I just finished reading Covenants with an eye towards our current Saga, and I was left with some nagging questions. I was hoping in this thread to solicit some advice from the forums on the topic of Rego Terram craft spells as they might be used to extend/create natural caverns or to mine.
Basically, I expect the players to, at some point, decide to try to use ReTe spells to either create/expand a mine or to expand some existing caverns. Now for a mundane miner this is back breaking work, fraught with considerable danger in this time period, requiring timber and luck (or rare engineering knowledge) to shore up tunnels, avoid aquifers and flooding, trapped gases and oxygen issues and the like. The crux of my questions are basically what of these issues apply to ReTe effects.
So, let's say a magus invents an ReTe spell, The Magus' Major Miner General spell for all mining purposes. The spell pushes back stone and earth, compacting it, allowing the magus to shape passageways through natural earth. Base Effect 2 (slightly unnatural or as a craftsman), +1 to include stone, +1 for "subtlety" (making passageways and rooms), +1 Touch, +1 Part, +1 Size. Level 15.
Assuming that spell is even valid 8), he should then be required to make a Finesse test against an ease factor +3 higher then the comparable craft. If a miner were going to attempt to do this, I would probably give him a very high ease factor (I'm inclined to say 12, possibly with bonuses based on materials used, precautions reducing botch dice of which there would be many extra), as mining in this period was quite hard (and in fact, as we think of it, probably didn't exist). It seems unreasonable, however, to impose the same difficulties on the magic. First there was a passage in Covenants saying that one casting of a spell amounted to about 1 days work of a single craftsman. That would seem to dramatically reduce the roughly 10 cubic paces of effect of the spell (but perhaps that's covered by the size modifier already). At the same time, I can not seem to decide whether a magically created passageway would require shoring beams and support. On the one hand it seems like it absolutely should as the mass is still present, only compacted, increasing the likelihood of a cave-in. Afterall, natural caves themselves do cave in even absent human intervention. On the other hand, however, there is an argument for the non-disruptive nature or manner of the rego magic not breaking the bonds of the stone with itself or the sediment, therein mitigating substantially the risks normally incurred by much more intrusive mining tactics, particularly, it would seem, if shaping through a vein of "solid" igneous rock for instance. If I were to buy that argument, I suppose I could just reduce the botch dice. If support beams were added as the work was done, that would seem to add further mitigation. What ease and risk factors would you impose?
As a secondary advice issue, let's say the above ReTe spells are being used inside a magical cavern. Now, I know many people are going to suggest that it all depends on my saga and why the caverns are magical, etc. My problem is that I lack a sufficient Ars vision to really have a feel for how working the caverns would effect the aura (or the regios) in the cavern complex. A passageway you only see if you've passed it four times, for instance, might not show up if the passageway it is suppose to appear in is expanded into a full cavern (or maybe it still does, only now it's in the middle of a giant room, making it much more obvious). Maybe you're already in the Regio when you see the side passage, I could play with the definition to avoid the question, but for pre-existing conditions that would seem problematic. Also, what happens to the Aura itself? Has anyone run magical caves that have gotten modified or expanded in game and how did you arbitrate the impacts of those expansions?