Discussion:

• 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 they cooled.
• The bloom weight recorded is a bit elevated when compared to other smelt records. The normal practice is to extract the bloom while white hot - and then strike over the surface. This removes the majority of clinging slag. The cold hammering used in this case did remove the majority of this, but certainly not as effectively as hot working. Normally striking over the bloom surface at a welding heat also compacts some of the lacy 'gromps' over the surface. The cold hammering could not do this effectively, so likely some additional material will be lost when the bloom is finally compressed into a billet.
• In general, this experiment, as a full scale test of a proposed Icelandic / Grass sod construction, can be considered a good success.
• The major modern intrusion to likely Norse / Viking Age method remains the use of high volume air via the electric blower.




Conclusions:




A) Effect on air delivery volumes:

ID = 30 cm : area at tuyere = 700 cm2
air required (at 1.2 to 1.5 l/m/cm2) = 840 to 1050 l/m
ID = 36 cm : area at tuyere = 1020 cm2
air required (at 1.2 to 1.5 l/m/cm2) = 1225 to 1530 l/m
With the start of the main sequence (charcoal addition) the available electric blower was set wide open. (That volume is thought to be 1400 l/m, but this has not been tested 'in line'.)
Another variable is delivery pressure, which in this case could not be modified. (Pressure effects how far into the charcoal mass of the furnace the air will penetrate.) The concern was that even if sufficient air volume was available, the air might not penetrate right across the furnace diameter.

B) Charcoal requirements :

Although the height of various furnaces varies, internal volume is a cube function.
Our typical furnaces require 4 - 5 standard buckets (about 1.8 kg) of charcoal to fill.
For this build the volume of charcoal needed was almost double - a total of 10 standard buckets.
This would certainly effect the 'hang time' of any individual ore particle. (The time the material was inside the reduction chemistry of the furnace.)
The consumption of a standard bucket of charcoal was inside the normal range considered effective on past bloomery iron smelts. The furnace ran a bit hot at first, at a rate of 6 - 8 minutes per bucket. This shifted (as expected) with the addition of the first ore charges. The average was about 10 minutes per bucket, increasing slightly with the later additions of larger ore charges. Ore charges started at 1 kg per bucket, and increased smoothly to 2.5 kg per 1.8 kg charcoal at the final stages.

C) Tuyere :

One effect that was not anticipated was the shifting of the tuyere angle over the course of the smelt. At the start of the smelt, the tuyere was set to the standard 22 degree down angle. However as the smelt progressed, this angle flattened out considerably. By the 2 1/2 hour point into the main sequence, the angle was reduced to 17 degrees down. Measured at the end of the smelt, the tuyere angle was at only 12 degrees down.
This appears to have been caused by the sods collapsing as they dried / organic materials burned away. The tuyere had been placed so that the interior end was resting on the stone slab lintel of the tap arch, but the outside end was only supported by grass sods.
This shifting could have been prevented by placing a small stone support on the outside end of the ceramic tube used.

(1) The reason for using the larger form was an initial intention to line the sod shaft with a thin clay layer. This was suggested by clay fragments found at Hals, ranging up to as much as 2 cm thick. With that clay liner, this furnace would have in fact had an ID closer to 30 cm. Time (and weather) limitations resulted in this clay liner not being added to this furnace. A successful test of a thin clay liner in an earth cut cylinder was made with 'Icelandic 5', May 2012




Photos:




Roughing in the smelter	Down the shaft	Smelter in action	Slag bowl
Slag Mass	Bloom

Raw Data:





CLOCK	ELAPSED	EVENT	AIR		CHARCOAL			ORE
					UNIT	COUNT	TOTAL	ADD	TOTAL
			gate	LpM	bucket		kg	Kg	kg
12:00		Start Preheat
		Inner surface crumbles
12:30	30	Start Charcoal - 5 Maple, 5 Oak	9		10	10
12:41	11	All Rough to fill	Max
12:50	9	Touch off volitiles
12:59	9	First graded charcoal			1	11
1:04	5				1	12
1:10	6				1	13
1:18	8				1	14
1:24	6				1	15
1:29	5				1	16
1:44	15*	Too much heat loss out of front. Packed it with dirt and sod. Possibly missed a charcoal bucket			1	17
1:49	5	Tried packing the front with mud, fire coming out between cinder blocks			1	18
1:59	10				1	19
2:06	7	First Ore
2:13	7				1	20		1	1
2:22	9	How do we know when the Sod is burning? How do we keep the heat?			1	21		1	2
2:32	10				1	22		1	3
2:43	11				1	23		1	4
2:57	14	Turere 17 deg. How do we keep the walls from moving?			1	24		1	5
3:05	8				1	25		1	6
3:13	8	Rocks are shifting. Front of dirt pile has lots of give. Front is still burning faster than back. Placing ore in the middle third will ensure better drop			1	26		1	7
3:29	16				1	27		1	8
3:37	8				1	28		1	9
3:48	11				1	29		1	10
3:59	9				1	30		1.5	11.5
4:09	10				1	31		1.5	13
4:18	9				1	32		1.5	14.5
4:32	14				1	33		2	16.5
4:42	10	Starting to pile the charcoal up into more of a mound			1	34
4:44	2	filled more to make the mound			1	35		2	18.5
4:55	11	Why slow times? Perhaps not enough air for the volume			1	36		2	20.5
5:08	12				1	37		2	22.5
5:17	9				1	38		2.5	25
5:30	13				1	39		2.5	27.5
5:39	9	Last charge of ore			1	40		3.33	30.83
5:46	7	Start of burn down			1
5:58	12				1

Reports of all of our iron smelting efforts along with more articles and information are available on the "Iron Smelting in the Viking Age" CD from the Wareham Forge.  Copies of the CD can be purchased here.

Text © Darrell Markewitz
Photographs © Darrell Markewitz
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