How to bake egg shells?

Hi
I am still having problems with one of my girls having thin shelled eggs, I have tried limestone flour etc but it didn't last long and have now just started giving them shell max booster and did get one good egg then another broken one, anyway I try to give mine some baked egg shells but I was just wondering how exactly to bake egg shells?
I put mine in the aga but for how long and can you over bake them as I often forget about them being there.  Also how small do they need grounding to?
Thanks
Pauline

Yes a aga will be great. I crush them down and give them back mixed with the layers. Do your girls have oyster shell mine have theres all the timse i put it into a plant pot and fix it to the wire mesh.

When I've baked egg shell I give them a whizz in the magi- mix.