>Here's a question. What happens when you take methyl cellulose powder on a plane? (Checked or carry-on)
I think it will depend on packaging. In a jar with a commercial label, particularly if it looks like it's unopened (particularly if it says wallpaper paste)? Probably nothing. In a little zip-lock bag that makes it look illicit? Probably not the best. Ultimately it is possible to test the substance, especially at international airports, which tend to have machines "onsite" (onsite being in the airport area, which could cover a lot of grounds). But it would also depend on how paranoid the local custom is, and how strict the country is with drug laws. You probably wouldn't want to risk it in Indonesia or Thailand where "minor" drug offences have major consequences.
