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Iron production weekend - 2004

Date: June 12 & 13, 2004

Location: Vinderheima

Premise: If they made iron -- then so can we using the same techniques

Conclusion: We can't do it this way either but this is much closer.

The Team:

Staff Dave
Recorder Neil Peterson
Leader Darrell Markewitz

Smelt_cd 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.

This year marked our third attempt to produce iron from ore. Once again we made some changes from the previous year. Once again we did not get any iron.

Again this year we had some visitors. Mark from the L'Anse aux Meadows, NL interpretative staff came out to act as an observer (so of course we put him to work). We also had a visit from Michael from the Farmers' Museum in Cooperstown. The museum website is here. He was on his way home after spending several weeks discussing and doing Japanese smelting. Michael organized and hosted the 'Pre Industrial Iron Symposium' in October.

In this year's experiment we reaffirmed our intention of targeting the reproduction of a NORSE method of iron production rather than making the SUCCESS of the experiment our priority. This choice controlled many of our design decisions this year. Our notes from these discussions can be found here. Some of the questions facing us can be found here. Of course this means that you may have to visit us for a few more years before you see an iron bloom from us.

Once again we would like to express our thanks to Royal Oak Charcoal who donated the charcoal used in this year's experiment. We have used Royal Oak Charcoal in all our experiments as they are a hardwood lump charcoal (pretty much what the Norse would have produced) and their quality is consistently high. From all of us -- Thank You!

The smelter itself was constructed differently this time. A circular smelter of clay/cobb was built. Around this base a stone box was added. The spaces between the two elements was filled with sand. Three weeks before the actual smelt we gathered to dig a local clay and build the smelter. Our target was a smelter 65 cm high with an average wall diameter of 10 cm. The internal dimension was to be 25cm. The bottom of the smelter was to be a shallow bowl with the air inlet (2.5 cm diameter) 10 cm above the bottom of the bowl and coming in on a flat angle. Our first mistake occurred here. The clay we dug was chunky and quite wet. Although we used a couple of techniques to break it up, the actual cobb we used in the build was neither as dry or as uniform as it should have been. This led to the smelter having a noticeable settling as it dried. The final dimensions of the smelter are listed in the table below. As these things always seem to work just as we finished building we found an article documenting a series of similar experiments back in the 1980s. Crow, Peter: Experimental Production of Historic Bar Iron, Historical Metallurgy v25, No 1, p27. We've included the numbers from his smelters in the table below for comparison.

Dimension Planned   Actual Crow
Wall Thickness 10 cm 10+ cm 20 cm
Interior Diameter 25 cm 25 cm 30 cm
Interior Height 65 cm 55 cm 60 cm
Diameter of air inlet 2.5 cm 2.5 cm 2.5 cm
Tube Angle 0 ~15 deg. down 30 deg. down
Inlet Height 10 cm 2 cm 20 cm

Photos of the Smelter build weekend

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Breaking clay with hand tools Crushing the fresh clay Breaking clay with power tools Mixing the cobb
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Digging out a flat base Setting the inlet and front wall Setting inner form on the base Inner and outer forms in place
Packing cobb between the forms

The day of the Smelt

June 12th. We began this process with a few overall area shots.

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Overview Final check Smelter top Bellows

During our morning setup an ore-roast was set up as we did not have a full 7 Kg of ore ready to go. The ore roasted was more of the same ore we used last year. The leftover roasted ore from last year was screened and weighed into 1Kg charges. When it had cooled sufficiently the ore roasted today was also broken up, screened, and set into 1Kg charges. A total of 7 charges were prepared. The ore was screened twice. Once with a 25mm screen to remove large chunks. It is worth noting that no ore pieces failed this test. We estimate that the actual largest piece was probably around 20mm. The second screening was done to remove fines. A 4mm screen was used for this purpose. This allows us to note that ore added for this smelt was between 4 and 25 mm in size.

In a similar fashion the charcoal was broken up and screened for size. Charcoal used was between 7mm an 25mm in size. The charcoal was also weighed into 1 Kg charges to allow us to track the usage.

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Setting up the burn Weighing ore Anne, Gus, Peter Sarah, Anne
Actual Time Elapsed Time Notes
1036h   We began with the warm up burn. We began the warm up with some softwood.
1051h T+0:15 The vacuum we were using for air was set to 1/4 and the opening into the furnace was noted as 10cm.
1053h T+0:17 While the warm-up continued we had our final planning meeting.
1104h T+0:28 The warm-up fire was switched to hardwood.
1153h T+1:17 Finished the first milk crate of hardwood in the warm up fire.
1214h T+1:39 First optical pyrometer measurements. Temperature inside was recorded as 2350°F (1288 C).
1234h T+1:59 Second milk crate of hardwood finished.
1243h T+2:08 Our old vacuum died. Oh well it happens.
1248h T+2:13 New vacuum brought on-line and air flow resumed.
1314h T+2:39 Time to begin the charcoal burn. The hardwood and ash was scooped out. New Air system was made ready.
1316h T+2:41 The Smelter was filled with charcoal (approx. 3.5 Kg), bellows were hooked up and pumping began.
1324h T+2:49 Live coals from the ore roasting were added to the top of the smelter
1330h T+2:54 Remains of fourth kilogram of charcoal added (approx. 0.5 Kg)

From this point the smelter was stirred/poked from time to time and small amounts of charcoal were added as needed. The experiment records only the time each Kg of charcoal was finished.
1341h T+3:05 Fifth kilogram of charcoal finished. Optical pyrometer measurement gives the internal temperature as 2380°F (1304 C)
1350h T+3:14 Sixth kilogram of charcoal.
1358h T+3:22 Bellows stopped.
1400h T+3:24 Bellows restarted.
1403h T+3:27 Seventh kilogram of charcoal finished.
1406h T+3:30 Optical pyrometer measurement: 2720°F (1493 C)
At this point the ore was added. Each charge of ore was added in two steps (each of approximately 0.5 Kg).
1410h T+3:34 Ore charge 1 - part one added
1414h T+3:38 More charcoal and ore charge 1 part 2 added.
1416h T+3:40 Eighth kilogram of charcoal finished.
1417h T+3:41 Ore charge 2 - part one added.
1422h T+3:46 Ore charge 2 - part two added.
1426h T+3:50 Tuyure rodded out.
1429h T+3:53 Ore charge 3 - part one added.
1431h T+3:55 Optical pyrometer measurement 2640°F (1449 C).
1431h T+3:55 Ore charge 3 - part two added.
1432h T+3:56 Ninth kilogram of charcoal finished
1440h T+4:04 Ore charge 4 - part two added
1441h T+4:05 Tuyure rodded out
1443h T+4:07 Tuyure rodded out
1445h T+4:09 Ore charge 5 - part one added. Tenth kilogram of charcoal finished.
1450h T+4:14 Tuyure rodded out
1452h T+4:16 Eyeball estimate of airflow volume was made. It appears that we are pumping 9L/sec (540 L/min) into the smelter.
1454h T+4:18 Ore charge 5 - part two added.
It was around here that we realized that the shallow air inlet was in fact giving us the problems we had been hoping wouldn't occur. An attempt was made to "save" the situation.
1505h T+4:29 Attempted to rod out the tuyure again. This time with a hammer and hitting it hard. We attempted to remove the pipe to clear the jam but it wouldn't move. At this time we decided to add a new air inlet on the back of the smelter 45 cm below the top of the smelter (hence 10cm above the base of the bowl.
1511h T+4:35 Back door was holed through and a slag pull was done.
1515h T+4:39 Eleventh kilogram of charcoal was finished.
1518h T+4:42 Another attempt to rod the tuyure.
1531h T+4:55 Another attempt to rod the tuyure. All of these attempts have produced only partial successes.
1540h T+5:04 Tap the back hole again.
1545h T+5:09 Move the bellows to the back hole. Front hole was plugged with clay.
1548h T+5:12 Twelfth kilogram of charcoal finished.
1552h T+5:16 airflow in the water measurement device attached is 4.5->6.5 cm of water.
1600h T+5:24 Thirteenth kilogram of charcoal finished.
1609h T+5:33 Fourteenth kilogram of charcoal finished.
1620h T+5:44 Fifteenth kilogram of charcoal finished.
1630h T+5:54 Rod tuyure
1641h T+6:05 Stop bellows, connect new pipe to back and change bellows
1644h T+6:08 Begin airflow with new bellows
1647h T+6:11 Rod tuyure
1656h T+6:20 Sixteenth kilogram of charcoal finished.
1704h T+6:28 Change to rotary air pump
1709h T+6:33 Seventeenth kilogram of charcoal finished.
1719h T+6:43 Eighteenth kilogram of charcoal finished.
1725h T+6:49 Nineteenth kilogram of charcoal finished.
1739h T+7:03 Rod tuyure
1749h T+7:13 Stop airflow and allow smelter to begin cooling.
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Adding wood Front view - air supply Burn with air Wood burning down
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Removing wood Preparing bellows Charcoal added Mark on bellows
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Airways Checking the airpath Looking down the pipe Preparing new airway
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Opening the back Rogering the tuyere Measuring for hole Plugging the new hole
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Preparing to shift Tapping the back Tuyere plugged Salg on radner
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Switching air source Wire testing the smelter

Day 2 - The Excavation

Sunday began with a photographic record of the site. It is worth noting that the smelter itself was still hot to the touch this morning (and the outside temp had been down below 10C the night before). After the record run we broke for breakfast and some discussion and then came out to disassemble the smelter.

Measurements of empty smelter. Above the original tuyure there was an area of was that had been burned away to a depth of 2.7 cm. This burned in area extended 19.5 cm up from the tuyure opening and had a maximum width of 15.5 cm. On the left side there was a burn in 1.6 cm in depth, on the right side a burn in of 1.5 cm depth existed. The distance across the top of the smelter from left to right was 23 cm, the base of the smelter was 27cm from front to back. The original tuyure opening began 46 cm from the top of the smelter.

More photos and begin some weights. We removed a large piece of slag earlier. The top of the slag was 53cm below the top of the smelter. The bottom of the smelter was intact and the slag was resting on the clay NOT on a layer of charcoal. There was no dead-zone and no place for the slag to go. The air path through the slag mass in these photos is obvious. Total bowl depth was 3 cm. The original tuyure was plugged with 5 cm of slag. For the accuracy of the following weights please note that they were recorded in pounds, ounces, and tenths of ounces and then converted to kilos.

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Side view Top view Tuyere Real view
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Left side Rear air port Removing rock Cobb smelter
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Extending crack Planning break Removing first piece Planning next piece
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Removing charcoal Brushing out pieces Removing more charcoal Removing more wall
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Closeup of wall Wall pieces Closer view Close up
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First slag Plugged tuyere Level of rear tuyere Level of slag bowl
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Pieces of wall Blowing out ash Clear of ash Piece of slag
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Location of slag pieces Tuyere Rear tuyere Recovering pieces
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Team at work Rear airway Slag mass Rear wall pieces
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Line of airway First airway exposed First airway Slag mass - bottom
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Slag centre removed Salg mass Overview Overview without slag mass
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Scaled photo Taking measurements Plugged tuyere Closeup of wall

Item Weight
slag removed from bowl 1.06 Kg
Solid blocks of slag with Iron Blebs 2.96 Kg
Deepest slags (near base) 0.69 Kg
Slag from near tuyure 0.23 Kg
Magnetically separated slag 0.31 Kg
Fill from the tuyure 0.09 Kg
Total Mass of Slag Recovered 5.34 Kg
Total Mass of Ore used 5.0 Kg


Time to burn kilos of charcoal
Once the first four kilos of charcoal had been added it was possible to see how long each new kilo took to burn. Those times are listed in this table:

Charge 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time NA NA NA NA 11 9 13 13 16 13 30 33 12 9 11 36 13 10 6

The smelter itself was a SOLID creation this year. If it had not been our intent to break it down for recording purposes we could have run a second smelt in the same body.

The biggest single issue this year was the lack of depth in the main bowl not allowing space for the slag and thus building up to where the air flow was significantly impeded.

In a nut shell - we pushed too much air. This created overly hot interior, which in turn created too large a volume of melted wall as fluid glass. This clogged the air intake. (That whole effect enlarged by slumping of inlet towards base during construction). Excess super heated oxygen literally vaporized the iron particles. The fluid glass slag drowned the air intake. Constant fiddling with bellows (plus break down of one) dropped interior temperatures and halted the reactions. Although an attempt was made to recover - the reaction had been halted and high temperature zone shifted to make restarting the reactions impossible.

Concepts for next year

This section is a little "free form" right now but will get some more work.
Smelt_cd 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.
      Updated: 4 Dec, 2007
Text © Neil Peterson, Darrell Markewitz, 2004
Photographs © Individual artists
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