Summary: Lignite blocks put in an iron bin and submerged in magma ignite and can't be extinguished, bin thenceforth sets fire to dwarves and instantly vaporizes water.
So I heard that you could turn blocks back into stones by putting them in a magma-safe bin; submerging said bin in magma, thereby melting the blocks; then recovering the bin and its contents and allowing the molten blocks to cool into stones.
I set out to test this today. It didn't seem to work.
It turns out there's a difference between a "dolomite" , which shows "Uses: Make steel bars, Make pig iron bars" and is a stone, and a "Dolomite", which has no uses and shows up in "globs" on the stock menu.
When I tried this, I put two Lignite blocks in my bin with some other stuff. Rather than melting, they caught on fire inside the iron bin and began emitting smoke.
As soon as I opened the test chamber to try to recover the bin and see whether I could persuade my smelter to take a big-S Sphalerite, the first dwarf to set foot in the chamber caught fire and died. I poured buckets on it for a while, but that didn't seem to change anything. The bin shielded the burning blocks from being extinguished by water.
I built a system to pump water into the chamber to see what would happen.
It instantly drained my reservoir, so I activated my fill pump from the cave river and started pushing more water in. The bin of doom is still instantly vaporizing all the water I pump towards it from a tile away. (Bin tile: Dry. Next tile: Dry. Second tile: Water [6/7].)
I just pitted a rhesus macaque child onto it, and it died in the heat, then quickly became bones.
I can't move the thing, but I could sure have some fun with it.
I'd like to see what happens if you make one on a retracting bridge and then drop it into a lake, or if you put it in a 1-wide passage that's the only entry to your fort during a siege or elf caravan. It could potentially have magma's advantages of burning goblins to death and destroying their narrow clothes, without the trouble of draining the magma when you want to collect the iron stuff, due to the flaming goblins running past the bin before dying.