The trick I use for trikmming these sorts of shapes is to make a chuck and trim them in one of these.
A chuck is a "donut" affair as you have imagined. It can be made from ANY clay actually and even bisque fired. At the co-op where I do most of my work we have several in various shapes and sizes that have been bisque fired. They don't have to be though. Read on...
Basically, there are two different ways of handling chucks:
If bisque fired - wrap a coil of clay around the edge that will come in direct contact with the item being trimmed. Sometimes you can get away with a piece of foam or even a paper towel or two. The purpose for this primarily is to protect the surface from getting gouged or in any way damaged.
If raw clay (way I prefer) make the chuck any way you need to have it fit the shape you are trimming. Gently taper the opening. This way the chuck will be self centering for the object you are putting into it or on it (Sort of like the principle behind a griffen grip). Take a stainless steel rib to the surfaces of the chuck to remove any wet clay and/or slop. I'll even take a torch to it a bit to be sure it's much drier than it would be as freshly thrown.
Invert your vase or other piece into the chuck.
And of course you can use the griffen grip and just set your piece up higher so that no weight rests directly on the rim. But I figure you don't have a grip or don't have ready access to one otherwise you'd have figured this out already! I actually really prefer making my own chucks for some shapes and the one you mention is one where I'd rather do it all myself.
Get a small level to be sure the bottom is in fact level. You don't want to trim it at an angle so that your piece will have a list to starboard!
Once you get the hang of this, it's really very easy and not at all time consuming. Hopefully others can and will also explain this process.
Hope it helps
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>From: Duane Archer >Reply-To: Clayart >To: CLAYART@LSV.CERAMICS.ORG >Subject: How do you trim bottom of fragile rim vase >Date: Mon, 15 Dec 2003 12:39:38 EST > > I have begun making eight pound vases with rims that are about 3/16 >inch in thickness. When I place the vase on the wheel (upside down) to trim >the bottom; The weight on the rims are enough to break or distort them. How >do I prevent this from happening. Do you experts make a tall donut of clay >to support the shoulder of the vase and keep the rim from touching the bat? Or >do you make the rim thicker to support the weight? I really like the thin >rim and would like to keep that as part of the vase. What are your >suggestions? Thanks in advance!!! Duane Archer, Tulsa Potter > >______________________________________________________________________________ >Send postings to firstname.lastname@example.org > >You may look at the archives for the list or change your subscription >settings from http://www.ceramics.org/clayart/ > >Moderator of the list is Mel Jacobson who may be reached at melpots@pclink