I think I understand your point, but I still think Ferrezt's idea is still worthy in his own right. If you would be as so kind to let me explain how I see it:
Where is it written that a CrIg spell need a tangible support to exist ? Given sufficient duration, would not a CrIg last in mid air ?
I think that is the heart of the intuition of Ferrezt, as I understand it. One could cover a magical effect (magical effect, spell, ot whatever, as long as it can have recognisable boundary) with fire.
I said there was no need for a Vim requisite, based on PoF. A spell like Blade of Virulent Flame (CrIg15, ArM5 p.140) would have been more appropriate.[size=75] (But it's still a good exemple ! Arc of Fiery Ribbons, for exemple, should not be Target: Group, but should have +2 size, since it's really one fire created that is the target, not many persons...)[/size]
It cover a metal blade with flame and has no Terram requisite.
I used the rational of this last spell to cover with fire a magical effect such as the Parma. The Base GuideLine for +5 damage is 5. ("...in unatural hape, such as a ring, ..."), the same guideline as for BoVF.
I think that a Rego requisite (w/ +1 mag.) to make sure the flame doesn't turn inward is reasonnable. (see Coat of Flame, same page) ( Maybe the req. is not needed if the effect doesn't penetrate the caster Parma?)
I don't see the need for a breakthrough for such an effect, since it could be allowed by the RAW, and doesn't seem to damage game balance.
It's just...original =)
But for now, I see no difference between such a spell and one another how would simply create a sphere of fire around the caster, for exemple.
Maybe Ferrezt could explain ?