Monday, October 08, 2007

Thanksgiving smelt - Draft report

This is just a fast overview of the smelt on Sunday October 7 - carried out by Neil and myself.

The smelter was constructed as had been discussed in earlier posts. Neil had gathered a donation of pre-mixed pottery clay (donated by Potter Supply House in Kitchener). This was cut into slabs roughly 6 cm thick, each trimmed to allow them to be stacked into the cylindrical shape of the smelter. The seams were 'mortared' using the wast 'smelter clay' that Selena has provided.

At the front of the smelter, the structure was built up from stone slabs. The lower section was raised to a level of roughly 20 cm. Two smaller pieces were laid on top of this, leaving a central slot about 5 cm wide by 7 cm high. On top of this was placed a large slab - 4 cm thick by roughly 30 cm tall, which was 30 cm at the lower edge and 20 cm at the upper. This slab sits over the zone of the smelter that is subjected to the highest operating temperatures.

The tuyere was mounted so that the tip of the steel pipe was set to just even with the inside surface of the smelter (the edge of the upper stone slab. For this experiment, the standard 22 1/2 down angle was used. Air was delivered via our standard blower, with the rate in the higher volume range that has proved successful in our earlier smelts.

The smelter was constructed on Saturday, and left for the moisture inside the clay to at least partially stabilize over night. This step turned out not to be as effective as was hoped.
The overall set up - during the preheat phase.
Because of the use of block clay (in replace of the standard cobb mixture) a longer than normal preheat sequence was undertaken. Split wood was burned using natural draw for about 1 1/2 hours. A low air blast was then applied for a further 25 minutes before filling with charcoal to begin the primary sequence. The higher temperatures created by the air blast to the wood drastically effected the clay. As the internal dampness flash heated to steam, serious spalling (in fact explosive shattering) of the clay bricks was the result. This so seriously damaged the top course of the clay blocks that this layer was removed and then replace with the sheet metal cylinder used in past smelts. This allowed us to maintain the normal working height of the smelter (adjusted total was 60 cm above the tuyere)

For this smelt, there was not a fixed base of charcoal fines established at an optimum level. Instead, the bottom of the furnace (packed earth) was allowed to accumulate a layer of ash and charcoal from the pre heat materials. In the end this would effect the position of the developing bloom.

Although the smelt was started with a reduced air volume, we fairly early on decided to return to more familure methods - so increased the air delivery to the range of 600 litres per minute. With this higher air flow the charcoal consumption was in the range of 8 - 5 minutes per standard bucket.
Neil taking an 'up the kilt' shot - nearing the end of the main sequence.
The ore used was the commerical hematite grit. To reduce the tendency of this smaller particle size to absorb excess carbon in the reaction zone, the ore was added as a single scoop sized slug, spread evenly over the top of the smelter each time (as opposed to layering it through out each charcoal bucket). It was decided to aim for a historic sized bloom, so a rough total of 11 kg of ore was used.

The primary smelt sequence (first charcoal to extraction) took about four hours.

A top extraction was undertaken, again represented the process we expected may have been used in the archaeological setting we are working towards. There was a clear knob of slag produced at the tuyere which was certainly melted stone from the front slab. This turned out to be a different composition than the normal slag bowl material - with a significantly different melting temperature.
The slag bowl and bloom had also formed somewhat lower in the furnace than has been the case in the past. Both these results made finding and extracting the bloom a bit tricker than in past experiments.

In the end, Neil pulled the resulting bloom. The weight was about 6 kg (roughly 45% return). As has been the case with other uses of the hematite ore, the exterior of the mass was fairly crumbly, but with a clearly solid core.
Remains of the smelter after extraction. The bloom is to the upper right corner.
Todays work is to excavate and record the structure of the furnace after it has cooled down. What is discovered can be compared to the archaeological evidence from the site at Hals.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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