Lost a miner already
I guess the mine and run method doesn't work in magma!
two ways to possibly prevent loss of a good miner.
1) use an engraver and create a fortification, rather then mining through the wall.
2) dig a trench in front of the punch through spot, and bridge across the trench to give your dwarf a spot to work from. Even a 1 tile wide trench will help a lot.
In both cases you should make sure the dwarf in question has something to do so they don't loiter.
1) Does magma take longer to go through a fortification than onto an empty tile? Otherwise what's the difference?
2) Does magma/water drop down from a bridge tile? Otherwise I don't understand what this means.
I'm surprised there doesn't seem to be a "working safely with magma" article on the wiki.
I never quite realised how few rocks are actually magma safe...
Oh BTW, is magma actually infinite? I ask because after breaking a few rocks the lava stopped being 7/7 everywhere and didn't seem to come back either.
Magma CAN be infinite, but it can also be in pools. It's normally in pipes in my experience, but if you fiddle with world gen, there might be a way to create more pockets.
I carve fortifications for both water and lava channels for three reasons:
1) Fortifications keep building destroyers out. Yes, they will swim through water or lava to get at your destructable floodgates & whatnot if they can survive in water/lava.
2) Seems to me that it does slow down the flow. I may be mistaken, but I do not believe so.
3) Anyone can be an engraver - you don't have to risk a miner, or a mining pick.
As for the channel and bridge thing. Here's what I mean:
LRRRRR
LRCCZRRR
LFB..........
LRCCZRRR
LRRRRR
L = Lava
R = Rock Wall
F = Fortification cut in wall
B = Bridge
C = Channel cut in floor
Z = stairs up from channel level in case dwarf dodges, falls in, and recovers
. = Floor
What happens here is that the lava will start spilling onto the bridge, but then has to spread left, right, and down to fill the area channeled.
I have heard of people simply using floor bars, rather than bridges. I have never tried that, but if you have a metal industry, or get bars in trade that you don't mind using, you could try that. Magma would fall straight through bars, and I'm pretty sure dwarves can walk on them.