Iron Production Experiment June 2007

Date:9 June, 2007, start time 9:45 am

Location: Vinderheima

Premise / Questions: 

• Re-use of an existing smelter after a winter - can it handle another smelt (#4 & #5 for this smelter)
  • The current clay cobb furnace was fired three times in 2006. With some minor patching the furnace will easily be able to withstand another sequence. We hope to match this furnace to the sequence of patching and re-use seen on excavated furnaces. Objective two relates to this process as well.
• Does running two smelts back to back work, and does it save anything
  • In the past we have seen that a large part of the charcoal consumed is used to bring the furnace structure up to operation temperature and to create the underlaying slag bowl. If manpower permits, we intend to extract the first bloom, then immediately re-fill the furnace and start straight into a second smelt sequence. A marked improvement in overall raw materials consumed is expected, but it is unknown how this second smelt sequence may develop.
• What does the debris field look like after a smelt
  • We intend to lay a clean layer of sand over our working area, and make a solid attempt to record both the work patterns and waste deposits that develop. Secondary to this will be collecting all the slags created to better compare total products against ore amounts.

Smelter Type:

Diameter:	9" (22.5 cm)	Stack above Tuyure:	16" (40 cm) with 12" (30 cm) metal collar it totals 18" (70 cm)	Tuyure Diameter:	1" (2.5 cm) I.D.
Tuyure Distance above floor:	5.5" (13.75 cm)	Tuyure Angle:	15°	Tuyure Penetration into stack (start/finish)	3/4" (1.9 cm) / N/A

The Team:

Leader	Neil Peterson
Staff	Darrell Markewitz Ron Ross
Recorder	Sam Falzone

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.

Smelt Totals

Total Charcoal	124 lb (56.4 Kg)	Weight of Bloom	900 g (1.98 lb)	Total Elapsed Time:	5:46 to last charcoal added, 6:08 to next smelt fully loaded with charcoal
Total Ore	51.24 lb (23.3 Kg)	Weight of Slag:	Not recorded	Bloom Quality	Good soft Iron

Side view	Top view	Dry smelter	In progress
In progress	Overview

Discussion At the beginning of the experiment, a number of things were noted that were expected to effect the progress of the two overlapping smelts:

1) The smelter structure had slumped forward slightly off vertical. This was largely due to the effect of wet conditions over the spring, softening the raw clay cobb that remained at the base of the structure. (From about the top of the tap arch level up the clay had been converted to a rough ceramic by the three earlier firings.) It was also possible that the effect of frost heave on the half banked side of the smelter contributed to the shifting. The slight lean of the furnace towards the tap arch was not expected to have a significant effect on the progress of the smelts. The angle of the tuyere, which had remained in place after the two November firings, was a matter of concern. The angle had shifted from the ideal 22.5 degrees down to closer to 15 degrees down. In past experiments, a shallow tuyere angle has resulted in the slag bowl forming higher in the furnace, with more potential to quickly fill with slag and drown the tuyere. A smaller bloom can be expected from a furnace with a shallow tuyere angle as well.

2) Most importantly, the rock iron ore was observed to have a higher content of waste stone, and was suspected being lower in iron oxide content as well. The rock ore remaining on hand was gathered by Darrell and Vandy Simpson from the secondary mine source discovered by Sauder and Williams in Lexington Virginia. Although a day had been spent at the mine with the Lexington crew gathering ore, the material on hand had been gathered (and backpacked down off the mountain) by Darrell and Vandy working alone (and undirected) the following day. (Earlier Virginia Rock Ore used had been much better quality material gathered by Lee and Skip from their primary source.) When working without supervision, there had been a specific attempt made to gather ore with various differing physical appearances from a number of different locations within the overall deposit. This in the hope that the quality would average out in the end. During the crushing phase it was obvious that there was much more plain stone included inside the pieces gathered than had been present in past material.

The combination of these two factors suggested that the smelter would produce more glassy waste slag than had been the case in the past. Also it was expected this would lead to more frequent tapping off of that same slag to prevent drowning of the tuyere. Overall it was expected that there would be significantly less available iron oxide, resulting in a smaller bloom and a lower total yield. Thanks goes to Megan Roberts, who did the lions share of the ore crushing, with Sarah Scroggie and Neil assisting on the earlier preparation weekend.

Our fuel source was once again Royal Oak Charcoal, supplied by yet a fourth generous donation of material from the company. (DARC thanks Royal Oak for its kind support by way of its donation of another 600 pounds of charcoal which made this and future experiments possible.) Sam gets credit for crushing the bulk of the charcoal required for the two smelts (using his impressive double mallet technique).

The first smelt was undertaken by Neil and Dr. Ron Ross, with Darrell guiding and troubleshooting (avoiding most of the work). Once finished with charcoal, Sam stepped in as record keeper over both smelts.

As is typical, although our intention was to start pre-heat for 8 AM, getting equipment set up and everything organized always delays the start, so actually the fire was started closer to 9:45. The preheat phase was fairly standard, with the switch to graded charcoal marking the official start of the smelt after 60 minutes. Over the first smelt the furnace ran evenly and maintained a textbook burn rate of 8 minutes per standard 4 lb charcoal charge. A total of 20 kg of ore was added for the first smelt. The course of the smelt followed what has come to be the fairly standard method : heat the stack / seed charges / main charges slowly increasing in volume / allow the last charge to settle then add a larger shock charge / burn down for extraction. As was expected with the high rock ore, more glassy slag was created than in our past smelts, and several tappings were undertaken.

Neil prepared and started a top extraction. It was quickly clear that there was an excessive amount of liquid slag at tuyere level, and he was having trouble locating the potential bloom. His pounding with the thumper was found to be just splashing into a large pool of liquid slag. Darrell quickly stepped in (working without much gear on and burning off a glove in the process) and located and loosened out the smaller than expected bloom mass. Darrell then grabbed it out with tongs and quickly moved it over to the consolidation stump, where Ron / Sam / Gus were waiting with large hammers. The bloom proved to be quite spongy, but was quickly worked down to a rough block a bit larger than fist sized. The finished bloom weighs only 900 gm. It still contains considerable slag and is expected to loose a significant amount of weight as it is later further compacted in the forge into a bar. The metal appears to be of workable quality however, expected to produce a good soft iron - if only a smaller amount.

Smelt Data

TIME		EVENT	ORE			CHARCOAL			AIR
REL.	ELAP.		ADD (scoops)	AMT (Kg)	TOTAL (Kg)	ADD (buckets)	AMT (lb)	TOTAL (lb)	SET.	VOL. (litre/min)
0:00		Start Preheat, Tap arch Sealed, Air started					4		A+½
0:43	0:43	Charcoal (graded)				1			C
1:00		Switch to graded charcoal
1:06		Increase Air flow							C+½
1:10	10	Charcoal				1	4	4
1:17	7	Charcoal				1	4	8
1:24	7	Charcoal				1	4	12
1:31	7	Charcoal				1	4	16
1:40	9	Charcoal				1	4	20
1:42		Ore - Seed Charge (full scoop)	1	0.4	0.4
1:48	8	Charcoal				1	4	24
1:48	6	Ore - Seed Charge (full scoop)	1	0.4	0.8
1:57	51	Increase Air Flow							C+¾
1:57	9	Charcoal				1	4	28
2:00	12	Ore	1	0.4	1.2
2:05	8	Charcoal				1	4	32
2:11	11	Ore	1	0.4	1.6
2:!3	8	Charcoal				1	4	36
2:19	8	Ore	1	0.4	2.0
2:22	9	Charcoal				1	4	40
2:29	10	Ore	1	0.4	2.4
2:30	8	Charcoal				1	4	44
2:35	6	Ore	1	0.4	2.8
2:38	3	Ore	1	0.4	3.2
2:38	8	Charcoal				1	4	48
2:42	4	Ore	1	0.4	3.6
2:45	3	Ore	1	0.4	4.0
2:46	8	Charcoal				1	4	52
2:50	5	Ore	1	0.4	4.4
2:52	2	Ore	1	0.4	4.8
2:54	8	Charcoal				1	4	56
2:56	4	Ore	1	0.4	5.2
2:59	3	Ore	1	0.4	5.6
3:01		Tap
3:02	8	Charcoal				1	4	60
3:06	7	Ore	1	0.4	6.0
3:09	3	Ore	1	0.4	6.4
3:10	8	Charcoal				1	4	64
3:15	6	Ore	2	0.8	7.2
3:19	9	Charcoal				1	4	68
3:21	6	Ore	1	0.4	7.6
3:25	4	Ore	1	0.4	8.0
3:28	9	Charcoal				1	4	72
3:29	4	Ore	1	0.4	8.4
3:31	2	Ore	1	0.4	8.8
3:35	4	Ore	1	0.4	9.2
3:36	8	Charcoal				1	4	76
3:38	3	Ore	1	0.4	9.6
3:40		Tap
3:43	5	Ore	1	0.4	10.0
3:45	9	Charcoal				1	4	80
3:46	3	Ore	1	0.4	10.4
3:47	1	Ore	1	0.4	10.8
3:50	3	Ore	1	0.4	11.2
3:53	8	Charcoal				1	4	84
3:53	3	Ore	1	0.4	11.6
3:58	5	Ore	1	0.4	12.0
3:58		Tap
4:00	2	Ore	1	0.4	12.4
4:02	2	Ore	1	0.4	12.8
4:02	9	Charcoal				1	4	88
4:04		Tap
4:05	3	Ore	1	0.4	13.2
4:07	2	Ore	1	0.4	13.6
4:10	3	Ore	1	0.4	14.0
4:10		Tap
4:12	10	Charcoal				1	4	92
4:12	2	Ore	1	0.4	14.4
4:13	1	Ore	1	0.4	14.8
4:15	2	Ore	1	0.4	15.2
4:17	2	Ore	1	0.4	15.6
4:19	2	Ore	1	0.4	16.0
4:20	8	Charcoal				1	4	96
4:21	2	Ore	1	0.4	16.4
4:24	3	Ore	1	0.4	16.8
4:26	2	Ore	1	0.4	17.2
4:27		Tap
4:28	2	Ore	1	0.4	17.6
4:28	8	Charcoal				1	4	100
4:30	2	Ore	1	0.4	18.0
4:32	2	Ore	1	0.4	18.4
4:35	3	Ore	1	0.4	18.8
4:36		Tap
4:37	2	Ore	1	0.4	19.2
4:37	9	Charcoal				1	4	104
4:39	2	Ore	1	0.4	19.6
4:41	2	Ore	1	0.4	20.0
4:44	3	Ore	1	0.4	20.4
4:46	2	Ore	1	0.4	20.8
4:46	9	Charcoal				1	4	108
4:47	1	Ore	1	0.4	21.2
4:49	2	Ore	1	0.4	21.6
4:51	2	Ore	1	0.4	22.0
4:52	1	Ore	1	0.4	22.4
4:53		Tap
4:54	1	Ore	1	0.3	22.7
5:12		Remove Metal Sleve after initial burn down
5:20	34	Charcoal				1	4	112
5:30	10	Charcoal				1	4	116
5:31		Shock Charge
5:37	7	Charcoal				1	4	120
5:46	9	Charcoal				1	4	124

After this we pulled the bloom and immediately refilled the smelter for the second smelt. The record of the second smelt is here.