I keep a pizza pan inside my oven, and I put rinsed egg shells there until I fill it up, or use the oven to bake something. I have found that 5 minutes at 200 degrees F make the shells brittle and they crush very well in a plastic bag. When I fill the bag, they go out to my chickens in a pan. I also use crushed oyster shell, but they will eat the egg shells first.