Saturday, August 03, 2019

Viking Age iron smelting air systems?

Those following me may be aware that I am participating in the Hurstwic 'Firing Up Ancient Secrets' project, set for August 23 - Sept 5 at Eiríksstaðir in Iceland.
My contribution is both my research into Viking Age iron smelting methods, and my (considerable) experience with an experimental archaeology approach towards understanding those systems. 
I have been doing a lot of background research and pondering about everything related to bloomery iron making during the Viking Age of late * 
The following is modified from a recent communication with a fellow experimental researcher:

Air systems are especially an area of investigation for me.
What I see as the problem is this:
- We can make blooms that almost exactly resemble the ones found in the archaeology. (Understanding there are not that many samples that have been found.) But to do this requires the application of large volumes of air, and high burn rates. (This based on the pioneering work of Lee Sauder from Virginia  )
- There is almost no hard evidence on what bellows might have looked like from the Viking Age itself. Two images from the period, no object remains what so ever. (Be very happy to be proven wrong - if anyone reading knows something!) This specifically true for possible iron smelting bellows - for which there are virtually no indications on type or size at all. Possibly some signs of post holes that might be supporting frames?

I note here that you most certainly can get some iron produced with low volumes of air. This has definately been proven many times by those working at European open air museums, using what are 'Early Iron Age' methods.
But I consider the quality of the blooms created a very important indicator in results.

My own reconstructions of the blacksmithing bellows, based on those two illustrations, creates a twin chamber unit which in working tests produces roughly 120 - 160 litres per minute (based on one stroke per second)
If you compare this to the 'ideal' air flow requirements, this is at best 25% of the required volume - if you are expecting to create a dense bloom that resembles the ancient ones. (For the 25 - 30 cm interior diameter most of us are working with, air in the range of 500 - 800 LpM.)

Most of us in the experimental community use all sorts of different measurements - or no measurements at all.
One standard is 'charcoal consumed over time'. Again, there is no consistent way this is reported, I see a lot of European workers using 'kilos per hour'. Here in North America (again thanks to Lee) the standard is 'charge amount per minute'. The ideal is usually quoted at '5 lbs over 6 - 8 minutes' (so make that roughly 2 kg).
Here we more typically are running at closer to an overall 1.8 kg every 10 - 12 minutes (so about 9 - 11 kg / hour) Our blooms are intentionally on the smaller size (normally 30 kg ore to about 5 kg iron).

The questions are :
What is the air volume produced by the various *theoretical* Norse type iron smelting bellows?

Ideally, to get any kind of understanding of this, individual teams need to undertake some sort of measurements.
- What are the physical dimensions of the bellows unit being used?
- What is the stroke count per minute?
- Is the actual production volume (when hooked into a working furnace) measured?

- What are the physical measurements of the furnace (especially the interior diameter at tuyere)?
- What is the average burn rate over the smelt?

- What is the yield?
- What is the quality of the iron produced?
(This last is so dependent on the quality of the ore being used - it may not prove a really valuable comparison!)

Right now I am working with a group attempting to experiment with a possible Icelandic based system. At present they are intending to build a large, single chamber bellows (more or less like one half of the known Norse type). This has been tested all of once - and I think is not the way to proceed into the Iceland side of the project.

There may be some element of just how the air flows into the furnace?
- A Norse twin chamber (larger smelting size) produces a flow that never stops, but with changes in volume as each chamber is pushed. The delivery pressure can also be modified each stroke by the force of the push. (Consistency a problem). Requires 3 - 4 workers.
Possible Norse Smelting Bellows - Vinland 3
- Multiple Norse twin chamber, small blacksmith size, linked to a central air bladder. The flow never stops, pressure modified by a weight on the bladder. We have tested this system out twice, but the main draw back is the larger number of workers required (6 - 8).
Blacksmith's bellows linked by bladder - SCA 50
- The single chamber being considered by Hurstwic has no historical examples (that I am aware of). It will produce air that starts and stops on each stroke. Pressure can be modified by the force of the push. (Consistency a problem) Requires 3 - 4 workers.

- A 'great bellows' (two stacked chambers, 'double action') is Medieval at best. This will provide a fairly constant blast, pressure consistent (modified by weight on the top delivery side chamber).  I see a lot of people using this post 1300 system  - and calling it Viking Age. Requires 3 - 4 workers
Settlement Era Great Bellows - Williamsburg
- Obviously use of a modern electric blower gives a constant blast (volume and pressure). Solves the labour problem!

Another extremely important element - which will effect the entire design of the furnaces, is the tuyere system itself. (To be discussed in a further posting)

* If regular readers have noticed a sharp decline in postings here over the last two months - this is the primary reason. 
- June was DARC's major demonstration at Upper Canada Village, plus my presentation at the ALHFAM conference.
- July marked a major construction project (upper deck structure and roofing) at Wareham
- August? 
* formal paper based on the ALHAM presentation to be written
* detailed research into iron smelting in Norway and Iceland
* writing a report relating that research to the Firing Secrets project
* preparing a lecture presentation (before the Canadian Ambassador to Iceland!)
* equipment load out and packing for the trip

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