Tim's right; I'd recommend consulting conservators about proper treatment of
sherds from privies and definitely not licking them.  We've watched various
substances migrate to the breaks of our privy and other sherds before we learned
to soak them all to remove various impurities (again, to speak euphemistically).
 At the very least I'd recommend soaking in distilled water and/or hydrogen
peroxide to remove organics.  I do believe that Donny Hamilton has posted useful
advice about such things on his Texas A&M web pages.

MCB