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[Bomepie]

I've been reading up on obsidian casting, and I'm wondering if it's possible to solidify magma being suspended by a bridge, then drop it.

Ideally I'd like to cast the obsidian for my next mega project by making a 50x50 obsidian farm at the top of the z limit, then drop each layer to ground level as it finishes. Is this currently possible?

[Merendel]

Unsuported obsidian that forms on top of a bridge will colapse and destroy the bridge.  Additionaly even if the bridge is open a cavein passing through its location will destroy the bridge.   Technicly you could do it but you'd have to rebuild the bridges each time.

Also 50x50 will be difficult to achieve as you would need to somehow support 9 bridges in the center makeing a clean drop impratical.  All my obsidian cast mega projects required building containment walls on each level.  at least they did if I wanted the magma to stay put long enough for the water to arive.  I could make a lopsided tower prety easy by just dumping the stuff willy nilly.

Personaly I prefer making them in 20x20 increments.  Pumpstack up to a drop area at the roof of the world.  4 magma safe bridges with wall around them, fill with magma.  Above that another 4 bridges, fill with water.  Build retaining walls on ground floor. Pull lava lever, pull water lever.  Send dwarves to rebuild retaning wall on new level while you close and refill the chambers.

[Bomepie]

So it sounds like I'm either stuck building all the retaining walls, or I'll have to collapse then re-floor a holding area for each level.

My understanding was that dropping the magma and water was primarily to ensure that there's no issues with water/magma residue clogging up the works. Do you know any methods besides dropping for ensuring a clean spread of magma and water for each stage? I'm building something stupidly big here, so I want to make things as cleanly repeatable as possible.

[Merendel]

Well the reason for the droping is so you can repeatedly reuse the same initial fill area so you only have to worry about constructing a single pump stack and the asociated flow controll.  Because lava/water falls strait down when droped it lands nicely in a retaining wall the same shape below the bridges without makeing a mess or haveing uneven spots.  Unfortunatly you still need the wall unless you dont mind spilt lava.   if you dont mind lava overflow you can skip the retaining wall as long as you can ensure the water is droped at a set time after the lava,  to long and the lava runs off or evaporates, too short and you may mix water and lava up on the Z lvl with the bridges deconstructing them.

If your dead set on doing a 50x50 in one go heres what I would do.  Build your ground level of the retaining wall(your mold)  Build a pump stack for water and lava up to at least the level above where you want the top to be.   Build enough floor/walls to pipe the liquids from the stack to the center of your mold.   Turn on the lava pump till you get at least 2/7 all the way accross.  Dont leave it unatended as you've got no overflow prevention.  Shut off the lava pump. once it stops driping turn on the water one.  The water should obsidianize the block below the outlet and then more water will spread off that block and onto the lava on either side obsidianizing that and so on.  you'll end up with mud covering everythign and water will spill off the sides before its done but it should still work.

However there is one catch.  your going to need alot of throughput on both water and magma or you'll end up with it evaporating before it can reach the sides.  Honestly I've never tried filling a 50x50 area in one go.   you will probably need 3 or 4 pumps pulling from different water sorces and presurizing the water feeding the base of the pump stack to ensure it always has a full 7/7 every tick it tryes to run.  Magma may take even more feeder pumps due to its slower spread speed.

To be honest I think you'd be better off casting a 20x20 tower (4 max bridges) and just repeating the proccess till you've got a 40x40 all butted up agianst eachother and finish it off with an L shaped ring to boost it up to 50x50 after that.

[itg]

The easiest way to do this is to just make a 50x50xZ block, where Z is the height you want, then carve the throne out of that. Just build a retaining wall up to the max height, then alternate pumping in layers of magma and water.

This probably isn't the most efficient method in terms of dwarf-hours, but it's very low on micromanagement, so it will probably save you actual gameplay time. Plus, the extra mined obsidian might be useful to you.

[jcochran]

Personaly I prefer making them in 20x20 increments.  Pumpstack up to a drop area at the roof of the world.  4 magma safe bridges with wall around them, fill with magma.  Above that another 4 bridges, fill with water.  Build retaining walls on ground floor. Pull lava lever, pull water lever.  Send dwarves to rebuild retaning wall on new level while you close and refill the chambers.

I agree about the 20x20, although 20xAnything is perfectly doable. But why have two sets of bridges when one will do?
Have 4 magma safe bridges with a wall around 'em and 2 of the wall segments being flood gates. One floodgate controls magma, the other, water.

1. Close bridge
2. Open magma floodgate, fill with magma, close floodgate.
3. Open bridge
4. Close bridge
5. Open water floodgate, fill with water, close floodgate
6. Open bridge

Since the floodgates are right at the perimeter of the bridge, you don't have to worry about any water and magma reacting and plugging up the works. And since all of the water or magma drops when you open the bridge, you will don't have to worry about any remnants messing things up there too.

[jcochran]

To be honest I think you'd be better off casting a 20x20 tower (4 max bridges) and just repeating the proccess till you've got a 40x40 all butted up agianst eachother and finish it off with an L shaped ring to boost it up to 50x50 after that.

Hmm. He could make a 50x50 retaining area below and for the bridges, use 20x50. A drop of that much magma would spread out to a layer of mixed 2/7 and 3/7 so evaporation shouldn't be a problem. But I'm uncertain how the casting would go when he drops water on it. Does a 3/7 tile of magma react with a 7/7 slug of water consuming all the water? Or will there be 4/7 water left over to spread out to react with more magma? Hmm.. Even if it does spread, I don't think it would work since once a tile gets to 1/7, the fluid stops spreading. So the most likely result would be solid obsidian below the 20x50 bridges covered with a layer of 1/7 water (and the associated mud) and an irregular perimeter of obsidian where the water flowed. There may perhaps be some unreacted magma in the mix too.
For a sure thing, cast 20x50 twice for a 40x50 area, then finish it off with a 10x50. And if he's sneaky, he can cast both 20x50 towers at the same time separated by a gap of 10 when he can cast second.

[Merendel]

I agree about the 20x20, although 20xAnything is perfectly doable. But why have two sets of bridges when one will do?

The only reason for 2 sets of bridges instead of only 1 is for speed of use.  If you go with a single fill chamber you have to wait for it to refill between drops.  With two bridges you can pull 1 lever, wait a few seconds pull the other lever  wait a few more seconds and pull them both agian and start the filling proccess while your building the retaining wall for the next level.  Filling the chambers can take a couple minutes depending on the size and how much throughput your pumps can produce.   If space or resorces are an issue sure 1 chamber with floodgate controll works just fine it just takes longer to operate.

Good call on the 20x50 though forgot that your only limited expanding in one direction with pairs of bridges faceing eachother.  However you still may run into issues geting enough throughput to fill them before evaporation kicks in on the edges.

He could make a 50x50 retaining area below and for the bridges, use 20x50. A drop of that much magma would spread out to a layer of mixed 2/7 and 3/7 so evaporation shouldn't be a problem. But I'm uncertain how the casting would go when he drops water on it. Does a 3/7 tile of magma react with a 7/7 slug of water consuming all the water? Or will there be 4/7 water left over to spread out to react with more magma? Hmm.. Even if it does spread, I don't think it would work since once a tile gets to 1/7, the fluid stops spreading.

Well as far as the magma goes it should work in therory. You probably wouldnt get an even 3/7 due to a few bits at the edge evaporating but it should be mostly 3/7 with a few tiles of 2/7 bouncing around once it all spreads.  I'm prety sure the whole 7/7 of water will be consumed on the first drop of water so you'll end up with a dry 20X50 hunk of obsidian and 30x50 hunk of 3/7 lava.   You could just build a second 20X50 water bridge setup on the other side of the area so you'd end up with a pair of 20X50 hunks of obsidian with a 10x50  strip of lava in the center.    If you floor in the 8x50 strip between the two drop basins (rember the walls take up some space) and wall in the sides you could rig up a couple of hatches or bridges down the center of that, flood that area and then open the hatches till enough water falls and spreads to obsidianize the middle strip.  You could even have the main water pump filling this area and use floodgates to fill either side basin. you'll have to wait for evaporation to finish prior to doing the next lava drop but then agian its going to take a while for the next layer of retaining wall to be built so it will probably finish by then.

[Bomepie]

Ok to give an exact idea of what I'm talking about here, the goal is to cast THIS:

Spoiler (click to show/hide)

If I carve from a single cast I'll have to make a 50x44x99 mold.

If I split this up then it would be:
Seat: 50x44x27
Each Armrest: 8x37x25
Backrest: 50x7x72

From what you guys are saying it's sounding to me like I may want to abandon the idea of any of the bridge stuff in favor of magma pumps at the top of the cast and a water reservoir suspended above the cast in the same shape exactly 1 z level deep. I could just run the magma pumps until I pass 2/7 on all squares at the bottom of the cast, then it would only be a matter of releasing the water as efficiently as possible.

Actually while I'm thinking on this, what happens when you drop magma onto a pool of water without letting it touch the sides? Would an obsidian square form, then cave in? Would the cave in destroy the water? Because I could definitely fill a cast with a little extra buffer room and just drop magma into the pool repeatedly as long as I don't lose all my water to the cave in action. I think the cave in would probably act like a piston, and even work to conserve water if done correctly. Or this could be done the other way around as well I guess, but I'll have access to water pumping much earlier than magma-safe pumping. I'm just wondering if I can get around the need for putting up and tearing down the cast by only controlling one component and not the other.

[Merendel]

It dosent really matter if you dump lava on water or water on lava you get basicly the same result.  I've got one question thats somewhat pertinant to your situation.  Are you going to try and do this above ground or below ground?   If below ground you can probably just use the area you mine out as your retaining wall, patching whatever holes the caverns leave in it.

Here's an idea.  Make your 50x50x49 or whatever tall mold.  Above the mold make 2 additional levels of mold where you move the walls 1 block further out on each side(52X52)  Fill the entire mold along with this uper level with water.  Technically you could use lava there instead but its generaly faster to fill a volume of that size with water.  A Z or two above the water level build a 20x50 set of bridges all linked to the same lever. The bridges should line up with the edges of the primary mold.   Fill the bridge area with at least 2/7 lava. Block off the lava input and pull the lever.  Lava will fall into the water, obsidianize with no support and cave in to botom of the mold as 1 big 20x50 slab.   Repeat 48 more times then build the next set of bridges to do the next colum.

The reason for the extra layers on top is so that the newly forming obsidian is not touching a wall and does not end up geting supported by the top of a previous pillar for the later 2 bridge sets you'll need for this project.  Also this method will be a pain if your doing it above ground as you've got to build the entire mold from the start,  50ish levels of constructed walls and the associated walkways for construction will be tedious in the extream to build.

[Repseki]

If above ground, couldn't you build the retaining wall level by level as you cast. Using the newly cast obsidian as access to build the next level up. Using a few constructed staircases on the outside of the retaining wall and placing the last few blocks from those?

Might take a bit of micro, but probably not as much as building the entire mold before you even start casting