Supposing a magus has already joined two pieces of metal or together - perhaps holding them in place with Rego - will a Muto spell D:Mom be sufficient to fuse the two parts together? Imagine briefly making them mallable like clay. Will it be necessary with a Rego(or even Creo) requisite to actually push/mold them together?
And yes, the finished product is limited by normal, mundane structural limitations, but a broken iron bar or stone column can be made as solid as before it was broken, no?
my opinion:
I think it's Muto only, though I' use duration concentration if it' a lot of connecting.
And it could look either like a bike frame, or as if it had always been just one part (whatever the magus decides when he invents the spell).
I was mostly thinking for masonry, two stone blocks properly aligned and with good, flat contact surfaces.
I hadn't thought about cosmetics. Perhaps if the two items light up nicely you get an almost seamless joining of the two, but if surfaces are more rough you might get a visible 'welding line'? Maybe this depends on the style of the magus' sigil?
since in 5th edition trun water in to ice and viceversa is domined by rego, shouldnt metal wield the same way, with Rego?
... I doubt he wants his greek column turned to lava though 
To answer the original question, I'd probably go for D:Conc as well, or a Rego req to make the surfaces mesh faster.
Water is dominated by rego because ice, steam and liquid water are all natural states of water. Liquid stone is much less common. No idea of what has happened now with hermetic Projects and the lava thing, but so far I would rule that "gaseous" or "liquid" stone is not a natural state, so it requires Muto to get that sort of change.
Up to you what the final appearence is. If 2 types of stone were being mixed, probably the line would be clearly defined. if you are fusing 2 sdtones that used to be together, you are probably likely to reach a seamless repair. Welcome to the hermetic blender!!
Xavi
Not really. Rego covers natural things, movements and also phase shifts. Water can be steam, fluid or solid ice - Rego shifts between them. But the ice will start to melt if the temperature is above freezing.
Rego also covers Craft magic - doing menial things, natural things - by magic. Splittting logs, quarrying stone etc. is Rego. As is working said materials into finished goods - although this requires high Finesse rolls.
So, one could argue that fusing two pieces of iron together is what a blacksmith naturally does by way of craft skills and a forge. A perfectly natural thing to do.
But to fuse rock together - nevermind that two pieces may have been one at some point - hardly seems natural. Also I'd like to be able to fuse a metal rod into a stone wall and things like that. So I'm thinking Muto territory.
You can certainly fuse two rocks together naturally, just heat them up (at temperatures comparable to those necessary to melt some metals). In general, Rego can achieve results that could be achieved by "natural" means without having to go through the intermediate steps; so Rego can fuse rocks together without melting them, just like it can produce cooked food (from ingredients) without need of a fire -- or a perfect, seamless glass case around a delicate piece of papyrus (from sand).
Thus all you need is Rego!
Hmm, good point. It's hardly as unnatural as I had imagined from the start.
I think I'll go for Rego then. I don't think it'll matter much to the magi I intend to use this.
Of course, what's "natural" in AM depends on what the medieval paradigm contained. I'm not sure if the fusing of rocks was a concept familiar to the medieval natural philosophers and, if so, what their thinking was.
Lava as molten rock, and clay being mutable rock certainly are. I reckon the added spice of magic makes it a reasonable result.
I tend to think of Rego as being able to achieve, via craft magic, anything that the best crafter could manage, and binding two pieces of stone near-seamlessly is certainly within that realm, albeit not actually fusing them into a single piece of stone - though at that point, the distinction starts becoming needlessly pedantic - the jewel is bound within the clasp to form one amulet, after all. Muto, to give claylike properties briefly, followed by a skilled potter or Rego requisites would, I feel, give you a true single lump with no seams or interstices at all. Both are about as difficult and give practically the same result and this which you'd choose would depend entirely upon the effect you want to go for in character and in setting.
I´d probably go with a Rego spell with a Muto requisite to allow for the most extremely wide variety of possible end results. Fusing stone by Rego only... Possibly yes, but you might want to fuse it while doing something else or something not possible without Muto so with a small addition you get a chance to end up with just about anything you could think of.
Yep! remember that the "whole" medieval understanding was based on Aristotle, Plato's aprentice, and one of its very first interest area was the underground ("geology"?) so that the concept of the rock's change of state from melt to solid where fully acceptated as natural. Even before, Hephaestus, was the god son of Era that ruled; blacksmith, volcanoes, fire, even technology advances, So he was the builder of the machinas from the clasical quote "Theus ExMachina".
So if any human being had Artes Liberales in the Mitic Europe it had been instructed in Aristotle. So probably knows about fused rocks.
Please correct me if i'm wrong 