Monday, July 03, 2017

Iron Smelting in the Celtic Age (three PLUS)

Continuing from the post on June 23...

Thijs van de Manakker returned my inquiry on some technical details of the smelt series seen on the videos documenting his process.
(Remember Thijs' primary language is not English! Any modifications / comments in italics)

1) The height of our furnace is 80 cm.
Interior diam bottom 38 cm. (a)
Interior diam top    27cm.

2) The slagpit is under the whole furnace.

(the third image shows the 'basket' covered with a roughly 1 cm layer of clay)

 3) One bellow is 36 Liter is 1/2 furnace content. (b) (**)
A furnace with two tuyères with Stiphout bog ore runs best at 17 strokes (of 2 bellows) per minute  17 x 72 Liter = 1224 L/min.
With that tempo we can charge every ten minutes: 1,5 KG (6 L) charcoal and 1,5 KG ore (2L), so  both charcoal and ore 9 KG /hour. (c)

4) The furnace on the film was not documented on paper, so I don't know the exact figures.
The average yield of our furnace is 15% raw iron . (d)
The highest amount of ore we ever could load was  103  KG .
Bloomforging gives 30% lost of iron, so bar yield of the furnace is 10%. (e)

5) Our blooms have  0,06 % Carbon. (f)
To get steel  we double an iron bar several times, to get rid of slag as much as possible, than the bar of 5 mm thickness  is packed with charcoal powder inside a  loam (mud)  cover and fired in a woodfire for 25 hours.

Thijs van de Manakker.
the notes / comments

(a) At 38 cm the interior cross section will be 1330 cm2
Our standard furnaces run between 25 - 30 cm ID, so 490 - 710 cm2
(see how even a small increase in diameter really will impact air requirements)

(b) My original WAG was really off on the math and actual reported numbers! I had estimated the furnace with a bigger diameter and a lower produced volume on the bellows.
Using the 'Sauder & Williams Method', the 'ideal' air volume for this furnace would be 1600 to 2000 LpM. 

(c) There is still no standard used throughout the experimental community on exactly how to measure and compare burning rates.
Kilograms of charcoal per hour / minutes per standard charge are good indicators.
Between the two, I suggest minutes per kg (charge) is the single best. During the smelt itself, this measure also gives the smelt master the best indication of just what is happening within the furnace as the smelt progresses. The consumption of standard charges will certainly vary over the course of any smelt event.
This makes this experiment with an average burn rate of 6 - 7 minutes per kg.
Our standard is to aim for 4 - 6 minutes per kg. (The impact of higher air volumes).

(d) Given an effective furnace design, and suitable air volumes being delivered, the next big impact on smelting yields is the quality of the ore itself.
So its hard to say if the yield reported is basically due to the iron content of the ore (not given here).
Our normal expectation is for 20 - 30% return from ore to bloom. Larger ore volume smelts typically produce higher yield results. I do note that our own efforts using human powered bellows typically yield down into the 15 % range as well.
(A hint to hopeful experimenters : For good results, your ideal ore should have +50% Fe / +65% Fe2O3. Sticking to a single ore type / source really helps your initial learning curve!)

(e) Bloom to Bar represents a loss that some people completely forget about in this whole process. Remember the objective of ancient iron masters was the * bar * - this the finished product then sold to the blacksmith (a separate craftsman).
Others working with historic methods have reported the same kind of results (roughly ore to bar = 10% return) The extensive pioneering work by Peter Crew should be mentioned.
I (shamefully) admit that I have not converted so many of my blooms up into working bars. Although I have been attempting to record some of the data of this progress systematically, this project lags (!). Generally I am getting much the same loss rate of about 30% from bloom to bar. Lee Sauder has told me his results are in the same loss range.
Our overall ore to bar results are a bit better, as the high volume air method normally used does increase the yield at the ore to bloom stage.

(f) My own data on iron content is far more varied - and honestly I would suggest less reliable. Although the potential sample size is good (see table o' blooms 2012), I have used a large number of quite different ores, with the expected variations in results. Our objective is always a 'nice soft iron', sometimes higher carbon (into the range of 0.5%) has been the result.

(g) Slap on the forehead time!
I have a copy of this paper (which I suggest all read) via Early Iron Production, Lars Nørbach (editor), ISBN 87-87567-40-7. I had read this back in my initial research work into bloomery iron smelting, well over a decade ago. (It is where our use of the 'thumper' came from!)
The volume however has a mistake in naming authors in the primary index, swapping the contributors of two of the papers. So I ended up missing this reference when I started researching this specific project.

(**) A later addition
I was thinking about what Thijs had said here.
My standard, based on Lee's work and example, has been to set air volume via the formula V = (ID area x 1.2 to 1.5) as LpM

I have never considered applied air against internal furnace volume.

The reduction process is a 'gas over surface' reaction. Although increasing air will create higher temperatures, the space inside the furnace actually involved in the reduction process is actually more important. (That is why the stack height is important. Too little and there is not enough space to utilize the reactive gases. Building too high does not greatly increase the furnace effectiveness.)

That being said - there may be some revealed wisdom in what Thijs has said.
Bears checking back with my own past furnace builds and smelts? I will be curious to see if bellows volume to furnace volume has any relationship...

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

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