I’ve known about sprinkling broken eggshells around young plants for a while, but until last year I wasn’t sure how to reuse my eggshells in such a way.
Last summer we spent a disastrous fortnight camping in Wales, the weather was awful. We did visit my Great Aunt in Swansea for the day however, who told me what she does with her eggshells. So simple and obvious really, but now I know it works I’m happy to make my own.
She keeps an old baking tin at the bottom of her oven and puts used shells in it. So when the oven is on, it bakes them.
I’ve found the smell of baking eggshells a little unpleasant, so I tend to keep the tray to bake in the other oven. When I remember, I put the tray in the hot oven after removing the cooked food so that the shells bake in the cooling oven. I usually do this twice, just to make sure they are brittle enough.
They then get crumbled and stored in a tub for use in the garden.
In terms of effectiveness against slugs and snails, well I wouldn’t rely on eggshells alone to prevent favourite young plants being eaten. But every barrier helps, and the eggshell pieces can always be dug into the soil after the season.
This photo implies we eat A LOT of eggs :-) On the contrary, actually. We had omlette tonight for the first time since the start of lent. The broken shells are about 8 months worth of eggs.
Living on the Veg 
It looks like a scene from Cool Hand Luke!
I’m going to set up a whole Indiana Jones style slug barrier incorporating the shells, copper wire, organic pellets and field mice with search lights and AK47s. If they get through that, a fat ball will drop from the bird feeder and chase them round the garden in a slo-mo-style tension scene, I say slo-mo-style because it’ll actually be slow, because it’s a snail and they’re not known for speed.