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Iron Smelt - Wareham, ON - Oct 9, 2011

Date:Oct 9, 2011

Location:Wareham, ON

See also: Darrell Markewitz's ironsmelting site

Team: Darrell Markewitz, Neil Peterson, Marcus Burnham

Premise: Pilot experiment for a slag pit furnace

Furnace Design

TypeNorse cobb short shaft, tap arch
Extractiondismantled
Diameter25 cm
Height56 cm
Volume27475 cm3
Tuyure Typeceramic tube
Tuyere height from base13 cm
Tuyere penetration into furnace5 cm
Tuyere angle22 deg
Base designslag pit (39cm depth, 28.5 tapering to 26cm diameter) cm

Burn Details

Bellow Typeblower
Avg Air rateunknown litres per minute
Total Charcoal Mass65 Kg
Avg Burn rate min per 2 Kg
Ore TypeBratton's Run rock ore
Total Mass of Ore48 Kg
Burn Duration5:45 Hours

Results

Mass of Bloom0 Kg
Bloom Typenone
Yield0%
Notesbloom? what bloom?

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.

Discussion:


The Slag Pit style furnace is an older type than the Slag Tapping furnaces that have dominated our work to date. The base of the furnace has a shallow hole or pit dug below ground, which is then filled with some kind of vegetation. In use, the vegetation at first supports the burning fuels in the furnace itself. The inital slag forms into the normal bowl shape below tuyere level. As both heat increases and slag accumulates, at first runnels of slag will drip down through the vegetation. Increasing heat first carbonizes the plant materials, then the weight of of the slag mass slowly settles into the pit. Designed and managed correctly, there is no need for additional tapping of slag. The upper furnace structure itself varies, ranging from smaller bellows blown to larger natural draw types.

The slag pit arrangement was common throughout Europe, roughly up to the end of the Roman period. The slag blocks created are almost industructable, and thousands remain. In Denmark alone, over 2000 have been found. Typically these are roughly the size of a bushel basket.

Conclusions:


It was clear that the failure here was with the selected ore body. This was considered questionable even from the start. A small amount of the DD1 analog, total about 6 kg, was addes about half way through the sequence. This may account for the runnels of dark slag (indicating iron present) seen at the rear of the block. Although it was hoped that this better quality ore might form its own small iron mass, it appears that this was not enough to overcome the generally low content of the bulk of the ore added.

It has been decided to retain the slag block as a reference sample. The area below the tuyere (where a bloom normally forms) is slightly magnetic. It may prove that a small amount of iron was produced, and remains trapped in the upper portion of the block.

Proof of concept was certainly delivered. The individual design of the furnace and the individual elements of construction did in fact operate as hoped. At one point the bowl was punctured by driving a rod down from the top, but other than this there was no significant problem with liquid slag blocking the tuyere. It should be noted that 'ore input against slag output' is very close to balanced, within the limits of a field experiment.

The furnace itself has minimal damage, and can be quickly repaired and re-set for another smelting event.

It is fullly expected that with the use of a propperly rich ore, the combination of slag pit with short shaft should function correctly. Other experimenters have advised that high air / consumption rates with their similar furnaces lead to cast iron production. Further tests would be required to make any valuable insights.

Photos:


thumbnail thumbnail thumbnail thumbnail
Filled pit Smelter Air system Taking measurements
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Slag in pit Closeup of slag Overall view Rear of slag
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Slag block

Temperature Data:



Holes were drilled through the furnace walls at roughly every 10 cm, starting at 10 cm above the interior base.
Measurements were taken using an industrial quality digital pyrometer (HH12B from Omega equipped with standard bare metal type K thermocouples).
The probes were inserted roughly 5 cm beyond the interior surface of the furnace wall.
Measurements were taken roughly every hour over the course of the smelt event.

Because the probes did not reach into the central core of the furnace, there is every possibility that the central furnace temperatures were even higher than what was recorded.
Our thermocoples failed (melted!) at roughly 1350 C. On several recordings, this temperature was reached.

Time Elapsed base tuyere plus 10 plus 20 plus 30 plus 40 top
    10 cm 20 cm 30 cm 40 cm 50 cm 60 cm 70 cm
12:06 :06   653 890 749 579 343  
13:05 1:05   1042 1335 1300 1145 1002 610
13:50 1:50 328 1051 plus 1350 1195 1189 1014 660
15:12 3:12 995 1226 1268 1293 1128 1011 608
16:32 4:32     1124 1265 * 909 700 719

Air Data:



Notes:

Two different 'wind surfer' type vane aneonometers were used. This primarily because a descrepancy between the recorded values for identical air flows had been encountered. It is not known if this is the result of wear in the older of the two units, or just because of functional inaccuracy with the type of equipment used.

The first use cycle was undertaken when the furnace was being used with a small handful of wood splints, so can be considered to be 'open'. The second series was with the furnace holding a full charge of ungraded charcoal (indicated as 'with load'). In a fullly functional furnace, the charcoal would be graded down to .5 - 2.5 cm pieces, plus additionally there would be some quantity of smaller sized ore particles in place in the shaft. Both of these elements would likely increase the load force required to deliver air via the bellows pipe.

Although the individuals taking part in this test were told to 'use a comfortable rhythm', both Marcus and Darrell had taken part in the Vinland 4 and Vinland 5 smelts - which where entirely bellows driven (using this same bellows). Darrell quite specifically attempted to match the same delivery as had been used for those experiments. In so much as his volumes appear to match fairly close to those of both Neil and Marcus (who also have bellows experience with this equipment), it may be reasonable that those numbers are fairly indicative of actual full smelt volumes.

Target Volumes LpM            
               
Smelter SIZE area 1.2 1.5        
cm cm2 LpC2 LpC2        
               
20 314 375 470        
25 491 590 735 typical dia.      
30 707 850 1060        
35 962 1150 1440        
               
TEST              
               
Operator   No Load     with Load    
    Gage A Gage B average Gage A Gage B average
               
Neil Speed 37 43 40 38 48 43
  Volume 313 363 338 321 406 363
               
Marcus Speed 28 35 31.5 40 42 41
  Volume 237 296 266 338 355 346
               
Darrell Speed 40 48 44 39 50 44.5
  Volume 338 406 372 330 423 376
               
Thomas Speed 50 55 52.5 36 40 38
  Volume 423 465 444 304 338 321
               
Gus Speed 39 35 37 37 39 38
  Volume 330 296 313 313 330 321
               
Group Average Speed     38     42
  Volume     322     352
               
Standard              
               
Neil's math 1 KPH is 1666.66667 cm/min    
  2.54 cm pipe is 5.06707479 cm2    
  1 cm3/min is 0.001 LpM    
  1 KPH is 8.44512465 LpM    
               
SPEED to VOLUME for 2.5 dia.          
KpH LpM x 8.45          
               
47 400            
53 450            
59 500            
65 550            
71 600            
77 650            
83 700            
89 750            
95 800 typical use          
101 850            
107 900            
112 950            
118 1000            
124 1050            
130 1100            
142 1200            
154 1300            

Raw Data:



CLOCK ELAPSED EVENT AIR   CHARCOAL     ORE  
          UNIT COUNT TOTAL ADD TOTAL
      gate LpM bucket   kg scoop kg
  total           bags    
12:00   start wood splints pre-heat              
12:30 :30 occassional ari via bellows              
13:30 1:30 blower with gentle air              
13:56 1:56 fill with ungraded charcoal     3   5.50    
                   
  event                
12:00   main sequence start 3.5 350          
12:06   bellows test conducted   325          
12:22 :22 blower reconnected 8 800 1 4      
12:30   heat to top of column              
12:37 :15       1 5      
12:42 0:05       1 6      
12:55   addition first ore - 1 kg amounts              
13:02 :011       1 8   3 1
13:10 0:08 end 7.75 bag charcoal     1 9 13.25 3 2
13:19 0:09       1 10   3 3
13:28 0:09       1 11   3 4
13:42 0:14 increased to 1.5 kg ore     1 12   5 5.5
13:49 0:07 reduced to 1 kg ore     1 13   3 6.5
14:01 0:12 end 9 kg / increase to 1.5     1 14 22.25 4 8
14:11 0:10       1 15   4 9.5
14:20 0:09       1 16   4 11
14:30 0:10       1 17   4 12.5
14:39 0:09 end 8.25 kg charcoal     1 18 30.50 5 14
14:48 0:09       1 19   4 15.5
14:57 0:09       1 20   4 17
15:06 0:09       1 21     18.5
15:15 0:09 increased to 2 kg ore     1 22   5 20.5
15:25 0:10 end 8.5 kg charocal     1 23 39.00 5 22.5
15:35 0:10       1 24   4 24.5
15:45 0:10 switched to DD1 ore     1 25   5 26.5
    rod from top, drops level              
15:49   air increased 9 900          
15:54 :09 air reduced 8 800 1 26   5 28.5
16:03 0:09 end 6.5 charcoal     1 27 45.50 4 30.5
16:15 0:12 increased to 3 kg ore     1 28   4 33.5
16:24 0:09       1 29   4 36.5
16:29 0:05 reduced to 2 kg ore     1 30     38
16:31   air increased 10 1000          
16:41 :12       1 31   4 40
16:49 0:08 end 9.25 charcoal     1 32 54.75 4 42
16:59 0:10       1 33     44
17:06 0:07       1 34   4 46
17:14 0:08 all ore added     1 35   4 48
    start burn down       35      
17:21 :07 end 8.25 charcoal     1 36 63.00    
17:26 0:05 last charcoal added 8 800 1 37 65.00    
17:38 0:12 increase air (short burst only) 9.5 950          
17:45   (approximate) - extraction              
                   
                   
    Pre-heat Time 2 hrs          
    Main Sequence Time 5:45 hrs          
    Total TIME 7:45 + hrs          
                   
    Total ORE 65 kg          
    Total CHARCOAL 48 kg          
                   
    Slag Block 40 kg          
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.
Text © Darrell Markewitz
Photographs © Darrell Markewitz
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