Do I take the original substance for the base individual, or the smaller one for MuTe like earth to glass? E.g. 1 cubic pace or 10...
Your target is the original substance.
There's the spell "Mimic the Christ's Miracle" (Apprentices pg 47) which turns water into wine, and in doing so reduces its volume due to the base individual for water being larger than that for processed liquids.
That isn't what I would have expected, which would be that you had to use the harder of the two relevant base amounts.
Technically, you use both :-/
For muto spells, you need both the Arts and Targets for both the original and the result.
So a MuTe spell to turn earth into glass is technically MuTe(Te).
Consider for a moment a spell to turn a man (size +1) or less into a bear (size +1). Clearly this is MuCo(An). And because the resulting shape is size +2, we'd need an extra magnitude for size. No-one would be really surprised at this.
That one caught me off guard too, and made me think about how Muto works. If I use MuTe to transform a stone statue into gold, or saphire for that matter, I'd expect it to retain it's size. But wouldn't that also take a number of magnitudes for size, as in the example above?
It's an interesting question. The core book isn't really explicit, but I never read EARTH THAT BREAKS NO MORE (p.154) as also shrinking the target. TRAPPING THE FIRE (p.142) results in a much smaller gem than the original fire, but this could be by choice rather than necessity. MISTS OF CHANGE (p.132) allows for a bear without extra magnitudes for size.