Some of the earliest steel was actually carburized iron, or wrought iron which was soaked at heat in a carbon rich atmosphere. When you etch this 'steel' it will show dark due to the carbon but the material is no more resistant to the acid than wrought iron. It can be hard to get a good etch for contrast because the two materials dont erode at signifigantly different rates. It can also be hard to get crisp looking patterns in this instance for the same reasons. At least that was my experience which was admittedly limited sample wise.
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