When I made my elliptical arch for my bathroom alcove, I used baltic birch plywood for the facing, and lined the inside of the arch with almost-solid blocking, then used two layers of dampened 1/4" drywall to bend to the curve. I then applied flexible plastic corner bead with glue, and mudded with durabond. The plywood was in place of any drywall. I did this (in an old house) in 2008 and the plywood/plaster tape joint has not cracked.
Plywood instead of drywall allowed me to get the curve exactly as I wanted it; drywall alone is too random and easily damaged.

Casey

The arch looks fabulous, as well as the bathroom! Thank you for the detailed and professional instructions. I'll definitely take into account this alternative.

Geeze, that is so beautiful Casey. And great instructions like MariaBarbara says.

Bendable 1/4" drywall requires some experience. I had drywallers who insisted on waiting for the right amount of rain as opposed to using a garden spray.

You can also use bendable MDF.

Or manmade castfaux stone as in the door arches below.

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sm: Nicely done dramatic bath!

You folks like Casey's bathroom? You should see the floor! Still one of my favorites of all time, Mr. Casey!

I have maybe 14 archways on my house. I kerfed the backside of plywood to make the ply conform to the curves. That was years ago. Now I'll use a bendable plywood. Instead of having the grain in the plies alternate, it all goes in the same direction, allowing for fairly tight and non-broken curves.

Around here it's sold under the name of "wiggle wood" and costs around $35 a sheet.

Haha, we call it "bendy board"
Thanks for the nice words, mongoct.
yes, my $20 sq.ft. (materials alone) bathroom floor. Those were the days. In retrospect I over-improved a bit.
Casey