I have 40 pounds of Drystall as well as white pumice bought from a nursery.
When I put it into a bowl of water it sinks. If you look at Drystall and
pumice under microscope, they are identical.
Your data point on Crater Lake is interesting, and I don't know why your
pumice behaves differently than mine. If I read the pumice raft idea
correctly, this just means that sometimes a large piece of pumice traps a
large air pocket internally.
Perlite, however, is a significant floater.
Sorry for top posting but I could not get my reader to quote your post
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