Phase 3C – Return to Icelandic / Smelt 98/D40 (3 November 2024)

(Please excuse the bad formatting here. This post taken from the soon to be published full report on the Icelandic series)

Condition of the Phase 3 furnace in October 2024

Over the three years between Phase 3B and 3C, the furnace had been left exposed to the natural weather cycles of rain and snow at Wareham. By late Fall of 2024, the sods had slumped considerably, and natural growth had covered the remaining surfaces. Any dried portions of the liner had washed clear, and especially the upper and thinner areas had visible fracturing from freeze and thaw effects.

A combination of slumping sods and loose earth washing down and forward into the extraction gap resulted in pushing the largely complete fused clay liner forward into that area. When the interior wall line was measured against vertical, the tilt forward angle was 20 °.

Measuring the tilt forward of the remaining furnace lining.

At first examination, the clay liner itself appeared fairly intact on the inside, although some major cracking was visible. There was the expected melted surface and slag coating in the area that had been above the tuyere in the previous two smelts. At the rear, a lip of slag remained where the last slag bowl had been broken clear and pulled away.

Interior of the Phase 3 furnace, before any modifications attempted.

The first step was to trim off the tall vegetation that had grown up over the sod cone. There was a considerable depth of loose soil that had accumulated to the front extraction gap and into the base of the furnace. This was carefully trowelled clear, working down to what had been the original earth base, the layer of remaining charcoal fines packing from Smelt 89/D34 making this easy too distinguish. Next the dirt that had fallen down between the stacked sods and the liner as it shifted forward along the rear portions was scooped out by hand. The hope was that freeing the remaining cylindrical fused clay liner would allow this to be moved backwards again to a vertical position.

  Broken sections of the clay liner, after the attempt to lift it back to vertical.

As it turned out, this did not prove possible, the liner proving more fragile that expected. It would break into four major and a number of smaller pieces, the rear and right side sections each about 25% of the original shape (seen at left and top above) These larger sections would be placed roughly back to their starting positions.

Description

It had been clear that a new front section of the furnace would need to be constructed, to repair damage from the last extraction. With the breaking of the remaining cylinder, it was decided to replace this entire area with stacked stone (gneiss), pieces remaining from the Icelandic Stone Block furnace series. Again the large slab of basalt was used to frame the extraction arch. The size and shapes of the available blocks with determine shape and positioning, with the gaps between individual stones were sealed with the previously created Icelandic manure cobb. The ceramic tube tuyere would end up being set below the lintel stone.

The repaired interior of the furnace for Phase 3C. Dark grey patches are fresh clay, the two sticks helping to support the major wall fragments while drying. Note the position of the tuyere, here below the lintel stone

  Completed build for Phase 3C, air connections in place.

Perhaps being overly ambitious, it was decided for this experiment to add an element which had been the main objective for three tests over the previous year. This was linking a motor driven sliding gate which would constantly alter the amount of air coming from the normal electric blower, creating a pulsing air delivery. The frequency of the alternate opening and closing of the gate could be modified, with a rate chosen that would mimic the 60 to 72 pulses per minute past experiments with the human powered Norse Smelting Bellows employed in the Vinland series by this team. In this case motive power was supplied by a corded hand electric drill, although running without fail over the 6 hours it was used, had a tendency to slowly speed up the number of pulses per minute, so needed frequent rate adjustment.

 
Positioning of the bellows frame to one side of the extraction / tapping space for Phase 3C. The mechanical sliding gate is powered by the red drill. Note the easy reach of the worker adding ore. Neil Peterson (R), Todd Hinkey (L).

To simulate an alternative placement for a large bellows unit, the wooden support frame seen was placed to one side, basically as it would need to be positioned to correctly mate with the tuyere connections. In this case those pipe fittings are set with air input at right angles to the line of the tuyere and view port, as seen in the image above. (Previous attachment when the smelting bellows itself had been used set the attachment at a 45 ° offset.) In either case, the offset would place the bellows to one side at the front of the furnace, allowing full access for tapping and extraction.

Furnace Build	repair and reuse of furnace 88/D33
Furnace ID	28 cm
Total Shaft Height	63 cm
Base to Tuyere	16 cm (+14 cm fines packing)
Stack above Tuyere	40 cm
Wall Thickness	4.5 cm (varies with patching)
Air Delivery via	ceramic tube, inserted 4 cm
Tuyere Angle	22 degrees down
Air Source	electric blower with pulsing air gate
Air Volume	525 E (varies considerably)
Burn Rate	7.7 MpK
Slag Control	tapping (4 kg tap slag added)
Charcoal Amount	50 kg
Ore Type	DD1 analog @ 54.8 % Fe
Ore Amount	17 kg
Bloom	0.97 kg (plus gromps)
Yield	5.7 % (as Fe 10.5 %)

For all the uses of the Phase 3 furnace, there was no problem for workers standing on the flat ground at the edge of the sod cone while working around the furnace. It was an easy reach to add ore and charcoal using the long handled scoop and bucket that was the standard equipment. Probing down into the furnace was possible by standing at the front extraction gap.

By mid way through the smelt, the impact of the reduced flow of air created by the pulsing blast became apparent. The charcoal was visibly burning much faster in the half of the furnace closer to the tuyere. There would be considerably more slag generated and tapped off over the course of this smelt, which later would be found due to excessive erosion of the stone blocks around the tuyere. The massive size of the slag bowl created led to the decision to extract from the top. Despite the known qualities of the red oxide analog used as ore, the end results were the poorest of the entire series, with only a 971 gm bloom and 5.7 % yield.

Phase 3C, at the start of the burn down process. The way the air blast was only effectively penetrating part way across the furnace, and heat effects were pronounced to the front of the furnace is clear.

On later examination of the cold furnace, considerable damage could be seen. The line of stone blocks at the front had melted back considerably

The patching of new clay had generally failed, allowing the larger plates of previously sintered furnace wall to shift and have breaks around the edges. This was especially case on the upper edges to the rear and right side (relative to the tuyere). The failure of the walls in these areas had allowed earth from heat effected sods to slump into the furnace as the charcoal burned down. It was felt that this furnace was too damaged at this point to allow for another use without a complete re-build.

  View of the furnace after smelt P3C. Excessive erosion to the stone front circling the tuyere is clear. (seen at the top of this image.)