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1 Sept 2018 - The Mother of All Furnaces

The goal of this day was to gather preliminary understanding of a rather radical interpretation of hearth QA from Ribe (a 50cm diameter round hearth).

21 July 2018 - The Build

Our intention had been to reuse a large bloomery iron smelting furnace for this burn by extending its height. Unfortunately the furnace was vandalized in the days leading up to our effort.

We regrouped and did a clean build from start to finish. The entire construction was cobb (clay, sand, and horse manure). The plan was for a furnace with a 50cm outside diameter at the base thinning as it got taller, 30 cm internal diameter, and a height of at least 1.4m.

A ring of bricks were placed in a circle with their inner sides forming a 30cm circle. In this way the walls could be build on top of the bricks. The bricks could be removed later to provide air ports. The walls were built around a standard propane tank. As the construction got near the top of the cylinder, it was pulled out, and the inside was patched. Working only from the outside it is difficult to ensure a solid construction, by taking the time to ensure a solid blending on the inside cracking can be reduced. The inside was then filled with a 50-50 mix of dry wood ash and coarse sand, and later (when we ran out of ash mix) charcoal fines. This serves both as a sponge to draw water out of the cobb, and also as a support for the structure as it grows. The tank is then placed on top of the filling and the process is repeated.

When we had the furnace fully formed we cut two ports into it on opposite sides. These were set at a height that we hoped would allow us to test "seated" and "standing" working positions. We then left to allow it to dry.

The final "wet" height ranged from 151 to 154 cm of which 9 cm came from the bricks at the base. The outside width at the top was 40.6 cm, with an internal diameter of 31cm. This makes the interior cross-sectional area 755cm2. The circumference at the top and bottom were measured to provide counter checks on the diameter: 135.9 and 177.8 cm respectively. This gives average diameters of 43.25 and 56.6cm and shows that single measurements of diameter have problems in the real world. This discrepancy is due to variations in both wall thickness and maintaining an even diameter (perfect circle) during the hand building process. The lower port was cut in at 94-95 cm (not quite horizontal - depending on which side) and was 12.7 x 5 cm. The upper port was at 117 cm and was 14.6 x 7-8 cm.

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Original furnace Vandalism The building form Early build
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Inside gaps Getting taller Full Height
Unfortunately we had planned on doing the burn that day, but the build process took longer than estimated, and all of us have busy lives. As a result it was a month later before Darrell could check in on the furnace and we realized we made a small mistake. Usually the packing is left in for a couple of days, then removed allowing the structure to finish drying. That step got missed here. The problem (that we should have anticipated) with it being missed is that the interior mix acts as a solid form. As the clay moves from "drier" to "leather hard" or dryer it shrinks - typically 10-15%. And since it couldn't contract against the packing materials - the walls cracked. In this case a spiral crack through the weakest points, passing from just above the base, running diagonally through the lower working port, and extending beyond it.

The widest part of the cracking was about 2 cm and extended fully through to the inside of the furnace. Darrell took the time to patch up the crack on Aug 25. This allowed the clay time to dry and shrink before our planned firing on September first.

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Crack from base Crack to top Measurement

1 Sept 2018 - The Burn

Burn day began with drilling out "light brights". These are 6mm diameter holes which can be used to insert thermocouples, or do visual checks. Then came the task of emptying the packing out the furnace. This was accomplished using a shopvac. It worked quite rapidly and the relative ease balanced the low cost (we will definitely have to replace the filter). We then removed four bricks from the base to act as air ports. The total area of those ports was 195 cm2. This gives a ratio of tuyere to cross-section of chimney of 195:755 or 1:3.8. The crack could easily lead to a "barrel fracture" pulling the crack open further and further. The first force is applied by the clay contracting as heat removes the last traces of moisture, with a second force applied by the expansion of the dry clay as it approaches high ‘sintering’ temperatures. In order to avoid this we added a few wire wraps around the furnace to add additional support (roughly every 20 cm). The working ports and crack were then sealed to maximize the draw through the bottom air ports.

We then loaded some small branches and paper into the furnace and ignited it. The goal was to burn this as a slow fire to drive out any residual moisture in the cobb and heat the furnace body. We ran this heat for 69 minutes and it was in this phase that our first surprise hit. The air draft in this chimney was quite impressive and highly variable. Our feedstock was small branches (smaller than a thumb diameter) about 30cm long. A bundle of branches about 20cm across would cause a massive shift in temperature and draft over a very short period - less than 2 minutes. In this video you can see one such cycle resulting in an increase of over 370° C in that short a period.

Different experimental groups have different traditions. As readers may know our iron smelting experiments all involve jiffy pop where other teams use hot peppers. Our bead furnaces did not use the jiffy pop as they are usually built in a way that would burn through the popper before the corn could pop. In this case with a build that was more like an iron smelter we decided to do the jiffy pop for luck. It turned out really well. A shame that didn't forecast the results of our burn test.

At the 69 minute mark we started adding charcoal. Our first goal was to find out how tall we could build the "stack" of charcoal. We did this by looking down from the top (with faceshields on) looking for that point where all of the charcoal was burning. Our hope was that this would build up to just under one or the other of the working ports. Sadly that wasn't the case.

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Vacuuming ash Measured holes Ready to burn One Laim Tall
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Jiffy pop A look down Charcoal light bright Flame light bright
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Ash buildup Stable pattern Inward flame Adding tuyeres
Extra Height

Time (Experiment)Time (elapsed)NotesCharcoal (buckets)Total Charcoal (buckets)Temp @ topTemp @ 140cmTemp @ 120cmTemp @ 90cmTemp @ 40cm
0N/AStart heat with small wood       
66   429° C    
2519Temperatures highly variable  209° C306° C214° C246° C 
4015Video Shot. External temps (from a point & click "laser") 55° C at base, 65° C @ 30cm, fairly consistent at 65 higher.       
1h 9m29First charcoal addition (approx 1.8 Kg/bucket)11     
1h 15m6    350° C   
1h 18m3First bucket fully red.12     
1h 22m4Bright red edges, black centre. 97° C on outside at top13     
1h 34m12 14 604° C580° C675° C800° C
1h 39m5 15     
1h 50m11    639° C634° C700° C560° C
1h 52m2 16     
1h 58m6Appears to be stabilized where we add more charcoal but by the time it has ignited it has dropped by a bucket.17 604° C   
2h 8m10Add larger amount to see if we can jump stack size310     
2h 12m4   555° C 500° C450° C755° C
2h 30m18Flames coming in from light brights.   630° C581° C546° C916° C
2h 36m6Light brights plugged, cracks plastered to increase lower air flow. Air ports rogered out due to ash buildup. Removed one more brick at bottom (54cm2 extra air inlet. Ratio now 249:755 or 1:2.9). Reduction flames at top of furnace turned to oxidizing.       
2h 45m9 313     
2h 53m8Extra Temps: 1010° C @ 30cm; 800° C @ 50cm, 655° C @ 60cm  487° C 435° C475° C810° C
3h 6m13Charcoal at 50 cm. Popping sound, extra air coming in from around lower port114 505° C   
3h 15m9Rogered air ports and temp immediately increased by 20° C. Ash is clogging the ports since it isn't hot enough to create slag to absorb the ash. So do you add more air inlets or add tuyeres to draw the air deeper inside?     440° C 
3h 30m153 more bricks removed (162 cm2 extra air ports. Ratio is now 411:755 or 1:1.8). Still burning at 50-60 cm from ground.    550° C550° C940° C
3h 42m124 pipes added to route air. We tried to get the outlet approximately 25% of the way in. The pipes were sealed with clay and sloped up as much as possible. 3 pipes were 3.5cm diameter, 1 was 2.5cm diameter. (208cm2 removed and replaced by pipes totalling 34cm2. Input area is now 228cm2 giving a ratio of 1:3.3)115     
3h 51m9 318  490° C434° C 
4h 6m15      240° C 
4h 10m4Rogered air ports, temp began to climb     179° C 
4h 20m10 220     
4h 25m5      260° C 
4h 26m1Rogered air ports     290° C 
4h 30m4      356° C 
4h 35m5Added metal collar to raise height to 185cm.     463° C 
4h 38m3      500° C 
4h 45m7 121   390° C 
5h 0m15      540° C 
5h 2m2Lower working port was unsealed, and opened. Charcoal level was just above the port.     560° C 
5h 3m1      1013° C 
5h 4m1      1093° C 
5h 5m1See extended note below     978° C 
5h 6m1Added a bellows to one of the tuyeres     946° C 
5h 7m1Put the large plug back into the port. Keep pumping bellows.     811° C 
5h 9m2      727° C 
5h 11m2Temp @ 30cm = 956° C       
5h 14m3Temp @ 30cm = 987° C       
5h 15m1Add blower, stop bellows, open working port       
5h 20m5At this point some simple beads were made.     720° C 

Extended Note At the 5h 5m mark point it was clear the massive temperature surge recorded in the working port over the last 5 minutes had come from feeding a lot of air right into the charcoal around the probe. This air was being pulled in though the working port. We had actually expected that the temperature would drop sharply when the port was opened due to the inrush of cold air. The difference between the expectation and observation was the height of the charcoal bed in the furnace. In this case the bed was high enough that the probe was inside the charcoal but in the area where the extra oxygen caused increased combustion. As the level of the charcoal dropped the temperature rapidly started to drop (see the measurements at 5h 6m and 5h 7m). It is also worth recording that the radiant heat at the working port was impressive and would take adjustment to get used to working in that area.

This point also marked the end of our planned activities. Because we had handy equipment we decided to see what impact a simple addition of a bellows might have. This was done with an eye to raising the temperature just enough in the port to allow beadmaking for those who had been patiently waiting for many hours to try. A blower was also added later to try and allow some bead making.


  1. Air ports right at the furnace base were a mistake. The accumulating ash needs some place to go. Also as individual charcoal pieces are consumed, they get smaller and smaller, reducing the gaps between them that allow air into the system. We should have known all of this. Ideally at least a shallow pit, extending below the air port level, would have provided a space for ash to collect into without blocking the air intakes.
  2. A ring of tuyeres, sticking in about 6-7cm on slight down angles would have moved the location of input air further into the interior of the furnace. Ideally this should create a more even burning of the interior volume.
  3. The system proved extremely sensitive to variations in air flow. Built as we did it needs a person constantly clearing the air ports. Granted this is a lower skill/effort job than operating a bellows but is the difference enough to matter? If they are clearing ports why not just put them on a bellows on a smaller furnace that is easier to build and uses less raw material.
  4. Burn rates on a bucket were similar to those we get with iron smelters. 10 minutes per bucket give or take. It should be possible to rig this system to accept a cycle of 3-4 buckets maintaining a working temperature the whole time. Of course we can make one of our smaller bead furnaces run at working temperatures for the same length of time using less charcoal (and less construction material) - so again why do the big build.
  5. Ash levels were very high during the warmup burn with wood, but overall they appeared lower when burning charcoal. This is a visual observation only we didn't run data collection during this pilot. Obviously if we run more experiments this is a useful measurement to gather. Possibly this is due either to the increased height, or to the reduced airflow. How this would relate to ash in the working ports is also an interesting question.
  6. Adding the tuyeres caused a drop in the working temperatures of abut 200° C. This was likely caused by reducing the size of the air ports from 54cm2 to 10 cm2 as we sealed up the pipes.
  7. The addition of the metal collar (33cm extra height) caused an increase of about 100° C. This was likely due to the enhanced chimney effect.
  8. Did we preheat it enough? We didn't take or record many external temperatures.
  9. Did we leave enough time for the system to react when we pulled 4 extra bricks for input, and later when we put the pipe tuyeres in place.
  10. It was fun building the monster, and applying skills acquired in our standard (30 cm ID x 65 cm H) to a more massive build. One improvement was the effective use of a small cement mixer for blending the cobb ingredients.
  11. Clearly there is a need to develop skills in establishing both the required temperatures, and maintaining both duration and consistency of these.
  12. With a construction cost of over $100 and 4 man-days, and a burn day cost of $150 in charcoal, and 3 more man-days we will likely prioritize other experimental sequences.
  13. Air flow in is a complex question needing to balance the draw both in terms of enough air to support he combustion, and enough pressure to move through the charcaol mass. The Mastery and Uses of Fire in Antquity by J.E. Rehder is the primary volume here supporting this with some math. We ran this furnace with ungraded charcoal (to match the diameters suggested in the book) as our normal graded chacoal size would be a tighter mass making air penetration more difficult.
  14. One related observation will effect the planned iron smelting experiment sequence intended for the Fall season. It was originally hoped this ‘tall stack’ furnace would effectively generate the required 1200C temperatures through a passive air draw. This temperature range is clearly not available with this specific furnace design. It may prove possible to modify the existing structure (air port system ? / base layout ?). The same consideration of materials and effort expended against likely results needs to be made.
  15. There is a difference between looking down into a 60cm bloomery furnace and a 150+cm furnace. Even when you use faceshields. The extra height puts the faceshield closer to the heat/flame thus reducing the time available for taking pictures or looking. Also it changes the angles. This means that unless you are careful that heat comes up under the faceshield and starts melting your beard. You have been warned.

Text © Neil Peterson, 2018
Photographs © Individual artists
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