very hard shell and strong membrane

[uncle bulgaria]

will chicks manage to get out of these shells, they are super strong i have trouble cracking them with the back of a knife. Only I am ready to set the incubator for spring but i never had such hard shelled eggs before. (buff Orpington)

 

[nuthatched]

I wouldn't set eggs from that hen. I'm guessing, but if you have to use a knife...

 

[Tonyroo]

You want good shaped eggs with smooth shell for incubation. Some shells maybe slightly thicker than others.

But this is ok, as the chick starts to develop bone structure it's using the calcium from the eggshell. By the time it hatches the shell is thin enough for the chick to break through from the inside.