As described in recent posts, the experiment on June 20 was to build and operate a full version of the type of iron furnaces at Hals. These are basically a cone constructed of grass sods, the shaft dug into the centre of that cone. The cone would have been framed in a box made of timbers. The space above the cone would have been levelled off with earth, creating a flat working surface around the top of the furnace shaft.
In a departure from the usual reporting method, here are a pair of illustrations of the smelting furnace during the later part of the main sequence. Illustrations by Margaret Gissing:
This represents a considerable (!) increase in volume over our standard layout (at closer to 25 cm).
• Stack height (at the start) was 50 cm.
• Tuyere was the standard ceramic tube (at 2 cm ID), set at 22 degrees down, about 6 cm proud.
• Air was supplied by the normal industrial electric blower, in this case wide open to maximum.
• A total of roughly 31 kg of slightly enriched DD1 ore analog was added.
• Total charcoal consumed was roughly 76 kg.
• Total elapsed time (from short pre-heat to extraction) was about 7 hours.
• Ore charging followed a fairly standard sequence, increasing smoothly from 1 kg up to 2.5 kg per charge.
• The consumption rate for a standard charcoal measure (graded maple at 1.82 kg each) was an average of 10 minutes.
• Initial extraction attempt was made via the top of the furnace. It was clear that there was a nugget of bloom present, but it did not prove possible to get a hook on to the bloom itself to pull it free of the slag bowl.
• At this point an attempt was made to extract via the tap arch. It became clear the slag bowl had solidly attached itself to the stones framing the tap arch.
• Tapping with a steel rod from the top suggests the bloom in place is a good size - perhaps as large as 15 x 20 cm. (Admittedly, this is a vague impression at best - and may represent wishful expectations!)
• A decision was made to leave the slag bowl with bloom in place. The overall intent of this experiment is to measure the functioning furnace so as to be able to gain insight how the excavated remains at Hals reflect the working furnaces there. This will be done best by more carefully examining / 'excavating' the complete furnace.
One thing that became obvious as the empty furnace cooled. The bare, dry earth walls started collapsing inwards as soon as the supporting charcoal burned away.
Expect a more detailed examination / photographs of both the smelt and the final furnace, shortly.
One minor correction. The dry earth walls did not collapse as the charcoal burned away (implies the charcoal was holding the walls up). In fact as the charcoal went down we saw only minor shifts in the walls (photos show this). If it had been the charcoal supporting it then as the charcoal kept settling it should have constantly pulled the walls down with it.
ReplyDeleteWhat was significant is that as the walls began to cool large amounts of the dirt started to fall. To me that implies how unstable the walls must have been.
Pretty interesting exploration of the least required to build a smelter. Neil's thoughts that this implies how unstable the [unlined earth] walls must have been isn't really a surprise. The turf used for the walls of the furnaces at Háls, according to the archaeological evidence, were lined internally with clay. Given how uncommon clay is in Iceland and how far it may have had to be carried to the site, and how small the *preserved* fragments are (after 1000 years of intense freeze/thaw cycles) there's no reason to suspect this was a thick layer of clay but this test seems clearly to have demonstrated without it that the intense heat of the smelt would have baked the soil walls dry, making them collapse...as one would expect. I'm actually mostly shocked how little collapsed in the course of your smelt, rather than that it did!
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