It’s been a few years, but they are white round pads, probably 3 inches wide. I’m not sure what’s on the pad, probably a solvent of some kind.

The pads go in a machine about the size of an larger microwave oven which I believe uses NMR to scan for nitrates and other kinds of explosive residue.

It was a pair of highschool level sumo robots, lots of wires and motors and gears.