Underwater fire? Bug or WAD?

[Ewierl]

The charcoal shield seems like a mistake because it's inconsistent- the spell doesn't work, but an item generating the spell does work. "They're different" arguments seem deflated by the way both item and spell produce an identical effect (when functioning).

One can, indeed, imagine those items working either way, in and of themselves. But consistency isn't a narrative "why?" question, it's a system question.