GeoArch

2007 Smelting Events

26th June 2007:

The opportunity has arisen to construct and operate a furnace for the "Four Parishes Heritage Group", based in the area around Highley, Shropshire. The furnace is to be built in the Severn Valley Country Park near Alveley and will be operated during National Archaeology Week. The furnace design will be similar to that used for "TimeTeam" in July 2003 (smelt 24). Today was the first day of construction. Although the build went well there was too much clay used in the bonding between the bricks and by the end of the day the upper part of the partially built wet furnace failed!

Failure of the part-built furnace

27th June 2007:

The furnace was rebuilt in a much more solid form. Fencing was placed around the site and the furnace will now be left to dry. Given the current atrocious weather the furnace will be left with a tarpaulin cover - to be removed by local volunteers or staff if/when the weather improves. The finished structure is large!

The rebuilt furnace

20th July 2007:

Our attempt to reach the furnace site resulted on us being stuck on the flood-bound M50 for 6 hours. We returned eventually to Cardiff, to see whether either the M50 or M5 would be passable tomorrow.

21st July 2007:

After a much delayed start we managed to reach Alveley after passing through some of the terribly flooded areas of the Severn and Avon valleys. The furnace had survived the bad weather well.

22nd July 2007:

The smelt 28 was undertaken. The fire was lit at 9.30 and good heat was obtained very quickly. The furnace was filled with charcoal, the tap arch closed and the blast applied by 10.05. The blast for this experiment was supplied by the bouncy castle blower and was conveyed through a piece of scaffold tube.

The smelt started well, and it quickly became apparent that we had a very high burn rate. In the early stages this may have been as high as 35kg/hr, but fell to (a still respectable) 14kg/hr later during the later stages. Ore charging started 1h30 after the initial fire-lighting and lasted for 1h30, during which 39kg of ore was added (the large furnace gave us the hope to aim for a large bloom!), giving an average burn rate of 26kg/hr. The burn down phase folowed the addition of just 6.9kg of extra charcoal, and was complete by 1:40. This gave a total smelt time of just 4h10.

The mass in the furnace separated to give an upper block (which is was hoped contained bloom), which as removed from the furnace with tongs and trimmed cold down to about 7.4kg. The top of the furnace was then removed to convert it into a smithing hearth (following the approach used in smelt 24). The blast was applied to the hearth at 14:45 and the "bloom" was given some 9 reheats and workings over a two-hour period, before it became apparent that the iron content was too low to justify further efforts.

This was a disappointing outcome for the smelt. In many ways it resembled the pervious smelt in this kind of furnace (smelt 24), producing little fluid slag and little bloom. It is unclear what caused the failure; perhaps the burn rate was too high, perhaps the clay lining was not siliceous enough to promote the formation of a fluid slag to protect any nascent bloom. Further work on the materials generated during the smelt may help to determine the cause of the failure.

smelt 28

28th September 2007:

Smelt 29 is going to happen as a rather spur-of-the-minute opportunity to participate in the Institute of Archaeology UCL's primtech event at West Dean. The smelt is going to be used to trial a version of the "Coated Tyle" furnace promoted by Skip Williams and Lee Sauder.

The furnace was constructed from three circular flue liners, 225mm internal diameter and 300mm tall. The lower of these has a tap arch cut from its base and a notch for the tuyere cut from the top. We tried to cut the holes in the basal liner using a hammer drill as suggested by Skip and Lee's design - but the liner was extremely tough and we had to resort to cutting the holes with an angle-grinder. The three liners were then stacked and coated in a thin layer of clay mixed with horse-dung. The shaft was then secured by wrapping chicken wire around the clay, fastening the join, then twisting random points with pliers to make the whole tight. The wire was then covered with a second layer of clay/dung. The blast will be conveyed through a 25mm diameter steel tube, smaller than the scaffold tube sued in hte last smelt.

building the furnace

29th September 2007:

The smelt 29 was begun with a wood fire, lit at 9.40. The furnace was filled with charcoal, the tap arch closed and the blast started 45 minutes later, using the large single-action bellows. During the pre-heat phase a burn-rate of about 5kg/hr was maintained. The top gasses ignited at 1h45 after the start and charging of the ore started 10 minutes later. The ore was initially charged with a ration of 1:2.5 (ore:charcoal), but after about an hour this was changed to 1:1. The burn-rate fell to about 4kg/hr at this point. The smelt proceeded very smoothly, with no blocking of the blowhole, and opening the tap arch showed now fluid slag in the furnace base.

The total charge of 11.8kg of ore took 3h12 to be added to the furnace. After the ore was finished 2.5kg of charcoal was charged (approximately half of the shaft content) and then the burn-down phase took a further 1:25. It was clear that removal of the bloom/slag mass would not be possible through the top of the shaft, so the shaft was toppled and broken.

An attempt was then made to compact the mass to extract any bloom from it. A smithing hearth was constructed from a horseshoe-shaped wall of bricks with a thin inner clay coating. The tuyere pipe was a section of scaffold tube. The large double-action smith's bellows were mounted on a temporary frame and produced excellent heat. Unfortunately the mass proved not to contain any usable iron (although small iron particles were present).

The initial discussions over the outcome of this smelt have raised several possibilities. One clear issue was the lack of a fluid slag: the main mass appears to have formed from a very viscous material which rarely formed prills descending into the furnace base. The lack of fluidity might be a result of a combination of both a low temperature (the low burn rate during charging may reflect this) and also a silica poor contribution from the ceramic liners.

On a more positive note, the furnace structure seemed very stable and once the technique is refined, it seems likely that the flue liners will be able to be used as a testbed and as an easy demonstration furnace.

The smelt would not have possible without the skills and hard work of various students and staff from the Institute of Archaeology, too numerous to name individually, but you know who you are...

soon after lighting the fire

the furnace was toppled at the end of the smelt

the base of the slag mass

attempting to compact the product

the smithing hearth