Serf's parma here, and I can't remember if this has come before:
If I want to change fire into stone, no problem, I'll cast a MuIg(Te) spell.
But what if I want to create a flame that is a flame, acts as a flame (burns, lights, moves...) but is also solid?
For example, if someone is determined enough, she can just push through a regular Wall of Flames, taking some damage. With a solid wall, the wall would still burn, but one couldn't get through it.
For this, is a MuIg effect enough? Or does it also needs a Terram requisite?
My first guess would be that it could be done with MuIg(Te) because Te controls solids ( from A&A). The logic behind it would be using Te to attract more of the particles that make things solid and heavy.
If you do not want to use A&A I think your MuIg would be fine to make the fire unnatural.
You could say that you're turning a wall of fire into a wall of stone that is as hot, bright and flexible as the original fire. So a MuIg(Te) spell should make sense, I think?
Base on the guildeline, it looks like a MuIg Base 10: turn a fire into an unnatural example of another element. A wall made of fire is definitely an unnatural version of a wall of stone, yet it maintains the solid attribute of stone.
It will require a size modifier of at least +1, if not +2 for size increase since the base size is a large camp fire (around a cubic meter or two).
Range: +1 for touch, or +2 for voice (much safer IMO).
Target: Either regular + size modifier, but +1 for Part would make sense
Duration: +1 For Diam, Mom is only good if wou want to set fire to what is touching the wall.
So Earthly Blessing of Mount Vesuvius: MuIg(Te) Base 10, Part & size increase +3, Voice range +2, Diam +1, requisit is free (based on Trapping the fire) 40
How much should be the damage bonus ? +15 like the Prison of Flames or more (because of the higher magnitude) ?.