Yet another use for old eggshells. Snails and slugs have a soft underbelly which makes it difficult for them to travel over crushed eggshells.
I have never had enough egg shells to be able to put this theory to the test but you could always ask a local cafe or restaurant if you could collect from them.
I have combined eggshells and hair to deter these pests and that has been successful, I do think it is a better slug catcher than many I have seen.
I always wash the eggshells in clear water before I crush them too much as the mucus from the eggshell may attract other pests. No point in getting rid of one vegetable garden pest only to end up with another one.