Lockdown has allowed many archaeology archives to be revisited and those of the South Trafford Archaeological Group continue to reveal some hidden gems. Amongst the material handed into the Group over more than four decades is a small piece of slag. This lump of waste was brought to the attention of STAG by Mike Redfern.
The slag comes from a field to the north of Bradford Lane in Nether Alderley, East Cheshire. It was found along with other pieces of slag in disturbed soil close to a field boundary above a stream around 2005. This site was first noted by antiquarian Charles Roeder in 1900, who discovered extensive iron slag deposits at the site after the up-rooting of a tree. It was Roeder who first suggested that this site was an iron bloomery.
In 1998 the Alderley Edge Landscape Project visited the site for a walk-over inspection (Cheshire HER Entry No 1454). Four mounds were visible along the field boundary above a stream. At one point along the stream bank was a hollow 2m wide and 1m deep surrounded by a slightly elevated mound c. 5m wide. This was interpreted as probably a collapsed shaft furnace. This comprised a cylindrical tower structure made from clay would have stood up to two metres high above the bowl of the furnace. A further field visit by the project in 2004 with local archaeologist Norman Redhead, excavator of a bloomery site in the Castleshaw Valley, confirmed the presence of large amounts of iron slag from bloomery waste spread over an area 90m long and in addition located several fragments of 14th century green-glazed pottery.
The slag fragment in STAG’s archives is 72mm by 46mm wide and 43mm deep and is a piece of tap slag exhibiting the classic ‘ropey’ morphology on its upper surface. This shows the flow line as it ran out from the shaft furnace tapping arch in a molten state before cooling. The shiny surface is due to the high content of silica. The section is crescent shaped which reflects the shape of the run-off channel, and the base is rough where it has been in touch with the ground. The slag is heavy. This is because it contains over 50% iron ore. The retention of some of the iron ore was necessary in the chemical reaction to allow the waste to liquate and dribble down to the bottom of the furnace where it was periodically ‘tapped’ out through the tapping arch. This slag belongs to a type of furnace called a bloomery, named after the nearly pure lumps of iron that were left after the smelt. Hand bellows were used to inject air (oxygen) and this is where the chemical reaction took place at c. 1200 degrees centigrade. The bloom would be taken away to be hammered on an anvil to remove pockets of air and adhering pieces of slag. It would then be worked into wrought iron bars for tool manufacture.
Processing at these sites usually involved additional processes such as a roasting hearth for preparing the ore for smelting and a secondary hearth for reheating the wrought-iron bloom which could then be hammered to remove impurities and turned into iron billets. This secondary process might involve re-heating the bloom many times. A charcoal clamp for storing the fuel for the furnace would also be needed and a wooden shelter for the workers. A nearby water source was used for washing the ore and quenching the bloom. All of these processes and features would be present at the Bradford Lane bloomery.
There are no excavated medieval bloomery sites from Cheshire. However, examples of excavated bloomery sites are known from seven sites in Greater Manchester: in the Castleshaw Valley at Cudworth Pasture and Spa Clough; at Chorlton Fold in Salford; at Cinder Hill in Farnworth; at Gadbury Fold in Leigh; at Holcombe Moor near Ramsbottom; and at Whitecarr Lane in Hale Barns, Trafford. Chorlton Fold, Gadbury Fold, and Whitecarr Lane have all been dated by ceramic evidence to the late medieval period, whilst the bloomery sites in the Castleshaw Valley, Cinder Hill, and Holcombe have been radio-carbon dated to the 12th and 13th centuries. Elsewhere in North West England, a bloomery site has also been identified through slag deposits at Pendle in Lancashire, by the Pendle Heritage Group. However, Cumbria, where there are extensive ironstone deposits, contains the largest grouping of known medieval bloomery sites in the region, with over 200 sites identified. This industry was supported by coppiced woodland which provided the raw material for charcoal burning, charcoal being the main fuel for the shaft furnace.
It’s not clear where the iron ore came from that was used in the Nether Alderley bloomery site, although the suspicion is that it must have come from the local geology close to the site or even from bogs (bog iron). The charcoal fuel for the furnace would most likely have been manufactured locally from managed woodland. The size of the spoil heaps at Bradford Lane suggests a site of some duration, and the excavated examples from Greater Manchester have shown that such smelting sites were used for several years, although on a seasonal basis. Thus, this small piece of heavy black slag provides a window onto the unexpected landscape use of this part of late medieval Nether Alderley; a contrast to the better-known copper mines that lie immediately to the north of the site underneath The Edge.