Bloomery Iron for Bladesmithing - some Questions and Answers (A)

Some Background:

I was contacted a couple of weeks back by a staff writer from a major magazine that focuses on contemporary bladesmithing:

I'm working on a story about knifemakers who have smelted their own bog iron. I was perusing the Bladesmith's Forum when I came across a post you wrote in January 2014 describing your experience with smelting. Would you be interested in talking to me for the story? I'm curious to learn more about the process of turning raw bog iron into blade and about the history of how smiths in the Iron Age worked with their material. 

There was a bit of back and forth. I was careful to explain that although I do consider my knowledge and experience with especially Northern European Early Medieval (Viking Age) furnaces to be considerable, my focus has been on the 'ore to bloom' phase, certainly not on the 'bar to blade' part. I suggested some further background reading off the (massive) documentation on my web site -  Experimental Iron Smelting. 

This lead to a list of specific questions.
Like usual, I started off like some absent minded college professor, pretty much delivering a compressed lecture on experimental archaeology, Viking Age history, the theory of direct process bloomery iron furnaces...
'This is just taking so long' says I. 
'Maybe you should not be trying to be so much the teacher' says she.
Yea
The guy wanted an *interview* - not a college level course...

But since I've written the stuff anyway, and knowing full well the final article is unlikely to do more than give me a line or two, I'm putting up the full 'answers' here...

2 (*). I'm interested in your historical reenacting. What is it like to smelt under historical conditions? 

Reconstructed Viking Age smelt at Vinland (L'Anse aux Meadows NHSC, 2010)

Leaving aside the larger questions of experimental archaeology and whether modern people can ever really duplicate the past (**).

Time:
The single biggest change I find in working in a 'historic' environment is the absence of modern time keeping. As early 21st Century North Americans, so much of our lives are controlled by our (obsessive?) use of time measuring devices. In iron smelting, reproducible and predictable results are framed in terms of burn rates, which are standard volumes against time.
My own team has used a number of 'ancient' type ways to determine time. When working with a hand powered bellows, using your heart rate is one possible measure. One of the team studies early music, and he wrote a couple of songs / chants to use to help regulate bellows stroke rates. Both of these methods were used in the final 'all Viking Age' iron smelt demonstration / experiment we undertook in 2010 at L'Anse aux Meadows NHSC for Parks Canada. We did have an observer / recorder keeping measured experimental data, but did not consult those notes until after the smelt was over.

Safety:
My standard principle is 'History stops when Safety starts'. 
You notice in the image from the Vinland experiment that although the team members are in Viking Age clothing and using historic type tools, we are still wearing modern safety glasses. Going blind from having a hot piece of slag or scale damage an eye is also historically accurate - but we are not completely crazy. Use of leather aprons and gloves has been standard for metalworkers since ancient times, if only to protect clothing, which was considerably more valuable before modern industrial textiles. Wool and Linen fabrics are actually considerably more functional around the furnace than modern synthetic materials. The main safety concern I have had over wearing historic clothing is in the footwear. Soft soled Viking Age shoes can be very slippery, and certainly provide little protection if you drop something on your foot!

Labour:
In operating a 'short shaft' direct process bloomery furnace, higher air volumes have proven to produce larger, more dense iron blooms. In fact, blooms most like those few found in the archaeology. You most certainly will get iron with less air, but those blooms tend to be smaller (less efficient) and much more lacy in composition.
There remains an open question on just what kind of bellows was actually used for iron smelting in the Early Medieval period, as there is nothing remaining in terms of archaeological evidence. The size of the reconstructed bellows created for the Vinland experimental series was largely guess work, but the measurements were based on the theoretical requirements for air inside the furnace used.
The effective pumping rate was one stroke per second, alternating between the two chambers. Individuals varied on their stroke force (delivery pressure), but averaged 60 - 75 strokes per minute. This without interruption, over the course of the entire firing sequence extending roughly 5 hours. We found that to maintain the needed consistency, we needed four individuals, working in roughly 10 minute shifts. This labour force needed to be at least semi-skilled to this task. This is a requirement totally separate to the needs of feeding and operating the furnace itself.
Its easy to see why the development of water powered bellows, starting in the 700's and moving across Europe through to about 1100 AD, had such a huge impact on bloomery iron production.
Of course there is also the labour involved in preparing all the required raw materials. Hundreds of pounds of charcoal (wood gathered, cut, baked, broken). Ore to be found, dug, roasted, broken. Clay to be dug, dried, screened, re-mixed. A furnace to be built.
All this just gives you a raw bloom. This still needs to be refined by hammering, folding, re-welding into a working bar. Its only at that point the bladesmith can start working his own art.

* Question 1 was related to contributing some images for the final article. I will be adding some of the ones I suggested as illustrations through my own series here.

** This is where I realized I was going out of control!
I had written the following as a mere lead in to what is above for question 2:

One of the huge problems in attempting to discover possible historic / ancient physical practise is the whole concept of 'can modern people really duplicate the mind set of historic people'. This actually is a topic of considerable debate in the museum and archaeology field.
Coupled with that is the whole problem of attempting to re-discover what at base is a completely 'lost' working tradition. Outside of Japan, there is no living progression of these skills down into the modern day. This is especially true for Northern Europe, where archaeological evidence is extremely limited, and the technology itself had shifted before any written descriptions of the original processes had been recorded.
Leaving that mainly aside, there are a couple of primary problems that impact any attempt to reproduce historic iron smelting methods:

- Outside of some very rare and isolated examples, there are almost no working traditions of smelting iron that extend down into the modern age. Japan is the primary exception to this. However, because ore type directly influences furnace design and operation, Japanese methods are quite different to those undertaken in Northern Europe. This is important, because most North Americans are patterning their furnaces around what are essentially late Iron Age / Early Medieval European 'short shaft' type furnaces. There are a number of modern makers specifically basing their work on Japanese models (Jesus Herandez would be one). Lee Sauder started with African prototypes, which he moved away from as he developed the base level understanding of the smelting process. (It was from this work and him that almost everyone else in NA learned the basics by the way.) Lee has recently returned to work on these systems.

- Not only is European process not represented as a surviving tradition, the exact type of small scale direct process bloomery that most of us are using today was mostly abandoned as a working technology by the middle of the Medieval period. The specific technology of processing raw ore into working metal bars has changed several times over the last 1000 years, which earlier methods replaced with newer systems that offered larger scale production, better efficiency and increased control of the results. The big shift from wrought iron to mild steel via the Bessemer process in the later 1850's being an example modern blacksmiths (should be) familiar with.
This is not quite a direct severing of ancient / historic process. Sometimes shifting realities will re-introduce an older working method. One of the best examples is the rapid growth of small to medium scale iron smelting in America just post revolution. Cut off from British industrial supply, the new nation had to rapidly and massively increase its production volume. Typical 'Yankie Ingenuity' was combined with abundant wood resources for charcoal,  particularly in the New England region with its many locations for water power, resulting in many smaller scale bloomery furnace operations springing up.