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Identifying the “blue substance” at the Roman site of

Vindolanda, Northumberland

Taylor, G1*, Hefford, RJW2, Birley, A3, Huntley JP4

1. School of Science, Engineering and Design, Teesside University, Middlesbrough, TS1 3BA.2. c/o School of Science, Engineering and Design, Teesside University, Middlesbrough, TS1 3BA.

3. Vindolanda, Bardon Mill, Hexham, Northumberland NE47 7JN.4. Department of Archaeology, Durham University, Durham DH1 3LE

*corresponding author, [email protected]

Keywords: Vindolanda, Vivianite, Preservation, Anaerobic, Iron

Abstract: Archaeological landscapes and environments are extremely complex and

diverse systems. Vindolanda, situated in Northumberland, UK, is part of the Frontiers

of the Roman Empire World Heritage site and has remarkable preservation of

artefacts of international significance including leather, writing tablets and bones.

Vindolanda is a waterlogged site and contains anoxic layers which, it is postulated,

aids in the preservation of these important artefacts. The anoxic layers contain many

chemical and microbiological properties which vary from context to context,

depending on what remains in the soil and what is introduced into the archaeological

environment through continued post-depositional processes. During excavations at

Vindolanda, as at other sites the presence of a ‘blue substance’ has been noted by

researchers for many years. This ‘blue substance’ has been generally described in

the literature as Vivianite (hydrated iron (II) phosphate), and has speculatively been

used as a signature of good organic preservation in archaeological contexts. In this

paper, a range of analytical instrumentation SEM-EDX and XRD has been used to

characterise the blue substance, taken from one well defined context to confirm as

Vivianite (Fe3(PO4)2.8H2O).

1. Introduction

The archaeological site of Vindolanda, situated in Northumberland is part of the

Frontiers of the Roman Empire World Heritage Site. The anoxic layers have

produced some of the most remarkable artefacts from the Roman world, such as the

Vindolanda ink on wood writing tablets, thousands of leather boots and shoes and

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even the preservation of fine textiles and other organic remains which seldom

survive in the archaeological record (Birley 2009). A common visual observation of

this exemplary preservation at Vindolanda has been the near constant presence of a

distinctive ‘blue substance’ which has been postulated as Vivianite. Although,

Vivianite has been recognised at archaeological sites its chemical and/or

microbiological formation has not been investigated in exhaustive detail within an

archaeological context (McGowan and Prangnell 2006). Thus, several questions

arise including under what conditions is Vivianite formed? What particular chemical

properties of soil are required? What microbiological processes are important

considerations? Furthermore, and most importantly what do the chemical and

biological properties tell us about the wider archaeological environment and post-

depositional processes in order to aid in the preservation and management of

important archaeological sites?

It is believed that understanding the complex chemical and microbiological

processes that impact upon the formation of Vivianite is essential to gain a more

nuanced appreciation of preservation mechanisms that take place on waterlogged

archaeological sites. Further understanding of preservation processes for artefacts

and the future care and management of precious, irreplaceable archaeological

resources and environments is crucial at Vindolanda and other World Heritage Sites.

Furthermore, it is hoped that a more comprehensive knowledge of preservation

processes will aid the understanding of effects due to land use and climate change

over time. After an explanation of the site formation processes the paper will explore

the most fundamental objective – characterising the “blue substance” that is often

observed within the deeper layers of archaeological deposition at Vindolanda.

1.1 Vindolanda

Vindolanda is a Roman archaeological site located on the Stanegate road from

Corbridge to Carlisle just south of Hadrian’s Wall in Northumberland, UK, see Figure

1.0.

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Figure 1.0:- Geographical location of the Roman Archaeological Site of Vindolanda,

Northumberland, United Kingdom. (Reprinted with permission of The Vindolanda

Trust).

The first Roman construction at Vindolanda predates Hadrian’s Wall and comprised

Roman auxiliary forts with associated extramural settlements, occupied from c

AD80-85 until the end of the Roman occupation and beyond (perhaps upto the end

of the 8th century) (Birley 2009). During its long history structures at Vindolanda were

repeatedly, demolished and reconstructed in a wide variety of different forms as the

garrisons and their requirements changed with time. Initially the forts were built in

timber (at least 6 wooden forts) but latterly in stone (at least 3 stone forts). As a

result of the rebuilding process, earliest occupational layers at Vindolanda are often

several metres deep with the last upper periods of use of the site, from the 3 rd

century onwards, generally exposed to a very different chemical and microbiological

environment as well as (until the 1930’s) damage such as ploughing and stone

robbing (Birley 2009).

The site is bordered by streams in small valleys to the north, east and south with

ground water coming down the hill mainly from the west appearing as springs. At a

number of points and in times of reasonable rainfall springs appear from the slope

and exhibit a red brown colouration with an associated “oily” sheen. A relatively

small plateau, located on the east side of the site, is bordered to the immediate west

by a depression in the ground. This depression was “in filled” regularly during the

occupation period. Most of the larger forts were built across this depression and

onwards up a gentle slope which moves upwards to the west. The remains of the

last stone fort can be seen today on the plateau and represent a small fort however,

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size, precise location and design of the stone forts varied with time over the lengthy

occupation of the site.

Figure 2.0: Aerial photograph of the Roman Site of Vindolanda (Reprinted with

permission of The Vindolanda Trust).

Due to the often-inclement weather of Northumberland, underlying geology and

geographical position Vindolanda can be very wet (Birley 2009). As a result,

occupants generally raised foundations of buildings when each new fort was

constructed. This repeated process coupled with a combination of Roman building

techniques, especially demolition followed by covering the site with clay and then turf

prior to rebuilding, resulted in Vindolanda becoming a layered cake of occupation.

Thus, occupational layers are often separated by largely impervious layers of

boulder clay. Importantly the clay layers are extremely effective at sealing out

oxygen (and probably oxygenated water) resulting in exceptional preservation of

organic and metallic artefacts (Birley 2009). In the deeper and usually anoxic layers

at Vindolanda it is very noticeable that a scattered blue colouration starts to appear

within a few hours of opening up an area and this can form on metallic and organic

as well as inorganic objects. It has long been assumed that the blue colouration was

due to Vivianite (Birley., per comms).

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1.2 Vivianite

Vivianite is unusual being a striking blue colouration (Read 1970) and thus distinctive

in an excavation environment. For the purposes of this short note paper Vivianite

(which is white in its anoxic form) is hydrated iron (II) phosphate (Fe3(PO4)2.8H2O)

which undergoes surface oxidation in the presence of light to create a mixed iron

(II)/iron (III) material. Once excavated and upon continued exposure the blue

colouration will progressively darken and may end up as amorphous iron (III)

phosphate, which is white/yellow in appearance. The nomenclature of the various

iron phosphates ranging from pure iron (II) progressively through to pure iron (III) is

somewhat confusing and the subject of some debate (Rodgers 1986). The terms

Vivianite, Metavivianite and Kerchenite are used loosely and it is not always clear

exactly what chemical composition is under discussion. This confusion is

compounded by the substitution of other elements (e.g. Manganese, Calcium and

Magnesium) into the material when formed in natural environments (Kloprogge et al.,

2003). The chemistry of iron in its two common oxidation states is extremely complex

and outside the scope of this short note paper however, it should be noted that many

other iron compounds can be formed, especially during the preservation of

archaeological artefacts (Wang 2007). For example, within the soil and in the

presence of excess oxygen and water, metallic iron will oxidise to the iron (III) form

and creates the familiar ‘rust’ orange/red colouration (Read 1970). Iron redox

processes under anoxic and reducing conditions are complex, involving abiotic

chemical reactions and/or microbiologically mediated processes (Melton et al.,

2014).

1.3 Vivianite and preservation

Iron artefacts from Hungate, York, UK showed a surprising lack of corrosive effects

despite being in waterlogged and anoxic conditions (Farrer et al., 1953). The lack of

corrosion was attributed to the inhibition of the activity of sulphate reducing

bacterium (Desulfovibrio desulfuricans) caused by the high concentration of tannates

in the soil (Farrer et al., 1953). Thus, the production of hydrogen sulphide, one of the

main causative compounds responsible for corrosion and deterioration of artefacts,

was not formed (Farrer et al., 1953). Furthermore, within the soil the presence of a

blue substance resembling Vivianite was noted (Farrer et al., 1953). Laboratory

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experiments using apparently corrosive soil further explored the role of Vivianite in

preservation (Booth et al., 1962). Booth (1962) demonstrated that nails coated with

Vivianite were less susceptible to corrosion, by restricting movement of ions in

solution (Booth et al., 1962). Although, the exact mechanisms of the role of Vivianite

in preservation of archaeological artefacts is still poorly understood, it is clear that

strict chemical and microbiological conditions must be present for Vivianite formation

(Rothe et al., 2016 Nanzyo et al., 2010).

This paper is the first stage of a wider and more comprehensive study into the

chemical and microbiological preservation processes at the Roman Site of

Vindolanda, Northumberland UK. Thus, understanding and unravelling the complex

chemical and microbiological processes that impact on the formation of Vivianite is

essential to gain a more nuanced appreciation of preservation mechanisms that take

place on archaeological sites. Preservation and not least the future care and

management of precious archaeological resources and environments are essential.

Furthermore, comprehensive knowledge of preservation processes will aid the

understanding of effects due to climate change and land use over time.

2. Method

2.1 Sample origin

During the excavation season (April to September 2016) at Vindolanda,

Northumberland, UK a range of samples were collected – the criterion for collection

was visual identification of a blue substance on the exterior of the artefact and/or

material. One sample was selected for analysis, as determined by the appearance of

a blue substance on a section of a rampart (see Figure 3.0) and collected for further

analysis. This rampart formed part of the internal packing against the southern wall

of stone fort 2 above the demolished wall of stone fort 1. The rampart was

constructed using turfs with these subsequently decaying to leave behind the mineral

components. The rampart contained (in parts) a blue substance which upon visual

inspection, comprised both individual conglomerates (of up to 1 or 2 mm in diameter)

and longer inclusions (a few cm in length and a few mm in width and depth). The

blue substance was physically extracted using a pair of tweezers.

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Figure 3.0 The rampart sample containing an approximately 1-2mm thick layer of

blue substance.

2.2 Scanning electron microscopy (SEM) and energy dispersive X-ray analysis

(EDX)

Sampling was undertaken along the long blue inclusion shown in Figure 3, and

analysed for morphology and chemical composition using a Hitachi VP S-3400N

SEM coupled with an Oxford Instruments EDX. The system was controlled by

OMNIC 7.3 software. Analysis was carried out in variable pressure mode, eliminating

the need of any coating contamination.

2.3 X-Ray Diffraction

X-ray diffraction measurements were undertaken by Dr Chris Pask in the School of

Chemistry, University of Leeds using a Bruker D2 Phaser Diffractometer with Cu

radiation (1.54060 Å) between 5 and 50° 2θ. XRD traces were analysed by Ms

Lesley Neve in the School of Earth and Environmental Sciences at the University of

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Leeds using the ICDD database and standard data from card number PDF 30-662

for Vivianite.

3. Results

The rampart sample was used to conduct the analyses of the ‘blue substance’ in this

study. As stated above the blue encrustations formed individual conglomerates (of

up to 1 or 2 mm in diameter) and longer inclusions (a few cm in length and a few mm

in width and depth). Image shown in Figure 3.0.

3.1 Microscopic analysis

The determination of the ‘blue substance’ was initially screened using low and high

power microscope analysis. The size and shape of the inclusions were not uniform

but rather a set of closely packed blue and off-white crystals. The results were

consistent with other previous published studies and geological literature for Vivianite

morphology (Read 1970). Scanning electron microscopy (SEM) analysis showed the

‘blue encrustations’ were distinct flat, monoclinic and oblong crystals of varying

sizes, the size of a typical large crystal was approximately 160 x 75 microns (Figure

4.0), the above morphology was consistent with published literature for Vivianite

(Read 1970)

3.2 SEM-EDX

SEM-EDX analysis of the blue encrustation from the rampart sample was performed.

The observed structural morphology was described above, see Figure 4.0, a (Egger

et al., 2015; O’Connell et al., 2015). A summary of the EDX results is shown in Table

1.0. and spectra provided in Figure 4.0, b. Iron and phosphorus were abundant in all

the samples. Silica, aluminium and manganese were present at much lower

concentrations. Aluminium and silica are present but thought to be due to impurities

from the soil. The morphology and elemental composition of the blue encrustation is

consistent with the published literature to confirm the presence of the blue

encrustations is Vivianite (Rothe et al., 2014; Rothe et al., 2016).

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Figure 4.0:- Physical and chemical properties of vivianite taken from the rampart

sample

a) Morphology

b) Elemental composition

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Table 1:- EDX results obtained from six blue encrustations samples from the rampart

sample. (data given as weight %).

Element O Fe P Mn Si AlVivianite 32.74 49.10 18.12 - - -Vivianite Rothe et al., 2014

- 43.0 23.50 3.3 - 3.4

Soil vivianiteKloprogge et al., 2003

- 51.33 41.25 2.61

Sample 1 45.34 41.44 8.31 0.36 3.01 1.27Sample 2 45.01 40.06 8.29 - 3.02 1.42Sample 3 47.56 39.58 11.36 - 0.74 0.38Sample 4 31.00 51.07 1.28 - 5.53 2.94Sample 5 23.36 33.95 0.13 0.19 3.70 1.65

Sample average (n=5) 38.45 41.22 5.87 0.28 3.20 1.53

Sample standard deviation (n=5) 10.69 6.20 4.90 0.12 1.72 0.92

3.3 XRD

The XRD trace was performed on a small sample obtained from the rampart sample.

The XRD trace showed approximately fifteen peaks which correlated with published

XRD data for Vivianite (Sameshima et al., 1985 ; ICCD card number PDF 30-0662).

Additional peaks could be seen and two of these correlated with published data for

quartz with one peak remaining unidentified. It is not surprising to find quartz

present as rounded granules containing silicon and oxygen are also observed by

SEM-EDX (Rothe et al., 2016).

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Vivanite Sample

Quartz standard

Literature standard

Figure 5.0 XRD trace from the rampart sample a) Black line - blue encrustation from rampart samples simulated pattern b) Blue line - literature standard single vivianite crystal (Ref ICSD 423390).

This combination of low powdered microscopy, elemental analysis by FTIR-ATR,

SEM-EDX and XRD can confirm that the blue encrustations separated from a

rampart sample is an iron (II) phosphate known as Vivianite.

4. Discussion

This study is the first of a series of ongoing investigations to articulate the biological

and chemical environment at Vindolanda in order to aid our understanding of

preservation processes for valuable artefacts. A combination of analytical techniques

including SEM-EDX and XRD have been used to study the compositional and

microscopic structure of a ‘blue substance’ from a rampart sample. The observations

and results confirm that the ‘blue encrustations’ located in the rampart sample can

be identified as Vivianite. Although, the rampart sample appeared to contain the

“purest” sample of the Vivianite it also contained quartz and other soil contaminants.

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The atomic percentages as determined by SEM-EDX indicated that the flat, oblong

crystal samples contained the elements and percentage composition of iron,

phosphorus and oxygen to be indicative of Vivianite. The size and morphology of the

crystals by both optical microscopy and SEM were comparable and matched to that

found in the published literature (Read 1970: Rothe et al., 2016). Many other

elements were also determined in small concentrations including silicon, calcium,

cobalt, manganese and aluminium. It was not surprising to find a small concentration

of manganese present as manganese can substitute for iron within the Vivianite

structure (Kloprogge et al., 2003). However, the small concentrations of calcium,

aluminium and silicon probably originate from the post-depositional environment. It

must be stated that a higher level of oxygen than expected from Vivianite alone and

a high level of carbon were also determined although, due to the nature of the EDX

technique the level of carbon (and oxygen) is likely to be over-estimated. At its

simplest the carbon and oxygen could be accounted for by the presence of

carbonate, acetate or related species. Alternatively, it is possible that polyphenols

(tannins) are also present at low concentration in the rampart sample (Farrer et al.,

1953).

The role and importance of Vivianite in these waterlogged environments should not

be underestimated. Firstly, the chemical characteristics of Vivianite means that it

acts as a buffer, maintaining steady-state conditions in anoxic sediments

(Walpersdorf et al., 2013) regulating and controlling phosphate concentration and

release within the environment and thus impacting on the preservation of artefacts

(Maritan and Mazzoli 2004). It has also been repeatedly reported that Vivianite is an

important sink for phosphate in coastal and lake sediments (Egger et al., 2015; Li et

al., 2012; O’Connell et al., 2015; Rothe et al., 2014, 2016). Subsequent

investigations will address the questions of why the chemical conditions at

Vindolanda (at least in some locations) appear to be favourable to the formation of

the Vivianite. Secondly, microbiological characterisation will seek to understand the

importance of sulphate reducing bacteria, Vivianite and preservation towards

artefacts (Farrer et al., 1953). Iron artefacts often corrode due to the metabolic action

of the bacteria producing hydrogen sulphide. However, under anoxic conditions

certain types of micro-organisms known as dissimilatory metal reducing bacteria

(DMRBs), can be active and these facultative micro-organisms use iron (III) as the

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terminal electron acceptor (when oxygen is in short supply) thus creating iron (II) (Li

et al., 2012). Thus, Vivianite is formed in anoxic layers due to the presence of iron

(III), phosphate and DMRB’s in the absence (or low levels) of oxygen (Miot et al.,

2009). Microorganisms are diverse and the role they play in Vivianite formation is

critical but there is still limited understanding of the important role they play in the

archaeological environment (Kip and Van Veen 2015). The microbial flora within with

archaeological deposits at Vindolanda will be extremely complex and will be the

subject of further investigation. It is most likely that there are metal reducing species

as well as iron oxidising species (Schädler 2009) as evidenced by the red brown

deposits appearing in the springs on the site. On bones it is assumed that Vivianite

formation is partly due to the proteins and amino acids acting as iron binding sites

(Johanson 1975). Therefore, mechanisms of formation extend beyond reactions with

hydroxyapatite but also include iron and phosphate (Mann., et al., 1998).

Furthermore, due to the fact that certain microbes can deposit a layer of protective

material on artefacts, these potentially have enormous biotechnological capabilities

(Kip and Van Veen 2015).

Vivianite is known to form in clay deposits and this is likely to be due at least in part

to the ability of clay minerals to bind both cations and anions. It has also been

suggested that clays may act as catalytic sites for the redox reactions of iron and

thus the formation of phosphate materials (Stuki 2011). The post depositional

environment itself should not be underestimated, the remarkable preservation at

Hungate was assumed in part due to the high concentration of tannates present in

the soil (Farrer., et al., 1953). Tannins and humic materials would be expected to be

found within occupation layers due to the great extent of leather working and also

due to the habit of using bracken as well as straw as a floor covering (Birley, 2009).

Tannates can inhibit the activity of bacteria, and includes tannic acid which is found

in oak bark, traditionally used as a mordant in the tannery process (Ford et al.,

2015 ). However, there have been suggestions that tannates may suppress some

bacterial types (sulphate reducing) whilst encouraging others (DMRBs) (Farrer et al.,

1953). There was an intensive culture of leather working at Vindolanda (Birley

2009), supported by the leather finds as well as written on two of the writing tablets

detailing the supply of goats and/or goats skins. Clearly, the Roman army required a

large quantity of leather goods, but many questions remain in respect to the kind of

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leather, number of hides, processing technology, species and extent of leather trade

across the Roman empire (Groenman-van Waateringe., 2009). Let alone whether

the leather used was produced at or very near to Vindolanda or simply brought in

from further afield.

Complex factors impact on the preservation of archaeological artefacts and these

are not fully understood in the waterlogged environment (Matthiesen et al., 2003;

McGowan and Prangnell 2006). It is clear that Vivianite formation can occur in a

shorter period of time (Cox and Bell., 1999). The preservation of artefacts such as

leather and iron in collections is of paramount importance, thus understanding the

preservation ability of Vivianite would greatly aid (Wang 2007; Zangarini et al., 2016).

Vivianite has been seen on a diverse range of archaeological artefacts and has now

been confirmed as the blue substance at Vindolanda. Further work is needed to

answer fundamental issues such as what are the balances of materials, chemicals

and biological agents in the soil surrounding the formation of Vivianite and how do

these help us understand the processes of archaeological preservation.

Conclusion

This is the first study that characterises Vivianite from a Roman site in the UK.

Extensive chemical consideration of the blue substance has been undertaken

leading to the more confident identification of the blue substance being Vivianite.

This will aid further investigations of chemical and microbiological processes within

the depositional environment. Unravelling these processes will aid understanding of

preservation processes and thus management of important sites of national and

international significance such as Vindolanda.

Acknowledgements

We thank the Vindolanda Trust who granted us permission to take samples for ana-

lysis. Thank you to Dr Chris Pask and Ms Lesley Neve, University of Leeds who un-

dertook the XRD experimentation and analysis.

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