In a moment of recklessness, I sent a duplicant to mop up some molten gold from my gold volcano. Little did I know that even though it's classified as a liquid while in the bottle (and oddly showing up in the raw metals instead of refined metals), I wasn't able to empty it in any way. I checked the bottle emptier which listed even magma but not the bottled gold. This is more of a problem if you've mistakenly moved it away from the area the molten gold will come out and it can't melt back into it.