Is the shape holding it in or do you think there was a hole in the plastic? You can try breaking the 'tufa in half to remove (use a hammer and chisel and give one sharp hit at the bottom on either side) and then glue/cement the two pieces back together.

Depending on the size of that boiler, whether it can taken outside, and how cold you are there in NY, you maybe could let it freeze a bit, then pour boiling water on the copper to make expand, do a little dance, bump it upside down, and hope it can pop free. Can you post a picture of it?
Good luck!

You probably won't be able to get the trough out of that copper boiler because it has straight sides. Perhaps you will get lucky if you pour hot water on it so that it expands a bit. Usually, a rigid mold, with straight sides is the kiss of death for getting a trough out (ask me know I know this lol)

Leave it alone for a few months. Simply set it aside and move on.

This just happened to me! We drilled a hole in the bottom of the pot and used an air compressor (be careful), pumped in a little air and shimmied the concrete pot out of the container!

Here is another trick -> I greased my mold, but it was still stuck. I placed it in a freezer. Overnight -> viola' - popped right out.

The curse of the vertical -- sided mold; certainly guaranteed to get a trough stuck lol One thing that can be holding it in is suction; punch a hole in the bottom of the mold to release it; and/or pour hot water on it or let it sit upside down in the hot sun and sometimes it will release. Good luck lol