Friday, June 26, 2015

Icelandic 6 - excavated

Monday, with the remains of Saturday's smelt expected to be cool, I 'excavated' the remains.
Top view (tuyere to left)
3/4 view (tuyere to left)
Neil Peterson's correction from yesterday (see comments) - that the earth was seen to collapse significantly more as the furnace was cooling, had continued overnight. The earth surfaces would fall away at the lightest touch. This would distort the inner surface from the shaft size present during the smelt itself.
Some idea of how much material collapsed is seen in the 3/4 view above. At the rear of the shaft (opposite the tuyere) as much as 5 - 6 inches of material had collapsed in. The 'average' collapse was bout 3 inches. This material was baked visibly to a reddish brown colour (all organics missing).

Loose debris removed to expose slag mass (tuyere to left)
 Eventually I was able to scoop out all the loose debris (ash, charcoal, loose slag pieces, collapsed earth).  For the archaeologists, this was done by hand 'polishing' the bottom surface of the shaft. I did not touch the walls themselves other than to remove a couple of the top stones (to keep them from falling).
As expected from the burn pattern during the smelt, there were larger charcoal pieces remaining in the back 1/3 of the furnace. (Some of these were larger pieces from the first filling with ungraded fuel - and some still burning slightly.)

To free the slag mass, the front area above the tuyere was cut away. First the cap stone was lifted, then the sod pieces over the tuyere down to the tap arch top stone were cleared.
Then the sods / earth from the sides were cut back to form an open slot, down to the top lintel stone of the tap arch. At this point the slag mass was removed as one piece. 

Front view (towards the tuyere)

Rear view - from the back of the furnace

Side view - Right from the front of the furnace

Top View (tuyere seen to left)
Field drawing with measurements (imperial / not to scale)

What remains is a very large mass - the expected bloom still locked inside the slag.
• The bowl itself has formed a bit lower than expected. The 'soft base' of charcoal fines was set at 4 inches / 10 cm below the lower edge of the lintel stone, but you can see the slag burned down a further 5 cm. This puts the bottom of the slag bowl roughly 10 inches / 25 cm below the bottom edge of the tuyere.
• The usual hot spot in an oval pattern around the tuyere has resulted in a wall of slag, fusing earth behind it. It is a circular pattern, roughly 7 inches / 18 cm above and about 8 1/2 inches / 22 cm to each side of the tuyere. This wall varies in thickness, thinner at the top and side edges, to about 1 1/2 inches / 4 cm at its centre.
• The slag bowl penetrated to about 9 inches / 23 cm from the front of the furnace. This left a gap about  5 inches / 13 cm from  the back wall. Some fragments of partially sintered, but reduced, iron were gathered from this rear area. It does appear however that the burning pattern inside the shaft did in fact pull ore into the heat zone (no ore fragments were recovered).
• There is a central pillar of slag rising above the normal flat surface of the slag bowl. This pillar is about 4 inches / 10 cm wide at the base and about 6 inches / 15 cm high, coming to a rough point.
It is thought that this slag was dragged up off the liquid slag bowl during attempts to hook the edge of the bloom during the failed top extraction attempt.
 • The ceramic tuyere has a final length of 6 1/2 inches / 16.5 cm, from a starting length of 8 inches / 20 cm. This represents only about 3.5 cm of erosion, more typical has been 4 - 5 cm in past smelts.

Almost all of the slag has been recovered. There are surprisingly few 'gromps' (partially sintered and reduced iron encased in slag that did not fall to incorporate into the bloom).
It is likely that most of the slag present is the result of silica mixed with iron from the ore. In this case there was no clay involved, which usually does erode to add to the slag produced.  One remaining task is to weigh all the slag produced.

At this point it has been decided to ensure the size and weight of the slag mass is recorded. To determine the presence of the likely bloom, it will be necessary to break up the slag to free the metal.  (This most likely to be attempted over the weekend - not an easy process with the slag mass cold!)
The hope is that the comparison of ore against slag and bloom produced will be of some use in understanding the production cycle estimated at Hals.

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

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