So, funny you should ask. I'll paint a picture of me squatted down with a rock in my hand and a pile of oyster shells on a flat rock at my feet. I often picture myself in a primitive manner while I am bent over aggressively bashing the oyster shells with a rock. It takes a bit of time, but i only have 5 hens so every month I'll crush like 8 shells. When i first started using them, the girls shells were like double shells! The oysters have lost their appeal and the eggs are now what i'd call normal instead of double shelled. Lol good luck! Before crushing, i rinsed well and air/sun dried the shells for about a week before offering. I think that when i first offered the oyster shells, they still had salt water and maybe some oyster residue still attached and thats why they were eating them more and resulted in super hard shells. I had to do something because they wernt touching the oyster shells from pro mana. I realized that the calcium from the pro manna bag isnt even real oyster shells. No wonder they didnt want it.