Forging the Bone Cover base level

Hello, i wanted to ask if the base level on this spell is correct.
Forging the Bone Cover
MuCo 5
R: Touch, D: Conc, T: Ind
This spell changes the inelastic nature of bone, making it elastic and able to be reshaped under pressure. The skull becomes as pliable as clay for the duration of the spell. Once this spell is cast, a human skull can be pressed into a book-shaped wooden frame. When the spell ends, the skull reverts to its inelastic property, but retains its new shape. If later used in an enchanted item, the reshaped skull still gives the magus the same Shape & Material bonus (ArM5, page 110). (Base 3, +1 Touch, +1 Conc)

Base Level Interpretation: A base level 1 Muto Terram spell will change one property of dirt. The base level to create dirt is Creo Terram 1, and the base level to create stone is base level 3. A base level 3 Muto Terram would change one property of stone. Bone is harder than dirt but softer than stone, so a base level 1 would not be enough, but base level 3 would. Thus: Muto Corpus base level 3, change one property of bone.
Hermetic Projects p.103

Woudn't changing one propriety of stone be the base guideline "Level1: Change one propriety of dirt" with a +1 magnitude to change stone as the book sugests? (The targets below are for changing dirt. To change to or from sand, mud, or clay, use the same level of magnitude as for dirt. To change to or from stone or glass, add one level of magnitude. To change to or from metal or gemstone, add two levels of magnitude. These adjustments only apply once, so that changing metal to metal is only two magnitudes higher. ArM5 p.154)

Bone is covered by Corpus, not Terram.

Sometimes in this game, it is necessary to look to other Forms when creating guidelines in addition to those provided in the rulebooks. IMO, there should have been a "change one property of" in every Form. IOS there eventually will be, when I "finish" the project.

If it were me designing the spell posted by @firegraid I would have probably also looked at Herbam and split the difference between wood and stone for bone. YMMV/SMV, as always.

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My answer was less helpful than it could have been as I misunderstood the intent. Lucas was quoting an existing spell and the explanation to justify the guideline that accompanies the spell. This actually looks like errata material.

I think whoever created Forging the Bone Cover actually used the same guideline as "Rock of Viscid Clay" as a parameter for comparison. It is MuTe 3, which if I understand correctly uses the Base 2 "Change dirt to another type of natural earth" to change dirt to clay, +1 to change from stone to clay instead.

However, the explanation in Hermetic Projects clearly doesn't conform with this. I also don't know if the explanation itself is logic, bone isn't covered by Terram, indeed...

But I think MuCo 3 "Utterly change the appearance or size of a person" is appropriate to make bone as pliable as clay, since changing the face requires modifications to bone structure. I would say the base level is correct, but not for the reasons stated in HP.

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This is the closest Guideline I thought of when I first looked in on this thread and I believe it is the appropriate one to base the spell off of. I agree with Rafael that the Base Level of 3 is appropriate, even if the given reasons in the book are not.

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