the ' Mother of All Furnaces - Iron '

or

'Roman' Passive Tall Stack

October 13 - 2018

The background to this experiment is a bit more contorted than usual.
My main smelting partner is Neil Peterson. His primary research area is Viking Age glass bead making, primarily the furnaces. In this there is even less archaeological evidence available than for iron smelting furnaces. (1)
One outstanding problem remains how to prevent excessive ash scarring on the surfaces of the beads produced using a number of potential furnace designs. This a problem we seem to encounter regularly, where the historic bead makers did not.

The latest in potential designs was to attempt a passive air system, based on the function of a 'tall stack'. Over a certain height (roughly 1.3 to 1.5 metres) the rising hot air will cause enough suction at the furnace base to ensure a constant draft, and effective burning of charcoal.

'The Mastery and Uses of Fire in Antiquity' by J.E. Rehder was our constant and best reference as we considered the possible designs for a 'Mother of All' scaled furnace. (2)

Our original intent was to take the standard 'short shaft' furnace built for the July iron smelting course here at Wareham (constructed by the crew of Hurstwic), and extend the height, roughly to double the normal height of 60 - 70 cm.

original plan

'Speaks with Fire' - end of smelt

As damaged...

But as it happened, I had a second major act of vandalism there at Wareham in early July, and the initial furnace, still in good shape after the iron smelt, was extensively damaged. (3)

So as it turned out, Neil and his bead team ended up constructing an entirely new furnace in later August. This was dried, then fired for the 'passive bead furnace' experimental test over September 1, 2018 - named 'the Mother of All Furnaces'.
In the end, the temperatures created where not as high, or as dependable, as hoped. In consideration after, this was most likely due to a simple error - of not leaving space *below* the air entry ports for the ash from the large quantity of charcoal burned to accumulate - without blocking those same air ports. There also was some questions that we might had of implemented Rehder's formulae on stack height / column cross section / air intake diameters even vaguely correct.

MoAF - after the bead furnace experiment

The basic MoAF bead furnace was 150 cm tall, with a cross section interior at 30 cm (with some variations). During the initial test, a metal collar had been added, increasing the entire stack height to 185 cm.
The highest temperature recorded was 940 C, but generally the furnace cycled through a lot of variation - and at lower peaks. Certainly the day long experiment had resulted in almost no damage at all to the interior or overall structure.

View down the interior of the furnace to the base, as seen before preparing the iron smelt experiment. (image by Neil Peterson)

As the image above shows, the internal temperatures of the initial firing only were high enough to just start sintering the clay into actual ceramic. The edges of the bricks supporting the furnace had some slag attached, with all the final charcoal fully reduced to ash, a layer about 2 - 3 cm thick.



Going back to Rehder, he gives an example of applying the (complex!) formulae to a theoretical passive tall stack furnace:

• - 40 cm ID / cross section .13 m2 (actually 1256 cm2)
• - 1.5 m stack height (above tuyere)
• - 3.5 cm average charcoal particle size 
• - 5096 mm2 tuyere cross section (so 51 cm2)
• - use of four tuyeres, each at 4.0 cm ID / with length of 20 cm  (4)

With the original MoAF burn, we had *not* run this math, but had relied on 'experience':

• - 30 cm ID / cross section .7 m2 (actually 706 cm2)
• - 1.5 to 1.85 m stack height (above tuyere)
• - .5 to 2.5 cm charcoal particle size (likely closer to 1.5 cm average)
• - 195 to 450 cm2 air inlet cross section (as rectangular openings in the base)
• - use of four to eight air inlets (altered by removing base bricks)

You can easily see that there were a large number of distortions from the system indicated by Rehder. One important question was : Did we actually make too large openings into the furnace, thus effectively reducing flow pressure into the stack?

So for the use of this furnace body for an iron smelt, we decided to conform more closely to Redher's example. Our cross section was fixed at 30 cm, which is 0.56 the area of the example. So we reduced our tuyere area by the same proportion.

• - 30 cm ID / cross section .7 m2 (actually 706 cm2)
• - 1.65 m stack height (above tuyeres)
• - 3.5 cm average charcoal particle size (described below)
• - 28 cm2 total tuyere cross section
• - use of four pipe tuyeres, each 30 mm ID / 18 cm long

As modified - MoAF-I

The charcoal sizes used were greatly increased. On some consideration, it was most likely that the small particle sizes used on the initial burn did not allow enough spaces for the effective passage of air through the system. Our normal practise is to take the raw bagged charcoal, then break it up with mallets over a 2.5 cm grid. This is passed over a second, smaller screen at roughly 0.5 cm (1/4") to screen out the fines. In preparing the fuel for the iron smelt, the rough was screened (not broken) using the upper grid only. This material thus had a lower size of 2.5 cm, with some pieces as large as 10 cm or more - in all hopefully conforming to the suggested 3.5 cm average.

Although not indicated in the layout drawing, the four tuyeres were placed to leave a roughly 20 cm height above the hard ash base. The tuyeres themselves were cut lengths of standard 'schedule 40' mild steel pile, so with a wall thickness of roughly 3 mm (1/8 ")

from the top, showing tuyere placement (image by Neil Peterson)

The four tuyeres were set at rough quarters around the circumference. Each was set so it would extend about 1/4 the way towards the centre of the furnace, a depth of about 7 - 8 cm. The intent here was to create as even a 'bubble' of air into the furnace volume as possible. The standard 22.5 down angle proven effected in earlier smelts was utilized for each.

There was better instrumentation on hand for this experiment. Neil had invested in a multi-input data recorder, linked to a computer lap top. A total of three small ports were drilled into the furnace wall, at + 10 / + 20 / +50 above tuyere level. High temperature thermocouples were installed in each, pushed into the furnace body about 10 cm past the interior surface. Linked to the data recorder system, this made for temperatures noted every 10 seconds over the main part of the firing sequence.

Showing the location of the temperature probes, one of the tuyeres is clear to the lower right.  (image by Neil Peterson)

 

 In addition, one larger circular viewing port was cut, on to the 'back' of the furnace (directly opposite of the position of the probes). This was set at 45 cm above tuyere level. The port was plugged with a removable cylinder of clay.

During the split wood pre-heat cycle.

As the furnace had already gone through a full firing cycle, the clay body was 'bone dry' and the normal pre-heating with small wood splints could be shortened, to roughly 45 minutes.
The first charcoal was added, initially only to the roughly +50 cm level. The side observation port proved of value here. For roughly the next hour, single buckets of charcoal were added, while the internal temperatures were monitored. (5)
It quickly became apparent that the probe inserted at +20 was giving inaccurate, consistently too low, readings. 
The exact burn rates in this period were hard to keep consistent, but this was as much because of the difficulty of determining the exact charcoal level as anything else.

After filling with charcoal - obviously incomplete combustion.

The internal temperatures certainly indicated that starting into the second hour after the addition of charcoal, the lower (reactive column ) area of the furnace was certainly reaching, and maintaining, 'smelting temperatures' of + 1100 - 1200 C.
It was decided at roughly + 1 hour 20 minutes to completely fill the tall stack. This was expected to 'crash' the temperatures, at least for a short time - and this is clearly seen in the temperature data.

At two hours into the main charcoal sequence, it was decided to start with ore additions (so roughly 35 minutes after fulling the whole stack to the top).
The main objective of this experiment was to see if and effective arrangement of tuyeres could create the high temperatures needed for actual iron smelting. In many ways this cause the actual production of a quality bloom was secondary. Although we were well aware of the 'Nissen Method' of adding a quantity of iron rich tap slag as the first charges (to quickly establish the working slag bowl), no slag was available to allow this step. Rather than use the well proven DD1 analog, some smaller amounts of other ore bodies on hand were used. In the end a total of 13 kg of 'Lexington Brown' limonite made up the main charges - an ore body shown to be lower in iron content. (6) There was a small amount of the DD1A analog, total of 5.8 kg, which was added as the final charges.
Once the whole stack was filled, the addition rate for a standard 1.8 kg bucket increased to an average of 30 minutes. It was decided that because of the known 'mixing effect' of the tall stack, larger ore charges would be added than usual. Here individual charges were initially 2 kg, later 3 kg each. (7)

'Down the Kilt' shot - through a tuyere at the later part of the smelt

Observations down the tuyeres certainly confirmed that iron ore was being reduced and was at suitable temperatures at the bottom of the furnace.
After the last addition of ore, the furnace was left to simply operate on its own. We calculated it would easily take another four hours to completely burn down.


(to come ... dismantling and results)


1) Iron smelting sites are both plentiful and distributed throughout the Norse homelands and their various expanded settlement regions. Although furnace remains tend to be just the base sections, these do indicate a number of construction variations on the basic 'Norse short shaft' type. (This the effect of locally available building materials primarily).
Actual Viking Age / Norse glass bead manufacturing sites, on the other hand, are quite limited - basically with only seven locations showing remains of bead making. Of these, only Ribe in Denmark contains remains that might be those of actual bead making furnaces themselves.
Neil has been undertaking a long campaign of experiments to attempt to build an effective (and efficient) small, charcoal fired, and bellows activated, bead making furnace. This work has been documented through a number of academic papers and presentations, as well as extensively on the DARC web site :
'Viking Age Bead Making'

2) This volume was published in 2000. I was presented a copy in 2007, which admittedly I did not read until much later. Although very much an academic treatment, and thus a bit hard for the more practical worker to sort through, there most certainly is a wealth of information presented. The volume covers the theory of operation of a wide range of historic furnace types - used for ceramics, glass and various metals. (A bit more study on my own part might have saved a *lot* of mis-steps over my own experiments!)
McGill-Queen's University Press, ISBN 0-7735-2067-8 

3) The result of this was the loss of roughly $50 in raw materials, but more importantly - roughly 16 plus 'man hours' of hard work. The report on file with the OPP gives the loss at $500.

4) example given on page 182

5) You will find the temperatures on the data have been colour coded :

•  + 1000 - 1050 C = light yellow
•  + 1050 - 1100 C = dark yellow
•  + 1100 - 1150 C = light orange
•  + 1150 - 1200 C = dark orange
•  + 1200 - 1250 C = light grey
•  + 1250 - 1300 C = dark grey 

The temperatures between 1100 - 1200 C are considered 'ideal' for the iron smelting process.

6) This material was some I had gathered back in 2006, with the assistance of Vandy Simpson. Although the area around Lee Sauder's home base outside Lexington Virginia most certain contains some very good quality limonite ore, the relative (lack of) skill of the gathering needs to be remembered. The last use of this material dates back to the 2007 - 2008 period, all the uses of this ore ended up with low yields.

7) Another factor here was the raw time - and old bones. This was the second iron smelt we had undertaken inside of four days! With charcoal additions at 30 minutes, combined with a later start, time was approaching 5:30 pm. Neil and I decided that 'proof of concept' was all this experiment was really intended. To that end, we basically used all the prepared ore we had on hand, rather than attempt the more usual 30 kg ore amounts normally used.