There existed many variations in iron composition both in terms of content and carbon depending on the ore and or smelting process. I think your guess is a good one. There was also carburized iron which was variously referred to as shear steel or blister steel. The iron would spark but would still pickle to show grain structure. I made some iron/blister steel damascus but couldnt get good contrast. At the time, I thought it because i was doing something wrong but later decided it was that the two materials had very similar (too similar) composition. You also could have a situation where a bar was over worked and you had carbon migration for steel to iron which would effectively make blister steel as part of the welding process.
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