You mean ONLY with more air?

 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).

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. *

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). 

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. )

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.

Icelandic Clay Mix Test - the full smelt

The following is a fast overview of the bloomery iron smelt at Wareham, June 19.

This experiment has the DARC team return to our experimental work based on elements of the 'industrial' Viking Age site at Hals, Iceland, originally excavated by Kevin P. Smith (1). The earlier work can be loosely divided into two phases (2) :

- Phase One / four experiments / 2007 and 2008 / testing individual design elements

- Phase Two / four experiments / 2012 to 2016 / testing use of turf builds, combining elements from phase 1

The current work, as Phase Three, centres on:

a) testing clay mixtures based on those from Iceland

b) testing durability of a turf (sod) supported furnace structure over time

An earlier blog post ("Sticking to It - a clay mix for Icelandic?") detailed the logic behind the clay mixture itself.

Part way through the build process

Based on experience from the earlier furnaces of this series, the wall thickness was set at 4.5 cm, measured against 'two fingers' as shown. An internal form was used, metal in this case, keeping the interior diameter consistent to 28 cm. (This replacing what could have been a wooden straight sided 'barrel' form historically). The clay was constructed to the rough top of the form (about 30 + cm), then the supporting cut sod strips were placed in a circle on the outside. The form was pulled upwards, with the exposed interior filled with a mix of half sand / wood ash. This method has been used many times, the ash mix both supports the thin clay and also helps to dry the walls. 

Built at 35 cm

After the first full course was established, a set of small stones, used like bricks, were placed to frame an extraction arch for the front of the furnace. One large piece of basalt on hand was used as a lintel to eventually support the tuyere and upper sod pieces.

With air system, at first charcoal addition

The build continued to raise the total stack height to 75 cm. The supporting sods were laid in five layers, to a height of roughly 70 cm (at the start of the smelt). The cone created was a bit irregular, extending out 72 cm to the left side as seen, to 56 cm to the right side. Although materials (timber and earth) had been labouriously gathered, it was decided not to box the sod cone and back-fill to a flat upper surface.

The extraction arch was 23 cm wide at the furnace wall, opening to 33 cm at the edges of the framing stones. The lintel slab sat on a slight diagonal, at roughly 20 cm high. There was additionally a small tapping arch cut, 6 cm at the base and 8 cm above the hard base. 

There was no specific attempt to match the air system to evidence from Hals, we chose to use our proven heavy copper tube, inset 5 cm beyond the interior wall. As supported on the lintel slab, and with our proven 22 degree down angle, this set the tuyere tip a bit high in the furnace. In turn this reduced the effective stack height to only 40 cm (normally considered a minimum distance). As this position would also leave a lot of space under the tuyere, a 'soft base' was created with wood ash from the drying fire and a layer of charcoal fines on top, raising the set base to 15 cm below the tuyere. The simplest method to support the normal viewing port and air input connection was to suspend it from a metal rod set well clear of the working opening in the sod cone. 

The primary smelting team :

Darrell Markewitz - smelt master

Neil Peterson - ore and records

Rey Cogswell - charcoal / ore

Kay Burnham - charcoal / ore

Richard Schwitzer - compaction

Kelly Probyn-Smith - safety

Travis Sweet - photography

Slag tap. Earth falling free of the upper sods can be seen as light reddish pieces.

Over the 7 1/2 hours of the main smelt sequence, it proved necessary to make at least four major slag taps to lower internal slag levels threatening to 'drown' the air blast. 

Later into the smelt, it was clear that furnace heat was seriously effecting the sod structure. Earth was baking dry, and pulling free from the root systems and falling downwards between any cracks. This would cause the whole sod cone to slump into itself by the end. There was no venting of furnace gases observed however.

Air flow had been purposefully reduced for this smelt, down to a range considered possible for a human operated bellows of Norse type. Although the long serving electric blower was used, air flow was initially set to roughly 700 LpM, increased about the mid way point to 800 Lpm. (3) This would create an average burn rate at 8 minutes per kg over the main sequence. 

Initially 5 kg of previously gathered iron rich tap slag was added to establish a working slag bowl system. This may have also contributed to the need for later tapping. A total of 25 kg of the DD1 analog ore (51% Fe) was used with combined average addition rate of 11.8 minutes per kg.

Pulling the bloom mass free. Image by Travis Sweet

The bloom mas proved much larger than expected! In pulling it clear of the furnace, the supporting lintel stone was pulled away, resulting in the upper sod layers at the front of the furnace also collapsing, exposing the front furnace liner.

Classic compaction shot - Darrell holding, Neil on sledge : Image by Travis Sweet

With loose gromps hammered clear, before cutting on the press : Image by Travis Sweet

I

To everyone's great surprise, after some first stage compaction and rough cutting, the resulting bloom, although a bit loose in texture, weighed in at 8.9 kg. This represents a 35% return from ore (4).

 

Furnace, just after extraction / compaction, with remaining slag bowl seen.

On a quick examination of the furnace (after our ritual post smelt Guinness) :

- The furnace still contained about 2/3 of the slag bowl, completely filling the interior save for a large piece broken away at the extraction arch to expose the bloom, creating a clear C shape. The depression towards the air blast side clearly frames where the bloom was pulled free. The slag bowl remaining is of a size and shape quite similar to those exposed at Hals.

- The front portion of the furnace has clearly broken away, when the supporting stone lintel pulled off, as the bloom mass was finally pulled free. The image above shows it's natural fall position. The sod that had been above this has pretty much totally crumbled away into it's containing dirt.

- There is considerable slumping of the encasing sod cone. The clay liner had actually lifted slightly several times while working the bloom free, but had remained as a solid structure overall. 

Largest of the roughly compressed and cut bloom pieces : 3 kg

to come : looking closely at how the clay walls survived the smelt

 

1) Smith, K.P., 2005, "Ore, Fire, Hammer, Sickle: Iron Production in Viking Age and Early Medieval Iceland", AVISTA Studies in the History of Medieval Technology, Science, and Art, Volume 4, USA

Also available as PDF on line : https://www.academia.edu/191535/Ore_Fire_Hammer_Sickle_Iron_Production_in_Viking_Age_and_Early_Medieval_Iceland

2) A overview of both the Hals site and these experiments is currently under preparation, co authored by Smith, Markewitz and Peterson (‘Now with 70% Less Clay! Experiments with Viking Age Icelandic Turf walled Iron Smelting Furnaces’) A short video overview was presented at the recent EAC 12 virtual conference, available on line : https://youtu.be/7Ltz5NG2BP0

3) For a furnace at 28 cm, the normal air flow would be set to closer to 900 - 1000 Lpm. see : Air Flow Rates

4) This impressive result may be the effect of the 5 kg of iron slag added as a first step. Our past results for smelts in the 35 - 30 kg ore range have more typically been roughly 5 kg (or less).  

'Sticking to It' - a clay mix for Icelandic?

 Part of our ongoing experimental series investigating a possible bloomery iron smelting furnace, as suggested by the archaeology of the site at Hals, Iceland. ( 1 )

The clay mix being tested was based on an 'in team' analysis of materials gathered by Michelle Hayeur-Smith from a natural deposit 'fairly close' to Hals. (Noting that it remains unknown if this was the clay material actually utilized for the furnace builds there). ( 2 ). Team member Marcus Burnham suggested a set of components available at Pottery Supply House, which would in combination approximate at least the chemistry of the Icelandic sample. ( 3 )

These individual components were dry mixed by hand, giving roughly 50 kg of the clay powder (hopefully enough for two builds with reduced wall thickness). ( 4 )

I made up a total of four small batches for next step testing :
- clay mix with water
- wet clay mix plus powdered slag (66 / 33 slag) - this 'copper shot' sand blast slag ( 5 )
- wet clay mix plus fine sand (50 / 50 by weight)
- dry mix / fine sand / shredded horse manure by volume (ie our current proportions using the EPK clay base)

In each case, the various mixtures of materials holds together well, without being too 'sticky', and all are nicely plastic.
It is not expected to have any physical problems building a furnace wall structure with any of these mixes.

The individual mixtures were pressed into small cylinders, filling a standard toilet paper cardboard tube. Each 4 cm diameter by 10 cm long.

- These were placed in a toaster oven set (very roughly) for 90C, over a duration of about 4 hours total.
I'm not sure that temperature was actually reached, because when I pulled them out, I could still handle each with bare (blacksmith's!) hands.
The centre part of the paper cylinders could be depressed - suggesting there was still water remaining. This thought to be the effect of the paper not easily passing moisture (?) .
- The cylinders were then placed in the rear section of my gas forge on a metal tray. (At top operating adjustment, this forge has been found to produce temperatures into the 1100 C range) The forge was lit, with a flow at a greatly reduced level.

Showing results of first (shortened) heat cycle.
Number 1 to left, through to number 4 to right

The material was heated enough to burn off the paper on the upper surface, but this remained solid and attached on the lower side. This suggests the the top had to reach at least 230 C, but the bottom surface (furthest from flame) did not get that hot.
You can see clearly that cylinder 1 - the straight Icelandic mix, has already crumbled.
Cylinder 2 - wet mix plus slag, has also become brittle.
Cylinder 3 (wet / sand) is showing cracking - but remains whole
Cylinder 4 (dry/sand/manure) remains unaffected (that lower crack seen was from
the initial packing of the material)
As it turned out, the forge propane ran out after a short time (20 minutes?). So in this it is hard to estimate just how hot the forge interior would have gotten.
The cylinders were allowed to sit in place inside the forge (so slow cool), overnight.

The following day I refilled all my propane cylinders, then repeated the test.
Again the tray was placed at the very rear of the forge as it was lit. A low propane rate to start. The material was left in place for about 30 minutes (until the forge interior was at full heat) then the tray moved forward to the centre, under the gas jets (where I normally would place metal to heat). ( 6 )

Temperature probe was inserted into the gap between # 3 and #4 (to right).

After 55 minutes total, a temperature reading was taken, the probe wire between the cylinders, tucked slightly underneath. Reading was 980 C.
At that point, you can see that cylinder 1 is composed of crumbled pieces.

The forge was then adjusted for higher temperature (increased gas flow and air combination).
The materials were subjected to another 85 minutes at this temperature, which was measured again to 1070C.

At end of second heating period (open door for image has cooled interior slightly).

You can see that the surface of cylinder 1 has slumped and started to fuse.
Cylinder 2 has developed some serious breaks.

The forge was turned off - and left overnight with the door closed to slowly cool. 

Cylinder 1 (dry mix with water) - has melted
Cylinder 2 (wet / 33% slag) has badly cracked
Cylinder 3 (wet / 50% sand) has developed a number of cracks, but mainly towards the surface. A bit of surface fusing (point directly under the gas jet)
Cylinder 4 (dry mix / sand / manure) shows no significant effects (again, bottom piece was separate at the start)

I took each of # 2 / # 3 / # 4, and applied pressure by flexing between my hands. I attempted to keep the force as close as possible - and applied to what I judged a 'reasonable expectation' level of pressure.
 

( Cylinder 1 - only the small disk seen earlier could be handled, the result was it easily crumbled )
Cylinder 2 - not much force was required to break along the large cracks - you can see from the dark coloured interior, this crack had broken completely through during the firing.
Cylinder 3 - held together effectively, despite surface cracks
Cylinder 4 - no effect to reasonable force (earlier separated piece shows internal texture)

The conclusion I draw :

Sample #1, our potential Icelandic clay mix alone, is not able to survive temperatures even at the lower end of expected iron smelting range. Combined with it's relative fragility even during the early heating cycle of a furnace, this material on its own is not deemed suitable for furnace construction (without modifications).

Sample #2, with 2/3 wet clay / 1/3 slag (by weight), was considered highly speculative at best. (Included mainly because I had the material!) This offering one possible version of 'Where did the silica come from?'. Iron rich slag could have been on hand (at least after a first smelt attempt). In the test, this mix proved relatively fragile. It might be possible functionally to use this mix, with greater care in mixing, construction and drying. Generally, this is considered quite unlikely to have been used.

Sample #3, with 50% wet clay / 50% fine sand, certainly would be an effective build material. First note is that the sand used here is of unknown composition, but likely Ontario granite based at about 70% silica (higher than Icelandic basalt based). It has been well demonstrated (Sauder) that high sand mixes can better withstand iron furnace temperatures, but at the same time do require more care during the construction phase to eliminate just the kind of surface cracking seen in the test sample. This is considered a possible effective mixture for the Hals build.

Sample #4, using our developed standard of equal dry volumes of clay mix / fine sand / shredded horse manure, was clearly provided the best properties of the four mixes tested. As proven in the past, the sand provides heat resistance and stability at temperature, the horse manure re-enforcement during the build and drying phases. Even with what is demonstrated as a significantly lower point clay material, this is considered the 'best possible' mixture to use for the upcoming full build test.

1) The original description of the Viking Age, 'industrial' level iron smelting site at Hals was provided by Kevin P. Smith in his 'Ore, Fire, Hammer, Sickle : Iron Production in Viking Age and Early Medieval Iceland' The earliest evaluation towards evolving a working system can be found in 'Working Towards a Viking Age Icelandic Smelt'.
Currently Smith, Peterson and myself are preparing a detailed description of the 2007 - 2016 experimental series, to flesh out our recent presentation 'Now with 70% Less Clay' which was delivered at the 2021 EAC12 Conference.

2) Unfortunately, for a number of reasons, an actual analysis of the any of the clay furnace walls recovered during excavations is available. Such tests were not undertaken at the time, not being within the scope of the original project there. At this point (after some 20 years!) is is quite unlikely to be able to arrange for this currently

3) With the kind financial assistance of Neil Peterson, who donated funds to cover the purchase of the clay materials indicated, plus charcoal to supply the next three full smelts in the current experimental series. 

4) Sorry. Due to problems experienced over the last several years of hard won information derived from the DARC team experimental work, I am intentionally holding back on listing the exact details here. A further paper is under construction, describing our newest interpretation of the excavations at Hals, and how this applies to our current experimental series over 2021 - 2022.

5) This material is composed of 'medium' sized particles, primary components are 53-60% (Iron Oxide), 32-37% (Silicon Dioxide). 

6) Readers need to remember that the camera records light and colour of heated surfaces differently from the human eye. See an earlier blog post.

Elora Sculpture Project - 2021

 

Need a break from COVID containment?

Consider a couple of hours walking around downtown Elora, with a visit to the 'River Walk' in Fergus!

 My own contribution 

An Undiscovered Plant containing a cure for cancer

Is found at site 16 - Fergus River Walk