How do I preserve this piece of birch?

When was this tree cut?

You need to make sure it has dried fully. Needs to dry for at least one full year. After that you can sand the center, seal it with wood sanding sealer, then apply 2 coats of oil based polyurethane. Coat bark area with the poly.

I agree with Kelli, you need to make sure the wood is completely dry before you coat it with the poly.

Add a few legs from old furniture. I do not care for the hairpin legs everyone is using. Old ones look better to me or attach 3 pieces of birch branch 2 or 3 inches in diameter and use as a table until you let it dry.

Hi Pauline, If not already dry-Put a strap around the circumference of the piece so it does not split. Ask at the hardware store, they aren't that expensive. A tree man who trimmed our trees and cut me two pieces told me to do that and I thought it wasn't important so I didn't. They both split like a pie with a piece missing. Hope this helps and my condolences to you for your dear husband.

The strap idea sounds interesting. As the wood dries out it will actually shrink. Not expand. Instead of using polyurethane I would use shellac. Birch full of oils just like pine. A primer coat of shellac will prevent the oil / resin from bleeding out.

I had a piece split so researched what to do next time- https://www.preservation-solutions.com/wood-stabilizer-prevents-cracks/