I have a character with a spell called Smite. Mechanically it is identical to Pilum of Fire in everyway. Cosmetically a pilar of blue white fire shoots from the heavens rather than normal fire flying from the casters hand.
would the character be able to read a book on spell mastery for pilum of fire and apply that xp to the mastery ability of this spell.
my instinct says no. Its a different spell. But the only change is purely cosmetic. So is it really a different spell or is this more like a sigil change?
Benefit wise, the only advantage is being able to use relatively common books for a custom spell.
If you define it as a different spell then I think that you you paint yourself in to a corner here.
Is there no way that it can be a casting sigil. perhaps with a flourish from Societates? Is the blue fire entirely unnatural or does it just look like a blue hot flame?
The bolt coming from the heavens (probably further away than Voice) rather than the magus seems to add functionality to the spell beyond the cosmetic. I might not argue that the level should change, but maybe the spell simply is different enough for the purposes of mastery. Or not. Instinct is probably the way to go.
I see two things making it distinct from Pilum of Fire.
It appears to be the sky-fire described in Art & Academe, rather than the normal flame you'd find on a torch or in a hearth. Blue fire isn't unnatural in Ars Magica, but it is a distinct kind of flame -- it's what gives the daylit sky its azure hue.
It's an attack from another direction. Pilum of Fire hurls a bolt of fire rather than simply creating fire on the target, and your spell shares a similar flaw but from above rather than from the caster. You could avoid a pilum of fire by crouching behind a low wall, while making this attack invalid would require ducking under some kind of ceiling.
There are some concerns that the spell appears to come from the heavens. It still only affects valid recipients within voice range. Are these differences cosmetic or does it allow the spell to do more than a Pilum of Fire? In my opinion, they are cosmetic. Pilum of Fire doesn't state that the pilum springing from your hand is a cosmetic effect, but Ball of Abysmal Flame does. If it's only the difference of a cosmetic effect, I'd be inclined to allow Pilum of Fire mastery texts to work.
I don't think the fire appearing to fall from the heavens or the color of the fire is more than a cosmetic effect.
I am with Jonathan.Link, all that differences are cosmetic, if the Voice range and the damage is the same, i think tahn still can be studied from Mastery books. Just like the mage with the blood stuff and his Aquam spells.
As stated in ArM5 p.87, under "Books on Spell Mastery", the spells are considered the same "if they have the same Arts, level, Range, Duration, Target, and effect."
The only question here is whether having the fire appear to come from the sky (and having the flames be blue) make it a "different effect."
I don't believe it does. The effect is still a direct damage to the target.