Apologies if my query is not clear.
I want to make a coving in situ, (as opposed to a cornice, which is normally cast "off site", then fixed with adhesive or screws, to the wall and ceiling.)
The traditional way of making a cove, is to fix a wooden batten to the wall, just below the lower edge of the planned cove. Plaster is then built up in the corner (wall to ceiling) and a specially made "runner" with a metal profile of the cove is "run" along the batten, scraping away excess plaster, leaving the cove. After several passes the cove is gradually produced. It's not rocket science, but requires skill, (which in my case is a bit limited).
It also requires knowledge - which is what I am seeking! - What kind of plaster should I use? Can I use ordinary finish plaster, or does it require anything adding to it, (e.g. lime)?
Hopefully this is a little clearer.