Furnaces with Shells?

(This is repeat of a contribution I made to an ongoing discussion over on Don Fogg's Bladesmith Forum...)

What appears under consideration here are thick walled furnaces, with an eye to increasing the durability of the structure.
I have worked a lot with various cobb mixes - blends of clay / sand / organics. Our (DARC) basic design here has been primarily chopped straw plus clay. I have built a lot of furnaces, as our primary investigations have been with various archaeological models.

From conversations with Lee Sauder, my understanding (??) is that the addition of sand to the clay increases the resistance to temperature of the finial wall material. Most importantly, the sand does not expand with heating near as much as the clay will, thus making the walls more stable at smelting temperatures. Careful drying of the constructed furnace is critical.
(A reminder to new readers - as water heats it expands as steam - something like 50 times the volume. If this steam does not vent slowly, cracking, even explosive spalling, is the result.)

Straw cobb works in a different manner. Our most typical mix starts with 50 % dry potters clay / 50% straw by volume. Add maybe an additional 10 - 15 % sand to stiffen the mix. The straw is chopped to roughly 10 cm (4 inch) or less pieces.
The individual pieces of straw in the mix carry out several functions:
- Being hollow, they give the steam someplace to expand into, reducing cracking effects.
- As the inner surfaces of the furnace reach smelting temperatures, the straw burns out. This leaves hollows in the clay matrix - in effect acting as an insulating layer. This actually does increase the overall resistance of the wall to high temperature.
- The outer layers of the furnace, although hot, are not high enough to burn away the straw. So the straw acts like pieces of rebar in concrete. Even if cracks will develop, the straw binds the gaps - holding the walls together.

Generally I would say that the high sand furnaces Lee builds (PDF) are more durable over the long term use. I think he had one furnace that ran over 35 individual smelts. The down side is that the construction requires considerable care and time to undertake correctly.
The use of the clay / straw cobb allows for much quicker construction. I have certainly done a build in the morning, then fired in the afternoon. I would generally say these furnaces are not as durable. The most we have used one furnace has been five smelts. This is more because of our climate in Central Ontario, working in an exposed smelting area. Clay cobb is much more susceptable to damage through the freeze / thaw cycle (mainly because of those internal voids). (Our normal pattern here is three smelts a year, June / October / November, so almost every furnace sees at least one winter cycle.)

Something Michael Nissen (from Ribe in Denmark) showed me when I visited with him in 2008 - the use of shredded horse manure as the organic mix. This results in a much finer texture to the cobb. It does still retain the heat resistance and strength of the straw mixes. Gather old horse pucks, which shred easily when rubbed between your hands. Mix 50 / 50 with the powered clay. I have not used this for a complete full sized furnace. It has become my standard mix for the smaller Aristotle re-melting furnaces, where it shows great refractory ability. Michael (at that point) was using a 'bellows plate and blow hole' system. This features a roughly 15 x 20 cm thin plate set into the front of the furnace - with a hole in it through which the air is blown. This is hottest part of the furnace wall, yet even 1 cm thick plates of horse cobb have demonstrated great durability.


I have constructed a couple of furnaces in metal shells.
One was a variation on our 'Econo Norse' brick teaching furnace. I managed to scrounge a 1/2 sized metal barrel (20 gallon) from the dump. This was used as an external shell for an arrangement of standard fire brick set in a hexagon pattern, three bricks tall. The gaps in the pattern were held in place with a mix of clay and sand - as much as a binder as anything else. Although heavy, the size is such I can lift the completed furnace by myself. The solid metal shell allows this furnace to be portable. The ideal way to use it is in combination with a concrete block plinth, packed with ash / charcoal fines (learned from Lee & Skip). See the smelt report : http://www.warehamfo...eport04-08.html

Last spring, I built a variation on Lee's successful design. This was his suggested mix of 50 clay / 50 sand. I had an old metal garbage can, which proved just the exact size required with the bottom cut out and turned upside down. At this point this furnace has only been fired once - but it remained in almost perfect condition after the smelt. You'll see in the image below it also uses a forged copper tuyere - again prototyped by Lee.
I thought I had a close up of the shrinking of the clay walls. Although I used the metal can as an exterior form, buy the time the furnace was completely dried with an internal fire, there was about 1 cm shrinking all around. Thats with an internal diameter of 25 cm and wall thickness of 5 cm. My mix was 'looser' than what Lee uses, so much of that volume might have been the extra water (??). This furnace is set on a circular brick base (again stolen from Lee). The intent is not to make the furnace portable, but to protect the walls from the effects of rain and more importantly Canadian winters (!). I have covered the entire thing with a old plastic 45 gallon drum for the season. See : http://warehamforgeb...tion-smelt.html

Darrell

PS - sorry about the order of the images - I pretty much pulled them down as I thought of it.
1) Three older furnaces, set out as an experiment in aging / weathering. The two at the rear are both clay /straw cobb construction.
2) My new Sauder style 'production' furnace, before firing.
3) Michael Nissen's furnace at the Heltborg Symposium, 2008. Body of straight dug clay, inset horse manure cobb bellows plate.
4) The 'Econo Norse in a Can' furnace - before firing

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Is there really a need for this forum?

 

Archaeometallurgy forum 

Forum for all aspects of past metallurgical activities

www.forum.archaeometallurgie.de

 A new discussion group, started by Bastian Asmus in early October.

He posed the question : Is there really a need for this forum?

When I posted in the Jiscmail list that I had started this forum I had some answers both privately and online if there really is a need for another resource.

I think that this is a good question and would like to discuss this in earnest.
The main criticism I received is that there are already too many resources available and that other, already existing social media could be used to achieve the same effect. By creating another one it is one more resource to have an eye on..

 The following was my contribution to that discussion :

There has been an evolution, as the internet as evolved, in this medium as a communications tool.

I may be able to make some bold (!!) observations, having been involved in metalworking aspects of using the internet since pretty early on:

At first it was the discussion boards? (Remember those?) Kind of like the live chat that you find associated with Facebook these days. Slow, so slow. Pretty much wide open. A bit like trying to talk at a party - everyone saw, anyone could drop a line in. Often frustrating because it was so slow in the transmissions. And no records kept at all of the conversations. You might be able to cut and paste out pieces into a file - but generally once sent, things were gone. You also had to almost find specific individuals by accident - you all had to be on the same board at the same instant to converse.
I was actually surprised to be able to find others interested in historic metalwork at the time (be the early 1990's). Having a group of people working on the same processes and source artifacts was amazing. And world wide!

Into the 90's - it was individual web sites. The amount and quality of the information presented varied a huge amount (and still does). The biggest problem (and still) is maintaining web sites over long durations. My own site has been at a stable URL since the mid 90's - and this is unusual. The huge advantage that with a little work, any individual could now publish information. For those of us working outside the academic stream, this was very important. Of course *quality* of information was very irregular (to be generous).

I was part of the group in North America, largely started by Mike McCarthy, lead by Lee Sauder & Skip Williams, behind 'Early Iron'. First several conferences, then an documentation web site (http://www.geocities.ws/earlyirongroup/). This never became as useful as it might be - largely because of the huge amount of work involved trying to manage the (yet another!) web site.
Attached to this was the original Early Iron discussion group. At one point it had something like 200 plus involved, world wide. The international aspect was its biggest strength, attempting to tie together what was happening in Europe with what was developing in North America. As such groups do, the interest would wax and wane. Tied to geocities / yahoo, there were problems with the access via e-mails. This was ideal for some, not so wonderful for others. It did allow for an archive of past postings.

More into the 2000 period, personal blogs became active. These are driven by interested individuals, so often can be fairly detailed in the amount of information included. Not necessarily the easiest to find, but generally a google search will yield topics of your interest. The best are fairly narrow in focus. They all suffer from limited volume. Most importantly they do not allow for easy discussion, but only represent a single viewpoint.

Don Fogg's Bladesmith Forum has become of late a very active discussion. This is largely because of the developing interest by practical, mainly American, knife makers in bloomery type materials. Some of the main voices in developing the techniques experimentally have become active there. The truth is that there are two quite different approaches developing. The earlier workers were very much concerned with *process*, many of the 'second generation' concerned with *product*. (My view at least.) This forum does allow for easy posting of images, and does maintain an easy to source archive. (The format is the same as what is being started here.)

Enter Facebook. 'Iron Smelters of the World' - Mark Green has been largely responsible for developing that venue. It has the advantage of being extremely casual. It has a certain ability to allow images. Its huge drawback is that it only allows for the shortest of text. It is often hard to follow single topics. There is preservation of the threads, but no easy way to sort or search those. Certainly something that has been extremely active, and pulls in a large number of people world wide. There are also a number of experimental archaeology FB pages available. Although there is much specialization easily possible, the truth is that FB is simply too fragmented.

When I was talking to Lee Sauder about this new discussion group, he did point out one of the greatest potential strengths. Of being able to hold information - search it - and allow for longer, more detailed postings. Of allowing for serious *discussion*, hopefully with a collection of academic researchers, skilled industrial people, experienced practical artisans and serious amateur researchers.
I have to agree with Lee (and echo Alan) that a blend of the 'rubber boot' archaeologist and the working metalsmith may prove the most fruitful. I have hopes. But I do notice there are a lot of 'views' here - and not that many submissions yet...

North Erie Shore - Bog Iron?

On 17/11/12 10:24 PM, Jonathan Martel wrote:
... I've been doing some research on bog iron in Southern Ontario and came across your blog posting from 2009.  I was wondering if you've had any luck since then in your search for bog iron sources?

Short answer - no.
If you search for 'bog iron' on the blog, there must be several dozen articles altogether. The 2012 searches in Newfoundland may be of most interest (with images of signs in a bog). You will also want to take a look at the most recent postings - describing some field work in West Central Ontario over the last couple of weeks.
The exact chemistry and geography that is required for a *workable* deposit of primary bog iron ore is very specific. There are a couple of possibilities in mechanism too. This is complicated by the fact 'bog ore' is a term that is used by almost everyone - but for radically different materials with quite different deposit methods. In short, if there is iron ore found any place near water, it gets called bog ore - regardless.
To complicate this, deposits may be from ongoing formation - or an ancient deposit, now uncovered.

I've found some historical sources indicating that bog iron was historically smelted in Essex and Norfolk counties.  Do you know if ore exists in any quantities whatsoever or have they largely been tapped out and not at all worth while?

Jonathan is referring to deposits - and furnace operations around Normandale Ontario, on the north shore of Lake Erie. See : https://sites.google.com/site/longpointsettlers/normandale-iron-works 
Link to the Ontario Historic Plaques information with a location map : http://www.ontarioplaques.com/Plaques_MNO/Plaque_Norfolk18.html

Those deposits were largely mined out in the middle 1800's . There were most certainly ongoing deposits in boggy / swampy ground around that area (Normandale - 1820 - 40 roughtly). These supplied both blast furnaces for cast iron, and also larger bloomery furnaces for making wrought iron bar.

Problem was two fold:
First, Victorian era iron furnaces are large, and require a large amount of ore to function - and a huge amount of charcoal to fire them. This demand quickly outpaced the natural production cycle of the primary bog iron ore. Net effect was the naturally 'stockpiled' material from ages before was quickly consumed.
Second was that the combination of clearing land for farming and cutting timber for charcoal production was coupled with the draining of swamps for fields. Combined, this altered the local geography, in effect destroying the swamps and bogs required for the chemistry of iron ore formation.
So in effect the limited natural resource was quickly consumed, while the landscape was altered to reduce the possibility that more natural ore could be formed. Commercial production in the Normandale area operated at best for a decade or two, before being abandoned.

No I will caution this next remark with saying that I have not personally walked the ground in that area. I have been told from those from that region (south of Brantford) that with the tensions between the First Nations groups and others - and troubles over land claims, that often local residents are not keen on unknown strangers walking their fields.
That being said, I have seen a sample of reported bog iron ore taken from the area. Of course there are all the problems with a sample gathered by someone generally interested in local history, but not with any personal experience with bloomery furnaces. Did the sample represent the kind of ore actually used in the Normandale furnaces? Or was it a piece considered too poor, even at that time, to be harvested and used?
Anyway, the sample contained a considerable volume of sand and small gravel. On a guess, I would say well over 50% of the volume was these impurities. It was hard to tell how much iron oxide was actually present in the roughly fist sized piece I was shown.
Certainly if that sample was representative of the modern day available iron ore, it would not be suitable for the kind of small size bloomery furnaces we are currently using. (Our Pre-1000, Northern European style furnaces require about 50% iron content (or better) to be truly effective.)


Note to Readers!
I certainly would love to be proven wrong on this! If your own field walking in Central Ontario discovers anything that looks like a purer primary bog iron ore, I would be most interested in knowing about it.

Is there IRON? - Part Two

(You are encouraged to first read the background article 'Is there IRON in these hills?')

First, remember the physical size of the Niagara Escarpment:

Now, take a look at the Ontario Roads map of the region around my home base in Wareham:

West Central Ontario, range circles (10 km) centred on Wareham

Did I say FLAT? What the ground around here looks like!

Over the late Fall, I have made a couple of trips that have included looking for possible iron ore sources. As I explained in the earlier posting, I really had no expectation of finding anything. But as my friend Lee Sauder (wisely) told me 'You ain't going to find anything - if you never look.'
I felt the only hope of finding anything at all would be at one of the rare places where erosion had exposed the base limestone of the Escarpment.

Four locations are indicated (yellow circles) on the map above, in the order I looked at them. For each I was looking for places where the under laying rock was exposed.

ONE - Belfountain

This point, just south of Orangeville and west of Highway 10. is where an ancient river had cut through the loose gravel cover to expose the limestone. There was enough water moving originally to both cut the rock, but also to throw refrigerator sized blocks around. Today there is a waterfall down one exposed face of this cut.

From the waterfall, we travelled about 200 m down the path that runs along the top of the cut on the north side. Eventually this trail runs down to the bottom of the cut. 
One feature there is a pile of larger stones, obviously cut from further upstream at some point in the distant past. These have been tossed into a pile along the south bank, in a fairly localized area. It looks like the force of water could carry them this far, but no further. These blocks are of a different limestone than those in the rock cut at the same area.

Showing the limestone face, part way up the pile of jumbled rocks (to the right here)

Kelly standing beside one of the large rocks showing iron staining

Iron staining on rock surface - bag is 4 1/2 inches

Iron staining appearing to form a line through the pile (?)

I was greatly encouraged by this - but this was not a source for ore!
On closer examination, what was happening was that these rocks had a smaller amount of iron contained within the matrix. As the water seeped into the stones, then later evaporated from the surface, there was a small amount of iron forming like a scale on the very surface. Although I did flake off a couple of fragments as samples, it was clear that the 'iron' was only a thin film on the surface. At best only one or two mm thick. More like pulling off a surface layer of rust flakes.

(Stay tuned for part Three)