What materials will hold microwaved heat the longest?

I like the rice ones. They smell nice.

Have I got the material for you!

There’s actually heating pads that you can put in the microwave. They stay hot for a pretty long time. If you’ve got radiator head (or you can get a radiator type heater), you can throw a blanket over it for 5 minutes or so then bask in it’s warmness. I do this but I wouldn’t actually recommend anyone else do this because surely somehow (even with radiators) you can set your house on fire.

If you try this, keep a fire extinguisher at the ready.

glass pebbles ;) either that or “black stones”
Both found in Michaels as well.

Harp’s suggestion is really exciting!, but flaxseed is also very good: smells nice, and stays hot for a long time, maybe an hour if you have enough volume, say 4 pounds. I’ve been throwing little home-sewn bags of it down where my feet are gonna go for years. zzzzzz

I have two rice bags that I have used for 5 winters. They still work great, even after 500+ heatings.

Just a question out of curiosity, since you all seem to be gurus at heating things up in the microwave. If I were to fill up a bean bag with, per se, rice or beans…how many times would I be able to heat ‘em up before having to dispose of them? =S