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)

Some always Remember

I normally wax philosophical for Remembrance Day. Although I have less 'time on the ground' than some who will read this , I do know my soul understands what my body had not experienced. That balance between the high ideals that drive and in the end preserve you, against the futility and raw terror of the action. All those statements of truths we knew, even young, but never really understood as now perhaps we now do. Of seeing the cycle endlessly repeated of 'Rich old man's war, Poor young man's fight'. Yet another generation of cannon fodder shoved into the meat grinder, only for the benefit of a mere handful of the Powerful. I don't think I have become wiser, but it seems more extreme and more obvious to me every conflict.

And so the Boys come Home. For a while.
We all know, in the darkness of the night, that this is just a pause in the endless cycle. This War will pass, like those before it. Certainly soon to be replaced with another Conflict. Those that sat behind desks will thump their chests and speak of Glory. Broken down veterans with empty eyes will be disreguarded on the street. Civilians will speak of the horrors, never considering the cost others paid for the soft lives they possess. Ever more Citizenship will be equated with Rights - never Responsibility.  Another generation will forget or disreguard the lessons so bloodily bought by the few.

I will raise a glass here, at that agreed upon time.
But this year, darkly...

African IRON - on YouTube

There are some available documentries of the last traditional iron smelting in Africa. These were recorded back in the late 1960s - early 1970's. Preview clips have recently shown up on YouTube:

Inagina: The Last House of Iron

Swiss archaeologist Eric Huysecom and cameraman Bernard Augustoni work with 13 master smelters to recreate the building of a traditional furnace for smelting iron in Mali. There has not been any traditional iron smelting in Africa since the 1960's, in part due to the importing of cheaper substitutes. The building of the furnaces and the work involved in the actual production is deeply entwined with ritual, symbolism and gender. This film describes in great detail every aspect of the event, from the selection of the site of the reconstruction - which is the oldest remaining furnace site in the region, last active in 1961 - to the final result. This is an important film for African Studies, Archaeology, Religion, Ritual, Technology and Gender.

a film by Eric Huysecom, (Geneva) and Bernard Augustoni
distributed by Documentary Educational Resources. Purchase: http://www.der.org/films/inagina.html
 

The Blooms Of Banjeli

The Blooms of Banjeli documents research in Banjeli, Togo on iron-smelting technology, its rituals, and the sexual prohibitions surrounding it. Including rare historical footage from the same village in 1914, it provides a unique technological record of the traditional method of preparing a furnace to smelt iron. This documentary offers an interesting approach to our understanding of the relationship between conceptions of gender and technology in traditional African society. The people of Banjeli liken the furnace to a woman's body, which is 'impregnated' by the smelter. The process of smelting is compared to that of giving birth, the furnace being the womb and the iron bloom, the newborn. a film by Carlyn Saltman with Candice Gaucher and Eugenia Herbert distributed by Documentary Educational Resources. Purchase: http://www.der.org/films/blooms-of-banjeli.html

Is there IRON....

... In them thar HILLS??

Or maybe not.


Although Iron, as iron oxide minerals, is one of the most common found on the earth, as anyone actually LOOKING for the stuff knows, it is really not that easy to find as usable ore.

An added complication is that those looking for iron ores for use in direct process bloomery furnaces need to have ores in concentrations far in excess of that which modern industrial processes can utilize. Ideally, the small Early Medieval furnaces that I am interested in require iron concentrations in the order of plus 50% for effective bloom creation.

- After centuries of human activities, many of the best ancient deposits of suitable iron ores have been already used up.
- Although primary bog iron ores have a continuous deposit process, the sad truth is that the exact chain of geology and geography needed to create concentrated deposits is very specific. Even small amounts of human activities can completely disrupt this formation chain.
- In many cases the reason ores may still remain in areas once exploited in the past is simply that those individual pieces of ore are not of high enough quality to have been utilized the first time around.

Obviously my first question would be 'Is there any usable iron ore locally?'

I live close to Dundalk Ontario, abut 2 1/2 hours drive NW of Toronto. On top of what is called the 'Dundalk Plateau', an area about as flat as a pancake:

The Niagara Escarpment is recognized as one of the world's unique natural wonders. Essentially, it is a landform -- a ridge of rock several hundred metres high in some locations -- stretching 725 kilometres (450 miles) from Queenston on the Niagara River to Tobermory at the tip of the Bruce Peninsula. Today, in Ontario, the Escarpment contains more than 100 sites of geological significance including some of the best exposures of rocks and fossils of the Silurian and Ordovician Periods (405 to 500 million years old) to be found anywhere in the world. 

from the Niagara Escarpment Commission web site 

Range of the Escarpment - Altered from Wikipedia

So - lets consider the ground around Wareham - my base in Central Ontario:

What this all means is that Wareham sits near the top edge of a gigantic, flat, thick, slab of solid limestone - a block a hundred metres thick. This was buried under plies of loose sand, clay and rock in later ages. About ten thousand years go, a wall of glaciers bulldozed off most of this loose stuff, filling in most of the cracks and pretty much scrapping the top off - leaving the landscape even flatter. There are some places around the edges that in the centuries since, rivers have washed out the loose materials, sometimes even exposing the top edges of the under laying limestone slab.
One feature of the remaining top cover over the solid stone plate is a bank of clay, roughly 10 - 30 metres thick. This is a fine red clay, those that remember 'Blue Mountain Pottery', popular in the 1960's and 70's, know the stuff. This layer effectively seals the aquifer inside the limestone from the surface collected water above. 

There are no exposed rock faces close to me. There are large valleys which cut into the plate of the Escarpment, to the east and north of Wareham. There are very few places the rock of the Escarpment are actually exposed however. Those valleys have sides covered with massive piles of loose sand and gravel. (Of course the flat areas around have farm fields on them, the contours of the valleys covered with combination maple and pine forests.) Getting to those valleys is a roughly 50 km / half hour drive north, closer to 30 km / 20 minutes to the east. Spotting any place where there is exposed rock is extremely difficult.


Now, our well at Wareham is a deep one (175 feet) down through the clay layer and into the aquifer of the Escarpment limestone. There certainly is iron dissolved in that water. It cloggs our coffee makers and even settles out as a thin film of fine red iron oxide if you left a container to stand overnight. Remember however the *surface* water is sealed off, entirely separated by that thick clay cap over the limestone. (Its why you pay for a deep well to begin with, as the surface water level fluctuates hugely over the course of the seasons!) So if there is elemental iron oxide in the deep water - it is only going to come close to the surface at locations where the base limestone is exposed. 

I certainly have wandered along the margins of creeks and swamps around Wareham, looking for possible primary bog iron ore deposits. A couple of places, I have spotted very marginal signs of a red iron ochre in the water. Never anything like an amount that could be collected, much less a deposit of actual bog iron itself. This is suggestive however, at least of the possibility.  
There are a couple of operations in the area that dig 'peat' - which in this case is really the organic rich sediments from the bottom of the swamps. These are not true peat bogs, with their chemistry of tannic acid. As these swamps are basically water filled pits into the under laying clay, there is no ability to leach elemental iron out of the rock underneath (this even assuming there is available iron in the rock itself).

Maybe along the slopes of some of the valleys?

(stay tuned for part two..)

 ,

Iron Smelt in Russia?

 (This is a much older piece that got caught as a draft somehow and I think never posted...)

This video clip was suggested by one of the Early Iron gang:



The clip shows a team from Russia (given the Cyrillic text) working at a large living history (Viking Age?) event.

I quite like the simplicity of the basic construction method. Use of bundled straw for interior form is elegant.

I do wonder at the purpose of the base construction.
The built up earth plinth may be primarily to raise the height of the furnace. This appears to be explained in commentary. Other than raising the furnace for ease of access, its hard to understand why.
Considerable care is taken with the construction of the base, with a layer of straw, covered with a clay cobb plate, this in turn with what appears to be a semi refractory layer. (Light coloured clay that appears to be mixed with charcoal fines.
This same light material is used as the inner layer for what looks like the first 10 cm of the furnace wall.

The interior diameter of the furnace looks to be roughly 20 - 25 cm.

Note the bellows size and style (medium size double action - Late Medieval) Depending on pump rate (hard to determine from the video) this equipment should be able to supply plenty of air volume.
Use of steel pipe for bellows tube / tuyere
Set at basically flat angle (only slight downwards)
Very shallow base distance below tuyere (perhaps 10 cm??)

The is use of round port for tapping. Appears to be at same height as the tuyere.
There was no tapping event recorded, and no tap slag visible in later parts of the smelt.

Total furnace height looks quite short, the entire furnace may achieve (barely) 40 cm total. Considering placement of the tuyere, this suggests a very short reactive column.

Charcoal is roughly broken for size, scoop from pile method for screening out fines.

Ore appears to be hematite / iron sand or oxide powder (brown ochre) ?
Laid in a large slabs rather than sprinkled through charges. Only two charges shown, and it would be important to know how much ore was used.

There is use of flux (a fine white powder - may be borax?) near end of smelt.
May be explained in commentary, but why?

Furnace is allowed to burn out and basically go cold.
Extraction is by breaking out rear wall to expose interior.

When the interior is exposed, the slag mass certainly looks considerably above the height of the tuyere. Was the smelt halted because the tuyere was blocked? (Commentary may explain?)

On the extraction, three pieces are pulled aside.
The first (which is the piece in the smelter's hand near the end) is holding heat in a manner that at least suggests there may be some iron in it. If so, it is extremely lacy and small. The second piece is dense and dark, and looks like iron rich slag. The last piece (seen again near the end of the video) is light coloured green, typical of an iron poor slag (melted furnace walls).
Obviously the comments of the team would be important to understanding their results.

It appears the smelt master has determined the pieces containing iron by look and weight. It might have been more instructive (for him and us) if these fragments had remained hot enough that they could have been hammer compressed

.

Shields, Wood and Grain lines...

Earlier, a discussion on Norse shield construction said something like: the
boards were split, as the Norse didn't saw their wood, implying that the
split wood was stronger as the grain wasn't cut.
..
If so, I'm curious as to how it worked.
Thank you,

Edward

 The following was prepared for the Norsefolk list this morning. May be obvious to those who work with wood - but I had the piece written...

I know those that know me well are likely laughing right now - Him, WOOD?
(First thing I teach my smithing students is ; "Metal Good, Wood EVIL".)

How does it work?
(*Real* woodworkers chime in and correct this!)

This might be a bit hard to explain with out proper diagrams - but bear with the use of symbols.

As has been described, a split plank comes from a clean, straight grained section of log. Wedges are used to break the circular diameter into half, quarter, eight - resulting in narrow 'pie' shapes.
The splits follow the natural lines of the grain - with the right piece of log, straight lines.
If you looked at the top of the piece - you would see a set of slight curved lines - the top edge of the grain.
If you looked at the side of your split plank, the ideal would be the side edges of each of those  grain lines
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This is extremely strong in *compression*.  If you push on the top of such a plank, the wood will bend before it ever actually breaks. Imagine the bow of a ship hitting a wave. The planks will flex rather than shatter.

If you had the technology for making saws (and the Norse could be considered NOT to have this *) you would be cutting a perfectly straight line through the log. You are slicing across a circular 'bulls eye' shape. Depending where the cut is, almost all the boards now have a combination of curved, diagonal cut, very few straight gain lines. This is especially seen on the side of the plank, with grain running pretty much all over the place.
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This is much weaker in compression. If you push on the top of such a plank, the wood can separate along the diagonal lines in the grain. Build that same boat out of equally thin *cut* planks, and when it hits the same wave, the hull will virtually disintegrate.


However, the question was about shields.
Now you are forcing a thin wedge (a blade) against the edges (mainly) of the plank.
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You see that in *either* case, the thin blade attempts to force into the lines of the edge grain. Some argument may be made that with a cut to the edge of a split plank - the board is actually *more* likely to break.
 But remember the shape of your shield - most of those blows are not going to come in exactly at 90 degrees to the end grain - but mostly coming in towards the side of the grain
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With the split board, the force of the cut is directly across the grain line - against its very strongest portion.
With the cut board, the force of the cut is applied to a possible *diagonal* grain line - a point of weakness.

(Since I think attempting to draw this would prove difficult bear with me)
Now cut two very thin planks - and stack them on top of each other. The line of the grain is offset by 90 degrees in the two planks.
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You see what you get with the split planks is that any angle of cut is attempting to slice through the strong side line direction of the grain
With the cut planks, almost any angle of cut is attempting to slice through the much weaker *diagonal* line of the grain.

Clear??

* In the case of the Norse, the metalworking problem of making a wood cutting saw in the first place is actually the limiting factor. The first plank cutting saws are wide, long, straight blades. Such a blade should be thin in cross section - to remove the least wood and so require the least force to move through the piece of timber. It needs to be absolutely dead straight - the smallest variation in thickness or warp (in any plane) will cause the saw to bind in the cut. The metal needs to have some flex in it, to withstand irregular sideways pressures likely during long hours of use. It also needs to be hard enough that the teeth do not instantly wear down to the point they do not cut.
Axes on the other hand, are much smaller cutting surfaces, and considerably more robust in construction. In smaller diameter VA forges, an axe blade can be pretty much all heated at the same time - not at all possible for a long pit saw blade.

(I have made many axes, enough swords - but has never even considered trying to hand forge a pit saw!)