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[madtentacle]

I just dug out a 5x5 130z-level magma piston just before realizing the bottom floor of the lower cistern sits just above the magma sea. Will the entire piston fall through the floor or will the magma flow tiles below the floor support the cave-in?
Could I perhaps save it somehow? Is there any construction that will support a cave-in so I could fill the bottom layer? I suppose I could always cast a layer of obsidian in the bottom layer if there is no other way.

[PatrikLundell]

If you only have a floor between the chamber and the magma sea the piston will smash through the floor. Furthermore, if the magma flow is covered by magma the whole piston will disappear without a trace (magma covered magma flow disintegrates anything falling onto it, except living creatures).

In addition to that, magma directly on top of magma flow (i.e. the lowest Z level) disintegrates itself as well as any water falling on top of it, as well as water running on top of it with magma flowing in from the sides (or, I assume, from above). To build something on top of the magma flow it will either have to be dry (i.e. at most 1/7 magma or water), or you have to use a specific technique with the construction of up stairs below down (or up/down) stairs on top of the magma flow and and then drop water on top of it from yet another Z level above. That will obsidianize the tile using the stair as some kind of nucleation core (or something...).

Thus, you piston has to fall onto a full Z level (or more) of rock to neither smash through the floor nor get eaten by the magma flow.

[madtentacle]

Thanks for the answer! Luckily I haven't filled the cistern with magma yet, so if I half fill the bottom layer with water first, it should turn into obsidian when I open the magma floodgate to fill it, right?
If that would work it feels like a pretty convenient solution. With some effort I could probably design it to cast obsidian on top of the piston to make it a resettable piston as well.

[PatrikLundell]

If you let magma in from the side into a room with water you'll end up with a wall of obsidian behind which the rest of the water lies. To get obsidian throughout you need to pour one liquid onto the other from above and have enough headroom to allow the poured liquid to flow to the sides to get to the rest of the other liquid (or use a retracting bridge, i.e. let water into a room above the obsidian farm, close the water inlet, and then dump it all straight down onto the magma to obsidianize it all at once).