The container can be glass, plastic or even a Ziploc bag (I've used them all). Just make sure that it is airtight.
After a short while (maybe 15-20 minutes), shake the container around a little to redistribute the jewelry. I've found that if you don't get the jewelry moving around, some of the silver may not oxidize very well.
This method of oxidizing sterling silver may take longer than using liver of sulfur, but it seems to work just as well - and you don't have to go outside in six feet of snow to do it!
If the egg gets cold before your sterling silver piece has oxidized as much as you'd like, remove the egg from the container and pop it into the microwave to heat it back up. Then place it back into the container and seal it up again.
I have used this process to oxidize ceramic beads dipped in silver also. When oxidizing ceramic beads dipped in silver, some of the pieces seemed to bubble if I laid them in the container. So what I do now is string them on cord with knots in between each bead to separate them. This seems to give better results.
Oxidizing Article by Pat Greer