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[Sizik]

. . . You could channel from above, of course, but dwarves are stupid, and I can mass designate ramps and then not have to look out for stuck dwarves.

Actually, you can't channel from above because of the bridges.

[Veroule]

I have found a 3 floor system to be the most efficient.  Counting from the bottom up,

Floor 1, magma fill area, you only need to get 2 deep on the magma.  Surround the chamber with draw bridges to act as a form.  You will have to have floodgates on a seperate lever, as you want to completely surround the room in draw bridges.  Those bridges will have to be down for your miners to enter.  When the magma has reached depth raise the  draw bridges to create a perimeter.  Plan to place an obsidian support in this floor while you are mining it out.  Be prepared with a lever to pull the support when mining has completed.

Floor 2, open space.  This particular floor requires nothing more than being open.  The construction of this floor involves mining it out.  Then channelling the perimeter.  You should use a grate as your dwarf's exit path, then retract the grate and never extend it again.

Floor 3, water area.  The floor space here should be made entirely of retracting bridges.  Pumps should be able to supply enough water to fill this area in short order.  You can use draw bridges instead of floodgates to block the water flow while the floor bridges are retracted.  The water dumps uniformly when the bridges are retracted which tends to leave very little water behind.

The procedure under this design is:
1. Open magma
2. Secure Fl.1 bridges
3. Secure Fl1. floodgates
4. Dump Fl.3 water
5. Open Fl.1 bridges
6. Begin mining on Fl.1
7. Reset Fl.3 so it fills
8. Build support on Fl.1
9. Link support to lever
10. Pull lever on support, restoring Fl.2 to open space

You can repeat at this point unless you want to secure all the stones.  I tend to use a rather sizeable chamber in this design to train spare miners and engravers.  I really wasn't concerned about the stones and was quite happy to have the cave-in move them into atom smashing position.

[Albedo]

Pardon for the threadomancy, but this excellent article seems like the spot to ask.

...I've found that magma fills a room so slowly unless it's being pumped that my miners can dig it out faster than it flows....

How slow is "slow"?

What's a good width for a magma channel to feed a farm? I know it's subjective, but I have no feel for how fast that sucker's gonna flow, or fill a form.  (My fortress is capped at 101 dorfs, but I'm going for the magma in the first summer.  But while "manpower" is not limitless for now, I do want to lead my target and plan ahead.)  The author of this article shows a farm with access to magma (a pipe or massive channeling effort) that's almost 30 wide!

Also, I have no bauxite (well, 3, that I brought with - hardly enough).  I was thinking of a 2-stage water drop system (via retracting bridges).  One is standard, to hit the main farm.  But the other, much smaller, would be positioned over the entry channels, and drop just a little water on them, to "plug" them, and allow the rest of the magma to spread out and do its thing.  Is this feasible?  The deciding factor is what happens to water that hits magma? How much is steamed off, and how much flows to the next magma tile?

How much water do I want to drop?  I doubt if I need 7/7, but is 1/7 enough?  (I'd rather minimize pumping/evaporation as much as possible due to the manpower thing.)

[Asteranx]

I have found a 3 floor system to be the most efficient.

What would happen if, theoretically, you used a bauxite retractable bridge as the floor of your magma fill area, had the obsidian form on top of it, mined (or ramped) it out, then retracted the bridge?

Could you set up a 4 floor chamber that drops the loose obsidian product into a lower chamber that can then be cleared out (or not... the stones would stack across multiple runs) while the bridge is re-closed and the magma starts refilling?

Or would the formation of the obsidian damage the bridge in some fundamental way?

If this works, you could put your masonry workshops around (or in... I don't know if falling stone would affect/clutter a workshop or a working dwarf) the drop zones without having to dedicate any of your haulers to clearing or moving the stones, or building large storage spaces for keeping them in.  Then, any time you want something made from obsidian, just queue it in those shops.

Personally, I haven't even set up a basic obsidian farm yet, so I'll be saving this for after I have the 3 floor version up and running, but maybe someone else has some idea of what would happen?

[Vanguard Warden]

The design I use is pretty simple, fast, and doesn't require bauxite to operate. I set up a two z-level high chamber with no ceiling to act as a big tub. Then, I have screw pump towers set up at the z-level above the tub (technically the third z-level) acting as big faucets for magma and water exclusively. I pull a lever and turn on the magma-faucet for a second or two, and enough magma will be pumped in to fill the tub to 3/7 or so on the bottom level. I do the same with the water faucet, and it solidifies all the magma, with the rest pouring out of a drain on the side of the second level. My dwarves take some stairs down to the access tunnel for level one and mine it all out. Designating the whole area for ramps will clear it all out without worrying about the problems dwarves have with channeling stuff. The small puddles of 1/7 water should dry out as you're mining and hauling everything, so after it's cleared out you can repeat the process.

You can pump magma with a screw pump fine as long as it isn't made of wood. It doesn't have to be magma proof unless magma actively spills onto the crank-end of it, which should never happen. You can even pump magma with a wooden pipe and corkscrew, as long as the block is made of stone. It's also the only way I know of to control the flow of magma with a lever if you don't have access to bauxite, by hooking it to an attached gear assembly.

[Grendus]

I have found a 3 floor system to be the most efficient.

What would happen if, theoretically, you used a bauxite retractable bridge as the floor of your magma fill area, had the obsidian form on top of it, mined (or ramped) it out, then retracted the bridge?

Could you set up a 4 floor chamber that drops the loose obsidian product into a lower chamber that can then be cleared out (or not... the stones would stack across multiple runs) while the bridge is re-closed and the magma starts refilling?

Or would the formation of the obsidian damage the bridge in some fundamental way?

If this works, you could put your masonry workshops around (or in... I don't know if falling stone would affect/clutter a workshop or a working dwarf) the drop zones without having to dedicate any of your haulers to clearing or moving the stones, or building large storage spaces for keeping them in.  Then, any time you want something made from obsidian, just queue it in those shops.

Personally, I haven't even set up a basic obsidian farm yet, so I'll be saving this for after I have the 3 floor version up and running, but maybe someone else has some idea of what would happen?

If the bridges were made of something magma-proof there's nothing that says why not. My guess would be the stones don't stack though, and it might kick up dust clouds. In theory you could even use the floor below it for crafting, just make sure it's cleared of dwarves before you drop the stones.

It would probably be more efficient as well, however I guess that Obsidian can be made faster than it can have goods crafted out of it, so the efficiency gain will be slight.

[zchris13]

I may destroy the bridge to have an obsidian wall formed on top of it.