How I did it.
Lath and plaster gothic vault ceiling
Washington, DC
The capital of.....taxation without representation.
First, I marked off a one inch line all the way
              around the ceiling.
I put on my metal lath using
              wafer head screws.
This is a small ceiling, so I could take a few short cuts. More on how to do a big one some other time.
First, I marked off a one inch line all the way around the ceiling. This gives me a point of reference to straighten the ceiling. I used 2 scrap pieces of 1/2" blue board for my chalk.
The ceiling was framed for me. More on how to frame these ceilings with channel iron in the future.
I put on my metal lath using wafer head screws.

I made sure the arrises (outside corners are called arrises) were well overlapped and round.

I formed the arrises with quarter inch masonite
              strips
Strips were packed with white mortar
I formed the arrises with quarter inch masonite
strips cut on a table saw. I aligned the strips with the columns and the center of the lights.
The strips were shimmed nice and round to
a half inch thickness using 2 scraps of masonite.
Strips were packed with white mortar, namely
molding plaster and lime. I did the whole ceiling out of white mortar, like plaster moldings. It sets fast, and I could finish everything as I go.
I filled in the side sections and
              troweled them smooth
scrap piece of therma-ply to form
              the arris
I filled in the side sections and troweled them smooth. I set my angles (inside corners ) using the chalk mark, and filled in to the arrises using a rod (straight edge).
The odd ball is this oval section where the ceiling turns the corner. I screwed on a scrap piece of
therma-ply to form the arris. I hope you can see
what is going on.
I packed in my arris between the therma ply and
              the lath
This cool "S" shape on the other side
              of the oval
Next, I packed in my arris between the therma ply
and the lath. After the mortar set up, I took off the
therma ply and I had a nice arris.

This cool "S" shape on the other side of the oval was done like the round sections.
I filled in the side sections and finished them 
The center sections are then filled and
              straightened with a 6 foot rod
I filled in the side sections and finished them, then
took off the masonite. I trued up the arrises by
scraping and chipping.
Now, all I have to do is fill in may center sections.
The center sections are then filled and straightened with a 6 foot rod. In the foreground,
you can see the funky shape where the ceiling
turns.
I filled in
              everything and troweled it smooth
A view of the
              "S" shape across from the oval
I filled in everything and troweled it smooth.
A view of the "S" shape across from the oval.
The next step was to brown coat and
              finish the side wall
the color finish
The next step was to brown coat and finish the side walls. Wood strips are used to form the arrises on the columns. We didn't want to use metal corner beads because this is color work, and I didn't want to leave the metal exposed.
We painted my white vault ceiling with transparent plaster bonder from USG, and put on the color finish.
Gothic vault ceiling in Washington,
              DC
Finished hallway. I didn't have to trowel the ceiling so smooth, seeing how it got another finish on top, but I did it anyway.
I did the vault by myself, and my employees did the brown on the side walls and the decorative color finish.

If you are wondering, the vault took me 7 days, working 4 hours a day.