I read somewhere on here they put their egg shells in the microwave for 30 seconds. feed it free choice if you have a rooster, they don't need as much calcium as it can damage their kidneys and liver - hope that helps
I save all my egg shells and then put them in a microwave for 2 minutes. I then put them in a small coffee grinder and it makes them into a powder that I mix with their feed or put in their yogurt and fruit as a treat. They never leave anything over and I think it is good for their egg production.