Lye usually works well.
 
It is sold in grocery stores for unclogging stuck drains.
 
Be very careful, as it is potent stuff.  Also it will react with Aluminum,
so it's not very good for that.
 
One thing I have done is to fill the pot about 3/4 with hot water. (I have
sometimes used lobster or big stew pots, enameled ones, to melt wax scraps
and cappings.)  Set it on a burner and let it heat right up to not-quite
boiling.  Stir the hot water around with a stick, sortof scraping the
sides.  Then just dump out all that hot water outside.  It will carry most
of the wax and goo (now melted) with it.  If you dump it into a big
container of some sort, eventually it will cool and you can sometimes later
salvage a thin layer of hardened wax from the surface.
 
OR, fill the container about 3/4 with hot tap water.  This time don't heat
it further, but drop in a couple tablespoons of lye INTO THE WATER,
carefully, and stir it up.  Let it sit for a couple of hours or whatever,
and then dump the liquid out.  The lye will dissolve most of the residues.
Then you can repeat the treatment if necessary.
 
Other than that, I don't know of any way to clean that stuff up.