Common microwave absorbing substances I'm looking to demonstrate to class of students how microwave absorption works, but I need something relatively common which I can easily make into a board. Does anyone have any suggestions for substances which I could use (currently my best idea is sandwiching a plastic bag full of gel or cheese - something viscous enough not to leak easily, but with a high water content - in between two boards, effectively making an absorbing plane to contain the microwaves) for this?
Oh, and for more information I'm using a microwave transmitter, receiver and oscilloscope for the rest of the setup.
Thank you for your time!
 A: When I do demonstrations of this type, I use a hand and also a wet sponge. I demonstrate that a dry sponge doesn't absorb microwaves, then wet it and show that the wet sponge does. If you want to be able to do the prep and assembly in advance, I would just put a wet sponge inside a ziplock bag. If you need a larger size, I bet wet paper towels inside a freezer-bag ziplock would also be a very strong absorber. You also might be able to find rectangular sealable bottles that you could just fill with water.
In comments, people have suggested ice. Actually, I believe ice is a poor absorber. This is basically work = force x distance. The water molecules are locked in place and can't rotate.
A: Maybe I misunderstand what you need, but why not use specialized microwave absorber products (carbon/foam/magnetic iron based), such as that at http://stores.cumingmicrowave-online-store.com/c-ram-lf-75/ - flat panel 61x61cm, 20dB absorption (with metal backing).
If $95 is too expensive, you can look for something less efficient or less broad-band. Please read carefully manufacturer's notes.
A: Plasmas absorb microwaves like crazy https://www.youtube.com/watch?v=G7lfzA7WzVI
