What is this?

It's a five-pound unmarked bag of white powder traveling across an international border. The Mexican customs agents pulled me aside and crowded around my suitcase. I couldn't help but chuckle when they pulled this out with furrowed brows. One woman picked it up and sniffed it. I explained to everyone that it was a chemical (soda ash) to help me dye fabric, but they pretty much ignored me and ran a test with this ultra-violet light gun. Phew! -PJ