Unpopular opinion: I stopped cleaning finds in the field after I scrubbed the patina off a bronze brooch near Colchester | Archaeology Discoveries - Chatter Central
Cleaning in the field is how you actually learn what you've got though. That brooch taught you a lesson, sure, but now you might leave mud caked on stuff for hours and miss tiny details or damage that shows up right away. A soft brush and some water won't kill most finds, you just went too hard on one piece.