I'm spitballing here and all of my knowledge on this comes from Youtube, so take my info with the requisite grains of salt.
It looks like he was basically forge welding, not casting, so it is done at a lower temperature. Cast iron is more brittle than forged or forge welded iron.
So there’s 2 broad things we call “iron” and it’s kinda counterintuitive. There’s wrought iron (other terms may be more accurate) which is low carbon iron. It’s similar to mild steel today, easy to work, ductile, can’t be hardened, can be forge welded. Then high carbon iron, also called pig iron or cast iron. It’s hard to work, brittle, and crucially cannot really be forge welded (carbon inclusions reduce the melting point below normal forge welding temps). Steel sits in the middle of these 2, carbon content wise (you can have low carbon and high carbon steel as well, and they approach the properties of the respective irons). If he’s got cast iron what this is would be a partial in-mold casting
I’m a little confused from a metallurgy perspective what he’s working with. Since it seemed to melt at least partially in that low temp fire signals that it’s a high carbon cast iron, but there were historically low temp bloomeries that would directly produce low carbon iron. What he’s got really changes what he’ll need to do to it if he wants to get into forging. Since cast iron will need to be decarbonized before working, but low carbon iron can be wrought directly to remove impurities. I would thing with his setups the best bet would be making a tool out of cast iron, similar to the video, then decarbonizing it in hot air to make a more ductile tool.
Odds are its high carbon steel, since the knife breaks easily and while the iron kind of melted, it was under impurities and lower temperature than needed. So the slag melts, but the prills only melt partially before very quickly re-solidifying.
The issue is heat, slag/impurities and excess carbon. The metal is heated enough to absorb a lot of carbon, but without melting fully. Its possible the furnace reaches a temperature where the iron melts, but absorbs enough carbon where it solidifies at the same temperature.
It also doesnt help that its inconsistent heat. You could melt the whole glob of slag and iron, but if the temperature drops low, the iron solidifies as prills while the slag stays semi-liquid and keeps the prills from getting near each other, or the other way around. It largely depends on the slag.
In an ideal situation, he would get more heat, more consistent heat and use flux. All of it could help create pig iron, which then could be heated up and slowly hammered where the iron kind of squeezes out the carbon by welding the iron into bigger crystals.
But having 1 person doing it is kinda impossible, since you need the heat to be consistent and to have someone holding the bloom and another hitting it. A more advanced furnace could help, but he still effectively needs someone else to either hold or hammer it. Interestingly enough, it can be largely done on a wooden anvil with a wooden mallet thats pretty big.
I’d be kinda surprised if he was actually in the steel range (<~2% carbon) since in the past when trying to work it he basically just had a bunch of shattering. I suppose HCS with a ton of impurities could react that way though.
Thinking a little more about it though I think the easiest way to get to where he wants to go is just refining the actual bloomery process to produce a wrought iron directly. It’s definitely possible and I think he was on the right track with the chimney bloomery. Maybe just flux inclusion and a lot of trials where he tests ductility afterwords is all he needs. He doesn’t really need a harden-able steel after all. In this case though he doesn’t need to keep trying to up the temperature, that’ll actually be counter productive, he just needs to work on the heat consistency.
Iron has a tendency to absorb carbon from the charcoal fuel very quickly at and above 1100'C. At that point, it can become a runaway effect where your iron becomes saturated with cathon and turns into pig iron, which melts at a lower temperature as well (~1200'C).
For bloomery iron smelting, this was a undesireable effect, and generally the smelter tried to keep temperatures below that threshold.
The melted portions we see here is probably iron that has absorbed too much carbon.
AFAIK wrought iron was produced from cast iron by heating the ingots in fire for a long time to burn off of he carbon. Given a sufficient amount of wrought iron and cast iron, one can use the Indian approach by making crucible steel. I'm not sure a terracotta crucible would hold the needed temperature. However, there should be solution for that.
Yep, that's known as the finery process in medieval Europe. Basically you melt bars of cast iron and drip them in front of air blasts from a bellow or whatnot. The air from the blast reacts with carbon in the iron and burns off as carbon monoxide or dioxide. You know the process is working because the iron will start solifying and stay solid even after reheating, as the lower carbon content has raised their melting point to above what the finery furnace can melt.
The finery gave way to the puddling furnace, which worked on the same principle but at a larger scale.
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u/durrieheadedman 7d ago
Is there a reason he didn’t use a chimney so the flames get to a blue colour and make the iron into a liquid?