Hi, everyone. I initially posted my inquiry on the Pottery board and have received some valuable information. Here is a link to that post, including photos:
http://community.ebay.com/t5/Pottery-Glass-Porcelain/Antique-Plaster-Ceiling-Light-Fixture-Medallion...
I'm concerned about doing a more thorough cleaning. It has a lot of aged grime on it, and need your expertise on how to clean it up without damaging any of the plaster and paint.
Thanks so much for your thoughts.