Tuesday, November 13, 2018

'Roman' - MoAF-I : Conclusions

Readers should refer back to recent postings on this topic :
the Mother of All Furnaces - Iron
'Roman' - MoAF-I : Results
What was learned?

1) Intake vs Exhaust vs Height
- The completed furnace, at 185 cm was certainly tall enough to allow for an effective drawing force into the stack.
- There were a number of factors that effected the relatively poor performance of the initial use of the MoAF - as a glass bead making furnace. Certainly one major factor on that first test was that the cross section ratio of intake ports to exhaust was certainly far too large. This may have been compounded by increasing the number of openings over the course of that burn.
- For the use of the same furnace body as an iron smelting furnace, the ratio conformed more closely to the theoretical model suggested by R.H. Rehder (1)
  Rehder's example was   1256 cm2 to 51 cm2 = 24.6 : 1
  MoAF-1 was          706 cm2 to 28 cm2 = 25.2 : 1

2) Charcoal Sizes
For the initial firing of this furnace, the charcoal was sized from .5 to 2.5 cm, our normal 'graded' fuel that has proved effective with our short stack iron furnaces.
With the MoAF-I experiment, this size was increased to 2.5 + / average 3.5 cm, as recommended by Rehder. This larger particle size proved effective in allowing air to flow upwards inside the furnace. Combined with the tall stack height, there was no problem maintaining the correct 'reaction time' for the falling ore, despite the larger gaps between individual fuel pieces.

3) Internal Temperatures
As can be seen on the temperature data, there proved to be no problem generating and maintaining the required temperatures to support the iron smelting process.

4) Type of Tuyere
The effectiveness of the mild steel pipe as tuyeres was a bit of a surprise, in terms of the lack of erosion / shortening of length over the entire burn. In the past, mild steel pipes have proven to melt off inside the smelting furnace, usually burning back towards the furnace wall. This in turn has resulted in excessive erosion of the furnace wall material itself - sometimes to the point of burning completely through!
The effect of the base height (below) may be an important variable here.

5) Base Height
Normally the position of the effective base of the furnace interior is controlled fairly carefully. Past experience has shown a space from base to tuyere of 10 cm is a minimum, with 15 cm the usual amount. This spacing is controlled by using charcoal fines or light wood ash packing in place. (This gap is required to leave room for the developing slag bowl, allowing accumulation of slag and the bloom - without 'drowning' the tuyere air blast).

For the MoAF-I experiment, it was decided to be very conservative in the placement of the tuyeres above the solid base of the furnace. The tuyeres were placed 20 cm above the base.
In the final analysis this large space proved to be an error. The slag bowl was found to form too low, dropping it down below the most effective heat zone inside the furnace. This in turn caused the partially sintered iron to cool below the effective 'welding temperature' needed to allow it to form into a solid bloom mass.

6) Ore?
Since the overall intent of this experiment was getting the needed temperatures, the actual addition of ore was almost an afterthought. Other project commitments had overwhelmed my available time to mix and properly dry a mix of our usual DD1 analog. So in the end we decided to make use of some materials on hand. This certainly proved to be of too low iron content, or not enough amount, to expect good bloom formation.

7) Bloom?
There certainly was effective reduction of iron oxide ores, with accumulation of the resulting iron metal. The overall lacy consistency of the resulting mass however does suggest a number of potential problems as the cause:
- Too small an amount of raw ore / of too low an iron content?
- Incorrect placement of the base below tuyere, cooling the iron below welding temperature to allow compaction?
- Generally a low air volume in the system, which has been shown in the past to result in both lower yields and less compact blooms.

Into the future?

At this point I have observed two other attempts at operating a full scale 'Roman' iron smelting furnace, 150 cm + internal diameter and 200 cm + taller. Neither of these attempts resulted in any effective iron production. There was a large amount raw materials required, and massive amount of expended labour for the construction. The amount of charcoal consumed was huge, with firings running days, rather than hours. All of this certainly beyond my resources here at Wareham.

The build for MoAF took a combined team of six, a full working day to construct. The materials required were over twice the amounts used for the typical 'short shaft' furnaces. As this particular firing was undertaken, the overall consumption of charcoal was in the same order as with the more complete sequence more typical.
It would likely prove quite possible to use ceramic 'flue' tiles for the upper stack portion of this type of furnace, which would speed construction considerably, and reduce materials costs.

Likely the most important factor that will influence continuing an experimentation series based on the Roman, passive tall stack builds was contributed by Neil Peterson:
'Just how many different historic periods do we want to investigate?'

1) 'The Mastery and Uses of Fire in Antiquity' by J.E. Rehder - 2000
McGill-Queen's University Press, ISBN 0-7735-2067-8 

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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