These heaters can be warmed in the microwave or the oven hundreds of times and stay warm in a pocket for almost an hour.
Flax Seed Hand Heater
Heat 1.5-2 minutes. No more than two minutes
Set oven to 200. Heat warmer for 5 minutes. Remove and shake. put back in the oven for 2 more minutes.
However you warm it, be careful not to overheat it.