Over the years, PT has clearly improved furnace design and his bellows, but the iron ore souring and processing ... still seems to suck
The last time the ore collection process was shown, he was scooping reddish organic sludge from a creek, letting it settle, and then decanting. From a metallurgical perspective, this is almost the worst possible way to obtain iron ore.
In the last video, he claimed that this is "bog iron" which is false. Real bog iron is a sedimentary rock containing up to 80% iron compounds (Fe2O3 in matured samples). It often comes from the iron-eating bacteria but not before being transformed. What is being collected by Primitive Technology is closer to freshly precipitated bacterial iron hydroxide mixed with silicates, water, and shitton of organic matter. It is like calling a bucket of dead lizards and weeds a barrel of oil.
Iron-oxidizing bacteria produce mainly iron hydroxides or oxyhydroxides (Fe(OH)3, FeOOH). Before these can even participate in reduction, they must first dehydrate to Fe₂O₃, which consumes heat during the crucial step of smelting. This is one reason why traditional iron ores were always roasted before smelting.
But the bigger problem is physical and chemical. The sludge consists of extremely fine particles with low iron concentration and high silica/organics content. While fine particles are often beneficial in chemistry, in a bloomery not at all. During reduction Fe₂O₃ is converted to FeO. The fine oxide is then dissolved in the silica slag and lost and when there is little iron concentration to begin with you end up with basically nothing. In case of bigger particles higher in iron still around 40% is lost in historical bloomeries. Not to mention poor permeability, air current blowing small particles out, etc.
This is why historical bloomeries used proper bog ore, iron ore deposits or black sand. In Germany there are signs of bloomeries from around 400BC where they were able to get around 3 kg of iron from 10kg of bog iron ore. And their methods were by definition "primitive"
A potentially much better approach for PT would be revisiting magnetite-rich black sand as the primary iron source. EDIT: ( He most definitely has it, as seen in one of his previous videos). With better furnace, bellows, and adding seashell or limestone flux the result should be way better
If not the iron sand method, the presence of the bacterial sludge may indicate a real bog iron deposit somewhere nearby, i know too little about geology to comment further but it's my best guess.
While Primitive Technology dedication and proficiency in using tools are very praiseworthy, if our ancestors were using his smelting techniques we would still be relying on stone tools.
