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
