Wednesday, September 06, 2006

Viking Age Smelt - September 2

Just a fast overview. The smelt was undertaken at 'Baron's Howe' - an SCA event hosted by Steve Muhlberger at Bonfield ON, just outside of North Bay. There was no power available on site, so all human powered equipment was required. The smelt was a continuation of the Dark Ages Re-creation Company's work into Viking Age smelting methods.

Team was composed of the primary DARC smelt group (Markewitz, Jarbeau, Peterson, Cook, Burnham) with assistance from various event participants for material preparation and bellows pumping (notably Russ Sheldon and Nick West).

The smelter was clay cobb, in this case roughly 100 lbs of commercial ball clay mixed with chopped hay and some local rough sand. A bundle of sticks was used as a form for the central shaft. Due to time constraints and fatigue, the cobb was mixed up considerably too loose, and significant sagging of the smelter was the result. The stones seen in the photograph were not originally intended, but were required to prop up the slumping base. The initial tuyere used was the standard high temperature ceramic with 2.5 cm ID.

Rough measurements of the smelter : 60 cm tall total
20 cm internal dia. at mouth
25 cm internal dia. at base
15 cm from tuyere to base

Air was supplied by the experimental 'Ubber Bellows' seen in the image. This is roughly based on possible Norse types - but enlarged to generate air volumes as suggested by early experiments (primarily Sauder and Williams). No definate measurements of the air delivery have been made to this point. Rough estimates are 90 l per stroke, with an average pump rate of 8 - 10 strokes per minute. With losses and the actual action of the system taken to account, a more realistic estimate is for a range about 500 - 600 litres per minute. (Peak delivery speed was measured at 70 kph.)

Ore was the Virginia Rock Ore, with about 12 kg of ore used. This was roasted and crushed to 'pea to rice with fines' as been done in past experiments. This amount is considered to be on the low end of what is required for a complete smelt reaction.

About 75 kg of charcoal was burned, the entire smelt running over about 7 hours including preheat.

There was a major failure in the air system at about six hours. This primarily due to exhaustion in the part of some of the bellows workers. The air was reduced, temperatures dropped and the slag froze inside the tuyere. Fast work with a slag tap, a replacement tuyere and switching to stronger bellows operators managed to get the smelt under control, but physical damage to the bellows itself occured. For these reasons, the 'shock charge' was omitted entirely and the smelt sequence cut short.

The end result was a roughly 1.5 kg bloom. Its appearance suggests it may be a higher carbon material. As with past smelts, the bloom was extracted through the top of the smelter.

The intent of this experiment was three fold:
First, the whole process was intended as a educational demonstration.
Second, some improvements had been made to the Ubber Bellows, and it was hoped that more experience would be gained related to this unit,
Third (and perhaps most significantly), the entire smelt was laid out to duplicate known features from 'House J / the Smithy' at L'Anse aux Meadows.

The smelt area was inside a rough approximation of the floor plan found at LAM. Our site was just slightly smaller, roughly 3.2 meters wide by 2.8 deep. As at LAM, the area was dug back into the side of a small hill. The size and placement of the smelter, stone anvil, and slack tube are roughly the same as indicated by the archaeology.

Of note is the debris field. First, there is a very distinctive void caused by the position of the smelt master,. For a right hander this is to the left side of the smelter (looking into the dugout mouth) There is also a lack of materials underneath the bellows, which must be placed to the right and forward of the smelter in this arrangement - to allow for room for the bellows operator.

1) This area shows larger pieces of unburned charcoal, perhaps with some unprocessed ore pieces. This represents spillage from additions to the smelter.

2) This area will have some of the above material, which then is covered with a quantity of partially burned charcoal and partially reduced and sintered ore fragments. This material is deposited when the balance of burning material is scooped from the smelter to extract the bloom. Material that was nearest to tuyere level will be to the top of this layer (smallest charcoal with the most ash, larger sintered ore pieces, closest to fully reduced).

3) This area will have considerable tap slag, ash and consumed charcoal fragments. This was just in front of the tap arch, and slag was pulled out here. Many of the pieces were pulled away while hot - creating tendrils of slag. Larger masses were tossed away from the work area entirely. (At LAM this would have been into the nearby creek.) In this experiment, this slag was dark greenish black, spongy in texture with gas bubbles making it quite low in density.

4) This area around the stone anvil contained fragments of the slag mass. The extracted bloom has considerable slag material attached, and light consolladation broke of egg to fist sized pieces of this material from the metal underneath. This slag (as in the past) is a medium gray in colour and relatively solid. Some smaller fragments of bloom metal would also be found here (any large enough to gather by eye were retained however).

Expect a more detailed field report to come, with additional photographs.

Neil and Karen have already posted up a number of their images from the weekend including the smelt at:

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

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