Warning : Often when you e-mail me a question, I tend to ramble on at length in reply. Having spent the time (for me typically an hour or more) on an attempt at a full consideration of that question, I will turn the reply into a blog post as well.
1) so you've shown the early medieval iron bloomery furnaces only produce accurate blooms when you push more air in than you can currently manage using what you think was the bellows technology of the time?
What a ball of wax that is!
I consider the effect of air a big disconnect, especially with what
has happened here in North America as interest in bloomery iron making
developed.
I consider myself one of the extremely small group that started the
whole thing, in the early 2000's. Lee Sauder and Skip Williams are most
certainly the very first to seriously undertake and repeat the process, and finally
come up with a system that not only produced significant blooms, but
consistently. Their objective was not historic method, but functional
results - how to get the best yields at the highest density. Modern
electric blowers were employed from the start. One very important
difference between North America and Europe is over here work with
bloomery furnaces has been primarily in the hands of blacksmiths - not
archaeologists or re-enactors. Lee's original point of inspiration was
African systems, for which there was some 'traditional' recording still
available. He would return to this interest into the later 2010's.
Making bloomery iron is an elaborate, expensive, time consuming, and
labour intensive process - with a very steep learning curve to positive results. So the resulting metal has a high 'investment'
value. Lee has set the 'selling price' of his bloom pieces at roughly
$60 CDN / kg (the few times I have been asked, I quote $100 / kg). This
is roughly ten times the cost of modern alloy steel bars. So for
blacksmiths, the only object type that can justify this kind of
investment in materials to object price is knives. Unfortunately, here
in North America, this has lead the entire Early Iron movement to become
dominated by bladesmiths. This in turn has lead to what I feel is a quite
unrealistic obsession with carbon content, an extremely modern
consideration of what is clearly a 'pre industrial age' material (and
processes).
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| The 'Gange o Fer' in 2004 : (L-R) Lee Sauder, Skip Williams, me, Mike McCarthy |
My interest started with Viking Age systems, most specifically sparked
by the single smelt event at Vinland by the Norse. c 1000 AD. As part of
the original 'Gange o Fer', it was so clear that there were many
individual variables effecting the dynamic inside a small scale furnace.
So the early years were simply testing variable after variable, in the
hopes of getting some understanding (and control?) over these both
individually and in combination. So my focus has never been either to
best possible yields or specifically 'quality'. If anything, my estimate
of 'good iron' is based on the ease of compacting the bloom to bar, then
the ability of that bar to be easily forged to object. Yes, we do end up
with some blooms to bars being higher carbon, and set these aside (as
the Norse would have) for cutting edges.
Through almost all of our experimental work, we have quite deliberately
aimed to making smaller blooms, in the 3 - 5 kg range. (Yield % climbs
sharply with larger ore volume additions!)
In Europe, much of the work with bloomery iron is in the hands of living
history sites and hobby re-enactors. What has been so frustrating to me
is the lack of recording. (See my piece in EXARC : 'Standardized Reporting...').
Lee has pointed out to me many times the overall difficulty of getting
any kind of effective measurements of air volumes, and that the only
uniform field reporting can be 'time of consumption'. Even there, it is
obvious to me that most people are actually reporting total charcoal
consumed / total time of smelt. This is actually only a vague average at
best.
One one major problem is the simple lack of historical accuracy I see.
If you are using what is at best a Late Medieval double chamber bellows
(to chambers stacked on top each other) you are NOT using 'Viking Age'
method.
Too often I see smelts described as 'Viking', which are using different furnace builds and ore types, than the known Norse archaeology. *
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| Our 'Econo Norse' teaching furnace : Brick, pipe tuyere, vacuum blower, using taconite. The only thing 'Viking Age' is the furnace diameter? |
Don't miss understand - people are most certainly getting iron blooms.
The times we have used a proven furnace layout and standard ore, yet with variations of a Viking Age type twin chamber (side by side) bellows, consistently our yields drop about 10 % overall, from an expected 20 - 25 % down to closer to 12 - 18 % return. The blooms also tend to be considerably less dense (so harder to work into bars, with more loss at this stage of the overall process).
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| Early twin bellows for smelting ? : 'Ubber-Bellows' for CanIRON V prep, 2005 |
- Obviously, one clear possibility is that the whole furnace layout and overall method we are using
is just not effective, and so may be entirely different than historic process. (This
might also be a simple as 'we still are screwing up'!)
- We are working with an Fe2O3 based ore analog at typically about 55%
Fe content. Natural primary bog iron ore is actually FeO-OH, which
potentially could be as much as 63% Fe. Natural ores vary considerably,
even from the same location, but the difference between both chemistry
and especially iron content may be a significant difference?
- We have certainly found a considerable 'learning curve' with use of
human powered air. It may be that we are just not working correctly with
this entirely. (One experiment using a secondary collection bladder may
be suggestive, but there is nothing from archaeology to suggest this
method. Latter Medieval illustrations which do show bellows use, don't
show bladders.
)
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| Norse 'blacksmith' size bellows linked to a bladder : SCA 50 event 2015 |
So key to this whole thing is a more correct statement :
'We don't get historic blooms when we use Viking Age suggested bellows.'
- There is not much data available on the actual measured density of the
existing artifact blooms.
- Others are certainly getting iron - but there is often no clear reporting of
actual yield or most importantly the quality of those blooms.
* What REALLY aggravates me is a most recent trend to individuals who are using 'Viking Age!' as a mere marketing label.