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

Friday, August 02, 2019

BIG 'Can O Worms'

...I would like to open that can of worms again and mention that last week we had some visitors from Germany who appeared in full "costume" at our doors. They had very little interaction with our staff and were rather standoff-ish with everyone. As a matter of fact most thought they might have been "real Indians". They actually made some of our staff uncomfortable, especially our own Indigenous staff as the attire showed a lot of skin as it were. Period correct yes, well done yes. Cultural appropriation? What would you do? How would you approach this. I watched and observed knowing that this is quite common in Europe. After some searches on the internet I found that in Germany alone there are some 40,000 people who belong to these "clubs" and spend weekends at a time in encampments or even longer. There seems to be differing opinions from Indigenous folk north and south of the border. Some think its ok some hate the idea.
Any one out there who may add a few worms to this can?

Yours in History

That message recently started some conversation on the ALHFAM discussion list. *

The following is (slightly edited) from my open reply.
Presented in the spirit of honest discussion.

Boy - Can O Worms hardly scratches this.
... I am going to ask for any First Nation's members reading to be patient, and try to pull back from their own personal frame of reference.

Correctly identifying Staff in any situation is a problem. Who speaks with authority?
In a classic 'stuff' museum, staff get identified through standard dress (uniform), or at least specific name badges. In our living history sites, historic clothes are the identifier. The visitor accepts information provided by those perceived to be staff. (For those working 'pioneer' periods : What happens when Mennonite or Amish visitors show up at your site?)

But spin this out a bit folks.
One very extreme situation provided in the discussion was people showing up to an American 'conflict' related site - with their 'period' firearms in their hands! This obviously represents huge problems in controlling behavior, especially public safety! (Honestly, I could only see this happening in the USA - with firearms. I have seen, any number of times, costumed visitors to Medieval Festivals waving swords around.)
I would bet many of us here have had the situation where they have visited another site (in street clothes) - and intentional or not, end up 'interpreting'. When you are passionate about history, about sharing information, it is hard to hear a question asked when there are no staff around - and not answer it. I wonder sometimes if I have a big (invisible to me) letter I tattooed on my forehead. (I was at the Yorvic Viking Centre, visiting from Canada in street clothes, and had a teacher ask me to explain a set of artifacts to her school group one time ??)
In the discussion following, a number of suggestions were given about how to differentiate between staff and costumed visitors. Honestly, I think the suggestions that shift into some kind of shaming are counter productive. (Issuing bright coloured sashes one suggestion for example). Consider how 'visitors' are signified at government buildings or in industry. I would suggest a stick on label or a lanyard with badge are typical - and this would be inexpensive. Most importantly this would be seen as a 'standard' approach by the individuals (and other visitors as well).

A second element here is about 'bad history'.
Every institution is attempting to control the quality of information, through some level of staff training, a pre-determined standard story. I'm sure we are all well aware of the 'set piece' delivery : "Welcome to the Irish Cabin, representing 1850..." (heard by myself, on Monday at a local museum).
So I would suggest one of the problems with visitors in costume is that the site can loose control of the information flow. Again, I bet many reading have been in the situation where we have heard poor, or outright incorrect, staff delivery at some other museum. How to contribute / correct needs to be handled with great care - but I'd also bet many of us have attempted this. (Honestly - it goes with the profession!)

The big can o worms is : 'Who speaks for the Dead'

'White Indians' : Interpreters at Jamestown Settlement (about 1998)
I state this in that manor quite deliberately. I am not attempting to be offensive. First Nations have real, ongoing, special, reasons to be more than normally sensitive to this specific example. The original situation described outlines first the problems of staff identification, then further, potential 'bad history'. There is this major third problem laid on top to consider.
Being honest - in truth almost * all * of us working as living history interpreters are representing things we are not. I expect no one reading was born before the Great Depression, likely very few before WW 2. We attempt to represent *history*.

I wanted to point this out specifically, because I personally undertake illustrating the Viking Age. This is in effect a 'dead' cultural set. I am Canadian by birth and culture, not even any Scandinavian lines. Am I allowed? By definition my activities are 'cultural appropriation'. This is fine because there are no living community to complain? Who decides on the accuracy of my portrayal?

'Famous Viking' : at L'Anse aux Meadows NHSC - 1996
I most certainly have experienced the 'furry Barbarian' visitor show up at our historic encampments. Good news for us is that these folks, although perhaps enthusiastic, so clearly do not visually resemble our working interpreters that no other visitors were confused. We had a group of 'O∂inist' (White Supremacist) followers show up at one presentation. They left disgruntled, largely because we were 'too boring', ie did not support their distorted views on extreme religion, slavery, and White Dominance.

In the case given that sparked this commentary, Germans especially have a long recent history of 'Native Re-creation', going back at least into the later 1800's. This has represented a true interest, with all the honest distortions possible when anyone attempts to re-create something not of their own birth - and so far removed from their own life experience.

Does any one cultural group 'own' that material culture?

* " The Association for Living History, Farm and Agricultural Museums (ALHFAM) is the organization for those involved in living history including living historical farms, agricultural museums, outdoor museums of history and folklife and those museums - large and small - that use "living history" programming. "

Monday, July 08, 2019

Elora Sculpture Project - 2019

Being a 'teaser' overview of this year's presentation.
Exhibit Guide - showing full names and locations
'Expecting' : Eva Ennist
'Urban Pulse' : Pamela Rojas
'Green Strata' : K.J. Ross
'Crow's Nest' : Holly Atkinson
'Marine Architecture Study A' : David Cross
'Aizpute' : Kip Jones
'Red Face Walking' : Mike Truelove
'Leader of the Pack' : Mike Hintermiester
'Temporary Shelter' : Susan Lehnen
'Grandmothers - Ode to Joy' : Rosalinde Baumgartner
'Capacitance Chamber' : Dylan Paczay
'Toy Giraffe' : Thoreau Bakker
'Dance Me To The End Of Love' : Beverley Cairns
'All The Things She Gave Me' : Theresa Pankratz
'Leap of Faith' : Tim Dolman
'The Shed' : Jane Longstaffe

'Standing Grove V' : Susan Rankin

My own contribution ' Tipping Point' has been discussed on an earlier posts here:

Note : The images of the various artist's work have been presented as close up - with the intention of provoking interest in the individual pieces. 
I encourage all to visit Elora and Fergus making the short walk around to view the sculptures for yourself. Both villages offer interesting shops and some very great restaurants - make an afternoon of it!

Saturday, July 06, 2019

Any PORT in a ...

Myself and a couple of others are in the initial process of fabricating a cupola furnace.  I am wondering about the glass for the tuyeres and remembered yours.  So I wondering what what diameter and thickness you used for your tuyere?  I assume it was high temperature glass.
Air piping - here fitted to a ceramic tube tuyere. The end fitting to the upper left is sealed with the clear lens under discussion here.
We had looked into special filter / high temperature glass at the beginning.
Getting a disk of filter glass the right size was going to be horribly expensive.

What we ended up with is a piece of simple, 1/8 thick plexi (plastic)
What we totally forgot - there is no heat at that point in the system!
With our rig, we have the input air coming in via a T or Y joint, located 'down stream' towards the furnace. This means the view port end is being washed with cold (ambient temperature) air.  The plastic was cheap, and we figured easy enough to replace if needed. That end port you saw has been through something plus 50 smelts at this date!

With the use of a copper tuyere - there is some heat transmitted back along that metal (which is how the copper keeps from melting btw). This effect is not significant with the ceramic tube tuyeres we use as out other standard.
I have one Y tube actually made of ABS plastic with a leather cover (used in historic demonstrations). So far that unit has gone through about a dozen full smelts! 
ABS plastic Y, leather covered - here fitted to a steel pipe tuyere

So - I would suggest you consider the tuyere material (may be steel pipe, set flush to the refractory interior lining on the furnace side?) Conduction down the tuyere itself is more likely your consideration.

There would be some value to using a dark colour plexi (would avoid the use of the welding lenses I have). I know you can purchase replacement plastic filters for welding helmets, something like $5 for enough to easily cut two end caps.

Friday, July 05, 2019

'so damn WET'... Ore Analog consideration

A potential correction on reported yields?

For a good long while, the iron smelt experiments have used an artificial preparation we call 'DARC Dirt'.
The origins of this preparation go back to 2008, when we started on the 'Vinland' series. Here at Wareham, there is no available natural iron ore of any kind. (As I have mentioned in previous posts, Wareham sits at one end of the massive block of limestone that forms the Niagara Escarpment - a block roughly 400 + km long and about 100 m thick!). Although 'primary bog iron ore' can be found 100 km north of here (up on to the Canadian Shield), I have never found any more than the faintest traces of iron locally.
Primary Bog Iron Ore - from L'Anse aux Meadows (early 1970's excavations)
For the Vinland series, we wanted to have a dependable quantity of an iron 'ore', which would also at least approximate the chemistry of that known to have been used at L'Anse aux Meadows by the Norse. Our solution was to mix up an analog, using commercially available 'Spanish Red' Fe2O3 oxide powder.
One huge advantage is that use of the analog has given us a very consistent baseline within our many variations on furnace design and air systems testing. There have been slight variations of this standard mix, usually by adding various amounts of forge scale (so Fe3O4) to 'enrich' the basic mixture.

DD-1 analog as added in a working smelt (Vinland 1)
Details of this mix  *

Chemical analysis
Fe2O3    81  %
SiO2      5  %
CaO      2.3 %
MgO2     2  %
Al2O3    2.6 %
Loss on ignition 7 %

As DD#1
Red Oxide          82 %
Wheat Flour          8 %
Water (after drying) ≠ 10%

Adjusted Fe2O3 content = 66 %

It is the WATER here that may be throwing off our reported yield numbers.
The iron oxide powders used as the basis for the analog mixtures are extremely fine. Originally, in an attempt to match the reported chemistry of the L'Anse aux meadows samples, whole wheat flour was added to create the 'organic' component. The physical process was to combine the dry materials, then add water to make a thick paste, roughly the consistency of butterscotch pudding. This was then ladled out on to trays to dry in the sun. It was found that the flour acted like a binder, producing loose lumps of finished 'ore', which also had about the same consistency (porosity) as the natural ore.

How much of the added weight was water?
Although sometimes a sample of the ore used in a specific experiment is pulled off to be completely dried (Usually baked in the gas forge) this 'loss on ignition' (LoI) is not usually recorded. Typical is about 10 - 13 % of the 'as used' weight.

This all occurred to me after the last smelt experiment ('Icelandic' stone block).
Both being somewhat rushed for time, and with an extremely rainy early Summer at Wareham, I had a lot of trouble getting that analog mix dried from the 'pudding' phase. On dry baking a measured sample, it was found that the LoI was actually almost double - at 18%.

I'm not sure exactly how best to apply is considerable difference (10 to 18 % LoI) to the recorded results. One thing would be to consider 'apples with apples', as most commonly the water weight is * not * considered in past recorded results.
- The difference in water weight on this last smelt is then about + 8%
- Reduce the ore weight to 'standard' (29.3 kg x 92% = 27 kg 'adjusted')
- Effects (increases) reported yield (7 kg from adjusted 27 kg = 26 %)

( * )  If you are interested in the long development sequence of our bog ore analog, the easiest way to read the many earlier commentaries is a simple search :'bog+ore+analog'

Thursday, July 04, 2019

Stone block furnace

Ryan (L) and Avery (R) - 2/3 through main sequence
Perfect 'chocolate covered cherry' image just as bloom is exposed (Image by Travis Sweet)

Initial compaction : (L-R) Ryan / Rey / Kip / Avery / Darrell (Image by Travis Sweet)
June 23
Furnace related to ongoing studies of possible historic Icelandic systems.
Granite blocks with clay cobb sealing.

Result was 7 kg bloom (about 25% yield)

Readers: I have been buried under a combination of research, project preparation, demonstration, academic presentation, newsletter editing - for the last month.
Up until now, there was a requested news blackout on the upcoming Iceland iron smelting project in August. Expect more entries related to all that, soon!

Monday, May 06, 2019

Introduction to Blacksmithing : May 10 - 12


A beginner's program, intended to introduce you to the forge and basic techniques as you make a number of simple objects. Includes discussion of correct hammer use, fast look at materials, history and contemporary work.

Instructor is Darrell Markewitz of the Wareham Forge (over 30 years worshop teaching experience).

Mention membership in OABA for a $25 discount!
Mention membership in SCA for a $25 discount!
Book with a friend and both get a $25 discount!

Details and Register online (Paypal) :

Other workshop courses available :

Monday, April 08, 2019

Metalworking in Cuba

Gates and Grills in Cuba

On short notice in February, I was offered free shared lodgings at a private owned B & B in Boca de Camarioca, Cuba. (1) I jumped at this, the first real vacation I have undertaken (outside of work related travel) in 30 years.  Boca is just down the northern coast road from the major tourist destination of Varadero, about two hours drive east of Havana. Varadero was originally a purpose built town for foreign tourists , at one time Cubans were not really allowed access. Boca still is where many of the actual Cuban workers in Varadero live. In recent years it has also become a place where ‘middle class’ Cubans have built retirement homes as well. So it is safe to say Boca still does not represent ‘average’ Cuba, but at least is a bit closer than the artificial image presented in Varadero.

Location of Boca de Camarioca, Cuba
It is safe to say that almost all Cubans have a much, much lower material standard of living than almost anyone in Canada. Under the extremely socialist system, everyone is provided with the most basics of food, housing, education and medical. But the quality of those things is definitely the minimal level. Wages against prices is almost alarming, from a Canadian standpoint. (2) Despite this, I found the people generally appeared happy, and certainly friendly enough. (Especially once they found I was Canadian - people seemed to well remember Pierre Trudeau’s support in the 1970’s.)

My idea for a vacation is a semi-active one. I normally like to keep my eyes open, observing as un-intrusively as possible. I spent hours each day, just wondering around, attempting to take photographs. This combination tends to mean not so many images of people, but more of landscape and buildings, with a fair amount of detail, is what I end up recording.

I kept my eyes open for any kind of workshops, but only saw the simplest, honestly very minimal, tools or equipment in use. An old washing machine motor, rope tied to a broken chair, used as a grinder, for example. The only time I saw an ox-fuel set, it was obviously being used for car repairs. However, it is instantly clear that Cubans are geniuses at ‘repair and recycle’. At least a third of the cars on the road were lovingly rebuilt and restored old Chevys and Fords - from the 1950’s and 60’s. Almost all doing daily duty as taxis.
Seen often - a lovingly restored old Chevy
Buildings were constructed of simple (narrow!) concrete blocks, on poured concrete floors, with flat slab concrete upper floors and roofs. Usually set close together on very small lots, with small courtyards at front and rear. (3) Cubans are clearly extremely ‘house proud’, even the simplest home was kept need and clean. ‘Guarantied Employment’ did mean that there were people tasked with street cleaning, but honestly I saw little garbage outside of clearly designated collection points.

One consistent feature was grill work over windows, and metal gates as the entrance through the low wall along the sidewalk at almost all homes. This was the entrance to Lourdes & Leo for example:
Street side entrance to the front courtyard
The use of grills included almost all the windows, certainly ground level, but also on upper floors. (The alternative was windows fitted with wooden slat ’storm shutters’.) Actual glass windows were very uncommon. The typical ‘gap’ in the grill patterns was closer to 6 - 8 inches, suggesting to me they ere intended not to prevent ‘reach and grab’ theft, but to stop physical entry through the openings. As an example, this was the *second* floor covering, on the house next to my lodgings:
The other grills on this house matched the design (this roughly 3 x 6 feet)
Those houses equipped with vehicle drives would also have full height grill-work gates. Most were designed to swing open in two leaves, but some were set to slide sideways. Many of the car gates were backed with simple galvanised steel sheet metal behind the grill work, primarily what looked like a retrofit :
Note the light construction of this two panel car gate
This sample, made of simple embossed sheet attached to the front of some framing, was an exception.
This home (right next to the beach) was certainly upscale compared to others!
One thing that is instantly obvious is : there is no actual forge work here.

- Almost all the grills are made of smaller sized flat stock, primarily 3/4 or 1 inch wide by either 1/8 or 3/16 thick. The white grill seen above is unusual in that it was made from 3/16 round rod, a material I only saw used a couple of times.
- Most all the grills were joined by welding cut bars, or simple cold bent scroll elements, together.
- One standard method is to twist the flat bars at 90 degrees at an intersection, then twist back to ‘edge on’. (Allowing for secure and simple welding)
- All the gates were painted, but in brighter primary colours. Green being the most common seen. Curiously, I saw hardly any use of black paint.
- Terminals were most commonly simple bevel cuts, or just left as square ends
Detail, note terminals, reason for the paint!
All of this taken together, suggested to me that the range of metal stocks available was extremely limited, stick welders in use, with only simple hand tools employed.

But within these extreme limitations, and what obviously are great cost constraints, there is clearly a good sense of design from these metalworkers.

This is an overall view of the detail above, which illustrates exceptional design inside the limits imposed.
At entrance to a small apartment building
Shows flow of lines, and balanced mirroring of shapes, from the upper to the lower sections. A balanced symmetry down the centre.
Elements from one section carried to the next - but not simple repetition of design.
Now I freely admit that my sample was confined to a very small area, and limited time. During my week long stay, I only saw one building from the ’Spanish Colonial’ period, and it in very rough condition. (Cuba’s ‘European Settlement’ goes back to roughly 1500 - but ‘Pre-Revolution’ elements are not well represented today.)

Still, I think the work I did see, even if in a relatively affluent area by Cuban standards, certainly represented the creativity of the Cuban metalworkers, despite serious restrictions on starting materials and limited tools.

1) Highly Recommended !
Hostal Lourdes & Leo /
This trip, the cost was roughly $28 CDN per night, for living room, one bedroom with two queen beds, small kitchen and tiny bathroom. Nice beach 5 minutes walk, 100 m from the main road. Wonderful hosts!

2) One example : Although school teachers are notoriously badly paid - even by Cuban standards, the cost of a simple pair of foam ‘flip flops’ was equal to roughly  * one week’s * wages for a high school teacher (paid roughly 25 CU / month).

3) I did see a reasonable amount of ongoing construction, mainly additional floors being added to existing homes at what seemed a snails pace. I found out that that same teacher’s monthly wage would only pay for 4 bags of concrete. The building blocks where commonly 4 inches wide, by roughly 18 inches (or more) long, with some only 3 inches wide. I rarely saw any metal re-enforcing in place in wall construction. Given occasional hurricane force winds, most certainly the slab roofs would lift, walls collapse - to the destruction of those living inside. This is in fact what happens on a regular basis in Cuba.

Saturday, March 23, 2019

Materials Test - Lyndhurst at CAMELOT

Although the Bloomery Iron Smelt undertook last Fall at the CAMELOT conference was primarily a public demonstration of the physical process, the concepts of Experimental Archaeology were also illustrated.

Additional Descriptions:
Ore Analog addition - main sequence
There was also a small experimental materials test integrated into the two day demonstration.

Regular readers may remember that I took part as a 'digger' volunteer at the ongoing excavations at Lyndhurst, Ontario, late August 2018. (1)
This illustration does not quite match the actual ground contours!

The original Lansdowne Iron Works is the first full scale commercial iron processing in what was then known as Upper Canada, dated to 1802- 1811. The complex included a blast furnace (producing cast iron) on the east bank, and a bloomery iron furnace on the west bank.

One of the main reasons for establishing a major iron processing facility has always been the local availability of the main raw materials : suitable iron ore and abundant wood for charcoal. A third element is the type and quality of the materials needed to construct the furnace itself. (2)

Of course limestone, which dominates the geography of the lower end of Southern Ontario, is not really suitable for construction of iron smelting furnaces. (3)
Lyndhurst however, is in the region of Ontario where the granites of the Northern Shield transition into the limestone covering Southern Ontario and New York state.

There was a thick deposit of clay also underlaying the modern backfill layer which has been placed over the ground level were the 1800 bloomery furnace was built. Clays, often modified with the additions of sand or organic materials has proved a very effective building material for furnace construction.

So - as part of my time at Lyndhurst, I gathered some of the clay from the furnace level, and some small fist sized stones from local rock cuts.

The raw clay was a medium grey colour, with what appeared a fairly high sand / silica content. Manipulating it, it felt more solid than 'slimy' (if that makes any sense to you). 
Lyndhurst stone sample
I had gathered two similar sized pieces of stone. The one used for the CAMELOT test was roughly 6 cm thick, 7 cm wide, on its highest point 8 cm tall. The thickness was consistent overall. (4)

Normally, the hottest part of our normal furnace build and operations is a rough oval that circles the tuyere. This extends roughly 10 cm to either side, 10 cm below and 15 cm above the centre of the tuyere. A ceramic tube tuyere was used for this smelt. (5)
Outside, Tuyere in place. Stone L / Clay R
Interior view (before tuyere installed). Clay (upper) is more obvious here.
You can see here that this test may not be quite as conclusive as it might be, as the two samples are placed just outside, and below, the 'hot spot' described above.
The surrounding wall material was the standard mix of high temperature clay / course sand / shredded horse manure. 

View of the interior of the furnace - cleared after the full smelt.
- In the image above, you can see how the tuyere has eroded back towards the furnace wall. 
- The stone sample shows less reduction than the regular furnace wall material that surrounded it originally.
- There is a bit more erosion to the sample clay than the regular wall material. 

The addition of sand and organics to the standard wall material mix is done specifically to reduce erosion and the effects of high temperatures. This does suggest that the Lyndhurst clay would prove effective for furnace construction.
During dismantling - The slag attached to the stone, and heat effects clearly seen.
The stone itself showed almost no damage at all - at least on a single smelt sequence. Again this suggests that the locally available stone would prove resistance to damage and effective for at least bloomery furnace construction.


At least as 'proof of concept' both the clay and rock samples gathered have endured a full bloomery iron smelt, at least within the dynamics of limited size and specific position. 
Ideally this limited test should be followed up with full builds, clay cylinder and stone block. 

1) The report on my observations of the dig - and my interpretation of the ground at Lyndhurst is still pending!
The excavations (test pits really) I helped with were on the west bank - bloomery iron furnace side.
One of the problems with close inspection of the original furnace construction is that the site of the bloomery furnace has been reused for two large scale commercial operations since. First was a flour mill, which involved the building of a more massive stone walled structure, which appears to have been constructed on top of the remains of the iron furnace. (This itself subjected by a major fire in 1811, which destroyed the entire complex.) Later there was a lumber mill constructed over the remains of the abandoned grist mill. This lumber mill was in operation into at least the 1950's, when another major fire destroyed that operation. Considerable earth fill has been added over each of these sequences in an effort to level the ground. The test pit that appeared to extend down to the level of the bloomery furnace was a good 2 metres deep below modern ground level.

2) This is apparent when you consider the wide variation of furnace builds seen throughout the Norse world. Although the slag tapping, 'short shaft' design is fairly constant, the builds range from clay (Denmark), stone slab / block (Norway), to grass sod (Iceland). See the extensive experimental series.

3) The working temperatures inside an bloomery furnace are in the 1150 - 1250 C range (or higher in a cast iron producing blast furnace). Limestones will decompose into lime (CaO + CO2) at about 900 C. You * might * be able to work around this by either using massive thickness and allowing for aggressive erosion, or using a sacrificial clay liner. (Note that is is very offhand - I have no direct experience here!)

4) I freely admit that my knowledge of stone types is limited at best!
The stones gathered appeared to be some metamorphic type.
(DARC does include a professional geologist, and I need to get him to examine the rock used.)

5) This with a fairly durable ceramic or the very fixed copper tuyeres in standard use. Tuyere set 5 cm proud of the inner surface, roughly 20 degrees down. Important is that this is also with a high volume air blast.
Past experiments has proven changing these settings can dramatically change the amount of erosion to wall materials.

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

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