User blog comment:Nagamarky/All Ironclads are Hardened or modded./@comment-2025464-20100620162103

Understood. I assume Impenetrable is the same way about spawning as I have a couple shields with it on. My guess is that the Ironclad is just like the Hellfire. It can only legitimately spawn with a specific prefix. (ie: combustible is the same as hardened.) That is my best hypothesis.