RomanArmyTalk

The title implies the question of the thread; what techniques will get to the low to no slag content of Roman iron and steel that we know of Romans using? The traditional explanation of hammering the iron while it was hot will only get it down to 5% before the slag is trapped (Simms) yet Roman Iron and Steel has significantly less if any slag when examined (Sims Roman Imperial Armor) so what technique would make up the difference? By what technique I mean what technique that we know Romans knew about.

Blast furnaces. I have no problem believing that the Romans developed this kind of technology.

I agree, this or possibly some form of mechanization.

I'm not sure that a mechanised hammer mill will reduce the slag content any more than manual hammers. It would just perform the bloomsmithing process more quickly.

Edit: but I'm pretty sure that manual hammering can reduce slag content below 5%. It just takes a lot of refinement.