4th RE-ARC Conference

www.rearc.us

My smelting partner Neil Peterson and I travelled the (long!) distance down to Re-Arc 3 last fall. It was absolutely worth the time and trouble. We greatly enjoyed ourselves (see the earlier review).

This is the session I have offered for this year's conference:

'Forging the Viking Age'

   Scandinavian culture was known for the quality of its metalwork,
   especially in iron. Just how did the tools available effect the
   creation of the object? Is there an effect from the qualities of the
   metals available themselves? Join artist blacksmith and Viking Age
   specialist Darrell Markewitz for this combination demonstration and
   hands on workshop session. A reconstructed 'sand table' charcoal
   forge, along with replicas of Norse blacksmithing tools, will be
   used. Those wishing to participate need to be dressed in natural
   fibre, long pants and work boots (jeans and T shirt ideal).

Course Iron Smelt - RESULTS

I ran a course here at Wareham over the weekend 'Introduction to Iron Smelting'
(see :  http://www.warehamfo...urse/index.html )

The two students were Jeff Evarts and Greg Eamon

The fellows built a very good clay cobb furnace and prepared most the materials on Saturday. The design was a pretty standard 'Short Shaft', about 70 cm tall and 28 cm internal diameter. The material was an even mix (by volume) of chopped straw, beach sand and dry potter's clay. The tuyere was a ceramic tube (kiln support), set at 22 down and inserted by 4 cm, 12 cm above a charcoal fines base.
Air provided by one of the industrial blowers the Early Iron group all uses, gated to roughly 800 lpm.

Charcoal was broken and graded maple and oak ( added separately), total of about 50 kg.
Ore was a mix of 'Barton's Run' limonite, DARC Dirt Analog and some rock ore from Quebec I had been given by Antoine Marcal. Total used was 30.4 kg.
The main smelt sequence ran over about 4 hours (plus burn down and extraction, maybe 7 hours total with the preheat). Our burn times were extremely consistent, averaging 9 - 10 minutes per standard charcoal charge. Ore additions started at 1 kg, peaked at 2 kg per charge.

The final bloom was extracted from the bottom of the furnace (rather than my more typical top extraction). With only two of us available for the smelt (Jeff took ill), labour was a bit short, so it was off to the hydraulic press!
This might have resulted in a bit more slag being forced into the surface of the bloom iron than would be normal. Later forging will tell on that.

The result of weighing after some rough compaction and cutting : total 10.4 kg. This represents a roughly 35 % yield - certainly great results.
The bloom does appear a bit crumbly, but the slightly lower temperature of the initial compaction (after running back from the furnace to workshop) may be part of the reason. There was no spark test made at this point.

Excellent work by our two latest members of the iron bloomery community!

Resulting bloom, partially compacted and sectioned



Furnace in foreground, Greg smashing ore in the back

Pushing (Hot!) Air...

 " My main question is what is the optimal air pressure and/or Volume. I realize too much air and you could have cast iron and erode the inside of the smelter, and not enough air, no Iron. "

Sauder & Williams have described their experience in a number of articles : 'A Practical Treatise on Bloomery Iron Smelting'
There are a couple of versions Lee wrote, ranging from a shortened verson for the Anvil's Ring, to a formal academic paper. Check Lee's web site (www.leesauder.com).

They found - and my own work and that of the stronger voices here certainly supports - the use of higher volumes of air. The short value is to use 1.2 - 1.5 litre of air per minute for every square centimetre of furnace interior cross section at tuyere level.
Most of us are using furnaces in the 25 - 30 cm diameter range. That 'magic number' works out to something around 800 lpm. I have a table up on my web site : http://www.warehamfo.../flowrates.html


I have done some in line pressure readings. Forgive the ruggedness of the instruments (pretty much cobbled together!)
The measurements I have taken show air pressures in the range of 3 - 5 psi with my own successful smelts. This may be more a curiosity than a useful indicator.
Does anyone else even measure that? How?
Think of the difference in hand feel between a blacksmith's blower and something like a vacumm cleaner - or a hand powered bellows...

Remember that its always better to have TOO MUCH air - rather than not enough!
You can (and we all do) reduce the operating temperature of the working furnace by adding increasing ore quantities in with your charcoal additions.

In terms of easy practical measures, again most of us are using a standard 'bucket' for our additions of charcoal. Most of us in North America are using a standard 'galvanized pail', which holds roughly 2 kg of charcoal.  Timing how long it takes to add this consistent measure gives a very useful measure of 'burn rate'.
Again for furnaces in the size range indicated, we have found the ideal burn rate is roughly 8 - 10 minutes per bucket / 2 kg.
Europeans usually will record 'kg per hour'. Using the Sauder & Williams method, that would mean roughly 15 - 12 kg per hour.
(You will see that typical European methods use only half to a third that amount. The results are predictably smaller yields and much lacier blooms!)

Of course, the specifics of your furnace design, type of ore, even charcoal type, can all alter this advice. There is a quite complex interplay between a very large numbers of individual elements for a truly successful smelt. Those with experience will certainly agree there is 'More Art than Science' at work here!

(Preparing to teach this weekend's 'Introduction to Iron Smelting' course at Wareham...)

Industrial blower with in line pressure and wind speed (volume) gages

About the Air...

Seth wrote to Don Fogg's Bladesmith Forum:

I have seen tuyeres adding to or helping to reduce wall erosion. What causes this and what am I looking for in a tuyere to help prevent it?

Right to start, we all need to remember that the exact working of any individual iron smelting furnace is a complex interplay between a very large number of variables! Your individual application of possible raw materials (ore, furnace, even charcoal) may certainly modify this basic level description. (!!)

Mike McCarthy proposed a working model for what is happening inside an iron smelting furnace during a late night discussion at Early Iron 1, back in 2004. I think his concept still stands up in the light of continued observations and experience:

McCarthy's original diagram - 2005

'The air blast inside the furnace makes a torus centered on the tuyere.'

My original notes on the discussion - 2005

This torus (bagel shape) is off centre, with the lower lobe smaller than the upper. (This due initially due to the effects of heat.) As the slag bowl develops, the shape is further distorted, with more of the air being deflected to the upper side of the tuyere.

Air Flow in a Working Furnace

The available air blast will effect the absolute size of the reactive area (reduction zone) inside the furnace.

Effect of VOLUME

VOLUME is important, because the reduction process of iron oxide both requires a certain volume of CO gas, but also a certain amount of effective time to take place. Things like the density of your ore, and its particle size, are contributing factors.
With low volume air, the core of the furnace may well be hot enough. But the individual ore particles may not have enough time at temperature and reaction chemistry to reduce. They may well fall to the bottom of the furnace without ever hitting an effective reduction zone.

Effect of PRESSURE

The PRESSURE at which the air is introduced into the furnace from the tuyere will determine the penetration of the air into the mass of charcoal within the furnace. At low pressure, the air remains close to the tuyere tip, which is also likely to put your highest temperatures closest to the furnace wall around the tuyere.
This is why using a rotary blacksmith's blower is often not found to be very effective for an iron smelting furnace. You may have large volumes - but with almost no pressure, the air simply does not penetrate the furnace.
Most hand powered bellows systems will certainly produce adequate pressures. Many European experimenters will use stones placed on the top plates of their bellows to control with fair consistency their air delivery pressures.

We have found that the ANGLE of the tuyere is critical to an effective iron smelting furnace.

Effect of ANGLE

If you place your tuyere too 'flat" (horizontal), what happens is that the air will not penetrate into the bottom of the furnace, leaving the effective heat 'ball' sitting too high. This in turn lets the developing slag bowl also form too high up in the furnace. The result is that you end up with a shallow slag bowl, with little room for a bloom to accumulate. As well, the liquid slag will also be sitting too high up around the tuyere, and quickly will 'drown' the air blast.

If you place your tuyere too steep, you move the air blast so it will be pointed directly at the developing bloom. Since you have oxygen at high temperature, it effectively acts like a cutting torch, and slicing your bloom apart as fast as it accumulates.

Effect of Angle on Bloom Formation

At Lee Sauder's Smeltfest research session in 2005 (if my notes are correct!) we specifically investigated the effect of tuyere angle. Through this series, we found the "best" angle for the tuyere to be 22% down from horizontal (plus or minus 5).

The next variable on the tuyere is how far the tip is inserted proud of the interior furnace wall by 5 cm / 2 inches. This helps to move the effect of the upper lobe of the torus of air away from the furnace wall.

Just to mess that element up, the material that your tuyere is made of will most certainly effect its durability over the life of the smelt:
Although using a piece of simple mild steel pipe will work - you are certain to find it will burn away quickly, moving the hot air to flow back against the furnace wall and start to erode it. 
Ceramic tubes are considerably more durable to furnace temperatures. Although these will burn back as well, our experience is that these endure long enough to prevent excessive wall erosion.
Experience has proved the most durable tuyere material is heavy forged copper. Lee Sauder has used the same copper tuyere for dozens of smelts will virtually no damage.


Putting the whole package together, you get something that looks like this:

Overall Diagram of Working Furnace

Of course you can elect to control the heat effects and wall erosion by modifying the design of the furnace itself.

Making the furnace body a flask shape, instead of a simple cylinder is one possibility. The ideal appears to be increasing the diameter at tuyere level to by the same amount that the tip of the stands clear of the furnace wall. (In effect a total of plus 10 cm beyond the diameter at the top.)

A second possibility (developed by Michael Nissen) is to insert a relatively thin plate into the heavier furnace wall - around the tuyere point. In practice, the highest temperature on the wall, thus the most erosion effect, occurs over a rough oval, 10 cm below and to the sides of the tuyere, and extending about 15 cm above it. The 'bellows plate' system relies on the atmospheric cooling off a thin cross section, in the range of 1 cm / 1/2 inch from a plate installed in that area.


Of course I do have to warn everyone that the working systems described above are most certainly not the only way to produce iron in a bloomery furnace!
A lot of combined experience has shown these elements do combine to make a very effective small scale bloomery however.