Using citric acid as an etching mordant

I'm looking for a mordant for etching metal plates (zinc, copper, aluminum etc) that could be considered less hazardous than traditional acids such as nitric acid or ferric chloride. I have tried phosphoric acid at very low levels, for 24 hour periods but the etch is shallow and uneven. I wondered about using citric acid ⇦ on eBay or Amazon [affil link] . Do you have any suggestions?

There are several good sites for non-toxic printmaking that should give you some good information about etchants. I would suggest you first go the MTSU (Middle Tennessee State University) Printmaking Discussion Forum and search the archives under non-toxic printmaking, Bordeaux etch, etc. Try Cedric Green's Non-Toxic site, Edinburgh Printmakers site, Nik Semenoff's site, and the Grafisk Esperimentarium. At the last, you can look at Henrik Boegh's new book, Non-Toxic Intaglio Printmaking. Links for many of these should be available at the MTSU site or use Google.