GeoArch

1999 Smelting Campaign Diary

5th March 1999:

The furnace has survived the winter well, but has been slightly damaged by rabbits. Repairs have been started and a rebuild of the tap arch is also being undertaken. The arch is being enlarged and stone-lined, to allow removal of blooms through the base of the furnace.

8th March 1999:

Repairs and alterations to the furnace were completed today. It was surprising that less than one day was all that was required to put the furnace back into operation after the winter break. In fact, the furnace repairs were just of the same order as the standard maintenance between smelts - only filling the rabbit hole, moving the blowhole back to the side and enlarging the tapping arch were substantial tasks.

13th March 1999:

Science Week Day 1:Smelt 11 was conducted as the first Science Week activity. The smelt revisited the single bellows smelt 6, but with the iron tube in place in the blowhole and with the heightened furnace. This was the first smelt with a new charcoal (local oak charcoal with a 10-40mm size), and with a new ore: a brown ochre from Clearwell Caves Iron Mine in the Forest of Dean. As with smelt 10 a 1:4 charcoal:ore ratio was maintained during the ore charging process (in the period 3:48 - 4:40). We attempted to tap at 6:42, but discovered the bloom had grown against the back wall (because the blowhole was slightly misaligned). The furnace was emptied, and the bloom removed hot and taken away for smithing by Toby Petersen. The bloom rather disintegrated in the forge, but one of the fragments was smithed down to a small, crude, piece of iron. This is the first smelt where we have smithed to iron on the same day, and been able to demonstrate the hot bloom to the public. The short process time and the economic use of charcoal added to the success of the day.

Bloom 11

15th March 1999:

Science Week Day 3: Smelt 12 was conducted. This was intended to be a repeat of smelt 11, except for the use of the same charcoal as during the '98 season and the correction of the blowhole misalignment. Unfortunately, after only a few minutes the middle board of the bellows disintegrated around the mounting pin. Rebuilding the bellows took a couple of hours during which time the furnace was blown by the small bellows. Given this problem, the production of a bloom was considered a success. The bloom was compacted by the blowing team down to a 1kg block, which unfortunately broke on its final reheat.

17th March 1999:

Science Week Day 5: Smelt 13 was conducted, using the new charcoal. The repairs to the furnace had to be undertaken immediately before smelting, and it seems one of the repairs failed early in he smelt, generating a large slag mass which interfered with blowing. A large 30 kg charge was used, but only a small bloom produced distally, together with a large dense slag block proximal to the blowhole. The last part of the smelt was conducted with the tap arch open, with very little effect on temperature. Aaron smithed the larger fragment of the smelt 12 bloom down to a crude bar.

19th March 1999:

Science Week Day 7: Smelt 14 was undertaken, using the new charcoal. Repairs to the furnace had been undertaken yesterday, and proved secure. 40kg of ochre were smelted using the 4:1 ore:fuel ratio. Working temperature was reached and charging started at 3:18, lasting until 4:50. The entire smelt was run with the tap arch open, and furnace bottom temperatures were high, and this assisted the flow of slag towards the tap arch. The smelt ran smoothly, although the "bloom" failed to bridge the gap over the tap-arch, and therefore it proved impossible to assist the flow of tappable slag. On removing the "bloom" it was discovered to have a tiny metal content, and was largely sintered ore. On reflection it is clear that the "blooms" generated on all the smelts this week have suffered from a low metal content. It seems very likely that the 1:4 ratio used is not allowing the ore sufficient residence in the furnace. Smelt 15 (on Sunday) will therefore have an adjusted ratio...

21st March 1999:

Science Week Day 9: Smelt 15 was undertaken, using the same parameters as smelt 14, except for a 1:2 fuel:ore ratio. This increased the total smelt time to 7:48 (from 6:39 for smelt 14). Working temperature was reached at 3:52 and charging lasted until 6:07, with a total of 40kg ore. Slag flowed intermittently through the open tap arch between 5:15 and 6:30, with a particularly large flow around 5:40-5:50. The "roofing" of the bloom and slag proved sufficient to support the charcoal for most of the last 2.5 hours of the smelt, with just a few collapses, which were cleared fairly quickly. The blowing rate was reduced between 5:50 and 7:00, because the gas pressure appear to be causing failure of the "roof". The bloom was removed to the smithy and a small part (approximately 20%?) was smithed down to finished iron (of which 250g was produced). An otherwise exceptionally successful day was only marred by the collapse of around one quarter of the furnace during manipulation of the bloom at the end of the smelt. A major rebuild will be required at the very least. However, given that we had already decided not to smelt for a period of 6 weeks to allow construction of new bellows, we now have the opportunity to review more fully the facilities required for the next stage of the campaign.

flowing tap slag bloom 15 being compressed collapsed furnace

29th April 1999:

The site for the first charcoal clamp (see also charcoal burning page)has been prepared over the last few days, and for the next three days Peter Richards will be on site to assist with its building and firing. The woodpile was almost completed today.

30th April 1999:

The covering for Clamp1 was put in place, and the clamp was finally ready for lighting around 16:30.

1st May 1999:

May Fair - day1. Clamp1 is burning steadily, and clamp2 has been part constructed as a demonstration of the internal structure.

2nd May 1999:

May Fair - day2. Clamp1 is burning slowly, and with appropriate care in reducing air flow at the end of the working day, we believe we can dispense with the night watch.

3rd May 1999:

May Fair - day3. Clamp1 continues its slow burn. Substantial tar deposits are accumulating around the brick air vents.

4th May 1999:

Clamp1 is starting to show some slightly browner smoke, indicating the progression of the process.

20th May 1999:

The next smelt is now set for Bank Holiday Monday (31st May), so a start has been made in repairing the damage to the furnace. It now seems possible that a new workshop will be created for us as part of the Woodland Fair (29-31st May), so we intend to use the furnace one last time, testing a lower fuel:ore ratio than the last two smelts of Science Week. It is now our intention that the new facility will be built in time for use during the National Archaeology Days at the end of July. Despite the severity of the damage to the furnace, we will repair it for the Woodland Fair, knowing that our mend only has to survive one smelt.

26th May 1999:

Furnace repair is progressing very slowly, following failure of the initial mend. Only 5 days to go until the furnace needs to be operational again...

31st May 1999:

Smelt 16 happened successfully; after all the worry the large mend stayed in place. The smelt employed Forest of Dean goethite ore, with the first 3kg being lump ore from Drybrook and the following 19kg was powder ore from Clearwell. The pre-heat phase was rather extended (4h50m) because of slight uncertainty over temperature (the recent mends had left the thermocouple tips buried in several cases). The charcoal (produced in clamp1) wasn't graded to a very fine grade, and this seemed to encourage the flame up the furnace wall above the blowhole. 54 minutes after the start of charging the first iron-rich slag showed in the blowhole and 10 minutes later a substantial tap slag flow appear through the charcoal in the tap arch. The tap arch was cleared and the flow continued, albeit slowly. A large block containing some bloom material collapsed into the void after about 30 minutes. Ore charging finished at 8hr, by which time there was no flowing slag. The charge was allowed to burn down for a further hour, before the bloom was dislodged with an iron bar. Bloom quality was rather poor, comprising a rather disjointed series of small masses. One piece of bloom was selected for compaction, and a series of around 8 reheats allowed a degree of compression.

flame on blowing wall flowing tap slag

24th July 1999:

National Archaeology Days '99 (1)
Today had been planned as the opportunity for a smelt, followed by smithing tomorrow. Unfortunately circumstances forced delay of the smelt until tomorrow. The day was usefully spent in setting up a pair of single action bellows. The bellows have been constructed cheaply (plywood, polythene sheet, canvas...) as a prototype. If they perform well, then "real" ones will be built later. These new bellows are based loosely on the designs illustrated by Agricola (1555: De Re Metallica, Book IX, p.365 of the Hoovers' translation), but reduced by about 50% (i.e. the boards are approximately 1m long, rather than 2m).

25th July 1999:

National Archaeology Days '99 (2)
The delayed smelt 17 took place today. The initial stages of the pre-heat and charging phases were accomplished using the maximum throughput of the new bellows. 3 hours into the smelt a 10 minute break was necessary to undertake some repairs, but otherwise the bellows stood up very well. As with several recent experiments, a slag flow to the tap arch was achieved starting 1 hour after the start of charging. However, within 20 minutes the flow had stopped and only a very sluggish movement of viscous material was occurring. Checking the temperature on the base of the bloom mass showed it was well in excess of 1300C - so the slag was therefore of the wrong composition. The blowing rate was slowed, to increase ore residence time in the shaft, and after 2 hours of a reduced blowing rate (with the furnace still burning at 8 kg/h) the tap slag started flowing again. This flow was maintained at a relatively low rate, allowing for removal of a large block 30 mins later, before the flow dried up after a further 40 mins, approximately 1h55m after the end of charging. When the flow had stopped completely, the blow rate was stepped up to expose the bloom, which was removed 10h3m after fire lighting. The bloom weighed 2.1kg, and appears of very good quality. Only a very mild compaction was given (2 reheats) before work was halted.

New bellows: foot operated, spring return

Smelt 17 in progress