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Do slimes need some floor to spawn in their spawn chunks, or can they spawn midair if I remove all the stone?

The Wiki is inconclusive, the notes marked with [citation needed] and [Verify]. A discussion from '11 notes they can spawn in solid rock (and die immediately) just fine.

My rough plan for an automatic slime farm is: dig a wide hole to bedrock at slime-spawning chunk using TNT-loaded dispensers, funnel them to a deep pond using water, cover the floor of the pond with hoppers. Slimes would spawn in midair above, fall, drown, and the hoppers would then collect the slime balls. With vertical walls and all side corridors (and surface) well lit, no other mobs should spawn.

That's the plan. Now the question is: will it work? Or do I have to build a bunch of spawn floors for the slimes and light them up so that nothing else spawns on them?

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The Mob Spawning section of the Minecraft Wiki goes into some detail about what the requirements are for a mob to spawn. The parts relevant to this question are:

  • If it's not a squid, then...
    • the spawning block itself must be non-opaque and non-liquid
    • the block directly below it must be opaque or upside down slabs / stairs and may not be bedrock
    • the block directly above it must be non-opaque
    • the spawning mob must not be obstructed by solid blocks, other mobs or liquids

There are other conditions on Slimes, but those are concerned with them being in the right chunks, rather than the precise blocks. So yes, for every non-squid mob the game tries to spawn, the mob needs to spawn on top of a solid block, with the block they spawn in, and the next one up, being non-opaque non-liquid, and with the block they spawn in being air so they don't fail the collision check. They will not spawn in mid-air.

Recommended practice for spawning slimes is a wide, flat floor, in a chunk they can spawn in, below level 40,

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    It seems, like the Minecraft version discussed here: minecraftforum.net/topic/385523-slime-spawning/#entry5511601 didn't check for the last point; Slimes still do need 2 blocks of space to spawn, but 2 blocks is not enough for the big ones to survive.
    – SF.
    Jul 18, 2013 at 12:09
  • From what I understand, (though this may be incorrect), it actually always checks that there's room for a slime to survive. The problem is that at that point, it hasn't calculated what size slime it will spawn - they're all the same mob internally - so it only ends up checking for a small slime. Jul 18, 2013 at 13:15
  • As I decompiled the source, the linked snippets are no longer there in 1.6.2; it is decided what size of slime to spawn early enough, and the function does call super.getCanSpawnHere() where appropriate already. Interestingly, the code suggests: Smallest slimes are allowed to spawn on Peaceful, which could be a great way to get the production underway without need to light any corridors - they should be fairly plentiful as all hostile mobs will despawn.
    – SF.
    Jul 18, 2013 at 13:28
  • Small slimes do not spawn on peaceful for pre 1.6 versions. I've done too much anti-slime work in my main area to know if it they would spawn in a post 1.6 release. Also, slimes will spawn above level 40 in swamp biomes.
    – John
    Jul 18, 2013 at 15:16
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    @John Slimes didn't spawn in swamps until recent versions. Did you test in the usual <40 slime chunk? Jul 19, 2013 at 16:54

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