A few points:
Magma can be channeled to wherever you want it. Usually it's in your interests to have your smelters and workshops near your living spaces and stockpiles. So don't build next to the magma, move the magma to you.
That said, a long magmaduct will discourage curious magma monsters from wandering by. If all we're talking about is magmen and fire imps, you're set. I've never had one climb down the magmaduct.
You need to use steel bars to guarantee that though - apparently fortifications melt. Constructed ones anyway, I don't know about carved ones.
It's possible that a fire immune building destroyer could jump in an exposed magma pipe, rush through your magmaduct, bust up your workshop, and climb out. This would require that he be able to pathfind through all that, which is unlikely, that he be super duper fire immune, and that he be able to climb out without a ramp.
On a side note, your best bet for digging that channel is to dig your block of workshops, dig stairs down off a short tunnel to the side, and dig a network of magmaducts below the center tiles of each workshop space. From that network of magmaducts, dig another tunnel one wide for a short way, then stairs up* and continue the tunnel to the magma. You want your magmaduct to hit the magma a few levels below the surface for pressure.
Now before breaching the magma, check to make sure where you can dig up stairs from the magmaduct right next to the magma. You want to get a place where one level above your magmaduct the magma is a little farther away. Smaller, or shifted over a little. Dig up in a safe spot, and clear the space directly above the one tile you need to breach to let magma through below. You're soon going to channel that out rather than digging in the magmaduct.
Now before you do that, dig a parallel passage that's one level above the magmaduct, back to your magma shop area. You should come in just above the stairs you build down from the shops to the network of magmaducts below them. This is your miner's escape tunnel.
So from the access tunnel above the magmaduct, breach the magma by channeling. Your magmaduct should pretty much fill up with magma for about 10-12 tiles right away because it's under pressure, but it won't slouch up your stairs to the access tunnel where the dwarf is. He will pathfind the safe way back, through the cool access tunnel, while the magma blorps along to eventually reach your magmaduct network under your magma workshops. Once he's out, wall up the stairs leading both up to the access tunnel and down to the magmaduct network. Nobody destroys walls.
*Digging up halfway through your magmaduct span comes into play now: you'll notice a slowdown of magma but if it's able to fall down a level it tends to pick up a little speed or something, and you have less chance of the flow petering out and cooling off at 1/7 all the time.
At this point, you should have a sealed magmaduct system, workshop spaces above it, and no holes anywhere. When you want to put in any workshop, channel out where the magma hole needs to be and plop the workshop right in.
The only flaw I've found in this method is when you have too little magma to fill the magmaduct. If that's the case, and you have just a shallow pool, consider just killing the few magma creatures and setting up your workshops way out there along with spare stockpiles for food, and assign beds and tables nearby for the foundry workers so they don't travel back to the main section. You might need to lock a door leading to your main fortress if they keep running over there for food. But far-off magma shops are still more efficient than chopping wood for fuel.
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Topic: Magma forges/smelters/kilns (Read 664 times)