Full Chisel Blog

February 4, 2013

Charcoal Iron & Steel

charcoal pile

Just read an interesting section in Material Culture of the Wooden Age, [1981] edited by Brooke Hindle, the article is entitled ‘Charcoal Iron: The Coal Mines of the Forest’ by Richard H. Schallenberg.  The article talks about many things including the process of bloomery and blast furnace iron powered by charcoal, comparing the differences between coal/coke iron and charcoal iron, the former containing sulphur and phosphorus introduced by the coke, these contaminants, together with more absorbed carbon contributing to a more brittle iron.

England and Europe with the exception of Sweden had converted to coal/coke iron processing in about 1815 due to the lack of wood to make charcoal while in America charcoal iron was produced until about 1945, due to the great abundance of wood for making charcoal.

‘Blast furnace iron was better than bloomery iron in all these respects.  It did have two serious drawbacks, however-the pig iron which poured from the blast furnace was hotter than bloomery iron and therefore contained more dissolved carbon, and the pig iron solidifies from a fully liquid state and therefore had a more crystalline structure than the bloomery metal.  Both these conditions made furnace pig harder and less malleable than bloomery bar, and thus it was a less useful iron in the manufacturing process of the day, which almost always were some form of forging.  Therefore, most charcoal furnace pig was ‘refined’ before being sent to market.’

‘Moreover, the hotter the iron, the more the dissolved carbon tends to form the chemical compound cementite [Fe3C], which is glass-hard and brittle.  Finally, the grain structure of coke and coal irons is coarser than charcoal irons, caused by the higher concentration of silicon in these metals.  Larger grain size makes the iron weaker, and also makes it more difficult to heat treat.’

‘To correct at least some of these problems, rather drastic refining techniques are needed for fossil-fuel-smelted irons.  The pig is melted, boiled, and the impurities burned out or chemically reacted with additives or refractory linings.  In the Bessemer and open-hearth processes, it is also alloyed to counter some of the bad properties.  Charcoal pig, however, lacked most of these drawbacks. And therefore did not need such extensive refining.  The charcoal pig was heated until it became soft and then was beaten under a trip hammer.  A certain amount of carbon was burned out in this way, as it was in puddling, but the main function of the continual hot working of the iron was to make it less hard and more ductile-the properties needed for forging.  That is, the heating of the pig iron ingot elevated the metal above what metallurgists call the recrystallization temperature.  If iron is mechanically worked above this temperature, the grain size is reduced, graphite particles are more uniformly distributed, and dislocations in the crystal structure do not produce hardening, as they would with working cold, rather relieve stresses and make the metal easier to work-i.e., make it more ductile.  Therefore, in the charcoal refining process the metal was not heated primarily to burn out the carbon, sulfur, and phosphorus, as was the case in puddling and Bessemer processing, but was heated so its internal structure could be rearranged above the crystallization temperature.’

‘Moreover, the repeated working of the refinery forge served also to spread thin filaments of slag throughout the mass of iron, giving the metal the fibrous, tough, shock-resistant and readily weldable properties characteristic of all true wrought irons.’

Better iron, better steel.  This method was used to make the Viking ‘Ulfberht’ swords.  I wonder how the Japanese made their iron and steel?

Stephen

 

5 Comments »

  1. Stephen,

    The Japanese didn’t have easy access to iron ore, their only indigenous source of iron was basically black sand. Same thing for the coal, it was mostly pine charcoal. They would normally recycle any broken piece of iron/steel laying around.

    The process would start with smelting in a forced air vertical furnace that would produce a large-ish bloom with various carbon content steel. The bloom would be broken up and the various bits would be sorted by carbon content ranges. Those groups would then be stacked and forge welded, beaten and folded to homogenize and remove the impurities.

    [gross simplification ahead, that is only one of the construction methods]The lower carbon bars that resulted were used for the back and core of the swords, the higher carbon bars would then make the sleeve that would be forge-welded to the softer core/back.

    The blade would then be differentially heat-treated using a “proprietary” mix of clay, ash, … in order to change the cooling rate when quenching the blade. This had several effects: produce a visible pattern in the crystaline structure of the steel, introduce the sword curvature, have a different hardness between the cutting end and the back (durable sharp edge, while avoiding brittleness).

    http://www.youtube.com/watch?v=-mmPMSK-pl8 Fast forward to about 36 seconds to skip the ads. Unfortunately, there’s no subtitles but you’ll get the idea.

    I hope this summarily answers your question,
    Michael

    Comment by Michael — February 4, 2013 @ 1:56 pm

  2. the 1945 date is extremely suspicious. I would suggest a misprint and it’s 1845. Even if wood charcoal was being used in 1945 it would have been in an extremely limited application.
    Ernst Schwarzkopf even remarks around 1910 or so that the mild steel then available wasn’t as nice for welding as old iron.

    Comment by joel — February 4, 2013 @ 3:10 pm

  3. Michael,
    I had heard that the fine Japaanese swords used Swedish steel.

    Joel,
    Not a misprint, the dates in the article were from the defeat of Napolean to the defeat of Hitler. The output was small but still preferred for some applications.

    Stephen

    Comment by Stephen Shepherd — February 5, 2013 @ 8:42 am

  4. PBS has interesting videos on both the
    Ulfberht, and the Samurai sword.

    Ulfberht:
    http://video.pbs.org/video/2284159044

    Samurai:
    http://www.pbs.org/wgbh/nova/ancient/secrets-samurai-sword.html

    Comment by Brian — February 18, 2013 @ 8:24 am

  5. Stephen,

    while there have indeed been a few instances where Japanese smiths used foreign steel, it hasn’t been very common in the grand scheme of things. You have to remember that the country was extremely isolationist from the 16th to the 19th century and that weapons got banned soon after the opening. The ban got lifted in the run up to WW2 and reinstated at the end of the war. Antique swords of “significant cultural value” get an exemption from the ban as “cultural artefact”, but swords made from foreign steel are strictly categorized as weapons. Many antique swords judged of insufficient quality are routinely destroyed every month still.

    Now, there’s an extra caveat: Japanese smiths making swords outside of Japan for the foreign market can use whatever they see fit but the sword mustn’t re-enter Japan or it will be confiscated/destroyed and the owner fined/jailed. Japanese smiths can also forge a blade blank from any metal they see fit and ship it abroad to be finished, as it isn’t legally a weapon while in Japan. There’s a lot of young Japanese smiths making ends meet that way, as the Japanese market has been winding down for decades and it’s hard to make enough money to live.

    Sample of some fine quality Japanese blade: http://www.aoijapan.com/tantobizen-koku-ju-osafune-sukesada I actually visited the forge complex where this knife was made 🙂

    Michael

    Comment by Michael — February 22, 2013 @ 6:46 am

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