Does the magma have to be connected to the volcano in order to build the forges on it? I don't understand... Also! If that's the case, couldn't you do this if there were pressure?
You don't understand how pressure works, do you? It'd PUSH RIGHT UP THROUGH THE FORGES.
Do you... Do you gave to be a dick about it...?
If you want to get all technical about it and talk about how pressure works... Water's all wrong at the moment. Not only that, but magma IRL has pressure. Soo...
Does the magma have to be channeled to a forge then it stops AT the forge? As I've stated, I've never used magma. So help me out without being a dick about it. WHY would it push through the forges? Aren't the dwarves smart enough to figure out a way to use it without it doing so?
EDIT:
Ahh! Okay. So I've looked up a fortress with magma. It seems that you put the magma underneath the furnaces. Well!
The magma won't push up through the forges if A) You seal it from the source with a bauxite-mechanism'd floodgate, B) You build above the source, C) You drop it down, then bring it up with steel pumps. Pumping water gets rid of pressure, so I'm sure it would do the same to magma.
Therefore, it won't always "PUSH RIGHT UP THROUGH THE FORGES". Especially not in capital letters.