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Kevin S.

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PostPosted: Mon 18 Jan, 2010 12:37 am    Post subject: Productivity of the Blast Furnace         Reply with quote

I know that the ancient/medieval Blast Furnace can produce a lot more metal than the ancient/medieval Bloomery Furnace. However, I need to know a more specific answer. Was it 4x, 8x, 10x, 20x, etc ?

Thank you for your answers
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Craig Johnson

PostPosted: Mon 18 Jan, 2010 6:14 am    Post subject: Variables         Reply with quote

Hi Kevin

That would be a very difficult thing to identify. The number of variables are very numerous and would all affect the outcome of a comparison on any two furnaces, let alone trying to figure an average for each style. Such things as quality of ore, size of furnaces, fuel type, skill of operators, construction of the actual furnaces and many more.

Trying to identify a definitive number may be impossible. Sadly we do not have the depth of information on the medieval production capability that one would like to make a solid comparison.

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Jeroen Zuiderwijk

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PostPosted: Mon 18 Jan, 2010 7:23 am    Post subject:         Reply with quote

Actually, a bloomery is a type of blast furnace. Blast furnace means nothing more then that air is blown actively into the furnace, as opposed to natural draft due to f.e. a chimney. The difference between a bloomery and a later blast furnace is that the latter produces pig iron, which can be tapped continuously. A bloomery needs to be oppened up to extract the bloom. So the bloomery is an interrupted process, while the blast furnace producing pig iron can run continuously. However, both can be scaled to different scales. You can make bloomeries that produce tons of bloom at one go, and small scale blast furnaces that produce just kilos at the time. For pig iron, as it is a continuous process, it lends itself well to larger scale furnaces that are more permanent. Large sized bloomery furnaces are less practicle, as you get large blooms that somehow have to be cut to smaller size, whereas the pig iron you can continously cast into small ingots. How large pig iron blast furnaces got I don't know. Usually bloomeries produce blooms in the order of serveral kg to several 10s of kg at the time. Wikipedia gives some known medieval blast furnaces, however it doesn't include sizes or capacity:
Jeroen Zuiderwijk
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Craig Johnson

PostPosted: Tue 19 Jan, 2010 6:44 am    Post subject:         Reply with quote

Yes Jeroen is correct, I fear I just assumed you meant a chimney style furnace versus the forced air variety. One aspect of this discussion is sometimes we assume today that we know how each style of furnace was run. The use of experimental archaeology over the last 30 to 40 years has increased our knowledge of this greatly but we still do not have a full picture. As an example about fifteen years ago I participated in running a Burgenland style smelting furnace. We set up a bellows system but due to the size of the furnace, 1+ meters by about 3 meters tall we found the draft of the structure ran the smelt with no additional airflow needed. In fact our biggest problem was we got the system to hot to early.

If you need a definitive number for comparison I am not sure how one would estimate that with any degree of confidence at this point. But I would say one can look at the experiments of the last few decades where the actual smelting has been done and begin to look at specific examples and begin to see what the result might approach when part of an on going industry in period for specific instances.

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Chris Arrington

PostPosted: Tue 19 Jan, 2010 10:26 am    Post subject:         Reply with quote

Craig and Jeroen are absolutely correct.

In general, the major difference between a bloomery and a blast furnance was capacity and height. This allowed it to retain higher temperatures, and increased draft (which increased temperatures also).

The Making , Shaping, and Treating of Steel is the definitive book on the making of steel. You can pick up older used copies on Amazon (linked) for cheap and its an interesting read. In fact, if you worked in the steel industry it was literally required reading. Or you can most likely find a copy in a library or thru inter library loan.

There is a whole section on the evolution of bloomeries to blast furnances, and describes quite a few of the major produceers from 1350 thru the 19th century.

I can not find any estimates of production from bloomeries in my edition, (there are like 8 editions thru the years), but it states that mid 1800's blast furnaces in the US typically produced between 1 and 6 tons per day.
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Jared Smith

PostPosted: Tue 19 Jan, 2010 3:20 pm    Post subject:         Reply with quote

If we allow all types of production, pig iron, wrought iron, steels of varying carbon content, etc. to be counted as "metal", I expect that the amount of "metal" yield depends less on the style of furnace than on it's overall size. Efficiency (having to reduce initial bloom versus desired final steel the first time through) would be a real factor though. I doubt you would do much better than a factor of 2 or 3 if comparing the better 12th century production with 15th century methods though.

Some of the 11th to 13th century natural chimney draft furnaces in South German regions were actually pretty large. They also used a reducing atmosphere oven to "bake" the cakes after initial production. Several tonnes were produced in batches. There were four or five of them known with dimensions in archeological examinations. What I can remember of the article I read was that their geometry just happened to give a good draft without mechanical assistance. (Hence they approximated a future blast efficiency through coincidence of having just the right geometry. I think Craig's experience indicates a roughly similar current day experience.) They produced enough steel to export throughout much of Northern Europe such that England, Ireland, etc. heavily depended upon importing their "cakes of steel" as preferred metal for cutting tools and weapon blades.

Absence of evidence is not necessarily evidence of absence!
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