We removed the grate tonight and sucked out all of the water from the pit, and attempted to suck out the clay. We made it about half the way down, but started to hit black water.
It is unclear if the water draining from the sink is backing up into this hole or not.
We left the wet/dry vac in the metalshop, because we couldn’t really think of a way to clean it. We figured that hopefully it would dry and the clay would just crumble out.