feeding egg shells

Two (I have 7)of my girls recently started laying!! It is very exciting. I had been collecting egg shells to bake and crush to supplement them. I'm not sure I really need to do that now because I noticed their shells are VERY strong and thick . However, does anyone know what temp you need to bake them at and for how long if you choose to supplement them with egg shells?? I do feed them some layer feed so they probably have been getting plenty of the nutrients they need! I'd like to feed them more table scraps and such once I can get used to saving some of that stuff to give them and see what they like.

Thanks for the information in advance. I am happy they are laying as I'm in Texas and it's been 100+ for it seems like forever, they are tough though!