Iron Production Experiment June 2007 #2

Date: 9 June, 2007, start time 3:53 pm (clock time 6:08)

Location: Vinderheima

Premise/ Questions: 

This is the second of two smelts to test these premises.

• 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	Dave Cox
Staff	Kevin Jarbeau, Ken Cook
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	92 lb (41.8 Kg)	Weight of Bloom	3100 g (6.8 lbs)	Total Elapsed Time:	3:25 to last charcoal added
Total Ore	52.6 lb (23.9 Kg)	Weight of Slag:		Bloom Quality

Discussion:

As soon as the first team moved out of the smelt area, the second team of Dave / Ken / Kevin got into action - the 'hot swap'. First the remaining ignited charcoal scooped from smelt one was dumped back into the furnace, with the remainder of stack filled with fresh charcoal and the air blast resumed. Because of the retained heat in the furnace, a couple of charcoal charging cycles were required to fully ignite the working height of the smelter and return to the desired 8 minutes per standard bucket burn rate. Again the standard ore charging sequence of seed charges / increasing volume of primary charges was undertaken.

Because the small size of the initial bloom had confirmed in our minds the low iron oxide content of the ore, it was decided to enrich the second smelt by adding hematite blasting grit. That material is over 90 percent iron oxide, but can often cause problems because of its actual lack of silica for slag formation. It was decided that mixing 50 % of the rock ore with 50% of the hematite might create a more balanced mixture of iron available inside the furnace. The method this team chose to employ was to alternate rock ore with hematite scoop by scoop, As the hematite is a bit more dense than the rock ore, the end result was slightly more of the hematite making up the roughly 25 kg total ore utilized over smelt two.

A significant secondary problem arose during the second smelt. The initial patching of the furnace wall in the tuyere area before the firing had been done with 'Blue Mountain Red' - a local ground clay which has a relatively low firing temperature. Soon into the second smelt, it was obvious that the smelter wall above the tuyere, but within the supporting earthen bank, had burned through. Dramatic slumps of the ash and sand packing mix on the tuyere side of the smelter indicated large amounts of this material was falling into the furnace. By keeping a quantity of this mix handy, it was possible to fill in the developing hole. Over the course of the second smelt the better part of a 20 litre pail of packing mix was consumed in this way. Mixing ash and sand and then bringing it to the kind of heat existing in the interior of a smelter (1100 C or more) results in the creation of glass. Huge volumes of light glassy slag were being produced from this source alone. Add that to the slag still in place from the first smelt, plus any new slag being formed from the addition of high rock content ore and it was obvious that slag volume was going to present a major problem. The team had to undertake a number of fairly large slag taps to keep the tuyere clear. Eventually the large volumes of glass being created via the packing mix (going through what later proved to be a fist sized hole) started to block of the tuyere.

Although not quite ready to extract the bloom in terms of an ideal sequence, a sudden restriction in air flow lead to the decision to pull the bloom. Because the smelter structure was not considered likely to be recoverable, it was suggested to make a bottom extraction through the tap arch. It proved difficult to pull out the bricks filling the tap arch, as repeated slag taps had created a large mass of solidified glass around them. Some close work with steel rod and hammer by Ken broke up and cleared the bricks and exposed the original base layer of charcoal fines and ash under the slag bowl. Kevin was then able to rake clear the bottom of the bowl. This was punctured, and hot liquid slag allowed to flow into the gap to soften the lower surface of the bowl. Next Dave was able to hammer clear some of the upper edges of the combined slag mass. Quickly moving to hook and pry through the tap arch, the bloom popped free quite suddenly, and Dave grabbed it with the tongs.

Again the bloom mass was moved over to the consolidation stump. With Neil / Ken / Sam on the hammers and Dave holding, the loose mother was knocked clear and the ragged edges compacted. The bloom was returned to the smelter body for another heat to allow for a second compaction (this time Ken holding and Dave now striking, with Kevin replacing Neil on the hammer). The end result was a rough cube of metal with a total weight of about 3100 gms. The second bloom is considerably more dense, more in line with what would be expected in a fully successful smelt.

Some Photos from the setup

Unnr shovelling ore	Dumping hot ore	Smelter layout

TIME		EVENT	ORE			CHARCOAL			AIR
REL.	ELAP.		ADD (scoops)	AMT (Kg)	TOTAL (Kg)	ADD (buckets)	AMT (lb)	TOTAL (lb)	SET.	VOL. (litre/min)
6:08		Refill smelter with Charcoal							C+¾?
6:20	12	Charcoal				1	4	4
6:28	8	Charcoal				1	4	8
6:37	9	Charcoal				1	4	12
6:37		Ore	1	0.4	0.4
6:46	9	Ore	1	0.4	0.8
6:47	10	Charcoal				1	4	16
6:54	7	Ore	1	0.4	1.2
6:56	9	Charcoal				1	4	20
7:05	11	Ore	1	0.4	1.6
7:06	10	Charcoal				1	4	24
7:12	7	Ore - Hematite	1	0.7	0.7
7:13	7	Charcoal				1	4	28
7:21	9	Ore	1	0.4	2.0
7:21	9	Charcoal				1	4	32
7:25	4	Ore - Hematite	1	0.7	1.4
7:31	6	Ore	1	0.4	2.4
7:31	10	Charcoal				1	4	36
7:37	6	Ore - Hematite	1	0.7	2.1
7:39	8	Charcoal				1	4	40
7:40	3	Ore	1	0.4	2.8
7:43	3	Ore - Hematite	1	0.7	2.8
7:47	4	Ore	1	0.4	3.2
7:48	9	Charcoal				1	4	44
7:54	7	Ore - Hematite	1	0.7	3.5
7:57	3	Ore	1	0.4	3.6
8:00	3	Ore - Hematite	1	0.7	4.2
8:00	12	Charcoal				1	4	48
8:02	2	Ore	1	0.4	4.0
8:05	3	Ore - Hematite	1	0.7	4.9
8:06	6	Charcoal				1	4	52
8:07	2	Ore	1	0.4	4.4
8:08	1	Ore - Hematite	1	0.7	5.6
8:10	2	Ore	1	0.4	4.8
8:12	2	Ore - Hematite	1	0.7	6.3
8:14	2	Ore	1	0.4	5.2
8:14	8	Charcoal				1	4	56
8:16	2	Ore - Hematite	1	0.7	7.0
8:17	1	Ore	1	0.4	5.6
8:19	2	Ore - Hematite	1	0.7	7.7
8:21	2	Ore	1	0.4	6.0
8:23	2	Ore - Hematite	1	0.7	8.4
8:23	9	Charcoal				1	4	60
8:25	2	Ore	1	0.4	6.4
8:26	1	Ore - Hematite	1	0.7	9.1
8:29	3	Ore	1	0.4	6.8
8:31	2	Ore - Hematite	1	0.7	9.8
8:32	1	Ore	1	0.4	7.2
8:32	9	Charcoal				1	4	64
8:36	4	Ore - Hematite	1	0.7	10.5
8:37	1	Ore	1	0.4	7.6
8:38	1	Ore - Hematite	1	0.7	11.2
8:41	3	Ore	1	0.4	8.0
8:43	11	Charcoal				1	4	68
8:43	2	Ore - Hematite	1	0.7	11.9
8:45	2	Ore	1	0.4	8.4
8:47	2	Ore	1	0.4	8.8
8:49	2	Ore	1	0.4	9.2
8:50	1	Ore	1	0.4	9.6
8:52	9	Charcoal				1	4	72
8:53	3	Ore	1	0.4	10.0
8:55	2	Ore	1	0.4	10.4
8:57	2	Ore	1	0.4	10.8
8:58	1	Gromps
9:00	2	Gromps
9:02	10	Charcoal				1	4	76
9:02	2	Gromps
9:09	7	Charcoal				1	4	80
9:17	8	Charcoal				1	4	84
9:25	8	Charcoal				1	4	88
9:27		Shock Charge	3	1.2	12.0
9:33	6	Charcoal				1	4	92