Crossover - Slag Rings in Archaeology?

The impact of an OAC project grant may extend well past the intended application.

My interest in bloomery iron started with the historic process of making the iron. Although there are some scattered living traditions (notably in Africa and India), these are fragmentary at best. For Europe, the technology of making iron has changed significantly since 1000 AD, with several pronounced shifts in method, equipment and type of metal produced. In attempting to re-discover what is a 'lost' tradition, modern researchers and practitioners are guided by very limited archaeological remains alone.
The best experimental archaeology may offer insights into how to interpret what may be puzzling artifact remains.

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These are some shots of slag rings recovered from two of our recent smelts (at Smeltfest 2012, Lexington VA, March 2012)

Lee Sauder has been using a heavy forged copper tuyere on all his smelts for the last several years. I'm not entirely sure just why he came up with this innovation. I believe it was in attempt to find a durable solution to the problem how the high temperatures inside the iron smelting furnaces were melting off the ceramic and steel pipe tuyeres then in use.
His tuyere was forged from a solid copper plate roughly 3/8 inch thick. First the piece was cross peened along the long axis to both spread and thin the rectangle into a triangle shape. Then the resulting form was wrapped into a cone. The finished cone is roughly 2 cm ID on the furnace end, about 4 cm ID on the bellows end. The piece is maybe about 40 cm long altogether. The closing seam is just butted together (not fused or entirely air tight).

Lee's clay 'medium shaft' production furnace.
The conical forged copper tuyere can be seen to the right

In use, what happens is that the heat the tuyere end is subjected to quickly travels back to the larger end exposed outside the furnace. The combination of radiation to the outside air, and rushing cold air down the inside surface, all combines to keep the tuyere end well before the slumping or melting point of the copper material. The result is virtually no effect to the the copper tuyere, even after many firing sequences. I think Lee has used this same tuyere for something like 30 smelts, with no damage at all!

As the rings would sit against the furnace wall

Inverted, showing the slag and ore fragments on the top surface

The slag will harden to a shell around the tip of the tuyere. These rings do not solidly attach fuse to the copper, normally hand pressure will break them clear.
You can see that both the internal and external diameters are indicated in the slag rings.
You can determine the upper and lower surfaces, with the heavier accumulation on the 'up' side of the tuyere in the furnace.
You can get some estimate of the tuyere angle. The slag has formed proud of the furnace wall, so if you assume the inner wall to be vertical, the inside surface does record the tuyere angle.

Inner surface (inverted here)
The inner diameter and thickness of tuyere can be determined.

Both the rings show cracking in roughly the same place. I think this is an effect of the cooling rate of the slag and the shape of the rings. One of the collected rings had in fact separated into two pieces ( the ring on the left in the images above).

One ring was broken into two pieces

We have worked with ceramic tube tuyeres as standard here for the last while. These are uniform, cheap and fairly durable. They also are quite obvious as a physical remain. Same goes for the iron (steel pipe) tubes we have also made use of. As the iron tuyeres are consumed with every smelt, I don't think that this material likely for VA process - just from a practical standpoint. (wasting iron to make iron?)

Copper tuyeres might be another mater. They would be 'relatively expensive' as objects, but because of their proven durability would be worth the investment for repeated smelt operations. The copper would be too valuable to discard, likely just being cut up as raw material for bronze production at the end of their smelting use. Any finds of copper cut to rings as a bronze related find? It would be the easy way to re-cycle the material.

Anyway, the slag rings are quite distinctive. Lee said he gets these every time. Worth a check against remains?? (Kevin Smith had mentioned that he had recovered some semi circular slag fragments from his excavation of an 'industrial' VA iron smelting site at Hals in Iceland. It will be interesting to see if these modern pieces in any way resemble his artifacts.)

We messed with using a copper tuyere a long while back, but at the time I did not have any heavy copper bar or sheet. The copper tuyere I made up was only 1/8 thick material, and did not transmit heat fast enough to keep the end from melting back to the furnace wall. This would have certainly produced some droplets of copper into the slag someplace. Perhaps another signature to look for in the archaeology?

(Modified from the initial posting on Hammered Out Bits)

Why Bloom Iron ?- three

(Day 13)

As is becoming obvious to those reading past postings, my main attraction to bloomery iron is a technical / cultural / artistic one.


My close friend (initially teacher and eventually workshop fellow) Lee Sauder was initially inspired to attempt to smelt his own iron because of a kind of Virginia Hillboy Ethic : " You should kill what you eat." As a working artisan blacksmith, he considered it important to the understanding of iron as a material to have experienced the creation of iron bar - from dirt. Lee had originally looked at 'traditional' African furnaces. There was some video that had been shot in the 1970's showing some old fellows undertaking one last iron smelt, from what they could remember seeing as young boys. (Compare this with Europe, where there was no living tradition at all.)
Right - Bloom Iron Vase - Sauder
Of course, the bug bit him big time. When I first met Lee (in 2002) he and his working partner Skip Williams had roughly 50 smelts undertaken (about where I am now). I consider Lee and Skip to be by far the most experienced iron smelting team in North America. Along with Mike McCarthy (and myself) they are at the core of the new 'Early Iron' movement.

My own interest in iron smelting started from a historical standpoint. The first iron ever produced in North America was made by the Norse in Vinland, circa 1000 AD. The site at L'Anse aux Meadows Newfoundland contains the remains of what was most likely a single use furnace. The estimated yield has been (very approximately!) calculated at about a 3 kg bloom.

Because of my work on the Norse Encampment living history program for Parks Canada, I was part of a small research working team in Summer of 2001, considering how best to represent this event to the visiting public. My very first attempt at smelting iron was made at that workshop. (And believe me, if there ever was a case of 'everything you know is wrong' - that certainly was it!)
Right - First Iron Smelt - 2001

From a purely technical standpoint, my interest was sparked by the historical traditions from early Europe. As with Lee and Skip, it was clear that I had to learn how to actually functionally *make* iron first. The clay 'short shaft' furnace that would become the standard model here in Wareham was based on archaeological evidence. However, modern equipment and 20th Century perceptions of science based methods would inform the techniques that were developed.
At this point, if I stick to known raw materials, and follow the established equipment set up and proven method, I can reliably produce a good quality iron bloom every time. An important direction to this continuing experimental work is to remove these modern elements, one by one. The exact methods used in ancient Norse iron smelting represent a completely shattered and unknown working tradition. Even the archaeology can only at best give the most general clues into the actual working methods used 1000 years ago.
Bloomery Iron represents a material functionally different than our modern metals. How you forge individual shapes leading into a completed object can be quite different than those you would use if utilizing modern industrial mild steel.

My interest in Iron as a cultural material remains tightly bound with a lifetime of exploring the history of Scandinavia during its 'Viking Age' - 800 to 1000 AD. The Norse were well known for their skill as metal workers. Their method of welding layered and twisted iron alloys together to create 'pattern welded' (or 'twisted composite core') sword blades created some of the most complex forged objects ever created. Although initially undertaken for purely functional reasons, they would raise the technology to high art.

Sword of Heroes - 2000 Detail
Two 9 layer twisted core rods with spring steel edges.

The raw difficulty of producing iron in the first place, coupled with the limits imposed by small anvils and use of simple charcoal forges combined to lift iron to a material suitable for high status objects. To the Norse, elaborately forged and elegantly designed iron objects were the equipment of kings and queens.
There is a drastic change in our modern frame of reference, where iron (mild steel) is so cheap that it is both plentiful and generally only considered a functional material. As an artisan blacksmith, I strive to change this popular perception of iron as material. The value of aggressively hand forged objects lies in the skill of the hands which had created it, not in the cost of the raw material.

Cauldron Hanger - Sutton Hoo, c 625 AD
(replica, the British Museum)

The Norse possessed a series of distinctive artistic styles, changing over time. My own personal design style, which I call 'Rivendale', has been deeply informed by the artifacts of the Viking Age.

The purpose of my OAC Crafts Project Grant is to allow me working time to blend these aspects of technical tradition, cultural framework - and artistic vision.

Why Bloom Iron ?- two

(day 5)

This will only be a fast post (and a bit out of sequence) as I'm off to Peterborough in a couple of hours for a public lecture :

Society for Creative Anachronism, Trent University, Peterborough Ontario
Lady Eaton College / Building 7, rm 208
7:30 PM

'Medieval Iron - an Overview'
A fast look over Iron as a material, iron work as a process, and iron objects of the Middle Ages. A focus will be to take a look at the kinds of objects of special interest and utility to the re-enactor. There will be a simple overview of blacksmithing equipment from the period - and what you would need to get started *historic* forging. Illustrated with images and replicas.

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The objective of my iron smelting work has been to produce a very low carbon metal. This most definitely was the primary objective in 'ancient' times. A low carbon iron metal is easiest to forge, so is the desired material for general blacksmithing work.

'Redemption' Bloom - November 2006

Modern Artist Blacksmiths usually use the term 'traditional' when they are talking about equipment, methods, and by extension materials, from the 'Industrial Age' - being roughly 1800 - 1900. This brackets the last of the large scale bloomery furnaces and the invention and growth of the Bessemer furnace, so the last decades of commercial wrought iron metal. (Generally commercial wrought iron was out of production and use in North America by around 1900. Actual wrought iron metal has not been produced in industrial quantities any place in the world since roughly 1975. See : Wrought Iron Work : What it IS - What it MEANS.

In the last post, I referred to 'historical' as the period which brackets the implementation of water powered machinery (in Europe) - being roughly 1000 - 1800. This is a great generalization, because there are a huge number of changes in the methods used to produce iron within those centuries. There is a gradual shift from small scale direct bloomeries to larger scale more 'industrial furnaces' over that period. High carbon cast iron will begin to be produced into the 1500 - 1600's, plus the introduction of coke (from coal) as fuel.

Almost by default, I'm then referring to the time from the first discovery of iron smelting up to the end of the Viking Age becomes 'ancient' - roughly 2500 BC - 1000 AD. As this is a massive time frame, there are certainly a number changes in iron making processes. All however are some variation on the small scale direct reduction bloomery furnace.

It has been with this series of ancient furnace technologies that I have been directly experimenting and implementing.

'Celtic Iron Age' : A re-enactor using a simple drum type bellows with a ground pit blacksmithing forge

Why Bloom Iron ?- one

(day 4)
The Project centres on the use of Bloomery Iron.

Right now I am in the 'shop re-organization and equipment set up' phase. This of itself may not be that interesting (although I will cover these aspects in later postings).

Bloomery iron is made by the direct reduction process, which is quite different than how our modern metals are made. This results in a metal which is also distinctively different than our modern alloys.

How modern mild steel is made:

The Bessemer Blast furnace : post 1855

How 'antique' wrought iron was made:

The Blast Furnace : post circa 1600
Specific product shown is high carbon 'cast' iron.

How 'historic' wrought iron was made:

Introduction of Water Power, post circa 1000
Actually a two step process, A 'finery' used for removal of excess carbon

How 'ancient' bloomery iron was made:

'Short Shaft' Bloomery Furnace, circa 600 - 1000
Product is spongy metal with slag inclusions.
(Image by P. Halasz)

The method that has been researched (through much trial and error) is based on this last 'ancient' furnace technology. The raw materials for this Project have been created using these methods.

There is a direct Canadian historical connection. The very first iron produced in Canada was made at L'Anse aux Meadows (Vinland) by the Greenland Norse about 1000 AD (late Viking Age). For more information on the early research into this specific process, see 'An Iron Smelt at Vinland'

The image above shows me working with staff from Parks Canada (Mark Pilgrim, left) and my own Dark Ages Re-creation Company (Dave Cox, centre rear) at L'Anse aux Meadows NHSC in 2010. A full re-creation of that original iron smelt was mounted, using all Viking Age tools and methods.


The other images above sourced over the open internet

Why? A Historical reference.

(Day Two)

So - why is important that someone is working with bloomery iron?

It is fair to say that to understand were we are going, there needs to be a consideration of where we have come from.

The following is altered from a much longer commentary I wrote this morning for the NORSEFOLK discussion group.

First (and most importantly) the standard metal used up to the Medieval Period is *bloomery iron*. This metal is soft, has a stringy texture with slag inclusions. Individual pieces would vary considerably in physical consistency. Carbon content would vary not only from piece to piece, but also *within an individual bar*. We modern smiths are completely dependant on mass produced, scientifically refined, industrially consistent (cheap!) metal alloys. These are produced using variations on the Bessemer furnace, only introduced in 1855.
There is a fuller commentary on 'traditional' versus modern metals on the main Wareham Forge web site : 'Wrought Iron - what it really is, what it really means'
(I get very aggravated by contemporary bladesmiths who have adopted bloomery iron making, building on the work of those who developed the current methods being used - and obviously not understanding them. Making bloom iron is *not* about alloy control, it is about creating a physical texture in the metal.)

Modern commentators looking at traditional practices often use the term 'ritually' in place of a better description 'based on experience'. Our concept of 'ritual' is most certainly far different than ancient / non Western concepts. 'What you do if you want things to work' - in our world we would call this science.
An experienced smith knows that when you quench different pieces of iron metals from orange in water, there can be changes in how it breaks when cold hammered. The exposed surfaces can have different colours and textures. Metal that is thus treated, then found to be brittle, have a surface of small crystals, and a bright, light grey colour - that material also makes for a hard / durable cutting edge.
(This selection of materials based on physical appearance is the core of the Japanese traditional method. Consider - How do you spark test for carbon content, a standard modern practice, in a world with no high speed grinding?)

This wide variation in the quality of the starting metal is vastly important when creating cutting edges. Examination of a large number of individual blades from the Roman to full Medieval periods has shown that the processes of quench hardening and drawing back temper were *not* universally applied by bladesmiths until much later than most would suppose. Although this fact seems counter intuitive to a modern blacksmith, my interpretation is that the variation in metal characteristics in bloomery produced iron is the reason.

Even a small 'short shaft' furnace is easily capable of producing raw iron blooms much lager than those typical of the few artifact blooms we have from before the introduction of water power (Europe, roughly 800 - 1100 AD). Early smelters were creating blooms in the 5 - 8 kg range, *limiting* potential size. This just because of the great difficulty of attempting to work larger masses of metal down to useful bars, with only stone anvils and hand powered hammers for tools.

'Redemption' Bloom - November 2006
6.8 kg, from 19 kg combined ore and gangue