If you talk about the base of the birdbath, try before you cut it in half. I used the same, oiled it very well and put a strong plastic strap down too, this helped to pull out the tufa from the mold after hardend (oil that too). To make it lighter and still strong, I put a plastic pipe in the middle, all the way down. If you leave the pipe longer, it will help to secure the base in the ground.

Andrea

I found an old plastic bottom to a bird bath, I cut it , taped it with duct tape, oiled it, and filled it with crete, and made a bowl for it with an old wok I found in the trash same day as I found the BB bottom. Worked out fine.
Putting the pipe down the middle sounds like a great idea.
bonnie

Here is a link that might be useful: plastic BB bottom