Qin Shi Huang's tomb, or go home...
Or a hat-making workshop. Bowler hats might even be better armour than a leather helmet for top-down attacks!
Programatically, I think the data structure to define flow-liquids (primarily magma and water) would need revamping. But as something more practical to do with barrels-of-blood, etc, it'd be interesting to have more than contaminent status fully available. But then it needs to handle blending. Water+magma is fudged into obsidian casting (and/or steam, depending on how you do it), but perhaps your goblet can have a splash of water in its whisky[1] or that moat can be made into one with various mixes of blood and perhaps the odd dash of vomit[2] without more problems than the various-loose-coinage restacking one.
Hmmm, probably a mercuric-magma mixing should make for an interesting ground-hugging mist, if that gets implementable in any real and accurate way (if it doesn't just dissolve in or compound up), which gives me an idea for a ziggurat with a nasty (albeit prolongued) surprise for those who wish to climb it. But melting other metals, at appropriate temperatures and in carefully calibrated quantities, could make for mixing chanelled rivulets into various massive amounts of more diverse and exotic alloys, governed by floodgates. Not necessarily to the extent of producing 18/8 austenitic stainless steel (perhaps for huge fortress-scale springs?), but a bit of clever property tuning and on a scale vastly larger than the current ore/bars in a reheated crucible...
But I diverge from the mercury. Perhaps I should have swerved into the subject of pottery statues, instead!
[1] Or whatever drink it would be best in. I don't even like my drams being on the rocks, which could at least be downed before much more than the cold leaches into the fine malt concerned, but that's just my personal preference and I imagine it'd be different for the kinds of whisk(e)y and blends that I already don't favour.
[2] Leaving the question of more "household wastes" aside, though 'improved'/more realistic dying and tanning would call out for Number Ones/Twos, as many have pointed out before. Even as Toady says he is disinclined to go thst way, as also pointed out before.