"Dave" wrote

Only significant use I've seen for eggshells is for feeding chicken hens.
Prevents softshell. No, not a hoax.

Agreed. Years ago my mother started keeping chickens and at first the eggs
just wouldn't harden, break under the weight of the hen, or dry in all sorts
of alien shapes.

A local egg farm finally gave her the answer. She'd break up the shells
(after use) a bit, rinse well in water, and let dry. Into a low oven for
maybe ten minutes and the shells would break up nicely by hand crushing to
be added to the chicken feed. It didn't take long at all until all the hens
were laying proper eggs.

OT, but somewhat related:
A friend of mine was surprised to find that her MIL, who'd been born and
raised in Mexico to missionary parents born and educated in the US, had been
using the above technique for years to add to hamburger dishes. Apparently
MIL's parents knew quite a lot about the importance of such things, but
couldn't buy vitamins and such in remote Mexico. Through their church, they
developed a lot of interesting ways to add the extras to their family's
food, and in the process taught through example nutrition techniques to
their constituents. Cool.

~~ Shelly ~~