Tag Archives: featured

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

2 Comments

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.