Could a MuTe spell change something made of stone, like a wall, in such a way that one could look through it or walk through it? In other words is opacity and/or solidity properties of stone that can be affected by Muto?
If so, what magnitude/level would such a spell have to be? Bear with me if there is an easy to find example of that in one of the source books. It's been a while since I played (4th edition at that time).
for a stone wall
base 3 ( the level 2 guideline with a one magnitude bump for stone and glass rather than dirt)
so level 10
Here's a spell from another thread to allow someone to walk through a wall by making a passageway through it.
If you wanted to drop the applicability to metal and wood containing walls it would be level 15, the right spell for one maga, (here, Andreva of Jerbiton) isn't necessarily the right spell for anyone else.
If you want to take the path of actually adjusting the material's opacity or solidity rather than changing it into another material (which in this specific case would be easier) there are these guidelines from page 154 of the core book that would work for you.
Or perhaps a better answer to your question would have been "Yes" because brevity is the soul of wit.
From the core book. Seemed to be a bit in conflict with the slightly or highly unnatural.
I suppose both modifications are highly unnatural. Would you add requisites? Imaginem for the opacity? Not sure about solidity... If it were about people passing through I'd say Corpus, but that doesn't seem right.
MuTe(Im) so the wall doesn't block species. MuTe(Co) so your wall is permeable to people, MuTe(An) to allow only vermin, MuTe(Au) to give your wall the permeability of a cloud (also you may have structural collapse when that happens). Magnitudes may vary based on that.
The Permeable Wall
Part of a wall, where the caster touches, becomes insubstantial, but not invisible. Objects and people can move through the wall, but it maintains its appearance and opacity. The permeable section is about two paces high, one pace wide, and a half-pace deep. Multiple castings may be required to open a way through a thicker wall.
MuTe20 Base 4 + 1 stone, +1 Touch, + 1 Diameter, + 1 Part.
The Transpicuous Circle
The part of a wall within a traced circle becomes transparent, and can be seen through as if made of glass.
MuTe15 Base 3, + 1 stone, + 1 touch, + 2 ring
I have a question related to the OP's post. What would the difference be between using Muto Terram to transform a stone wall into a transparent material, and using Perdo Terram to destroy the opacity of a stone wall? They seem like similar effects, but the Perdo version is far more difficult.
My vison of the setting is that hermetic magic is a creation of magicians rather than the rules of nature. From this point of view it is completely reasonable that the spell could be done using both perdo terram and muto terram and that doing it one way is harder than doing it another.
Metacreator notes the spell at lvl 15, but the design says: Base 4, +1 Touch +2 Ring, Base 3 is "change dirt so that it is slightly unnatural" so it seems the +1 mag for rock is in there. But should the Target be Circle? The description talsk about making "a circle of wall transparent".
The interesting question about Window of Singular Direction is: How thick a wall will it affect? If it does use T: Circle one interpretation is that one views it as a half-sphere so the depth the spell affects is the same as the radius of the circle. But this is not RAW.
And speaking of MuTe spells and lacking +1 mag for rock, what about Rock of Viscid Clay? It uses Base 3 without added magnitude for rock, but it is meant to affect rock.
If you look at other Perdo guidelines - see PeAq 15 "Destroy one property of a liquid (like alcohol's ability to intoxicate or sea water's saltiness" on p123 or PeCo 40 "destroy one property of a person, such as their weight or solidity" on p133 - other forms allow you to destroy individual qualities of an item, but it is much more difficult than with muto magic.
MuTe lists at level 1 "change one property of dirt" so it must be able to change properties. The listed spells frequently use base 3 "change dirt so that it is slightly unnatural". Changing stone to glass would be the level 2 "change dirt to another type natural earth" but with +1 magnitude for affecting stone or glass.
If you use PeTe to destroy stone's opacity then you only destroy its opacity. Otherwise it is still stone, not glass. If you did this to, as an example, granite, it would still be rough, heavy, and strong, with a significantly higher melting point than glass. If you destroyed granite's solidity it would be permanently liquid (unless the Pe effect were limited in duration rather than being moment duration), where if you used MuTe you could change it to a liquid, pour it in a mold, then return it to being a solid.