So, how does it work in ME when you dig out metal ore (e.g. iron), how is this material classified? Rock or metal?
I'm afraid I don't even know IRL, mineralogy isn't really my thing. A superficial search seems to indicate the ore is considered a rock in modern terms. But in ME?
Is metal ore a rock which you process to extract the metal, leaving slag behind?
Is it a rock with bits of metal mixed in, forming a hybrid material?
Is it already classified as a metal, but just not a very good metal?
The question arises in our Rhine saga where our covenant owns and operates an iron mine. Our Verditius magus wants to build a device easing the process. We're considering either using MuTe ala Rock of Viscid Clay or perhaps a PeTe to destroy the 'hardness' property of the rock.
But do we need to affect rock or metal?
And will the Perdo version then ruin the metal as well?
I would treat ore as rock. The metal in the ore is not considered seperate from the ore, so any spell is simply affecting the ore (rock). There is no metal yet.
Note that a PeTe that destroys the hardness of the rock may very well ruin the metal produced from it. If the hardness property is destroyed, it is destroyed for all of it.
That is not to say that a PeTe effect cannot be used to make mining easier. Simply breaking up the rock in small pieces is certainly possible. That is not the same as destroying its hardness. It would be very similar to Fist of Shattering or Obliteration of the Metallic Barrier, which shatter the object. So (Base 4 destroy dirt, +1 stone, +1 Touch, +1 Part) would work well to shatter a section of stone 1 cubic pace in size (the base Individual). This gives you the same level as Rock of Viscid Clay, so a matter of preference (Mu vs Pe) in how you work the material.
I think it would count as a rock which you process to extract the metal. After all, City & Guild p134 mentions brass being produced from copper and calamine, so people think of the special ingredient to make brass as something in the ore rather than as zinc metal.
I would personally go for using ReTe and treating metal ore as a rock you process to extract the metal. As Rego Craft allows you to process raw materials without using intermediate steps so if you can turn hides to leather, you can turn ore to metal and slag with no use of fuel or production of toxic fumes (useful if you want a lead mine). Rules-wise, if you count are as rock with bits of metal mixed in, separating it out should be about the same difficulty-wise for ReTe. Thinking about it, you may need to target metal as you want to separate the metal as a finished product rather than just change rock.
So a rock of viscid clay wand, target part, to soften up some rock, dig it out with a spade and leave it by your smelter. When the rock of viscid clay expires, run it through the magic smelter.
The smelter might have "The mystic furnace" - ReTe base 2 (control dirt in a slightly unnatural fashion), +2 affect metal, + 1 touch, +1 target part - total level 10, up to a cubic foot of metal (1 Terram base) will be separated from a metal-bearing ore that is touching the smelter. I suggest slightly unnatural as you are working it, although it is possible for metals like gold to naturally enrich and come out as flakes in a river. You could also use it to separate out metals from alloys, if you want to go hunting for scraps of silver or break down brass to get epic quantities of a strange metal little-known in Mythic Europe (even if they are making it in India).
We're taking rock/ore, and make it soft enough to work with a shovel using MuTe. Pile it in a wagon or wheelbarrow or... whatever, and take it out of the mine.
Then we use the Mystic Smelter (above) to extract the metal from the more, with pure rock as left-overs (as opposed to slag). We'll probably do this after the Muto effect has worn off.
This "left over rock" can be made soft for use as building material, should one so desire, or just be carted away.
Yeah, but do we want the mystic smelter? Isn't the purpose to make things easier but still inconspicuous?
We have copious amounts of charcoal available, so if the heavy work like digging and lifting can be done easier and faster we can still produce fair amounts of iron with less work and fewer people, without it looking (too much) like magic.