For discussion, see two pages every practicing field archaeologist should read, "Protocols: Ceramic Artefacts and Skeletal Material," in Archaeology Meets Science: Biomolecular Investigations in Bronze Age Greece, eds. Y. Tzedakis, H. Martlew, M.K. Jones, Oxbow 2008 [ISBN 1-84217-238-7, WorldCat] 236-7, the upshot of which is: when handling ceramics, don't use plastic bags, don't use plastic gloves (if gloveless, use "hands that are free of potions and lotions"), and don't wash with acid.
"Archaeologists can render science useless unless as excavators, they handle their material in the right way...
No one, not any archaeologist or excavator, can be criticised for the way ceramic or skeletal material was excavated, cleaned, or stored, until contaminants started appearing in the organic residue results and were traced back to their sources. Excavators did what they thought was the best thing, and it was, until science came along and changed the rules.
From now on, however, there is no way an archaeologist can escape condemnation if he/she wilfully allows information that has been stored inside a pot or a bone, to be destroyed.
It is now an absolute obligation for excavators to think about the future application of science to archaeological subjects, and to prepare and archive artifacts accordingly, i.e. in such a way that the chemical signals that have survived hundreds or thousands of years, are not contaminated or destroyed through thoughtless handling and storage."
(And to make this post strictly relevant to the blog as a whole... pages 273-280 of the above-quoted book describe organic residue analyses of a Canaanite jar; a Myceneaean Vapheio cup, crocus-painted askos, and pithos; and local carinated cup and barbotine jug from the settlement of Punta d'Alaca on the island of Vivara near the Bay of Naples. Among the residues identified were vegetable oil flavored with a woody herb/bark, herb-flavored unresinated wine, olive oil flavored with an herbal extract, and an herbal mixture possibly flavoring milk or cream.)