Cirencester Abbey Grounds 2017 survey

16th October 2017

This week sees the start of a resistivity survey of the Abbey Grounds, Cirencester. The first days work extended over much of the excavated area of the Abbey and some areas immediately to the north.

17th October 2017

The survey today covered an area to the east of the Abbey buildings. Large apparently ditched enclosures appeared in the east of the survey.  Do these correspond to the enclosures in the middle distance on the Kip drawing? The modern and medieval lines of the culverted Gunstall Brook can be seen in the centre of the area, with a possible building immediately outside the line of the culvert, but parallel to it.

18th October 2017

Today’s survey included buildings to the NE of the cloister, with the probable culverted Gunstall Brook running below them.  To the east, running from the SE corner of the survey towards the bandstand is an unexpectedly prominent set of features dividing the area of the Abbey from the wetter alluvium to the east. Is this a ditched boundary or a former natural channel?

Some poor quality data from near the bandstand will probably need replacing in due course, but otherwise another  very productive day!

19th October 2017

A very wet day, but good progress was made. There are a couple of patches of poor data, but otherwise the weather didn’t affect things too much.  The survey included much of the central pathway shown on the Kip drawing, passing through the various enclosures and over the various ditches/stream towards the icehouse, with the modern bench by the lake (the small gap in the data) right on its line.

20th October 2017

A minor surveying glitch meant the day had a slow start. One of the fixed probe markers had also been removed by a dog overnight, but a reasonable data match between days was still obtained. There now appears to be a second old stream channel just west of the edge of the modern lake. The impression of a series of land divisions (fields/gardens?) crossing the old valley floor is increased. Although some of these figure on Kip’s engravings, they may have a much older origin.

Thank you to all those who have contributed to the survey this week. I’m looking forward to getting the whole picture next week!

Images of current data

The images below show the data as a standard greyscale image (with black=10ohm to white= 90ohm measured resistance) and as a high-pass filtered image (black=-25ohm to white= +25ohm to emphasise the anomalies) collected so far.  This area includes parts of the Abbey in the southwest, together with various other structures of the Abbey period and later, as well as the post-medieval formal gardens.

Early medieval smithing at Gelligaer

A watching brief, undertaken for Caerphilly County Borough Council, at an extension to Gelligaer cemetery about 150m NE of the second Roman fort, Gelligaer II, produced a rather elegant and unusual smithing hearth . The hearth was associated with a small drainage gully and a couple of post-holes. The feature comprised two hollows, linked by a worn central section into an elongate figure-of-eight shape. One bowl, approximately 700mm in diameter, formed the anvil base. This contained two large slabs of local stone. It was unclear whether these were used directly as an anvil (their surfaces showed no damage), or whether they were packing for supporting or levelling a wooden anvil block. However it was used, this bowl contained a large quantity of hammerscale.IMG_1534 (small)View of the smithing hearth from the SW before excavation; anvil to right, hearth to left.

IMG_1542 (small)View of the smithing hearth from the NE before excavation; anvil to left, hearth to right.

The other, northwestern, bowl (650mm in diameter) formed the truncated remains of the hearth itself and lay 400mm away from the anvil base. The hearth contained abundant charcoal (almost entirely of oak) and 2.4kg of slag, including two complete, large smithing hearth cakes of approximately 900g each.

A piece of  small diameter branchwood charcoal was selected for radiocarbon dating – and it returned a date, which was not Roman, but cal. AD 610-670. This is an outstanding result, because it represents the first  direct evidence for 7th century occupation at Gelligaer itself. Interestingly, a structurally very similar hearth was recorded in the 1970s near Pontarddulais (50km west of Gelligaer) – and it gave a similar date of cal. AD 410-660.

Gelligaer is a large upland parish, centred on the village with its two Roman forts, medieval church and motte.  In the early medieval period it was the centre of the commote of Uwch Caiach, part of the cantref of Senghenydd, in turn part of the Kingdom of Glywyseg, later known as Morgannwg. The motte may be the Castrum Cadwallon, held by the English crown in 1194 and possibly previously the home of Ifor ap Meurig (Ifor Bach; 1100-1170), Lord of Senghenydd and his son Cadwallon. 

Gelligaer is reputed, in some accounts, to have been the birthplace of St Cattwg, a 6th century saint, whose parents are supposed to have been Gwynllyw (ruler of Gwynllŵg (Wentloog), the lowland area west of the Usk around Newport) and Gwladys (daughter of the Irish King Brychan of Brycheiniog). Both parents, so the stories tell, later became hermits and St Gwladys is commemorated by a small  medieval chapel on the hills immediately north of Gelligaer. These largely mythological characters formed a part of the foundation stories of the early medieval kingdoms and monasteries. The 6th-7th centuries are represented in the archaeological record of Gelligaer by inscribed stones, one on the Roman road north of Gelligaer, inscribed TEFROIHI, TEFSOIHI or NEFROIHI, and one to the northeast at Brithdir, inscribed TEGERNACUS FILIUS MARTI HIC IACIT.  These inscriptions give a glimpse into the world inhabited by the early saints, the world also of the smith who used the hearth at Gelligaer.

 

Changes are afoot…

The long delayed update to the main GeoArch website (www.geoarch.co.uk) is now, finally, happening. This will entail some switching of content between the more formal business site and this one. Heading over here will be the old experimental archaeology blogs (well, they weren’t known as blogs then – the first smelting diary started several months before the first use of that term…), while the GeoArch report library will travel back to the business website where it belongs. I guess its going to take a little while to shake down…

The High Island Monograph is out…

Great to see another project from a few years ago reach final publication!

Georgina Scally’s High Island (Ardoileán), Co. Galway: Excavation of an Early Medieval Monastery, Archaeological Monograph Series No 10, Department of the Arts, Heritage and the Gaeltacht. 347pp.

Lots of detail on monastic hardship on a windswept rock in the Atlantic. Just 35 euro from Wordwell…

High Island0001

Ynysfach – unravelling the refinery process

Part of the site of the Ynysfach Ironworks (a separate, but integral part of the great Cyfarthfa Ironworks of the Crawshay family) in Merthyr Tydfil was excavated by the Glamorgan-Gwent Archaeological Trust in 2011 prior to the development of  ‘The College Merthyr’ (a new tertiary education facility developed by Merthyr Tydfil County Borough & The University of South Wales).

The excavations provided a unique opportunity for examinations of the structures and residues associated with the refinery process. Refineries were developed in Merthyr Tydfil in the late 18th century as an intermediate process to allow the newly-developed Cort's puddling process to convert  grey cast iron smelted from the local sedimentary ironstones into wrought iron. The process involved remelting the pig iron under a strong air blast and then casting it into thin white cast iron slabs by chilling in an iron trough constructed over a cistern of water and with water sprayed onto the molten iron.

Refinery hearth setting Ynysfach Ironworks - a setting for a refinery hearth in the forground, with the cistern (which would have had the tapping trough above) in the background

Slags produced during the process flowed off the surface of the iron and were collected for disposal. An accidental flow of slag into one of the drains allowed the confident attribution of the slag to the refinery process. The slags were characterised by a very high phosphorus content, leading to the development of phosphoran varieties of the common slag minerals as well as various phosphate phases. In particular phosphoran fayalite occurred widely, with compositions ranging up to the theoretical maximum degree phosphorus substitution.

BSEM image of a refining slag
BSEM image of a refining slag. The phosphoran fayalite (lower) is the dominant phase. Ti, V, Cr -rich spinels (lower right) are also common. The interstitial area (upper) contains various phosphatic phases (dark) as well as wustite (bright) overgrown by phosphoran iscorite (pale grey).

Young RIP 2013_20 (small)
As well as the macroscopic slag flows, the refinery process produced microresidues of two types: small (<40μm) dense spheroids of iron or iron oxide and larger, 100-2500μm diameter, hollow spheroids of slag comprising magnetite dendrites in a glass or micro-crystalline matrix. The spheroids were probably generated by the turbulence of the air blast on the molten iron and slag in the refinery furnace respectively. The spheroids form a major component of the deposits in the site’s many drains.

These new data, together with reappraisal of contemporary accounts of poor quality puddled iron before the introduction of refining, suggest that control of phosphorus, in addition to the control of silicon, may have been a major factor in the need for the refining process.

(further details may be found on the Glamorgan-Gwent Archaeological Trust's website - http://www.ggat.org.uk/)

Wolf spit: tungsten and medieval tin smelting

The origin of 'wolfram', the alternative name for tungsten (retained in chemistry through the use of the symbol W for tungsten), was because the tungsten-bearing minerals were held in medieval times to devour tin during tin smelting. Rich deposits of tungsten occur in the Hemerdon Bal Mine in Devon (the 4th largest deposit known in the world), on the edge of Darmoor, and currently being reopened as the Drakelands Mine. The wolframite of the deposit has a similar colour and density to cassiterite, the tin ore mineral once widely exploited around Dartmoor. The medieval tin smelting site at Brownie Cross lies 4km NW of Hemerdon Bal. It was discovered and excavated by Cornwall Council’s Historic Environment Service, Projects team, which had been commissioned by South West Water to undertake a programme of archaeological investigations along the route of the 13.5km-long Avon Water Main Renewal Project in April and May 2009.   Radiocarbon dates indicate activity within the late 13th or early 14th centuries and the site is interpreted as a manually blown 'blowing house'. The slag from Brownie Cross is mainly in the form of fragments of thin sheets, probably scraped from the surface of the tin in the float. Some slag samples are comparable with tin slags described previously from SW England, but most show elevated concentrations of both tin and tungsten. These slags bear inclusions (particularly close to the sheet bases) of tungsten metal, tin oxides, wolframite and hard head (tin-iron intermetallics). The body of the high-W slags are glass with an Fe-Mn-Mg tungstate, outside the recorded compositional range of natural wolframite. The most highly tungsten-enriched slags also commonly show a relict emulsion texture, with the tungsten rich component showing extreme enrichment of tin with respect to silica, reflecting the low yield observed by the german medieval smelters which led them to use the name 'wolf rahm' meaning wolf's spit or foam.

Slag showing emulsion textureBSEM image of highly tungsten-enriched tin slag from Brownie Cross. Variation in the size and density of the Fe-Mn-Mg tungstate crystals reveals the texture of an emulsion of slags differing tungsten enrichment. Field of view 4.5mm.

Tungsten-rich tin slagBSEM image of the basal part of a slag sheet, showing film-like development of tin oxide (mid grey), tungsten spheroids (white) overgrown by Fe-tungstates. The body of the slag above has elongate Fe-Mn-Mg tungstates in a tin- and tungsten-rich glass . Field of view 160 microns.

Iron Age iron smelting furnaces in Berkshire

Iron Age furnaceIron Age iron smelting furnace, Wokingham

The excavation of some Iron Age  iron smelting furnaces near Wokingham for West Sussex Archaeology is a major current project. The furnaces have a wide flat floor, they are over a metre in diameter and have a broad arch with a large raking pit.  They therefore have some morphological similarities with the later Roman domed furnaces of SE England.

The site has two furnaces sites. One was almost completely destroyed by a modern pit. The other has an early furnace that was demolished and filled with slag and the debris from its superstructure, with a replacement furnace constructed over it and just 0.5m further west, so that it was able to use the same raking pit and slag dumps. The two furnaces are aligned and bounded by short lengths of shallow ditch.

Iron Age iron smelting site near WokinghamIron Age iron smelting site near Wokingham

 

The first furnace cleared of its contents

The first furnace cleared of its contents

 

Update 17.8.14:  Excavation of the first furnace showed that it had a similar form to its successor. The clay wall of the pit was vertical and horseshoe shaped in plan. The arch was effectively the entire width of the pit and defined externally by a pair of large furnace-bottom slag blocks, oriented vertically.

Welcome to GeoArch!

Welcome to the new GeoArch blog site! This site will host descriptions and discussions of on-going projects, comment on developments in archaeometallurgy and general news of GeoArch activities. As the site name suggests, the main area of interest will be archaeometallurgy, but I am sure some of archaeogeophysical work (and indeed other aspects of archaeological science) will creep-in as well.

Tim Young