Using ReTe to make caves

I'm just about to start playing in a new Ars Magica campaign, and the first story arc is going to focus on the establishment of a brand new covenant, in the Lake District of northern England. I'd looked at learning the CrTe spell which lets you create a stone tower, but I was wondering how you'd go about making a ReTe spell which would excavate a cave system in a natural cliff face.

My PC has the Elemental Magic virtue (so adding additional elemental requistes doesn't increase the magnitude), and so one option which has occured would be to find a natural system of limestone caves with a stream in them, and use a ReTe(Aq) type spell to direct the natural water flow to sculpt out the caves for us.

My two big questions at the moment are: 1) Does this qualify as controlling stone in an slightly unnatural manner (Base 2) or a a very unnatural manner (Base 3)? Manipulating a stream (if I went with that option) would seem to be Base 4 (control a liquid in a forceful but calm way). 2) How do I incorporate the size of the cave into the spell? Do I work off the base Individual for Terram of 1 cubic pace of stone, and if so, how do I increase this for a Terram spell (presumably Room and Structure both apply very nicely to existing caves; could I raise the magnitude to Structure and make a nice big cave system that way?).

Essentially, can anyone clarify exactly how you'd use Rego Terram to make a habitable cave system, either from solid rock or by enchancing an existing cave system?

Many thanks in advance.

Tricky. I think, in canon, what you are trying to do is a PeTe effect, much like "Rusted Decay of Ten-Score Years", where you are trying to speed and control erosion. You might look at "The forgiving Earth", with +1 magnitude for stone. Break the stone apart, with the water washing the stone away, with a finesse roll to shape the rooms. Yes, that could work.

According to the Covenants book (p 50), "Rego magic can make any change which a mundane craftsman could perform... (even without tools, time or skill)..." - which would cover carving out a cave.

And since the main restriction on Rego Craft Magic is Finesse and the quality of the final product, and "a cave" can be abjectly crude and still be perfectly functional, I think the only limit, therefore, is Target, time, patience, and a place to dump the waste stone.

(Unless the SG/Troupe just don't want to go there.)

Such spells are often spontaneous, but nothing prevents a formulaic version. (If you do it right, it will not only create the cave, but rough building stone as well - the spell focusing not on creating a hole, but on creating the stone blocks. Even without much Finesse, if you turn half to rubble, what do you care?)

IMS we just did that. We alos negotiated with the local noble to provide him with stone for his castle under construction. The result is that we create a nice place to live and get benefits from crewating the covenant that go beyond a simple roof over our heads :slight_smile:


I think if you want to use Rego, you don't bother with the Aquam and just use ReTe craft magic to dig a mine. My big fear though is that without sufficient craft skill, digging the cave may result in a cave-in, damaging your maga's reputation immeasurably among the survivors. Likewise, a PeTe effect without a Rego requisite may lead to a collapse.

Off the cuff I'd say something like this would work:

Carving the Convenient Cave
ReTe 20

This spell carves out a single continuous "room" or tunnel in an area of stone or earth. The room/tunnel may be of any shape desired, but must fit within a cube 10 paces on a side. The resulting "room" has stone arches and support pillars where a mundane crafter might have put wooden supports, as long as there is sufficient source material for them. Digging in sand or a bog or other area where the spell can't reach the bedrock or other source of stone results in an excavation that is very likely unstable and may collapse immediately. Assuming the area is suitable, the caster must make a minimum finesse roll of 12 or better to ensure that the structure is well designed and safe. A particularly complicated floor plan may require a higher finesse roll. A failure is obviously unstable, and needs additional magical or mundane work to be made safe. A botch looks perfectly safe until it is too late. The removed material is piled behind the magus, soft material is piled into a large flat mound, stone is cut into rough blocks suitable for use in building.

Finesse roll: 12 (Base 6 for mundane crafter, +3 for craft magic, +3 for work that would take a mundane a month)
(Base 2, +1 Stone, +1 Touch, +1 Part, +3 Size)

Notes: I am away from my books, so the size may be way off.

What does the Rego requisite do to prevent a cave in?

I'd think, if anything, an Intellego requisite, to know how to avoid the cave in (tho' that's a bit after-the-fact as well).

Allows some of the material to be used to create bracing and supports around the area you are removing.

Ah - a combination of Perdo and Craft magic - right you are!

But the problem of ignorance remains - how the mage knows where to create these (unless they are somehow factored into the spell itself, an "over-engineering" function of sorts. Meh.)

There is no ignorance problem. Terram is the Hermetic knowledge of how to manipulate, create, destroy, etc things made of stone. Terram is the knowledge that you need for this job.

(Emphasis added)

From this it appears that Rego craft magic has the necessary knowledge "built in" under the RAW.

However, I think it's a bit of a handwave to say that the wizard doesn't actually need to know what he is doing. A magus that has no idea how leather armor is made can pop off a spont ReAn and convert a cow corpse into a cuirass without the first notion of what that transformation involves or how it could/would be accomplished. Rather, I think that a more logical system would be that Rego Craft magic requires the magus to have the relevant Craft skill, otherwise he doesn't know what to do with the magic. Regardless of how skillfully he can wield the it (i.e. pass a Finesse check), if he doesn't know what to do with the magic, it is not very useful. To my mind Finesse applies to how artfully an effect is brought about, placed, or manipulated. Although somewhat unwieldy, I can see Craft magic requiring two checks (a Craft check for "do you know what to do" and a Finesse check for "can you actually do it") for each use of Craft magic.

All of that aside, the rules on Craft magic are pretty clear that you don't need skill, just Finesse.

Yes, all Hermetic knowledge has "built-in" the knowledge to use it. Hermetic knowledge is the knowledge to use magic according to Hermetic theory.

What is wrong with that?

A magus who has a score in Rego and Animal does know something about how to convert a cow into leather armour. The magus knows how to do that transformation via Hermetic magic. That's what he learns (amongst other things) when he studies Rego and Animal. His Rego and Animal Art Scores measure how much he knows about, and how good he is at, controlling things and doing things to animals using Hermetic magic. The fact that he does not (or does) know another way to do the transformation is irrelevant. He wants to use Hermetic magic. So all he needs to know is Hermetic magic.

In your saga perhaps, but not from anything the rules say.

"Wrong"? Nothing - if it's your saga. If it's someone else's then they have to make their own decision.

But it's not about the transformation - it's about the design. The mage gets what they want - but do they always know what they want?

What might be wrong with it is that there is a lot of knowledge that can work in addition to magic, and your ruling overlooks that. For instance, Architecture - a mage can build a castle, but will it look good? Engineering - Will it really be strong enough? Strategy - Will the defenses be wisely placed? Maybe - maybe not.

If two magi try to Rego a Castle and they both have the same "Magic Knowledge", but only one has these skills - will the castles be equally good? They'll both be what the individual mage wanted - but will each know what the best thing to make is? Will they both know enough to want the right/best thing they can produce?

You can handwave that - but that's exactly what it is, a handwave. Nothing wrong unless you feel it's wrong, and then that's the only reason you need.

Um, the rules in Craft magic don't discuss carving out a cave, they address "making" things.

Finesse is needed to turn out a high quality finished product, in the shape the mage wants. But Finesse will not warn a mage if a cave-in is imminent, nor if they ceiling they have planned is insufficient to hold its own weight.

With high Finesse, a mage can make a eggshell-thin glass staircase or suit of plate armour. But no level of Finesse will stop the inevitable if someone actually tries to use it.

The skill tells the mage if what they want to build is structurally sound. Finesse just lets them make it look good until it collapses.

Now, that can be inconvenient, and so SG's/Troupes may want to ignore that detail - fine. But a mage digging out a cave with Rego and Finesse is no diff than a PeTe spell or a team of mundane miners - except the miners probably know what they're doing from experience.

Thanks very much for the suggsetions everyone - this seems much better than making a tower, as it doesn't cost vis and gives us a bunch of stone to use ourselves or sell off. My PC has a decent amount of Intellego as well as Rego, so I can survey the existing rock beforehand, but finding a miner to advise on what to carve sounds like a solid plan. Time to go check with my GM on exactly how he wants to handle the carving a safe and nice cave aspect.

There is also of course the option of using MutoTerram to change a cubic area of stone to liquid.
The original spell "Rock of viscid clay" need only tiny modifications for this, or can be used as is, together with previously mentioned water magic to move the clay aside.

I think the rules concerning craft magic in Covenants do have something to say on the topic; I believe they take the view that Finesse rolls should be involved with craft magic to determine the quality of the result. As a result, I would take a middle ground, and say that the Finesse roll a magus makes after casting a cool "make a cave" ReTe spell would probably be what determines how structurally sound/aesthetically pleasing the cave system is. (Plus the magus' sigil, of course.)

I had considered using Muto, but I'm starting with a Muto of 0 and a Rego of 7, so... :slight_smile:.

Aesthetically pleasing, certainly. And "exactly according to plan" as well.

But Finesse won't stop a mage from making a rookie mistake in the plan, one that's not obvious until the deed is done. Like trying to make a violin from balsa wood - the mage can do it, but when s/he tries to string it s/he's going to be in for disappointment when it crushes. Now, you can say that Herbem would teach a mage to know that, and that Terram would teach a mage to know how to avoid/spot cave-in potential - that could work.

But I, for one, just can't see Finesses in and of itself doing the job, not as it's described anywhere.

I've told the story before, but I'm reminded of a player/mage in one a saga who decided that the bottom 20' of "Mystic Tower" were being wasted, and so designed their version of the spell to be 100' above ground. (As described, it's 80' tall w/ a 20' deep foundation.) They didn't create it on bedrock. The SG was kind, and it started to lean over the same day they cast it, before anyone had a chance to move in.

The fact that a mage can do something doesn't mean they've thought it all through, nor understand the more "subtle" points of the end result.

ArM5, page 77 middle column, says: "Your Scores in the Arts represent your aptitude for working with the various types of magic". That is, a magi's aptitude for doing things with Rego Terram magic is his Rego Terram Score. His Profession:Architect Ability Score is not relevant.

Remembering about foundations is hardly specialist knowledge. It sounds like a terrible SG that would let a player's character spend a season (or more) researching a pointless ritual. I think that a sensible SG would just tell the player that their character realizes this is a very stupid idea. Or (I think better) rule that while mundane, medieval craftsmen cannot create a tower that stands properly without foundations, a magus using magic can. Because it is magic.

Read what it says about Creo on page 77 in ArM5:
"...magic can heal a natural thing even if the caster has no idea what is wrong...a magus can only create something that he knows about...[but] A magus need not be able to create an artificial item by mundane means; he only needs to be somewhat familiar with it."

There is no need for a magus to understand in detail how something is done mundanely in order to use Hermetic magic to do that thing.

The rules on Craft Magic in Covenants are very problematic, in that they strongly imply that no mundane skill is needed to create an object. I have always ruled, however, that without mundane understanding of the object you are creating, what you create may be elegant and perfect, but may not account for unknowns. You can create a Platonic form in abstract, but if you summon a wizard's tower over unstable bedrock or build caverns in unstable shale, there will be long-term implications, as the magic performed its role perfectly, but the magus was ultimately misguided by his own ignorance in getting himself there. You can find spells that go both ways, but I would suggest the approach of needing mundane knowledge to contextualize magical effects--no matter how perfect you rmind control spell is, if you don't understand people and Church politics, there is going to be a problem, and similarly for crafts (perfectly-assembled ship that was not appropriately seasoned, anyone?).

I have always ruled that knowledge of the Forms give you also mundane knowledge as well as non mundane one. A magus with rego herbam 15 knows how to work a field and the yearly cycle of crops. Thinking otherwise is just nonsensical to me. We have used Int + Form/3 rolls in the past when determining if a magus knows something in that area. Magic is also about knowing mundane proprieties of the stuff you work with, and how it will interact with its surroundings.

Craft magic has some issues (basically too high finesse factors) that bother me much more than what you mention.