she smashes seashells in the springtime

I had picked up a couple of lots of someone's beachcombed shells. They were a couple of dollars and I took the big ones for toad hides.
These small ones aren't nice enough for any art projects so I'm breaking them up.
It's spring and I've got trees to plant so I'm going to add some to the planting holes along with some biochar.

I've found seashells remarkably hard to bust up. Are you just hammering the hell out of them? Do you have any tricks to share?

Why break them up? They would be lovely as mulch just the way they are....

There's a trick. Just put them in a fire for a few minutes, and you can crush them easily in your hand. The fire doesn't even have to be particularly hot. I guess it's a matter of both heat expansion and burning off any non-mineral stuff that might be holding them together. It smells a bit... special when they start heating up, so I'm fairly certain something in them burns.

I did just hammer away. Some were pretty soft since they had clearly been in the water a while before washing up but they all were not too hard to smash. There is some scrap leather under it all and the brick with holes worked great. I could just brush bits down into the holes and keep going and the leather made it easy to collect and move to the the container.

I'm using them as a calcium source, not mulch. The compost pile at our old place had come with a ton of oyster shells in it. Since they were there, I used them and would often find roots clutching them when clearing the annual beds so I started breaking them up and that seemed to help the tomatoes especially.