1.1. Background

Although glass was widely used in Roman Britain, it becomes much rarer in the Late Antique period and the vessels have a noticeably different appearance, in both style and colour [1, 2]. The glass itself was made on a vast scale in the form of large slabs at glass manufactories in Egypt and the Levant coast [3–5]. From there the glass slabs were broken up and exported widely to workshops across the Mediterranean and Europe where it was re-melted and made into vessels for further distribution [6].

The key ingredient in this glass was 'natron', a source of sodium-rich minerals that formed in the Wadi Natrun in Egypt [7], which was combined with sand. The colour of natron glass was due largely to iron-rich impurities in the sand but was commonly modified by adding compounds of antimony or manganese to influence the oxidation state of the iron dissolved in the glass. The redox conditions were further influenced by the melting conditions (temperature, duration and atmosphere of heating) and the degree of recycling, since recycled glass (cullet) would often contain more impurities and have an altered oxidation state relative to freshly made glass [8]. By the Late Antique period, transparent glass usually contains manganese rather than antimony as a colour-modifier [9] and has a stronger green or olive hue relative to Roman glass because the iron content of the glass increases significantly [10].

1.2. Archaeological glass

The archaeological glass studied here is a style characterised as Atlantic tradition glass by Campbell [11–14], which is known from sites along the west coast of Britain and Ireland, and was imported from the Bordeaux region of western France. Atlantic tradition glass is pale yellowish to amber coloured, often decorated with opaque white glass trails (Group C), but may be undecorated (Group D), and in the form of cone beakers or small bowls. Some of these fragments display an amber colour with purple streaks. The glass studied here comes from Tintagel, a high-status settlement on a promontory in the far southwest of Britain, and from Whithorn, in the southwest of Scotland, another high-status settlement which became an important place of Christian pilgrimage in the Late Antique period. A variant of the Atlantic tradition is found at Whithorn, the 'Whithorn' tradition (Group E), and may have been manufactured there. The composition of the glass is a natron type known as Foy 2.1, which is thought to originate in Egypt but has been identified across Continental Europe and the Byzantine Empire, including Cyprus, France, Spain, Italy, Germany, Bulgaria, North Africa, Egypt, and Britain [15–22]. Foy Type 2.1 is noticed particularly in the 6th and 7th centuries AD but is thought to have originated in the late 5th century. It is typically yellow/greenish, but there is also an iron-rich variant, which has a deeper green colour and is mainly noted from the mid-6th century onwards.

1.3. Influence of manganese and iron on natron glass colour

Previous studies have investigated the range of colour in natron glasses [23–25] with a focus on Roman and Late Roman glass of slightly different composition by using X-ray absorption near edge structure (XANES) and optical spectroscopy to characterise the oxidation states of iron and manganese.

The combined presence of iron (Fe) and manganese (Mn) produces glasses with a wide range of colours (from green to amber to purple). In the presence of iron, manganese can either act as a purple colourant when present as Mn3+ or can oxidise the bluish reduced iron (Fe2+) to its yellowish oxidised form (Fe3+), resulting in its almost colourless form (Mn2+). The reaction that would occur between Fe and Mn is the following:

In XANES spectra of iron in glass, the energy position of the pre-edge peak and the main edge have been found to be strongly influenced by the oxidation state (shifted to higher energy for Fe3+). Besides the pre-edge intensity varies as a function of the site symmetry. A shift to higher energy for the pre-edge and the main-edge is also observed at the Mn K-edge with increasing Mn oxidation state [23, 24, 26–29]. However, determining a link between glass colour and Mn speciation (2+, 3+ and 4+) in ancient glass has proved more complex than for iron; whilst distinctions are clear in mineral samples with different oxidation states [30–32], studies of ancient and reproduction glasses have consistently failed to detect much variation in Mn K-edge spectra using XANES in glasses of different colours [23, 28]. Analyses suggest a prevalent presence of the almost colourless Mn2+ dominating the XANES spectrum, and possibly hiding the additional presence of the pink Mn3+ form, even in strongly purple glass. De Ferri et al [23] showed that XANES failed in detecting Mn3+ in a Mn2+ rich purple glass, while Mn3+ is more clearly detected by UV–VIS spectroscopy. One of the advantages of the use of optical absorption spectroscopy is the determination of both Fe and Mn ions simultaneously, although their quantification is problematic due to the difficult assessment of the ions site geometry in glasses [33]. The same observations were made by Schalm et al [34] and Arletti et al [24], who emphasised that the higher oxidation states of Mn are likely unstable at the temperatures used in glass production. Capobianco et al [28] investigated a wide range of flesh-tone to purple glass in medieval stained glass and confirmed that the vast majority (at least 95% of the total Mn) of manganese in the glass is Mn2+ and that the remaining small amount of Mn3+ shifts the colour from light pink to purple. It was also underlined that purple glass can be produced only in unusually oxidising conditions. As demonstrated in reaction (1), Fe2+ and Mn3+ cannot be present in glasses at equilibrium, leading to the hypothesis that a short duration of melting and fining is necessary to promote an oxidised redox state of manganese in out of equilibrium conditions. The time taken for melts to reach equilibrium, the influence of pot size and geometry and furnace atmosphere, the addition of reducing agents, and the use of different raw materials are discussed by Bidegaray et al [25] and the impact of some of these issues can also be seen in the experimental melts recreated for this study.

1.4. Aims

The Atlantic tradition glass is visually very distinctive; it is thin, good quality and has an unusual pale yellow to amber colour. Conversely most contemporary glass from Anglo-Saxon sites in Britain and north-western France is more often shades of blue, green or brown, despite being made from the same type of glass [18, 35]. The colour and appearance of the Atlantic tradition glass was intentionally created by the glassworkers and these properties were valued by the communities on the West coast of Britain.

This paper investigates the colouring technologies used by glassworkers in 5th–7th century glass imported to Atlantic Britain by correlating the glass colour with the iron and manganese ratios and their oxidation states. In particular, this study focuses on: (1) the influence of different glass production parameters (addition of reducing and oxidising agents, effects of mixing and re-melting glass batches produced in different redox conditions and changes in melting duration) in experimental glass samples; (2) the variation of Fe and Mn oxidation states in archaeological glass with visible heterogeneity in the form of purple streaks; and (3) the optical character of some of the archaeological samples.

Experimental natron-type glass and archaeological glass from Tintagel and Whithorn (UK) were analysed using different synchrotron-based techniques, including bulk Fe and Mn K-edge XANES spectroscopy, micro-XANES (μ-XANES) and micro X-ray fluorescence (μ-XRF). The Fe and Mn distribution and oxidation states in the samples was mapped by μ-XRF and μ-XANES, respectively.

2.1. Materials2.1.1. Archaeological glasses

The archaeological glass samples studied here belong to the Atlantic tradition glass and they were recovered from archaeological excavations in Tintagel and Whithorn (UK) (table 1). In some of the archaeological glass, there were visible purple bands, probably from incomplete mixing, which suggested some alteration of the glass colour just prior to blowing the vessels (figure 1). In addition, some Atlantic tradition glass appears pinkish/yellow in reflected light but greener when viewed in transmitted light (figure 1).

Figure 1. Optical images of cross-section of amber glass with purple streaks from Tintagel (T1) (a) and Whithorn (WH1) with a white opaque decorative trail visible in the upper surface (b); yellow/pink fragments from Tintagel (c) and Whithorn (d) in reflected (left) and transmitted light (right) displaying a greener colour in the latter.

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Table 1. Summary of the archaeological glass samples from Tintagel and Whithorn. The vessel number refers to the classification in [36].

Sample IDSmall finds number/vessel numberProvenanceAtlantic tradition glass groupColourT15040TintagelGroup DYellow with purple streaksT22025TintagelGroup BLight greenT33099aTintagelGroup CYellowT45006TintagelGroup BGreenT55096TintagelGroup CYellow/pinkWH116125/Vessel 22WhithornGroup CYellow with purple streaksWH214429/Vessel 69WhithornGroup EYellowWH314753/Vessel 67WhithornGroup EYellowWH41362/Vessel 55WhithornGroup DYellow2.1.2. Experimental glass

In addition to the archaeological glass samples, a series of experimental glasses with known compositions (following the chemical composition of the archaeological glass from Tintagel and Whithorn) were made up under controlled conditions by glassworkers Mark Taylor and David Hill (table 2 and figure 2). These batches investigated three variables:

(1)  

Composition: Test glass batches S1 and S2, each 150 g, were melted for 2 h in a gas furnace at 1300 °C in ceramic crucibles with lids. The glass was then cast onto a surface and cooled in air. These batches contained different ratios of iron to manganese oxides (table 2).

(2)  

Redox: Small test glass batches S1, and S1a–S1d, each 150 g, were melted for 2 h in a gas furnace at 1300 °C in ceramic crucibles with lids. The glass was then cast onto a surface and cooled in air. These batches all had the same composition, S1, but in batches S1a to S1d different amounts of KC4H5O6 (a reducing agent) were substituted for K2CO3 (table 2).

(3)  

Mixing: Multiple large batches with composition S1 and S1d were made in a pot without a lid, each 1 kg in total, melted overnight in a gas furnace using a gas/air mix at 1300 °C (renamed P1 and P1d). These were cooled and crushed to chunks of 1 cm or less. These were mixed and remelted (M1 and M2) to create the amber colour of Atlantic tradition glass. M2i contains a higher proportion of P1 to P1d than M1.

(4)  

Heating duration: The second mixture was heated for a day and sampled after mixing (M2i), after 1 h (M2ii) and after a day (M2iii).

Figure 2. Small test batches of experimental glass with different amounts of reducing agent prepared in lidded crucibles (S1–S2), (a) large batches melted in open pots (P1 and P1d) (b) and mixes for glass blowing to create the amber colour of Atlantic tradition glass (M1–M2iii).

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Table 2. Summary of the experimental glass and their characteristics.

Sample IDReducing agent (wt%)MnO2/Fe2O3DescriptionS101.43Brown glass melted in crucibleS1a0.101.48Light brown glass melted in crucibleS1b0.201.44Green glass melted in crucibleS1c0.151.46Light green glass melted in crucibleS1d0.131.48Brown glass melted in crucibleS202.08Dark brown glass melted in crucibleP101.45Dark purple glass melted in potP1d0.131.39Yellow/green glass melted in potM1/1.40Yellow/green mixed glass melted in potM2i/1.40Purple mixed glass melted in pot and sampled after mixing and meltingM2ii/1.38Purple/amber mixed glass melted in pot and sampled after 1 hM2iii/1.40Amber mixed glass melted in pot and sampled after 1 d

Both the archaeological and experimental glass specimens were mounted in resin and polished to a ¼ micron finish to be analysed.

2.2. Methods2.2.1. 3D digital microscopy

The samples were observed by 3D digital microscopy, using the Keyence VHX7000 3D digital microscope at different magnifications.

2.2.2. Laboratory bulk XRF spectroscopy

The compositions of the archaeological and experimental glass were determined using a Bruker M4 Tornado bench-top energy dispersive μ-XRF spectrometer. At least three points were analysed for 200 s livetime, with the XRF being operated at 50 kv, 200 µA, in a vacuum with no filter. The results are given in the supporting information (tables S1 and S2).

2.2.3. Scanning electron microscopy combined with an energy dispersive spectrometer (SEM-EDS)

To study the sample morphology and evaluate the presence of nanocrystals or droplets, or weathering layers responsible for the optical properties, the samples were also analysed by SEM combined with an EDS, using a FEI Inspect F, with an Oxford Instruments X-Act SDD and INCA software. The machines (XRF and SEM-EDS) were calibrated using sets of glass standards.

2.2.4. Colorimetry

Colorimetric measurements were performed using a X-Rite Ci62 portable spectrophotometer with a D65 illuminant. Five measurements were carried out on each glass fragment and the data are presented according to the CIE L*a*b* standard colour system.

2.2.5. Synchrotron-based μ-XRF mapping, bulk and μ-XANES spectroscopy at Fe and Mn K-edge2.2.5.1. Acquisition of bulk XANES data

Fe and Mn speciation measurements of the experimental natron-type glass and the archaeological glass were performed at beamline ID21 of the European Synchrotron Radiation Facility (Grenoble, France) [37]. The energy of the X-ray beam was defined using a Si(111) double crystal monochromator, and scanned from 6.53 to 6.68 keV, with steps of 0.4 eV, for the Mn-edge and from 7.1 to 7.25 keV, with steps of 0.4 eV, for the Fe-edge. The monochromator was calibrated using the first inflection point of the manganese and iron foils (maximum of the first derivative at 6.5515 keV and 7.1271 keV respectively). Macro-XANES spectra were collected using an unfocused beam (300 µm square), in transmission mode for the reference powders, and in XRF mode for the glass samples. For homogenous samples, five points were analysed, and the spectra were normalised and averaged to enhance the signal-over-noise ratio.

The samples were mounted vertically, at an angle of 62° with respect to the incident beam. XRF was collected using a single energy-dispersive silicon drift detector (SGX, 80 mm2) at 28° of the sample surface.

In addition to the glass samples, reference mineralogical compounds containing iron and manganese in different oxidation states and local geometry were analysed by bulk Fe and Mn K-edge XANES spectroscopy. The minerals containing Fe2+ and Fe3+ used as references are: almandine (Fe3Al2(SiO4)3 with Fe2+), olivine ((Mg, Fe)2SiO4 with Fe2+), hematite (Fe2O3 with Fe3+) and magnetite (Fe3O4 with one Fe2+ and two Fe3+), following references considered in similar cases [23, 24, 26].

The minerals containing Mn2+, Mn3+ and Mn4+ used as references are: rhodochrosite (MnCO3 with Mn2+), manganite (MnO(OH) with Mn3+), and pyrolusite (MnO2 with Mn4+). These reference minerals were finely ground and spread as fine layer on a tape.

The glass samples were analysed embedded in resin in cross-sections.

2.2.5.2. Evaluation of artefacts in XANES data

Before starting the systematic analysis of the samples, preliminary tests were carried out on one of the experimental glasses in order to assess radiation damage caused by the beam to the sample. Experimental conditions were optimized to reduce the dose on the samples: XANES spectra acquired in continuous mode; individual spectra acquired on many different points, rather than cumulating many spectra on the same points, ending in an acquisition time of only ∼1 min per spectrum; use of a fast shutter to expose samples to beam only during data collection (in particular closed at the end of each spectrum, when the X-ray energy is sent back to initial value); detector as close as possible to the sample. Once these conditions were set, radiation damage was assessed on two experimental glasses, by consecutively acquiring five XANES spectra on the same point (figure S1). At Fe K-edge, the second spectrum only slightly differs from the first spectrum (shift of the edge towards lower energies; no clear modification of the pre-edge intensity and position), but the next four spectra are similar. At Mn K-edge, no difference is observed among the five consecutive spectra. Based on this result, and despite radiation damage is very low, it was decided to acquire XANES spectra first at Fe K-edge, then at Mn K-edge.

A second possible artefact in this experiment is due to the fact that the XANES spectra were measured in XRF. In this configuration, spectra can be easily distorted by the self-absorption effect depending on the concentration of Fe and Mn. The importance of self-absorption effect was assessed by comparing raw spectra and spectra corrected using the Athena software [38]. Taking into account the matrix composition, no significant changes in the spectra were observed in the range of Fe and Mn concentrations considered here (∼<1%), indicating that self-absorption is here sufficiently low to be neglected (figure S2).

2.2.5.3. Data processing and analysis of XANES data

Analysis of XANES data was done in different ways. After normalization using the PyMca software, spectra were compared and the variation in intensity at two specific energies (7.136 keV for Fe and 6.562 keV for Mn) has been used to qualitatively evaluate the change of Fe and Mn speciation in the different glasses. These energies were selected by considering the highest values of the standard deviation of the spectra collected from the glass samples and are indicated below by a vertical dashed line in figures 3 and 4.

Figure 3. Fe (a), (c) and Mn (b), (d) K-edge spectra of reference minerals and experimental glasses produced as melts in small crucibles (a), (b) and in larger pots heated overnight (c), (d). The dotted vertical lines represent the energies giving the highest deviation between the spectra shown here (7.136 keV for Fe and 6.562 keV for Mn).

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Figure 4. Fe (a) and Mn (b) K-edge spectra of the archaeological glass from Tintagel (T1–T5) and Whithorn (WH1–WH4), showing more reduced Fe in green T4 and T2 but little detectable difference for Mn between glasses.

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In a second step, XANES spectra were compared using principal component analysis (PCA) with a workflow using the Orange software [39]. PCA was applied on the second derivative of the XANES spectra. PCA scatter plots allow comparing tens of spectra from glasses and references and identifying clusters of spectra. Averages were then calculated over these clusters.

Finally, the analysis focussed on the pre-edge feature of the Fe K-edge XANES spectra, which is particularly sensitive to the valence state and local geometry. The weighted peak position (centroid) shifts to higher energies for increasing ferric to ferrous ratio [30]. The pre-edge peaks of the averaged spectra were fitted with two Gaussians functions as described in the literature using the Larch software [40, 41].

2.2.5.4. Data acquisition, processing and analysis of μ-XRF maps and μ-XANES spectra

The glass samples were also mapped at 7.2 keV by µ-XRF to study the distribution of Fe and Mn in the samples, especially in the archaeological fragments with purple streaks. For these detailed analyses, the beam was focused to 0.23 µm × 0.80 µm (v × h) using a Kirkpatrick Baez mirror system. The PyMca software was used to fit the µXRF maps, to separate the contribution of different elements, and normalise them with incident intensity [42].

To study the dichroic character of the samples, from the μ-XRF maps, points of interest were selected for the acquisition of μ-XANES spectra of the surface of the glass. In particular several points (every 1 µm) were investigated within the first 20 µm from the surface to compare the oxidation state of Fe and Mn on the surface and in bulk.

Finally, in order to qualitatively map the evolution of Mn speciation from the glass surface to the bulk, the same µXRF map was acquired at a few specific energies in the pre-edge and edge region (from 6.55 to 6.61 keV) and different regions of interest were selected. In the maps presented here, the following energy ranges were selected: E1 from 6.566 to 6.573 keV and E2 from 6.556 to 6.563 keV. The integration of the intensity over these energy ranges was calculated and used to produce the maps.

3.1. Influence of different glass production parameters on the experimental glass samples

The experimental glass samples were prepared following the chemical composition of the archaeological glass from Tintagel and Whithorn (tables S1 and S2).

Fe and Mn K-edge XANES spectra acquired from the experimental and archaeological glass fragments show some features that can be linked to the bond distance, oxidation state and site symmetry of these ions. The full spectra are shown in figures S3 and S5, while the close-up of the pre-edge region is displayed in figures 3 and 4.

The comparison of XANES spectra from glasses and from references show that most iron is present as Fe3+ and most manganese as Mn2+. The most significant differences in the spectra occurred in the pre-edge region and just above for Fe, and in the main shoulder in the Mn edge.

The Fe K-edge XANES spectra of the small experimental glass batches produced in lidded crucibles (S1–S2) and the reference minerals indicate that by increasing the amount of reducing agent (KC4H5O6) in the melt, higher amounts of reduced iron (Fe2+) are produced (especially in samples S1b and S1c) compared to the samples without reducing agent (S1 and S2) (mostly visible in the shift of the edge to lower energies, but also in the presence of a small peak at 7.128 keV in the pre-edge) (figure 3(a)).

From the Mn K-edge spectra of the glass (S1–S2), the addition of the reducing agent in the mix does not lead to a significant modification of XANES spectra (figure 3(b)).

By comparing the results obtained from the small glass batches S1 and S1d and the corresponding larger glass batches made in an open pot (P1 and P1d, respectively), some differences in the spectra can be observed however (figure S4). Iron and manganese are in similar oxidation states in P1 and S1 (large overnight and small test batches respectively), while higher amounts of Fe2+ and Mn2+ concentrate in large overnight batch P1d compared to small test batch S1d (figure S4). Measuring errors, particularly in the amount of reducing agent added, are more likely to have a significant impact on glass colour and ion speciation in the small batches so the results for the larger batches (M and P series) are considered more reliable. There will be other differences caused by the longer melting duration and larger open pots used for the M and P batches. This highlights the important roles that the geometry and size of the melt container and melting duration play in the modification of the glass redox [25].

Fe-XANES spectra of the batches produced in large open pots without and with reducing agent (P1 and P1d respectively), mixed in different proportion and remelted (M1), and heated for different durations (M2i, M2ii and M2iii) are shown in figure 3(c). They are similar with iron predominantly present in its oxidised (Fe3+) form. P1d and M1 show a slight shift of spectra to lower energies, while M2i–M2iii spectra are intermediate between P1 and P1d-M1.

Regarding Mn, all the glass samples mainly contain manganese in its reduced form (Mn2+), but the glass prepared with a reducing agent (P1d) and the ones obtained by mixing (M) are shifted to lower energies compared to the purple-coloured P1, indicating a higher Mn3+/Mn2+ ratio in P1 (figure 3(d)). In addition, remelting and reheating the mixed batches (M2i) for 1 h (M2ii) and 24 h (M2iii) results in less Mn3+ in the glass, as the longer melting duration promotes the conversion of Mn ions (in excess compared to Fe) into Mn2+.

3.2. Fe and Mn concentration and speciation in the archaeological glass from Tintagel and Whithorn

The average of Fe and Mn K-edge XANES spectra collected from the archaeological glass fragments from Tintagel (T1–T5) and Whithorn (WH1–WH4) are shown in figures 4 and S5. They are very similar, indicating that iron and manganese are present in a similar oxidation state in all the yellow samples, with mainly Fe3+ and Mn2+. The Fe pre-edge and edge position shows that the green glass samples (T2 and T4) are slightly more reduced than T1, T3, T5 and WH1–WH4 (figure 4(a)).

In the yellow samples with purple streaks (T1 and WH1), different XANES spectra were collected in the yellow and purple areas to assess any changes in Fe and Mn oxidation states. No detectable differences could be observed in neither Fe nor Mn XANES spectra between these areas in the same sample (figure 4).

However, considering the limitations of XANES with glass samples discussed above, the presence of a small and subordinate amount of Mn3+, which would be responsible for the purple colour of the streaks, could not be discounted [23, 28]. Previous studies indicate that even a small amount of Mn3+ can produce purple to light pink colours [43]. In particular, an Mn3+/Mntotal ratio of about 4% was found in purple glasses and considered to be the main agent influencing the colour saturation [28].

The graphs in figure 5 show the intensity of the edge position at 7.136 keV and at 6.562 keV for Fe and Mn, respectively, in the XANES spectra acquired from the experimental and archaeological glass fragments in relation to the Mn/Fe ratio. With intensity values higher than 0.22 at 7.136 keV in the Fe spectra, the glass appears green (T2) and (T4), as more reduced iron is present (figure 5(a)). With intensity values lower than 0.92 at 6.562 keV in the Mn spectra, the glass is purple (P1) due to higher amount of Mn3+ (figure 5(b)).

Figure 5. Graphs showing the intensity at 7.136 keV for Fe (a) and at 6.562 keV for Mn (b) in the XANES spectra acquired from the experimental and archaeological glass fragments and the Mn to Fe ratio. The coloured arrows qualitatively indicate the relative concentration of Fe and Mn in different oxidation states and the associated colour. The black dashed circles indicate data collected from the same sample in yellow and purple areas.

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The intensity at 7.136 keV and at 6.562 keV in Fe and Mn XANES spectra, respectively, is in the same range in both the yellow experimental and yellow archaeological glass, indicating that Fe and Mn are mainly in the same speciation in all the glasses. Compared to P1 (a purple experimental glass) however, Fe and Mn are more reduced in the yellow/amber archaeological samples. As evidenced by the data obtained from M2i, M2ii and M2iii, the Mn3+ content in the glass decreases by reheating the same batch of glass, as more manganese gets reduced to reach equilibrium conditions, according to equation (1). The colour of these fragments changes from purple (M2i) to amber with purple streaks (M2iii) after reheating for 24 h.

These data suggest that the yellow ancient glasses were produced by mixing green glass with small amounts of purple glass containing Mn3+. This ensured that the final glass would have a yellow/pink colour instead of green, and also explains the presence of purple streaks in some of the yellow archaeological fragments.

Except for T4, which has a Mn to Fe ratio lower than 1, the ratio of Mn to Fe in the majority of the experimental and archaeological glass is between 1.4 and 1.9 (tables S1 and S2). Some amber-yellow archaeological glasses (T5, WH3 and WH4) display a higher Mn/Fe ratio (from 2 to 2.5) (figure 5). Despite having a higher content of Mn, the majority of manganese is present as Mn2+, and so the colour of the glass is still amber/yellow. The results obtained from the yellow archaeological glass samples with purple streaks (T1 and WH1, highlighted in black dashed circles) indicate that the Mn/Fe ratio is higher in the purple streaks as the glass has not been fully homogenised. Although the majority of the Mn is present in the same oxidation state in the yellow and purple areas, some Mn3+ must persist in the purple regions due to the additional Mn and non-equilibrium conditions, causing the difference in colour in these areas.

This result was confirmed also by elemental maps obtained by SR-µXRF (figure 6), showing that manganese concentrates in the purple areas and streaks.

Figure 6. Optical images of glass from Whithorn (WH1) and Tintagel (T1) and µXRF intensity maps of Fe and Mn.

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PCA was used to further compare the individual spectra acquired at Fe and Mn K-edges on both the experimental and archaeological samples and better highlight their differences, taking into account the entire white line region, rather than the absorption intensity at a specific energy. The first component for Fe XANES represents only 38% of the variance, showing the high heterogeneity of the dataset. Conversely, PC1 for Mn spectra represents 64% of the variance, and indeed reflects a much lower dispersity of the dataset.

The position of the Fe reference materials in the PCA scatter plot shows oxidized iron (Fe3+) concentrates in low PC1 while references with Fe2+ shifts to high PC1.

In the PCA applied to the Fe spectra, three main groups were identified (figure 7(a)):

(1)  

Group Fe_1 (purple circle): experimental glasses produced in larger pots and heated overnight.

(2)  

Group Fe_2 (orange circle): yellow/amber archaeological glasses.

(3)  

Group Fe_3 (green circle): green archaeological glasses.