrument nozzle and the sample surface. The intervals for sampling are by eye and ruler, and therefore will not be perfectly consistent. The moisture content of the cores was low as they had lost some moisture during storage, but they were not completely dry when analysed. The moisture content was essential for seeing the stratigraphical changes within the samples, as without it all samples are rendered to a hard white lump. Without measuring the moisture content, the known disproportionate effect this property has on individual elements cannot be quantified.

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These, and other factors named here, render the analysis semi-quantitative, despite calibration using SRMs. Figure 4.10 contains a summary of the analytical process.

Figure 4. 10. Flow diagram of the sample processing and analysis for cores from Heimdalsjordet.

81 4.3.6 Kaupangveien

Process

The general method for Kaupangveien is broadly comparable to Heimdalsjordet, as previous experience had highlighted effective approaches and lessons in consistency had been learned.

All ten cores were taken from an excavation surface, where the topsoil was mechanically removed and archaeological features identified as negative features in the subsoil. Therefore, cores were treated similarly, being cleaned, recorded, and analysed directly. Sample intervals were 2 cm for archaeological layers, and 4 cm for non-archaeological layers. As topsoil was not present, this consisted of subsoil only. Analytical time totalled 300 seconds for all filters, in mining mode (Cu/Zn). The SRMs were identical to those used for the Heimdalsjordet analysis, as was the frequency of analysis.

Time and variation

Sub-samples and bagged individual samples from the smithy were dried at 40⁰C for 24 hours in single-use containers.

The issues with the sample window, the error in measuring intervals distances on the cores and avoiding large inclusions were largely similar to Heimdalsjordet, although there were no mottles from hydromorphic processes in the soils to avoid. As with the Heimdalsjordet samples, the first and last 1 cm of each core were avoided.

In situ readings were not processed further, as the data is inherently varied and incomparable.

Challenges specific to Kaupangveien

The subsoil exposed by topsoil stripping at Kaupangveien was medium to coarse, moderately sorted sand representing the former back beach from shoreline retreat (see chapter 7). The substrate under this was marine clay silt, becoming increasingly gleyed and laminated with depth, but most cores did not reach these depths and thus the substrate in the cores was beach sand. The majority of cores were archaeological material, with a high organic content and a sandy loam matrix. Upon opening the cores, the poorly consolidated sands lost their form, as did the non-archaeological part of the core. Interfaces were very sharp, and the archaeological deposits, stabilised through the organic content and matrix, proved possible to analyse directly.

However toward the base of these deposits, the weight of the instrument began to crumble the remaining core. Therefore, many sections of the cores had to be sub-sampled rather than recorded directly. Where the depth of the archaeological deposit was thin (<10 cm), the entire core was sub-sampled at 2 or 4 cm intervals (cores 1936, 1938, 1939 and 1942, all from the wall ditch).

82 Limitations of the data set

As stated in the previous section, the analytical method was inconsistent due to the collapsing of the cores. Therefore the comparability of the data is questionable and semi-quantitative. In the final section of this chapter, data comparing direct core analysis to sub-sampled material is presented and evaluated.

From this experience, the efficiency of method of directly measuring cores is dependent upon the soils and sediments present on the site.