Hermetic Projects has some pages about lava, although I believe there are strange bits and some holes in it (attempted to design a Lava Maga based on it, abandonned the idea)
IIRC, its base damage is +15, making full immersion (which is what CrIg spells do, mechanically speaking) +60 damage.
Based on what I remember of the guidelines and exemple spells, creating Lava is CrTe(Ig), affecting lava is Muto/Rego Aquam (Ignem).
For your project, I agree with Johnatan Link: A CrIg spell won't instantly melt rock into lava.
I see 2 possibilities:
1)
IIRC, Blade of the Virulent Flame will melt a blade in half an hour, with a fire doing +5 damage. I'd use this as a base.
Problem is, Ars damage is non-linear, so +10 dam isn't twice as hot as +5 damage. Still, let's ignore it for simplicity's sake.
Likewise, let's assume that stone melts as Iron.
=> You can say that a "fire" doing +15 damage will melt stone in 10 minutes, for exemple. I'd have damage increase for touching it increase by 5 from nothing to +15. So, say, nothing the fist 2 minutes, then +0 for 3-4, +5 for 5-6, +10 for 7-8, +15 for 9-10, and afterwards, the lava is fluid enough that you "sink" in it, which will increase damage.
You can do the same thing with higher magnitudes.
2)
Assume that a CrIg spell doing +60 damage would melt rock instantly (damage equal to full immersion in lava). That’s base 55.
A base 50 spell would take one turn.
Every magnitude lower double the time.
So base 45 would melt rock in 2 turns
Base 40 would melt it in 4 turns
…
Base 25 (+30 damage, as BoaF. I’ve seen people here in whose sagas, this was the highest degree of heat currently achievable by hermetic magic*) would melt it in 32 turns
…
Base 15 (+20 damage) would melt it in 128 turns, which is about… 12 to 13 minutes.
Damage is easier to compute: Just assume full immersion, whatever. You’ll always have radiant heat.
At half the duration, the spell does +30 damage. At ¼, it does +15.
So your base 15 spell could do something like:
3 minutes: +15 damage: The rock is really hold, it hurts and inflame shoes.
6 minutes: +30 damage: The rock is melting, people are knee deep in lava
9 minutes: +45 damage: mostly liquid lava, people have more than half their body immersed
12 minutes: +60 damage: fully liquid lava and immersion
- consult with your SG on this. If it isn’t, don’t bother. If it is, an easy way for your character to become famous as a pyromancer would be to engage in (IMO quite easy) research to expand CrIg guidelines so as to create hotter fires. Burn, baby, burn!